Degassing of deep groundwater in fractured rock

Transcription

1 WATER RESOURCES RESEARCH, VOL. 36, NO. 9, PAGES , SEPTEMBER 2000 Degassng of deep groundwater n fractured rock around boreholes and drfts Jerker Jarsj6 and Georga Destoun Dvson of Water Resources Engneerng, Department of Cvl and Envronmental Engneerng Royal nsttute of Technology, Stockholm, Sweden Abstract. Deep groundwater contans dssolved gases that may come out of soluton f the water pressure s lowered. Water pressures are often decreased down to atmospherc pressure when water s wthdrawn from deep boreholes and drfts n the bedrock, for example, n nvestgatons regardng nuclear waste dsposal. Groundwater degassng may then contrbute to the development of a local, unsaturated zone around the borehole or drft, whch may affect nflow and, as a consequence, the outcome of hydraulc and tracer tests n fractured rock. Laboratory experments wth gas-saturated water n rock fracture replcas have demonstrated that degassng causes consderable hydraulc conductvty reducton under certan condtons. Degassng has also been hypotheszed to be the cause of observed flow reductons n the feld; however, supportng feld experments have so far been lackng. We report results that consttute the frst feld support of the development of an unsaturated zone and, as a consequence, hydraulc conductvty reducton due to groundwater degassng around a borehole. The borehole tests were conducted approxmately 450 rn below the groundwater table at the 3,sp6 Hard Rock Laboratory n southeastern Sweden. No hydraulc conductvty reductons were observed at gas contents of about 1%, whereas a 50% reducton n hydraulc conductvty was observed at a gas content of 13%. Formal hypothess testng, based on all avalable feld and laboratory degassng tests, supports degassng as the actual cause of the observed hydraulc conductvty reducton at the hgher gas content. We also show analytcally that hydraulc conductvty reductons due to degassng may occur at much lower gas contents around drfts than around boreholes. 1. ntroducton [B ckblom, 1991; Banwart et al., 1997]. Borehole pumpng tests and pressure recovery tests are standard technques for the The quantfcaton of flow and transport propertes of frac- determnaton of the hydraulc propertes of fractured rock tured rock has receved much nterest snce the 1980s because (see, e.g., de Marsly [1986] for an extensve revew of sngleof the potental fnal storage of radoactve waste n the bed- well hydraulc test methods). n addton to such hydraulc rock n several countres. The concept for the storage of hgh- characterzaton carred out n boreholes, drft nflow measurelevel radoactve waste preferred by the Swedsh Nuclear ments are also mportant for the hydraulc characterzaton of Waste Management Company (SKB), for nstance, nvolves larger rock masses. Water s collected n dtches that cross the the constructon of a fnal repostory n grante bedrock at drft floor, and the total nflow versus tme s measured for rn depth. For research and demonstraton purposes a specfc drft sectons. full-scale underground research laboratory (the sp6 Hard When hydraulc testng s performed several hundreds of Rock Laboratory (HRL)) has been constructed n the bedrock meters below the groundwater table, saturated condtons are n southeastern Sweden [see Banwart et al., 1997]. Through generally expected to preval. Durng a hydraulc test seres n both modelng and expermental studes conducted at the the Strpa mne n Sweden at 385 rn depth, however., Olsson 3,sp6 HRL and other stes n deep bedrock, such as Grmsel [1992] and Brgersson et al. [1993] observed that the nflow to a and Wellenberg n Swtzerland and Yucca Mountan n Ne- drft was a factor of 8 smaller than the nflow measured at the vada, the hydraulc and transport propertes of the (fractured) same locaton before the drft excavaton, usng sx boreholes bedrock have been nvestgated under a range of condtons. formng a rng. Olsson [1992] and Brgersson et al. [1993] sug- Phenomena and processes that may affect these propertes gested that the reduced nflow could have been caused by the around deep repostores have hereby been addressed, for ndevelopment of an unsaturated zone around the drft due to stance, evaporaton and thermal loadng effects [Fnsterle and degassng of groundwater n the low-pressure zone near the Pruess, 1995; Walton, 1994; Pruess et al., 1990], mcrobal acatmospherc pressure boundary at the drft wall. The volumettvty [Kotelnkovand Pedersen, 1997], and excavaton effects rc gas content, that s, the volume of gas at atmospherc pres- [Emsley et al., 1997]. sure condtons per total water volume, ranged between 2% One of the man goals for the research and development and 4% at the ste. As opposed to the drft case, water preswork at sp6 HRL concerns the qualty and approprateness sures around the boreholes were consderably above atmoof dfferent methods for characterzng bedrock propertes spherc pressure because of borehole pressure regulaton thus Copyrght 2000 by the Amercan Geophyscal Unon. Paper number 2000WR /00/2000WR $09.00 preventng degassng. The potental occurrence and mpact of degassng, relatve to other phenomena that may have contrbuted to the observed 2477

2 2478 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER nflow reducton n Strpa (such as changes n the rock stress ng the Strpa observatons [Olsson, 1992; Brgersson et al., condtons due to the presence of the drft), could not be quantfed based on avalable data. The observatons n Strpa thus rased mportant questons about to what extent groundwater degassng may be expected to affect hydraulc property 1993]) s then used to address the overall objectve of ths study: to nvestgate under whch condtons degassng may be expected to cause flow and hydraulc conductvty reductons around boreholes and drfts n the feld. Wth ths purpose we values that are determned from drft nflow measurements at formulate a hypothess regardng the condtons requred for large depths below the groundwater table or from hydraulc the occurrence of groundwater degassng, whch we then test tests n boreholes. t s concevable that degassng can arse as usng all avalable, relevant laboratory and feld data. a drect result of lowerng of water pressure that occurs n connecton wth hydraulc testng; however, degassng may also nfluence the hydraulc condtons even before the test start. At 2. Statement of the Problem sp6 HRL, for nstance, the drft s subject to atmospherc 2.1. Basc Relatons pressure, and many boreholes used for characterzaton pur- The extent of the zone around a borehole, or a drft, where poses ntersecthe open drft. As a consequence, atmospherc groundwater degassng may occur s lmted to the regon pressures preval durng drllng, whch mples a possblty of where the water pressurepw s lower than the bubble pressurep,: degassng and an unsaturated zone development smlar to that hypotheszed n Strpa. f degassng of deep groundwater s a pw<pb. (1) sgnfcant factor n controllng the hydrology around bore- n the followng we wll refer to the zone around a borehole or holes and/or drfts, t s essental to know ths when exper- drft, where (1) s satsfed under water-saturated (sngle ments are planned, executed, and evaluated. phase) condtons as the low-pressure zone. The extent of the Wth regard to the prevalng volumetrc gas content n deep low-pressure zone X o w s then a measure of the sze of the groundwater, measurements at varous stes n Sweden have regon where groundwater degassng can possbly occur. n shown results that are smlar to those of the Strpa mne. For addton, dfferences n propertes between dfferent gases mexample, at the sp6 HRL (at m depth) the gas ply that the volumetrc amount of gas that wll be released for contents at atmospherc pressure condtons ranged between a specfc water pressure lowerng (below p,) s gas specfc. 0.1% and 5% [Geller and Jarsj6, 1995; Nlsson, 1997; Pedersen, Provded that equlbrum prevals between a gas phase and 1997], and the gas contents at Laxemar (located about 1 km lqud phase, the relaton between the partal pressure of the from Asp6 HRL) at m depth ranged between 2% gas pa and the concentraton of the gas dssolved n the lqud and 5% [Pedersen, 1997]. Further, the data show that there s Cat follows Henry's law: a consderable spatal varablty n gas contents wthn each ste. The groundwater gas content n two dfferent fractures pg = HCgt, (2) some 20 m apart may, for nstance, dffer by a factor 2. At all where pa s the absolute gas partal pressure n klopascals, H the above-referenced stes, ntrogen s the domnatng gas, s Henry's law constant for the gas n the lqud n kpa m 3 occupyng approxmately 80% of the total gas volume. Some of the ntrogen may have an atmospherc orgn. However, n mol-, and Cgl s the molar concentraton of the gas dssolved most cases the observed gas contents exceed the solublty lmt n the lqud. The decrease n dssolved gas concentraton A Cgl at atmospherc pressure, whch mples that more ntrogen s due to a water pressure lowerng to P w < P, = P a then added at some depth, possbly through mcrobal processes becomes ACg -- (Pb-- pw)/h. Ths decrease n dssolved gas concentraton can be related to a correspondng evolved volu- [Pedersen, 1997]. At the Wellenberg ste n Swtzerland a varablty n gas metrc gas content A 0 a = A Va/V w (gas volume comng out of soluton per total water volume) through the deal gas law as composton and content wth depth has been observed, wth the more shallow groundwater beng domnated by ntrogen pb -pwrt and the deeper groundwater beng domnated by methane A Oa = Pw ' [Natonale Genossenschaft fr de Lagerung Radoaktver Abf lle (3) (NAGRA), 1997]. At m depth the formaton water s generally close to fully saturated wth methane at formaton pressure (wth local exstences of a free gas phase [NAGRA, 1997]), mplyng even hgher volumetrc gas contents at atmospherc pressure condtons than for correspondng depths at the Swedsh stes. To address the potental degassng problem, the focus of an expermental feld program was drected toward borehole tests, because they are easer to control and may be conducted at much lower costs than drft experments. n ths study, we where R s the gas constant and T s the absolute temperature. Thus the bubble pressure Pt, can be estmated from (3) by measurng the evolved gas volume AV e from a total water volume Vw when the water pressure s lowered to Pw. For radal nflow to both boreholes and drfts the lowpressure zone s stuated n the vcnty of the borehole/drft (hereaftereferred to as the well), as a result of steep hydraulc gradents caused by convergng flow. Gven the hydraulc head qb = H 0 at the outer boundary (dashed crcle n Fgure 1) at dstance r 0 from the well center and (k - Hw at the well face report and nterpret a seres of feld degassng experments (dstance r w n Fgure 1), the steady state hydraulc head proconducted at Asp6 HRL, whch nclude the frst feld support fle around the well s [Bear, 1979] of the development of an unsaturated zone and, as a conse- n (r/rw) quence, hydraulc conductvty reducton due to groundwater rk(r) = Hw + (Ho- Hw) n (ro/rw)' degassng around a borehole. n addton, we summarze an (4) earler feld degassng experment at sp6 HRL [Geller and where (H 0 - Hw) s the drawdown at the well, r s the radal Jarsj6, 1995] along wth a seres of laboratory degassng exper- dstance from the well center, r w s the well radus, and r 0 s ments [Jarsj6 and Geller, 1996]. The complete set of avalable referred to as the radus of nfluence (Fgure 1). The water laboratory and feld studes of groundwater degassng (nclud- pressures n a fracture plane at an angle of degrees from the

3 JARSJ0 AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER 2479 Fracture plane z # # \ Borehole Horzontal plane Fgure 1. Schematc vew of a borehole ntersectng a fracture. horzontal plane s gven by pw(x, z) = pg[rk(x, z) - z sn ng flow, regardless of the value of. At 300 m depth (assum- ( )], where p s the water densty, g s the mass gravty, and x ng well rad between 0.03 and 3 m and a radus of nfluence and z extend n the plane of the fracture; the coordnate system of 150 m) the hydrostatc pressure dfference s 5% or less of s shown n Fgure 1 wth the x axs beng horzontal. Equaton the pressure dfference caused by the drawdown. Thus fracture (4) can be transformed to Cartesan coordnates order to orentaton s not expected to nfluence the occurrence of deexpress the pressure profle around a borehole ntersectng the gassng to any measurable degree and wll therefore not be fracture plane accordng to consdered further n the followng comparson. pw(x,z) = pg Hw + (Ho- Hw) ln(x/x2+z2/rw) n (ro/rw) - z sn ( ) 2.2. Example llustraton The mplcatons of (1),(3), and (5) for the case of degassng of deep groundwater around boreholes and drfts are llusrw x/x 2 + z 2 ro. (5) trated n Fgure 2, whch shows a comparson of the estmated Equaton (5) mples that n mmedate vcnty of the well, pressure feld n a horzontal fracture ((5) wth = 0) around wthn one meter's radal dstance from the well bore, for a borehole (sold lne) and a drft (dotted lne) at 300 m depth, example, the hydrostatc pressure dfferences are neglgble n assumng borehole and drft rad of 0.03 and 3 m, respectvely. comparson to the pressure dfferences caused by the converg- Fgure 2 llustrates that the pressures around the drft are

4 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER 2000T! ;T:- Borehole: radus=o.o3m 1600 s=3m % N2 gas (bubble pressure < 1000 kpa) oo 2o0 0!,,,,,,,,! Radal dstance from borehole/drft wall (m) 1.5% N2 gas (bubble pressure 100 kpa) Fgure 2. The pressure feld around a borehole n comparson wth a drft. sgnfcantly lower than those around the borehole at the same radal dstances. f one assumes that water releases 1.5 vol % ntrogen gas as the water pressure s lowered to atmospherc pressure (whch corresponds to a ntrogen bubble pressure of nfluence r o (of 150 m), because of the logarthmc relaton n (4) and (5). The actual extent of a developed gas-contanng zone Xac may, n fact, be smaller than the full low-pressure zone X]ow, 100 kpa accordng to (3)), the radal extent of the low-pressure because of the knetcs of the bubble nucleaton process [see, zone (where the water pressure s lower than the bubble pressure) s X]ow = 0.4 m for the drft, whereas X]ow s only 0.01 rn for the borehole (Fgure 2, bubble pressure llustrated by dashdotted lne). Fgure 2 also shows that the extent of the lowpressure zone, X ow, around a borehole s ncreased by about 50 tmes to 0.5 m when ncreasng the bubble pressure 10 tmes to 1000 kpa, whch corresponds to 15% evolved N 2. The correspondng area of the low-pressure zone (A]ow = r(x]ow + e.g., Parlar and Yortsos, 1989]. Further, the evoluton of a separate gas phase s expected to ncrease hydraulc gradents and water pressures n the vcnty of the borehole relatve to the sngle-phase case, whch would also reduce the actual, steady state, gas-contanng zone Xac relatve to the full lowpressure zone X]ow. Ths expected change n local water pressures under degassng condtons s llustrated schematcally n Fgure 3 for lnear flow condtons. The bold lne n Fgure 3 rw) 2 -- rr2w) s then enlarged by 350 tmes. Ths calculaton llustrates the expected sngle-phase pressure dstrbuton unexample llustrates that smlarly favorable degassng cond- der water-saturated condtons between the fracture outlet tons, n terms of the sze of the low-pressure zone, may be obtaned around drfts at lower gas contents as around boreholes at hgh gas contents. The results llustrated n Fgure 2 are relatvely nsenstve to the assumed value of the radus of (x = 0), where the water pressure equals the atmospherc pressure P atm, and the outer boundary (x = L), where the water pressur equals po. The bubble pressure pb s llustrated by the dotted lne, and the low-pressure zone extent at satu- Po Pb -- two-phase (water+gas) --sngle-phase (water) Patm Xgc Xlow 0 Length (x) L Fgure 3. llustraton of the expected effect of a local gas-contanng zone Xgc on the pressure dstrbuton for lnear flow condtons: a comparson between sngle-phase and two-phase condtons.

5 JARSJ(3 AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER 2481 rated condtons X ow extends from x = 0 to the ntersecton between the bubble pressure lne and the sngle-phase water pressure lne. The thn lne n Fgure 3 llustrates the water to nvestgate whether the lowerng of pressures down to atmospherc pressures n a borehole ntersectng a water-bearng fracture would lead to degassng and formaton of an unsaturpressure dstrbuton that s expected as a result of a gas-phase ated zone n the vcnty of the borehole. Furthermore, the development wthn the low-pressure zone and a local decrease possble effect of ths unsaturated zone on the fracture conn fracture transmssvty due to unsaturated water flow wthn ths zone. Local transmssvty reducton mples a steeper pressure gradent n the vcnty of the fracture outlet, resultng n a reducton n the gas-contanng zone under two-phase condtons, relatve to the low-pressure zone under waterductve propertes was to be quantfed. However, there were no ndcatons of sgnfcant hydraulc conductvty reductons due to degassng the plot hole test [Geller and Jarsj6, 1995]. The test was conducted n a fracture wth a transmssvty of approxmately 10-6 m 2 s -, and the observed flow ratesaturated condtons. pressure drawdown relaton was lnear all the way down to 2.3. Degassng Hypothess Formulaton There are at least two factors that may contrbute to a consderable hydraulc conductvty reducton as a result of degassng. Frst, the occurrence of bubble trappng mples that gas may accumulate n the fracture, such that the local degree of fracture gas saturaton (.e., gas volume per total fracture atmospherc borehole pressure, demonstratng the lack of any nfluence on flow rate. Besdes the conclusons regardng degassng, the observed lnearty thus also showed that the changes n stress condtons, due to the lowerng of borehole pressures, dd not nfluence flow rates (and hence effectve conductvtes) to any measurable degree. The local volumetrc groundwater gas content, measured onste durng the testng, volume) s consderably greater than the evolvng volumetrc ranged between 0.5% and 1%, whch s relatvely low. gas content A 0 a (see Gardescu [1956] for a detale descrpton The plot hole test thus ndcated that at these relatvely low of the behavor of gas bubbles n capllary spaces). Second, the nonlnear relatve hydraulc conductvty functons for unsaturated fractured meda mply that hydraulc conductvty decreases consderably wth ncreasng degree of gas saturaton [e.g., Pruess and Tsang, 1990; Fourar et al., 1993; Persoft and gas contents, degassng does not cause sgnfcant hydraulc conductvty reductons around boreholes. Through a seres of laboratory tests n transparent rock fracture replcas the hydraulc conductvty was measured under degassng condtons at hgher gas contents [Jarsj5 and Geller, 1996]. These tests Pruess, 1995]. However, nether of these two factors can pos- were conducted under radal flow n order to smulate the sbly contrbute to hydraulc conductvty reducton unless a separate gas phase forms wthn the fracture pore space. Further, the sze of the fracture zone where gas forms and the local hydraulc conductvty changes must be suffcently large n water flow around boreholes and drfts. For volumetrc gas contents above 3% a consderable gas saturaton was developed n the fracture, and, as a consequence, the transmssvty was sgnfcantly reduced by a factor of 5. relaton to the total fracture sze; otherwse, the effectve hydraulc conductvty of the entre fracture wll reman essen- 4. Descrpton of New Feld Experment tally unchanged. Snce X ow (see Fgure 3) provdes an upper lmt for the 4.1. Expermental Procedures actual sze of the developed gas-contanng zone Xac (Fgure An njecton-wthdrawal degassng test sequence was de- 3), t s reasonable to hypothesze that X ow must exceed some sgned to detect degassng effects n the feld at the 3,sp6 HRL crtcal length measure n order to make the condtons for gas-phase development favorable and the hydraulc effects of groundwater degassng sgnfcant. n ths paper, we wll test and conssted of a seres of constant pressure tests (CPTs) and pressure recovery tests (PRTs). Durng a CPT the borehole pressure s kept approxmately constant at a target pressure, whether there s such a correlaton between the estmated usng a back pressure controller, and the flow rate s monvalue of the low-pressure zone extent X ow and the actual outcome of a seres of feld and laboratory experments n terms of whether or not hydraulc conductvty reducton has been observed n each experment. The reason for ths formal tored. Durng a subsequent PRT the borehole s closed, such that the flow rate s equal to zero, and the borehole pressure s montored as a functon of tme. The new njecton-wthdrawal test sequence was smlar to the plot hole test sequence [Geller hypothess testng s that alternatve explanatons, besdes and JarsjS, 1995] and contaned three phases. n phase 1 the groundwater degassng, are concevable for experments where hydraulc conductvty reductons have been observed. Such alternatve explanatons nclude fracture deformaton due to changes n effectve stress durng the experment and/or turbulence effects. However, none of these explanatons would result n the hypotheszed correlaton between the X o w value and the expermental outcome n terms of the occurrence or absence of hydraulc conductvty reductons. The X ow value has relevance only for the degassng explanaton (see Fgure 3) and s meanngless for all other explanatons, mplyng that f a correlaton s found, t consttutes a clear support for the degassng explanaton. flow system was characterzed for sngle-phase condtons by a seres of tests at borehole pressures above the estmated bubble pressure. n phase 2, two-phase flow condtons were allowed to develop by reducng the borehole pressure to, or below, atmospherc pressure. The change n hydraulc conductvty was measured. Fnally, n phase 3 repeat tests were performed above the estmated bubble pressure to observe whether hysteress effects occur n the resaturaton of the system to sngle lqud-phase condtons. The man dfferences between the plot hole degassng test sequence and the njecton-wthdrawal degassng test sequence s that for the latter, we elevated the evolved gas content of the 3. Summary of Prevous Expermental Results groundwater by (1) njecton of gas-saturated water pror to/ durng the hydraulc testng and (2) lowerng of borehole pressures below atmospherc pressure. Gas-saturated water was n December 1994, Geller and Jarsj6 [1995] conducted a plot hole degassng test n a borehole approxmately 300 m below ntroduced to the fracture both through njecton-wthdrawal tests n the same borehole (sngle-well test confguraton) and the groundwater table at, sp6 HRL. The man objectve was through a contnuous njecton n one borehole whle perform-

6 2482 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER ng hydraulc testng n a neghborng borehole (dpole test These tests at undsturbed pressure condtons ndcated that confguraton). n the present set of three feld experments, the groundwater flow through the closed boreholes was relatwo sngle-well tests were conducted at dfferent locatons, tvely hgh wth values of 3.1 x 10-7 m 3 s -1 n P2 and 2.8 x along wth one dpole experment. Detals on the feld condtons for the CPTs and PRTs that were performed durng test phases 1 to 3 of these three feld experments are gven n secton 5 and n Fgure m 3 S -1 n P4. The shut-n borehole pressures measured n P2, P4, and P8 were approxmately 2000, 1000, and 400 kpa, respectvely. Ths s consderably lower than the hydrostatc pressure of approx- Fgure 4 (top) shows the two 300 L tanks used for saturaton mately 4000 kpa that can be expected at these depths as a of the njecton water wth N 2. Before saturatng the njecton water wth gas, we added a conservatve tracer (amno-g) n order to be able to quantfy the dluton of the njecton water durng the subsequen testng. The concentraton of other gases n the njecton water was mnmzed through the applcaton of low pressures at the top valve of the tank, and then N 2 was bubbled through the tank at a hgher, constant pressure for result of the nearby open drft. However, t should be noted that the shut-n borehole pressures are well above the estmated natural bubble pressures of kpa, and hence the drft s not expected to cause degassng the regon around these boreholes. nterference testng furthermore showed that the fractures ntersectng these boreholes were hydraulcally connected, whch enabled the dpole test confguraton. a mnmum of 48 hours. Durng the njecton phase of the experments (Fgure 4) the N2-saturated, traced water was njected n the dfferent rock fractures at a constant flow rate. 5. Results 5.1. Durng the wthdrawal phase (Fgure 4, bottom) the borehole Water Pressures pressures were kept approxmately constant usng a back pressure controller, and the flow rate, the borehole pressure, and the atmospherc pressure were montored. n the dpole test the njecton phase and wthdrawal phase were carred out smultaneously n two dfferent boreholes. The volumetrc amount of gas flowng out from the test secton was deter- Fgure 6 shows the test sequences of the sngle-well testng n borehole P2 (Fgure 6a), the sngle-well testng n borehole P4 (Fgure 6b), and the dpole testng usng P4 as njecton hole and P8 as wthdrawal-test hole (Fgure 6c). Each test s represented by a bar n Fgure 6, and the tests are presented n chronologcal order. Open bars ndcate njecton of gasmned both through drect measurements of the accumulated saturated water (NJ), damond-patterned bars ndcate presamount of gas n gas traps and ndrect measurements based on tracer concentratons. sure recovery followng njecton (PR), dagonally patterned bars ndcate constant pressure test (CPT), and sold bars n Ste Descrpton dcate pressure recovery test (PRT). Phases 1, 2, and 3 n Fgure 6 refer to the test phase classfcaton gven n secton 4.1. For the Fgure 5a shows an overvew of the *sp6 HRL faclty wth the 4500 m long access tunnel that ends at 460 m depth below sea level. A detaled ste and hydrochemcal descrpton of the *sp6 HRL s gven by Banwart et al. [1994]. The ste used for the degassng experments s located approxmately 450 m below sea level (ndcated n Fgure 5a) and contans nne boreholes, each 56 mm n dameter. We used three of these bore- CPTs the y axs of Fgure 6 ndcates the constant pressure that prevaled durng the tests. For the PRTs, the PRs, and the NJs the y axs ndcates the approxmately constant pressure condtons that prevaled at the end of these tests (see the paragraph below for further detals). The degassng tests performed at atmospherc pressure are denoted "CPT-atm," and those performed below atmospherc pressure are denoted "CPTholes, denoted P2, P4, and P8, for the degassng experments. vac"; the atmospherc pressure s ndcated by a dashed lne. Fgure 5b shows the orentaton of these boreholes relatve to the drft (top vew). Addtonal orentaton and nclnaton data The steady borehole pressure at no-flow condtons was estmated from the average borehole pressure (P o) at no-flow for these boreholes are lsted n Table 1. condtons durng the last part of the PRT. Ths pressure s The geometry and connectvty of the fractures ntersectng llustrated by a dashed lne n Fgure 7, whch shows a Horner the degassng test holes were estmated through pressure and flow measurements, tracer experments, and geologcal and geometrcal data avalable from core loggng, borehole camera plot of a typcal PRT (PRT1 n borehole P8). The Horner tme, shown on the x axs, s defned as log [(tp 4- At)/At)], where tp s the borehole producng tme (precedng the borehole loggng (Per pont P 220 borehole TV), and tunnel observa- shut-n, here equalng the elapsed tme snce the start of the tons. Four water-bearng fractures ntersect the three test boreholes, of whch two parallel fractures ntersect borehole P2, and boreholes P4 and P8 are ntersected by one waterbearng fracture each. The locaton of the borehole packers was adjusted to nclude these man hydraulcally actve strucborehole test sequence) n seconds and At s the tme n seconds elapsed snce the borehole shut-n (.e., after the start of the PRT). Thus the Horner tme value decreases wth ncreasng tme from the test start and equals zero after nfnte tme. The pressure p * at a Horner tme of 0 n Fgure 7 llustrates a tures. Table 2 shows that the transmssvtes of these fractures hypothetcal pressure that would preval n the borehole after were of the order of m 2 s -1. Further, Table 2 lsts the nfnte tme for radal flow n an nfnte aqufer. n fnte fracture orentatons as well as the dstance between the bore- aqufers, there wll be devatons from the dotted lne shown n hole-drft ntersecton and the borehole-fracture ntersecton Fgure 7 (e.g., constant pressure boundares mply that the well (see also Fgure 5b). The dstance between the dpole test pressur eventually wll reach a steady value p, satsfyng P o -< njecton hole (P4) fracture ntersecton and the wthdrawal p < p *). An analyss of all the PRTs conducted n boreholes hole (P8) fracture ntersecton was 1.08 m. The tracer experments were performed as decayng pulse njectons, crculatng the tracer n one borehole at a tme wthout applyng any excess pressure and thus mantanng the natural gradent. The flow n the ntersectng fractures was P2, P4, and P8 showed that the average P o/p* rato was The value of unty ndcates that there are no consderable, systematc trends n the late-tme transent pressure data. Hence the PRTs were conducted for a suffcent perod of tme for po to be a meanngful estmate of the steady well pressure estmated from the dluton wth tme of the crculated tracer. at no-flow condtons.

7 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER 2483 Borehole / Pressure,, m t, _ : /Transducer JECTON Vacuum Pump _ ressure Transducer Gas Regulators\ x Water n]e Drve Gas Bubble Ga: x ump Water Outl Ntrogen Tanks, Gas Saturaton WTHDRAWAL Pressure Transducer Logger Steel ' Flowmeter ] L/ Regulatng " U Outlet Fgure 4. The expermental equpment (constructon drawng by Geosgma AB, Sweden).

8 2484 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER packer was mantaned throughout the testng n P4, and the -.':.,:.,... :.:. -'...'.',' ,.....'::-. sold bars of Fgure 6b show that the changes n P o pressure ß ' '.'",.',. :[. '.., ¾?? :,".'.: '.:: * T.v2.5'....*..,,:,: ================================================== ß... ß.-'::: :. :::"..':.,..'...:_... e., '. :_7..','.,q,.4f),....g:......:'.'.. ';2-%-' 2,?,.'.'-'-"-;*,.' ':...-.:.-ex::::.:.:.:.:.:.:.:.:.:.:.:.:..'.:. :::.x.:.... :::::::::::::::::::::... between dfferent PRTs were smaller n borehole P4 than n borehole P2.... '",-,,.,.,-...,.:. ;.,,.,...q::,: : ::.,. ',-..,., '.,T,'......::.::.:.:::: ::...:...,.,..::. Fgure 6c shows the pressure n the test hole (P8) for the.. " ' ' :.:,Z.<..:."...< ø " - '-e <½:. -' :?...'.'.'::::.. :.>,.:-:-:-:-. =================================================================================== '-:-:.-'.:,:..:4<.:-'--'-.:.:-:.?...'.:.:..'..'::.:.:..'...:...:.:.:.:.;.2.'.-;. -:.:.:. ::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ß.. -: :.:, ø..,._ :, '.,,..::X:.>;.:.:..:.:...:.:.:.:;.:..- ' =kq.".'>=?..':!,'.'.'?.!=[=!!=!=e=5==... -.:.:.:.:e.;::_'.:.:.:.:.:.: :..:.:.:.:.:.:.:.: '-.'.-[:.=:;E=:;=.:.-.:.-.:=M.-E=E.?!..E::.:!=C5=:: :?...=..:::?.=..!=5=E..Z:..:.:.:.:.:.: :.:: -:.:-:.:':-:.:':.:.;':.:.:.:':.:..':'.-..'>:'.".{':':':- dfferent dpole tests. The damond-patterned bars labeled... "BEFORE" and "AFTER" show that the P8 pressure at no. :?.;^.:.. :: :..:;.....`.. :[:::::[::::[:[...:!::::[:[!:::[::..::: :.:::.:. :!:::[: ::!:?!:::::!.!::::!:!:! ::: :::[:!::5: :[:5:: : :5::?.::[?..:[.'.-: flow condtons was about 400 kpa both before and after the ß : ;" ß.?..,.:,,. ' =====================================================================.....::.: '.o'.'1'?..'": 'X..":q.:':..'.:.:.5.".":--'.;.:-?..:--'-.'. ::::.:';:.;.,-.'...-;. :::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::: : ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: -.',.'- 1.._';..' :;.2:...- :.-..:.;.---:.---;.;.:.:.:.:... : :.: : :g..:.. '. -.:'-:.:. ::':-': ' :::::::::::::::::::::::::::::::::::::::::: :::.:.2.:.:.:.'.: :.'-:.:.:.:.:-:.:.::'..:. :::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::: dpole testng. The njecton of gas-saturated water n borehole. ß :..? E......! -:5... -? :-:;?..? z-?.-? [!5!!! : : [!:!!![! :!! 2!!!!!::: ":"... P4 started whle P8 stll was closed. As the njecton was nt-,:;!."..½:?.:'4::::.'.. :::::::::::::::::::::::::::::::::::::::::::::::::::::..:-:..::..::::::.::.' '... ============================== :5::!:!:!:!: :::::::::::::::::::::::::::::::::::::::::::::::::::: ated, the pressure n P4 tself was elevated from the no-flow value of about 900 kpa to an approxmately constant value of 2600 kpa, whch was mantaned throughout the dpole test sequence. The actual dpole testng was then ntated by conß..:::::::::::::::::::::::::::::: :. : :... :... ::..:.....::...q?:...:...:::... ; q:.:..%.. ::::: :.:g :. :::::::::::::::::::::::::::::::======================================= ductng CPT1 n borehole P8 and ended by conductng PRT4 :::. [: : j. : :..'::.::. j: ========================================================================================== :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: (Fgure 6c). The sold bars of Fgure 6c shows that the pressure at no-flow condtons n borehole P8 was elevated to about ' : :;½.?.-?q? :: :?':_-"' :':":':':"... (a)... Y _ The sold bars n Fgure 6a show that P o was lower for the last two PRTs conducted n borehole P2. These changes n the P o value are accounted for n the followng comparatve analyss of the dfferent test phases by the use of drawdown as a parameter nstead of the absolute borehole pressure (Pbh) (the drawdown beng defned as Po - Pbh). The packer was exchanged at two tmes durng the testng n borehole P2, and the borehole was subject to atmospherc pressure condtons for several hours. These events are labeled "OPENBH" on the x axs of Fgure 6a. The condtons durng the packer exchanges were smlar to those prevalng durng a CPT at atmospherc pressure; however, the flow rate could not be montored durng the exchange. The open bar of Fgure 6b shows that the borehole pressure ncreased consderably durng the njecton n borehole P4, n contrasto the njectons n borehole P2 (Fgure 6a). The same 1700 kpa durng the dpole test and that ths value was approxmately constant throughout the testng Gas Contents Hydraulcall The evolved gas content A 0 a (.e., evolved volume of gas per volume of water) was measured at the borehole outlet usng a acnve secnons gas trap. n addton, water samples were obtaned for the analyss of tracer concentraton n order to estmate the degree Drft Jal of dluton of the njected, gas-saturated water. The pure n- Drft centre P8 P4 P2 jecton tank water (wthout dluton) was N 2 saturated at pressures hgher than the atmospherc pressure, such that the es-... lm tmated A O a value accordng to (3) was approxmately 17%. (b) The measured A 0 a value for the pure njecton tank water was 14.4%. The dfference between the estmated and the mea- Fgure 5. (a) Overvew of the sp6 HRL and the locaton of the expermental ste and (b) orentaton of the test boreholes sured value may be due to the loss of a small volume of the relatve to the drft. evolved gas bubbles through escape past the gas trap. Table 3 summarzes the evolved gas contents for the tests n boreholes P2, P4, and P8. The gas contents of the outflowng water were relatvely low, about 1% or less, after a few hours of water wthdrawal from the test boreholes P2 and P4, as shown n Table 3. These low values are consstent wth the tracer concentraton measurements, showng a low concentraton of njecton water flowng out from the boreholes P2 and P4 (1% or less after a few hours of wthdrawal followng the gas njecton, Table 3). Hence these low gas contents are due to dluton of the njecton water at the tme of the wthdrawal phase. n contrast to the low gas contents prevalng durng the njecton-wthdrawal tests, the gas content of the outflowng water durng the dpole test performed n boreholes P4-P8 was hgh (13.0 _+ 1.8%, Table 3). The tracer concentraton analyss showed that ths was the result of low dluton of the gassaturated water that was njected n borehole P4 smultaneously wth the testng n borehole P8 (.e., approxmately Table 1. Coordnates of the Borehole-Drft ntersecton, Borehole nclnaton, Borehole Drecton, and Borehole Length Borehole Y, m X, rn Z, m nclnaton, Drecton, b Borehole ( 56) (East) (North) (Depth) % deg Length, m P P P Coordnates are gven n local Asp6 system. Negatve numbers ndcate slope downward. bborehole drecton s gven relatve to sp6 local north.

9 JARSJ AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER 2485 Table 2. Data on the Water-Bearng Fractures ntersectng the Test Holes ntersectng Transmssvty, a Dstance, b Strke, c Dp, d Structure Borehole m 2 s- x m deg deg Drft wall P2, P4, P Fracture ' P2 7 X 10-9 (' + ") Fracture " P2 7 X 10-9 (' + ") Fracture P4 4 X Fracture V P8 4 X atransmssvty estmated usng equaton (A2) wth ro = 150 m. bdstance s between the borehole-drft ntersecton and the borehole-fracture ntersecton; see also Fgure 5b. cstrke s relatve to J sp6 local north. dzero dp mples horzontal fracture. 90% of the water wthdraw from borehole P8 conssted of the fracture durng the degassng test below atmospherc pressure gas-saturated njecton water (Table 3)). Hence the tests n n P4 could not be montored because of ar nvason n the boreholes P2 and P4 provde new data for smlar degassng borehole, possbly caused by partal desaturaton of the part of condtons as those prevalng durng the plot hole test [Geller the fracture between P4 and the drft. and Jarsj6, 1995], whereas the dpole testng n boreholes P4-P8 provdes feld data for smlarly hgh gas contents as those prevalng durng the laboratory tests of Jarsj6 and Geller [1996], that s, consderably hgher gas contents than for the plot hole test. Throughout the dpole degassng test, gas-saturated water was njected n borehole P4 at a rate of 1.25 x 10-6 m 3 s - at the same tme as the hydraulc (wthdrawal) tests were performed n borehole P8. The relaton between the steady flow rate and the borehole pressure n borehole P8 was approxmately lnear under sngle-phase flow condtons (phase 1, 5.3. Flow Rate-Pressure Relatons CPTs 1-4), as ndcated by the open squares n Fgure 9c. Durng the CPTs the borehole pressure was regulated, Lnearegresson showed that the coeffcent of correlaton R 2 whereas the flow rate transents were montored. Statstcs on between the sngle-phase CPTs and a straght lne was as hgh all of the performed CPTs above atmospherc pressure n bore- as Further, Fgure 9c shows that both the degassng test holes P2, P4, and P8 show that there was no measurable df- (CPT-atm, ndcated by a sold square) and the sngle-phase ference between the flow rate 50 mn after test start (Qso) and PRT1 (ndcated by a cross) devate from ths lnear relaton. 150 mn after test start (Q so); the Q so/qso rato was For a relable expermental nterpretaton t s mportant to Furthermore, the rato between Q so and the flow rate n the determne the sgnfcance of these devatons statstcally. end of the CPTs (on average 350 mn after test start) was as Specfcally, the PRT1 observaton (cross n Fgure 9c) s hgh as Fgure 8 llustrates the flow rate durng the found to be nconsstent wth (.e., sgnfcantly dfferent from) dpole testng n borehole P8 (these flow rates are responses to the sngle-phase CPT observatons (open squares n Fgure 9c), the regulated borehole pressures of the dpole test sequence ths wll ndcate systematc measurement errors or (changes shown by the dagonally patterned bars n Fgure 6c). Fgure 8 of) ambent condtons that are unaccounted for n the sngleshows that the flow rates n P8 decreased rapdly durng the phase nterpretaton. Ths would then also decrease the conffrst mnutes of the CPTs and stablzed thereafter. The excepdence n the evaluaton of the degassng test (CPT-atm, n ton s the degassng test at atmospherc pressure (CPT-atm), whch two-phase flow condtons are plausble, sold square n whch wll be dscussed n more detal below. Owng to the fact that the flow rates at the end of the CPTs n general dd not Fgure 9c) and the subsequent repeat tests under possble exhbt any consderable or systematc trends, they wll be two-phase condtons (CPTs 5-7 and PRTs 2-4). A more determed steady flow rates n the followng. taled nvestgaton of the potental phenomena that may have Fgure 9 shows the relatons between the pressure and the nfluenced the degassng test result wll not be meanngful steady flow rate for the sngle-well testng n P2 and P4 and the unless the CPT-atm flow rate devaton from the sngle-phase dpole testng n P4-P8. The sngle-phase (phase 1) tests are lnear regresson lne s statstcally sgnfcant. ndcated wth open squares n Fgure 9, whereas the degassng n order to address these ssues formally, we have ndcated and repeat tests (phases 2 and 3) are ndcated wth sold n Fgure 9c the confdence nterval for the ftted, sngle-phase squares. Apart from the borehole pressures at no-flow cond- regresson lne (as the nterval between the dashed lmts) tons, the P2 borehole pressures exhbted a lnear relaton wth along wth the lmts of predcton (dotted lnes) for the sgnfcance level of The 95% confdence nterval accounts for the steady flow rates (Fgure 9a). No flow rate decrease due to random errors n the observatons. The observaton degassng was observed for atmospherc borehole pressure errors m- (about 100 kpa) nor for borehole pressures below the atmospherc pressure. n borehole P4 the relaton between borehole pressure and steady flow rate was also lnear down to atmospherc borehole pressure (Fgure 9b). No sgnfcant flow reducton due to degassng was observed n P4; the small flow rate decrease at atmospherc pressure (for phases 2 and 3, sold lne and sold squares) s wthn the measurement varablty, as seen when comparng the phases 2 and 3 test observatons wth the other observatons shown n Fgure 9b. The flow n the ply that the regresson lne (sold lne n Fgure 9c), whch s based on lmted avalable observatons, does not necessarly concde wth the actual lnear flow rate-pressure relaton. Furthermore, the nterval between the lmts of predcton ndcates the range of flow rates, whch wth a probablty of 95 % would contan an addtonal (new) observaton, provded that the expermental condtons are the same. Fgure 9c then shows that the null hypothess (that PRT1 falls wthn the lmts of predcton) cannot be rejected at the

10 2486 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER [] njecton (N J) g Pressure recovery (PR) [] Constant Pressure Test (CPT) ß Pressure Recovery Test (PRT) Phase (1) Phase (2) <,,-, 1500 (a) Sngle-well testng: borehole P2 Atmospherc pressure H Phase (3) > ooo ooo- ooo o o o o Borehole tests (chronologcal order) [] njecton (N J) [] Pressure recovery (PR) [] Constant Pressure Test (CPT) ß Pressure Recovery Test (PRT) 25OO (b) Sngle-well testng' borehole P4 Atmospherc pressure 2000 Phase (1) Phase (2) J< Phase (3) 1000 " 500 *ø n Borehole tests (chronologcal order) Before/after the dpole testng [] Constant Pressure Test (CPT) ß Pressure Recovery Test (PRT) 2000 ß ' ß m ***, Phase (1) (c) Dpole testng' boreholes P4-P8 Atmospherc pressure hase Phase (3) 0 ***' /' 0 n u_ 0 Borehole tests (chronologcal order) Fgure 6. Sngle-well testng performed n (a) borehole P2 and (b) borehole P4 and (c) dpole testng performed n boreholes P4-P sgnfcance level. The PRT1 devaton from the snglephase regresson lne s thus statstcally nsgnfcant. The correspondng null hypothess for the degassng test (CPT-atm), however, can be rejected at the same sgnfcance level. Hence we can conclude that at the 0.05 sgnfcance level, the flow rate devaton durng the degassng test s sgnfcant and cannot be explaned by randomness. Note that ths concluson remans unchanged even f PRT1 s ncluded n the lnear regresson.

11 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER : ' p* 6.E Horner tme, og((tp+at)/at) Fgure 7. Horner plot of a pressure recovery test (PRT1 n borehole P8). ß ß ß r, 4.E-07 E 3.E-07 "2.E-07 1.E-07 O.E+00 Numbers to CPT# refer Tme (ran from start of dpole test) Fgure 8. Flow rate versus tme durng the dpole testng n n Fgure 9d we fnally show the phase 3 repeat tests CPTs borehole P and PRTs 2-4 (sold squares), whch followed after the degassng test. We then see that even though the borehole pressures were ncreased agan durng the phase 3 tests, the apparent value should therefore not be used as an absolute flow rates were stll lower than durng phase 1. The borehole unsaturated conductvty value but rather as a measure for pressures durng the repeat CPTs were well above the bubble comparson wth the saturated hydraulc conductvty value pressure of about 1000 kpa (Table 5) for a tme perod that estmated from phase 1. exceeded the length of the degassng test CPT-atm (see Fgure n order to make a consstent comparson of potental df- 8). The fact that the flow was reduced durng the last repeat ferences n (apparent) hydraul conductvty values between CPT (CPT 7, see Fgure 9d) mples that the redssoluton of test phase 1 and test phases 2 and 3 for all the conducted the gas phase was slower than the formaton of the gas phase. degassng tests (.e., the tests n boreholes P2, P4, and P8), As for the formaton of the gas phase, the decreasng flow rates regresson lnes were ftted to the drawdown-flow rate relaversus tme for the degassng test CPT-atm n Fgure 8 may tons representng these test phases (see the appendx). Table reflect the accumulaton of gas bubbles wth tme n the frac- 4 summarzes the estmated ratos of the hydraulc conductvture, showng that the reducton n transmssvty due to de- tes of the fractures before and after the tests at atmospherc gassng does not occur nstantly as the borehole s opened. Ths borehole pressures on the bass of the slopes rn of the regresreasonng holds provded that degassng s the reason for the son lnes and (A4) n the appendx. For comparson, results observed flow reducton n the frst place, whch s an ssue that from the plot hole test [Geller and Jarsj6, 1995] are also ns further addressed n secton 6. cluded. Table 4 shows that the coeffcent of correlaton R 2 was As Fgure 9d also suggests, the dfference between the phase hgh, whch mples that the flow rate, ndeed, exhbts a lnear 3 flow rates and the phase 1 flow rates s smaller for hgh relaton wth the drawdown. The hydraulconductvty ratos borehole pressures. Ths may be an effect of the consderable for all sngle-well tests show devatons of the order of 10% volume reducton of the gas phase, whch can be expected as from unty, whereas a consderably lower value of approxthe water pressures ncrease. Accordng to the deal gas law the mately 0.5 was obtaned for the dpole test, mplyng that the volume of gas should be reduced to only 10% of the orgnal apparent fracture conductvty was reduced to half of ts prevolume as the borehole pressures ncrease from 100 kpa to vous value after conductng the degassng test at atmospherc 1000 kpa. The resultng degree of water saturaton (volume of borehole pressures. water per total pore volume) should then be ncreased correspondngly. Hence the unsaturated hydraulc conductvty that can be evaluated for phases 2 and 3 represents only an appar- 6. Degassng Hypothess Test ent conductvty value that s nfluenced by the varable degrees Flow reductons have been observed both n the feld results of water saturaton durng the dfferent stages of phase 3. Ths reported here and n earler laboratory and feld tests when the Table 3. Evolved Gas Contents Durng the Testng n Boreholes P2, P4, and P8 Test Sngle-well test n P b(2.4) c Sngle-well test n P4 0.1 e Dpole test (P4-P8) 13 _+ 1.8 b Mean Value (and Standard Devaton) of Evolved Gas Relatve Concentraton Content A O a, % of njecton Water, a % <1 d <1 d 90 _+ 8 avolume of njecton water per total water volume was determned through tracer dluton measurements; 100% njecton water mples 15% N 2. bvolume of evolved gas per volume of water at atmospherc pressure was measured on ste wth a gas trap. The value s relevant for the entre test sequence. CMeasurement was performed at an absolute pressure of 36 kpa; 1.1% extra ntrogen s released for a water pressure lowerng from atmospherc pressure to 36 kpa (absolute). dvalue was below the detecton lmt of the equpment. evalue was measuredurng the degassng test at atmospherc pressure; the evolved gas content was too low to be detected before and after the degassng test.

12 2488 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER 1.6E-6 -- (a) Sngle-well testng' borehole P2 1.4E-6, 1.2E-6 E 1.0E-6 ' g. --D--Phase (1) (CPT1 to PRT1) + Phases (2)-(3) (PRT2 to CPT6) - -.Phase(3)(contnued; CPT7 to PRT4), 8.0E-7 : 6.0E-7 0,, 4.0E-7 2.0E-7 O.OE+O Pressure (kpa) 3.0E-7 T 2.5E-7 E 2.0E-7 (b) Sngle-well testng' borehole P4 --D--Phase (1) (CPT1 to PR1) Phases (2)-(3) (CPT-atml to CPT5) Phase Phase (3) (contnued; PRT4 to CPT-atm2) (3) (contnued; PRT6 to PRT7) 1.5E-7 1.0E-7 O 5.0E-8 0.0E Pressure (kpa) Fgure 9. Flow rate versus borehole pressure for the sngle-well tests performed n (a) borehole P2 and (b) borehole P4 and for the dpole test performed n boreholes P4-P8 durng (c) phases 1-2 and (d) phases 1-3. water pressure has been lowered to atmospherc pressure (the dpole test and the experments of Jarsj6 and Geller [1996] and Olsson [1992]). There s also expermental evdence on the absence of flow reductons for correspondng water pressure lowerngs (the sngle-well tests and the experment of Geller and Jarsj6 [1995]). Provded that the observed behavor can be flow (Po), the bubble pressures (Pt,), and the borehole pressures durng the degassng test (Pt,h) for the new borehole tests reported n ths study, the plot hole test [Geller and Jarsj6, 1995], the Strpa observatons [Olsson, 1992], and the radal flow laboratory degassng tests [Jarsj6 and Geller, 1996]. The estmates lsted n Table 5 are based on smplfed fracexplaned by occurrence/absence of groundwater degassng, ture geometry and boundary condtons, such that (4) and (5) then some degassng-related condton must have been favor- are vald. For the laboratory tests the radus of nfluence (re) able n the former experments and not so favorable n the was 56 mm. For the feld tests the calculatons are based on the latter experments. The outlned basc degassng relatons (see secton 2) suggest that the extent of the low-pressure zone X ow should affect the magntude of degassng effects. We wll therefore determne n the followng test whether or not there s a correlaton between X o w and the actual outcome of all avalable experments and observatons (n terms of the occurrence or absence of an observable flow reducton). The null hypothess Ho s then that there s no such correlaton, and rejecton of He supports our assumpton that observed flow reductons have been caused by degassng. n order to perform the hypothess test, we use the data on the evolved gas contents for the dfferent tests (Table 3) as a assumpton that r e equals 150 m n all cases. However, the low-pressure zone extent s relatvely nsenstve to the choce of re, because of the logarthmc relaton n (4); see Jarsj6 and Destoun [1997] for further detals. For the dpole test the extent of the low-pressure zone around the wthdrawal test hole n the drecton of the njecton hole was estmated usng superposton. Specfcally, we estmated the hydraulc head dstrbuton around the test (wthdrawal) and njecton holes, d)t(rt) and d)(r), respectvely, by supermposng the (4) solutons for the two boreholes over the dstance of 1.1 m (r - r t '- 1.1) between them. By supermposng boundary condtons, we arrved at a resultng head dstrbuton ( TOT(rt)-- bass for the estmaton of correspondng bubble pressures 4t(rt) + 4(rt = r - 1.1) correspondng to pressures of 107 through (3). Table 5 summarzes the borehole pressures at no kpa at the wall of the wthdrawal well (r t = rw), 2600 kpa at

13 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER ,0E-7 8.0E-7 A 7.0E-7 t E 6.0E-7 5.0E-7 4.0E-7 O 3.0E-7 2.0E-7 t (c) Dpole testng' Phases (1)-(2) n boreholes P4-P8 ";" " ' ' ß Phase (2) (cpm-atm) ["--..._"'Q-'L',,,, X PRT 1 1- " "Q'"' Regr. lne, phase (1) CPTs _. : _ "'Q.','-., J... Confdence lmts, alpha=o.05 J... :""" "'-.".7'. 7.7_ " '" -' ' of Lm predt cton, alpha=o E-7 O.OE+O Pressure (kpa) A 4.0E-7 4.5E-7 t 3.5E-7 (d) Dpole testng' Phases (1)-(3) n boreholes P4-P8 1 3 Numbers refer to CPT # t 3.0E-7 E 2.5E-7 2.0E-7 1.5E-7 O ll_ 1.0E-7 5.0E-8 -El-Phase (1) (CPT1 to CPT4) - -Phases (2)-(3) (CPT-atm to PRT4) 6 4 O.OE+O Pressure (kpa) Fgure 9. (contnued) 1800 the njecton well wall (r t = r,), and 1000 kpa at the outer boundary (whch s about the observed shut-n borehole pressure at no flow and no njecton n borehole P4). The estmated steady state extent of the low-pressure zone (X ow) for the sngle-well degassng tests was at the most about 2 cm from the borehole wall, as shown n Table 5. The value of X ow for the dpole test tabulated n Table 5 refers to the dstance from the wthdrawal test hole n the drecton of the njecton hole. Ths s the drecton of the hghest pressure ncrease, mplyng that X ow may have been larger than that n other drectons. Nevertheless, Table 5 shows that the extent of the low-pressure zone durng the dpole test was at least 0.4 m or an order of magntude greater than the correspondng extents for the sngle-well tests. The correspondng area of the Table 4. Estmated Hydraulc Conductvty Ratos Before and After Lowerng the Water Pressure n the Boreholes to Atmospherc Pressure Test Sngle-well test n P2 Sngle-well test n P4 Dpole test (P4-P8) Plot hole test (sngle-well test) c m Durng Phase 1 Before Test at Atmospherc Pressure, a m 3 kpa - s X 10 - ø(3, R ) 2.35 X 10 - ø(2) b 3.28 X 10 - ø(4, R 2 = 0.989) 4.40 X 10-8 (3, R 2 = 0.992) m Durng Phases 2 and 3 Durng/After gphase2-3 Test at Atmospherc Pressure, a m 3 kpa - s - gphasel ' (Equaton(A4)) 4.59 X 10 - ø(3, R 2 = 0.996) 2.19 X 10 - ø (4, R 2 = 0.874) 1.75 X 10 -m (4, R 2 = 0.992) 3.96 X 10-8 (4, R 2 = 0.922) athe number of CPTs and R 2 values are wthn parentheses. bthere are too few observatons to determne R 2. cphase 1 ncludes CPTs 1-3 and PRT 1, and phases 2 and 3 nclude CPTs 4 and 6-8 and PRT 2 of Geller and Jarsj6 [1995]

14 2490 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER Table 5. Borehole Pressures at No Flow (P0) and Durng the Degassng Test (Pbh), Estmated, Steady Evolved Gas Content Durng the Test (A0a), and Correspondng Bubble Pressures (Pb) n Addton to the Estmated, Steady State Extent and Area of the Low-Pressure Zone (Xlow and A low), Assumng Smlar Boundary Condtons as Those Prevalng Durng the Tests Test or Observaton P0, kpa Pt, h, kpa bog, vol % p,, kpa Xlow, m Alow, m 2 Reduced nflow? Sngle-well test n P2 a no Sngle-well test n P4 a no Dpole test (P4-PS) a >0.4 e 0.5 e yes Plot hole test (sngle-well test) b no Strpa drft observaton c yes Laboratory tests d yes aths s the feld test reported n ths study. bths s the feld test reported by Geller and JarsjO [1995]. CThs s the feld observaton durng the Strpa smulated drft experment, reported by Olsson [1992]. dthese are laboratory tests wth CO 2 gas n replcas of natural rock fractures, reported by JarsjO and Geller [1996]. evalues are calculated wth a well pressure of 2600 kpa n P4 and 107 kpa n P8 and a separaton dstance of 1.1 m. low-pressure zone, A low = *r(xlow + rw) 2 - rrrw, 2 was 100 to 2000 tmes hgher n the dpole test than n the sngle-well tests, where degassng dd not cause observable hydraulc conductvty reducton. Further, Table 5 shows that the value of Xlow for the laboratory tests (where degassng was the certan cause for observed flow reductons) and the Strpa drft observaton (where degassng was one of the plausble causes for the observed flow reducton) were also greater than for these sngle-well tests, where no flow reductons were observed. Hence, n all three experments where degassng ether dd certanly cause the flow reducton (.e., the laboratory test), or was the most lkely cause for the flow reducton (.e., the dpole test), or was hypotheszed to have caused observed nflow reductons (.e., the Strpa observatons), the value of Xlow s greater than for the three tests where degassng dd not cause any sgnfcant nflow reductons. The probablty for ths Xlow outcome to occur randomly, that s, to occur even f there s no correlaton between observable flow reductons and the low-pressure zone extent Xlow, s only 5%. Hence our null hypothess must be rejected at the 0.05 sgnfcance level, whch supports the assumpton that degassng s the cause for the observed flow reductons. 7. nvestgaton of Alternatve Explanatons 7.1. Turbulence Effects The occurrence of turbulence can mply nonlnear relatons between borehole pressure and flow rate. The reported degassng tests were performed at low borehole pressures, mplyng hgh pressure gradents and relatvely hgh flow rates. Hence we nvestgated the possblty of the onset of turbulence at these relatvely hgh flow rates. Specfcally, we explored tur- bulence effects as another plausble explanaton for the observed nonlnear relatons between borehole pressure and flow rate durng the dpole test. We wll therefore compare the condtons prevalng durng the dpole test both wth the condtons prevalng durng the other degassng tests (where lnear flow relatons mply no or neglgble turbulence effects) and wth studes specfcally addressng turbulence n fractures. The bass of ths comparson s provded by the Reynolds number (Re), defned as Re = Dr/v, (6) where D s the hydraulc dameter, v s the pore water velocty, and v s the knematc vscosty. n analogy wth Fourar e! al. [1993] we use the relaton D = 2ah, where ah s the hydraulc fracture aperture. Further, we estmate v as Q/(2 rrra h), where Q s the volumetrc rate of borehole/well nflow and r s the radal dstance to the borehole/well center. As ndcated by the resultng expresson Re = Q/(rrrv), Re ncreases wth decreasng values of r. The hghest value Re = R ema x occurs at the wall of the borehole/well, that s, at r - r w. The crtcal Re value for whch turbulence effects start to evolve dffers from medum to medum. n porous meda t s commonly assumed that turbulence causes consderableffects for Reynolds numbers greater than 100, whereas t s assumed that no turbulence effects wll occur for Reynolds numbers less than some value between 1 and 10. Further, for flow n ppes the crtcal value of Re between lamnar and turbulent flow s around For lnear flow n fractured rock, expermental results revewed by Romm [1966] showed an onset of turbulence for Re values between 10 and 100 n rougher fractures and between 100 and 2000 n smoother fractures. Table 6 summarzes the values of r w, Q, and Rema x for the degassng borehole tests that have been conducted n the feld. Table 6. Estmates of the Maxmum Values of Reynolds Number (Remax) rw, Range of Tested Q, Borehole Test mm m 3 s - Range of Rema x Lnear p - Q Relaton for Tested Q range? Sngle-well test n P2 28 Sngle-well test n P4 28 Dpole test (P4-P8) 28 Plot hole test (sngle-well test) 42.5 Laboratory tests x x x x x x x x x x yes yes no yes yes

15 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER 2491 Also, sngle-phase flow experments that were a part of the laboratory degassng tests n rock fracture replcas [Jarsj6 and lkely cause for the 50% reducton n hydraulc conductvty that occurred durng the dpole testng. There are no ndca- Geller, 1996] are ncluded n Table 6. The hghest flow rates occurred durng the plot hole test, resultng also n the hghest tons that turbulence ths reducton. or fracture deformaton contrbuted to value of Rema x of However, the relaton between borehole pressure and flow rate was found to be lnear, ndcatng that turbulence effects were absent or neglgble n the plot hole test. n contrast, durng the dpole test, where a nonlnear pressure-flow rate relaton was observed, the values of Rema x were only between 1.1 and 4.5, whch s about the same range as for the P4 test and below the range for the P2 test; the pressure-flow rate relaton was lnear n both P4 and P2. Furthermore, n both the laboratory experments of Jarsj6 and Geller [1996] and the experments summarzed by Romm [1966], lamnar flow condtons were observed for hgher, or consderably hgher, values of Re than those prevalng durng the dpole test. Hence the Re value evaluaton yelds no ndcatons of turbulence beng a factor contrbutng to the observed flow rate reductons durng the dpole degassng test. We used the sngle-well and dpole feld expermental data, along wth other exstng laboratory and feld data on groundwater degassng, n order to test the correlaton between the estmated extent of the low-pressure zone (X ow, where the water pressure s lower than the bubble pressure) and the actual expermental outcome n terms of the occurrence or absence of hydraulc conductvty reducton. Formal hypothess testng at the 0.05 sgnfcance level showed that such a correlaton exsts, supportng degassng as the actual cause for observed hydraulc conductvty reducton at the hgher gas content. The results thus mply that degassng of groundwater may, ndeed, cause hydraulc conductvty reductons n the vcnty of boreholes; however, they also show that ths wll only occur for gas contents that are consderably hgher than 5%. For comparson, the gas contents m below the surface at 7.2. Fracture Deformaton the,3,sp6 HRL n Sweden ranged between 0.1% and 5%. An alternatve phenomenon that may, n prncple, lead to However, hgher gas contents have been observed, for nmeasurable hydraulc conductvty reductons s stress-nduced stance, at the Wellenberg ste n Swtzerland. For drfts the fracture deformaton caused by ncreased effectve stresses theoretcal degassng analyss shows that degassng may cause when lowerng the borehole pressure to atmospherc pressure. hydraulc conductvty reductons for lower gas contents than However, such effects are beleved to have neglgble mpact for boreholes. on the present dpole test results for the followng reasons: (1) Laboratory experments [e.g., Barton et al., 1992] have shown that nonelastc, rreversble rock fracture deformaton s much Appendx: Hydraulc Conductvty hgher durng the frst loadng-unloadng cycle than durng the subsequent cycles. The test hole (P8) pressure had been lowered to atmospherc pressureseveral tmes before the start of The followng analyss s relevant for the case that steady condtons are establshed n the experments. The presence of constant pressure boundares n the feld mples that steady the dpole test, mplyng that the man part of any rreversble condtons wll be reached after some tme. For nstance, the deformaton of the fracture ntersectng P8 should have oc- degassng ste at 3,sp6 HRL s located below an sland, where curred at these occasons and not durng the degassng test. (2) The E modulus for the sp6 rock s suffcently hgh (69 +_ 5 GPa [Xangchun and Shaoquan, 1997]) not to cause any consderable changes n fracture aperture, gven the actual water the surroundng sea provdes an approxmately constant pressure boundary. An evaluaton of the conducted PRTs and CPTs showed that these tests were carred out for a suffcent perod of tme to reach steady condtons (see sectons 5.1 and pressure change of 6 x 10-4 GPa durng the dpole degassng S.3.). test (Fgure 9c) and assumng lnear elastc behavor of the fracture aspertes. (3) The observed lnear relatons between flow rate and borehole pressure measured n the fractures The fracture hydraulc conductvty (K) can be determned from the steady state borehole nflow rate-borehole drawdown relaton accordng to ntersectng boreholes P2 and P4 show that the changes of the pglo n stu rock stresses due to a water pressure lowerng down to K= rn 2A atmospherc pressure dd not at all nfluence the transmssvty of these neghborng fractures at the 3,sp6 HRL degassng test ste (see Fgures 9a and 9b). n = 1, (A1) 8. Summary and Conclusons Pg K = rn n (rw/ro) n = 2, (A2) pg n = 3 and rw<<r0, (A3) K= m4,r w Deep groundwater contans dssolved gases that may come out of soluton f the water pressure s lowered. Ths mples the possble development of unsaturated condtons and, as a consequence, changes n the hydraulc condtons. Through expermental and theoretcal analyses we have nvestgated the effect of groundwater degassng on the hydraulc propertes of where n s the flow geometry (n - 1 mples lnear flow, n = 2 mples radal flow, and n = 3 mples sphercal flow), p s the densty of water, g s the gravtatonal constant, r w s the well (borehole) radus, l o and ro are the length and radus, respectvely, of nfluence (.e., the dstance from the well to the outer rock fractures under dfferent condtons. boundary where the pressures are not nfluenced by the open- Two sngle-well feld tests were performed at relatvely low evolved gas contents of about 1%. n addton, a dpole test was conducted, where the natural gas content at the ste was eleng of the borehole),a s the cross-sectonal area equalng the length of the borehole tmes b, the aperture wdth, and rn s the slope of the steady state borehole nflow-borehole drawvated to approxmately 13% through njecton of gas-saturated down relaton (m s (Pa s)- ), wth the drawdown beng defned water. No reductons n hydraulc conductvty were observed durng the sngle-well testng, whereas degassng s the most as the dfference between the steady state borehole pressure at no-flow condtons (.e., at the end of a PRT) and the borehole

16 2492 JARSJO AND DESTOUN: DEGASSNG OF DEEP GROUNDWATER pressure durng wthdrawal (durng a CPT). For n = 1 the Emsley, S., O. Olsson, L. Stenberg, H.-J. Alhed, and S. Falls, ZEfracture plane s assumed to be parallel to the borehole, for DEX--A study of damage and dsturbance from tunnel excavaton by blastng and tunnel borng, SKB Tech. Rep , Swed. Nucl. n = 2 the fracture plane s assumed to be perpendcular to the Waste Manage. Co., Stockholm, borehole, and for n = 3 the borehole s represented by a Fnsterle, S., and K. Pruess, Solvng the estmaton-dentfcaton problem sphere of radus r w. For n = 1 and n = 2 the fracture n two-phase flow modelng, Water Resour. Res., 31(4), , transmssvty T s obtaned as T = Kb. For rw = 28 mm (.e., Fourar, M., S. Bores, R. Lenormand, and P. Persoff, Two-phase flow n smooth and rough fractures: Measurement and correlaton by the radus of the test holes P2, P4, and P8) the approxmate porous-medum and ppe flow models, Water Resour. Res., 29(11), (A3) yelds a K value that devates less than 0.2% from the , exact soluton, gven an r o equal to (or greater than) 25 m. Gardescu,.., Behavour of gas bubbles n capllary spaces, Trans. Equatons (A1) to (A3) show that the fracture hydraulc Am. nst. Mn. Metall. Pet. Eng., 86, , conductvty K s proportonal to m, the slope of the steady Geller J. T., and J. Jarsj6, Groundwater degassng and two-phase flow: Plot hole test report, SKB spo HRL nt. Coop. Rep , Swed. state borehole nflow-borehole pressure relaton, regardless of Nucl. Waste Manage. Co., Stockholm, flow geometry (n). Gven a specfc value of m, the flow Jarsj6, J., and G. Destoun, Condtons for fracture transmssvty regeometry needs to be determned n order to determne the ducton due to degassng of groundwater: Analytcal expressons, absolute value of K, snce the flow geometry determnes the numercal smulatons and analyss of laboratory and feld data, SKB Prog. Rep. HRL-97-03, Swed. Nucl. Waste Manage. Co., Stockholm, proportonalty constant between K and m. However, the steady state analyss ams at determnng the relatve decrease Jarsj6, J., and J. T. Geller, Groundwater degassng: Laboratory expern K due to degassng, and, assumng that n s the same before ments n rock fracture replcas wth radal flow, SKB Prog. Rep. and after degassng condtons, the rato of K values after HRL-96-12, Swed. Nucl. Waste Manage. Co., Stockholm, (durng test phases 2 and 3) and before (durng test phase 1) Kotelnkova, S., and K. Pedersen, Evdence for methanogenc Archaea and homoacetogenc Bactera n deep grantc rock aqufers, FEMS degassng wll be equal to the rato of m values after and before Mcrobol. Rev., 20, , degassng, regardless of the n value, Natonale Genossenschaft fr de Lagerung Radoaktver Abfflle (NAGRA), Geosynthese Wellenberg 1996: Ergebnsse der Untergphase2_3/gphase 1 -- mphase2_3/mphasel. (A4) suchungsphasen und, NAGRA Tech. Rep. NTB 96-01, Wettngen, Acknowledgments. Ths study was funded by the Swedsh Nuclear Waste Management Company (SKB), Sweden. We gratefully acknowledge contnuous help wth the expermental work throughouthe test perod from Bengt Gentzschen, Geosgma AB, Sweden. We thank Kent Hansson, Andreas Wahlberg, and Peter Andersson from Geosgma AB for ther contrbutons on expermental ssues and the staff of the 3,sp6 Hard Rock Laboratory ste offce for ther assstance. We thank Jl Geller and Chrstne Doughty from Lawrence Berkeley Natonal Laboratory, Calforna, and Olle Olsson from SKB for deas and suggestons on the performance of the tests. Paul Marschall from NAGRA, Swtzerland, generously provded valuable comments and nformaton. Furthermore, the exchange of nformaton regardng the expermental ste wth Anders Wnberg (Conterra consultants, Sweden), Lars Brgersson (Kemakta consultants, Sweden) and John Gale (Fracflow consultants, Canada) of the TRUE resn njecton group was of great value durng the plannng of the plot njecton-wthdrawal test sequence. We thank the TRUE resn njecton group for enablng us to perform the experments on ther ste. References B ckblom, G., The sp6 Hard Rock Laboratory A step toward the Swedsh fnal repostory for hgh-level radoactve waste, Tunnelng Underground Space Technol., 6, , Banwart, S., E. Gustafsson, M. Laaksoharju, A.-C. Nlsson, E.-L. Tullborg, and B. Walln, Large-scale ntruson of shallow water nto a vertcal fracture zone n crystallne bedrock: ntal hydrochemcal perturbaton durng tunnel constructon at the. sp6 Hard Rock Laboratory, southeastern Sweden, Water Resour. Res., 30(6), , Banwart, S., P. Wkberg, and O. Olsson, A testbed for underground nuclear repostory desgn, Envron. Sc. Technol., 31(11), 510A- 514A, Barton, N., A. Makurat, K. Monsen, G. Vk, and L. Tunbrdge, Rock mechancs characterzaton and modellng of the dsturbed zone phenomena at Strpa, SKB Tech. Rep , Strpa Proj., Swed. Nucl. Waste Manage. 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Nucl. Waste Manage. Co., Stockholm, G. Destoun and J. Jarsj6, Dvson of Water Resources Engneerng, Department of Cvl and Envronmental Engneerng, Royal nsttute of Technology, Brnellvfgen 32, SE Stockholm, Sweden. (j erke aom.kth. se) (Receved May 10, 1999; revsed March 30, 2000; accepted Aprl 28, 2000.)