Proceedngs World Geothermal Congress 2015 Melbourne, Australa, 19-25 Aprl 2015 Hydraulc DTH Flud Hammer Drllng as a Sesmc Whle Drllng (SWD) Source for Geothermal Exploraton and Drllng Predcton Poletto F. (OGS), Wttg V. (GZB), Schlefer A. (OGS), and Bracke R. (GZB) (GZB) Internatonal Geothermal Centre, Lennershofstrasse 140, 44801 Bochum, Germany; (OGS), Treste, Italy volker.wttg@geotherme-zentrum.de; geotherme@geotherme-zentrum.de Keywords: Geothermal drllng, hydraulc hammer, sesmc whle drllng, sesmc predcton, DTH hammer, water hammer. ABSTRACT Hydraulc downhole flud hammer systems provde an nnovatve, feld proven technology for fast, hard rock drllng from slm to regular sze holes down to potentally any depths. Ths technology performs va rotary-percussve mechansm, also generatng hghenergy downhole axal sgnals for sesmc-whle-drllng (SWD) purposes. In ths work we descrbe a sesmc-whle-drllng experment usng a down-the-hole (DTH) hydraulc water hammer as the drll-bt source n a shallow well drlled at the GZB test ste. The man objectve of the test was to nvestgate the performance of ths type of drll-bt sesmc source for 2D and, n perspectve, also 3D reverse VSP purposes, to analyze ts sesmc emsson n relaton to the drllng parameters, and to experment and evaluate techncal aspects for further development of an automated SWD system ntegrated nto the hydraulc DTH hammer drllng system. We present results wth real data obtaned durng the experment and by deferred processng, and analyze the related drllng condtons. We show that the processed sesmc sgnal contans clear and wdeband sesmc events. Fnally we dscuss perspectves for the development and mprovement of the DTH-SWD ntegrated system, to optmze measurements and mplement automated acquston parameter control for regular full-scale SWD surveys. 1. INTRODUCTION Drll-bt sesmc-whle-drllng (SWD) source has been used by ol and gas ndustry as a borehole sesmc technology to provde usable data whle and after drllng n order to descrbe n detal the geologcal and geophyscal characterstcs of the subsurface around the well and ahead of the drll bt (Poletto and Mranda, 2004; communcaton by Maln, 2012). The sesmc and geophyscal measurements provded by ths technology and method help to better understand and predct a geothermal reservor and to adjust whle-drllng the possble way of mnng/reachng t. The SWD data provde structural nformaton to better locate possble fault zones and ther drectons, and to dentfy other anomales n the geologcal structure, whch s possbly desrable and mportant n the 2D and 3D area around the well (Poletto et al., 2011; Poletto and Mranda, 2004). Wthout nterference wth the drllng actvty, the typcal drll-bt SWD technology so far uses, as ts source, the nose of standard drll bts. These tools may be more favorable than roller-cone bts workng by teeth ndenton and axal loadng and unloadng percussve acton and, n some cases, less favorable polycrystallne damond compact (PDC) bts, workng manly by shear acton (e.g., Poletto and Mranda, 2004; Poletto 2005a,b). In the conventonal crosscorrelaton-deconvoluton SWD approach, the method uses sesmc recordngs around the well, together wth reference (plot) recordngs at the top of the drll strng or at bottom hole n the proxmty of the bt (Poletto et al., 2014). Poletto, Corubolo and Comell (2010) showed that usng plot sgnals for crosscorrelaton s benefcal also when usng the drll-bt source by sesmc nterferometry (e.g., Wapenaar, Draganov and Robertsson, 2008; Vasconcelos and Sneder, 2008) SWD approach. In standard SWD applcatons by roller-cone bts and nearvertcal drllng, surface plot measurements can be n general successfully utlzed to record the drll-bt sgnal. Advantage of the approach usng only surface plot sensors s that no recordng tools are requred n geothermal hgh-temperature (HT) wells, where the use of downhole electroncs and recordng tools tends to be problematc and can be a crtcal ssue. In the process of gettng sesmc sgnals from the vbratng bt, the sesmc sgnature of the drll-bt source vares dependng on the type of bt, lke roller cone or PCD, as well as drllng condtons, performance of the drll rg and ts drller (Poletto, 2005a; 2005b; Poletto and Bellezza, 2006). To montor these condtons, qualty control durng automated SWD s performed drven by drllng parameters, whch are typcally transmtted to the SWD system by the mudloggng or drll-control unt avalable at the rg ste (Poletto and Mranda, 2004). In ths paper we present SWD conducted wth DTH water-hammer drllng technology developed for hard rock and deep drllng purposes. New drllng technologes are, amongst others, the key to economc explotaton of deep geothermal reservors (Vollmar, Wttg and Bracke, 2013). The drllng speed or rate of penetraton (ROP) of classc drllng technologes, e.g., roller cone and lately also PDC bts, suffers greatly n deep and hard formatons. Thus, the goal s to develop tools wth hgher ROP and low wear to reduce drllng and trp tme and cost. DTH hammers usng ar have successfully proven hgh ROP for a long tme n shallow drllng down to approxmately 300 meters. However, to reach greater depth and constant effcency, the workng medum must rather be a lqud, due to the compressblty and very hgh neffcency of ar. The use of drll mud for better borehole control s requred as a possble soluton. Today, there are only few hydraulc DTH water hammers commercally avalable based on operaton wth clean water. Moreover, t s not yet possble to drll wth mud at constant low wear of the hammer parts. Due to ths ssue, new mud-drven hammer workng prncples and prototypes are stll beng developed and tested at GZB n Germany and prvate ndustry worldwde. Usng a downhole DTH flud hammer wth fxed or possbly varable frequency for drllng produces a sgnfcant amount of axal percussve nose. Ths type of percussve drllng system acts as an ntense SWD source wth relatvely constant percussve frequency n a process that creates a seres of pulses, each of them havng a wdeband spectrum. When the hydraulc drver wll be able to be operated wth a sutable set of drllng percusson frequences, ths hammer technology may eventually allow runnng varable frequency sweeps at a gven locaton, and thus may be an mproved tool and source for SWD data producton and, consequently, for subsequent data nterpretaton after processng. 1
In recent years GZB n Bochum has done ntensve work on testng and developng new DTH flud and mud hammer systems (e.g., Wttg and Bracke, 2012), whch shows very promsng results to be used for SWD loggng purposes n prelmnary drllng tests. Recently, prelmnary testng has been conducted at GZB laboratory ste together wth OGS (Treste, Italy), makng use of the OGS long standng SWD experence and technology adapted for geothermal purposes. Fgure 1 shows a schematc layout of the DTH SWD experment. The objectve was to demonstrate the use of the hammer drllng tool as a source for SWD loggng equpment, for 2D and 3D sesmc purposes. For ths purpose, the DTH flud hammer drll rg was nstrumented by plot sensors, and a sesmc recordng lne was dsposed n the proxmty of the drllng ste, durng the drllng of a shallow depth nterval from near surface down to approxmately 140 m depth. The analyss and the control of the data requred the adapton of the SWD system connected wth the drll-rg control unt for DTH drllng parameters. The ntegraton of drllng and SWD wll enable geothermal exploraton to be much more accurate, further targetng the rght reservor locaton (dentfy sweet spot ) by fndng and vsualzng fault zones and other anomales much more precsely va borehole sesmc data. Furthermore, by provng sesmc sgnals from substructures underneath / ahead of the drll bt t also makes for a much safer and more economc drllng operaton. In ths paper we descrbe the method, show the results of the prelmnary DTH SWD test, and dscuss perspectves for ths ntegrated technology. Some man aspects n the tunng of the SWD and DTH ntegrated technology are related to the specfc drllng technology based on hydraulc drver and fast drllng condtons, enablng to operate wth hgh rate of penetraton (ROP) and requrng to montor the sgnals wth approprate recordng parameters. 2. HYDRAULIC FLUID HAMMER DRILLING TECHNOLOGY The DTH water hammer technology used n the framework of the SWD test presented n ths paper was orgnally developed and bult by Wassara AB n Sweden, a subsdary of LKAB, the nternatonal mnerals group based n Sweden (Wttg and Bracke, 2012). Ths drllng technology s based on ntensfed flow through water/lqud, drvng a downhole pston, resultng n a percussve drllng acton of the drll bt. A rotaton acton s added by turnng the complete drll strng from surface va the drll rg and ts top drve. Thus, the workng prncple of all current DTH water hammer systems s a heavy-weght pston nsde a hammer housng, beng powered by a flow through lqud under certan hydraulc energy, meanng flow and pressure (Wttg et al., 2015, n press). The contaned hydraulc energy of the drllng flud wll then be converted va dfferent knds of valve and control mechansms nto a fast axal upward and downward moton of the pston, creatng a percussve acton by drectly strkng the drll bt. The percusson frequences currently vary between 30 and 70 Hz. As the pston transfers ts energy drectly onto the drll bt placed underneath, the bt n turn crushes and destroys the rock. The flud runnng through ths downhole hammer powers the pston, cools and lubrcates the bt, stablzes the borehole and, most of all, carres up the cuttngs from the rock-breakng process. Ths process of crushng rock and carryng up cuttngs/debrs needs to be n good equlbrum n order for the hammer drllng process to work well and effcently. Fgure 1: Schematc confguraton (not to scale) of DTH water-hammer SWD experment usng the drll bt as a sesmc source. Plot sensors nstalled on the drll rg s top drve record sgnals transmtted through the drll strng and drllng equpment. Recevers of a nearby sesmc lne record drect (black contnuous arrow) and reflected from formaton nterfaces (black dashed arrow), drll-bt sesmc sgnals. The SWD acquston system hosted n a the GZB lab was connected to the drll-parameter control system (red arrow), and to the telemetrc lnes to acqure the plot-sensor sgnals as well as the sgnals of the lne of sesmc recevers (blue arrows). Ths drll acton ncreases sgnfcantly the rate of penetraton (ROP),.e., drll speed, wth respect to conventonal rock bt rotary systems. Ths s a prmary target to mprove the drllng system, useful for geothermal drllng by fast drllng performance purposes. At the same tme, ths type of drllng acton provdes an effectve drll-bt axal source usable for sesmc whle drllng purposes. The drll system wth ths confguraton s usable wth any drll rgs and drll targets. The DTH drll rg used durng the SWD experment was mounted on a moble truck rg (Fgure 2a). The rg s equpped wth dgtal drll-parameter controls systems, provdng man physcal drllng parameters sampled n tme every 3 s durng all the drllng phases. These parameters nclude man drllng and hydraulc parameters, such as weght on bt (WOB), torque etc. The target of the ntal drll test by hydraulc hammer acton was approxmately 150-200 m depth. Fgures 2b shows n detal the bottom-hole assembly (BHA) of the hammer ncludng the bt used durng the experment. The bt s an nsert bt wth 7 ¼ n. dameter. Ths tool was used n a drll ppe assembly composed of 6 n. outer dameter drll ppes. Each ndvdual drll-ppe length s four meters. Maxmum drlled depth durng ths prelmnary SWD phase was 140 m. 2
Fgure 2: a) Vew of the fully moble, track type drll rg, ftted wth a DTH flud hammer drllng system. Ths rg may operate down to approxmately 1-1.5 km depth. The drll rg s equpped wth an electronc drllng parameter loggng and montorng system. b) Close up vew of the downhole water hammer utlzed durng the experments. At the bottom of the hammer, the drll bt wth hard metal nserts s clearly vsble. The bt tself has no movng parts, but rotates durng percusson drllng wth the complete hammer and drll-strng assembly. 3. ACQUISITION LAYOUT AND PARAMETERS Durng ths prelmnary test, n order to have basc ndcatons for the ntegrated use of SWD wth the DTH flud hammer system, the decson was made to perform a quck survey usng a portable SWD technology, wth a sutable, however lmted number of sensors, and adequate flexblty to adaptng the SWD system and the montorng of the parameters under dfferent operatonal condtons encountered wth ths new type of drllng system. 3.1 SWD acquston lne The SWD acquston system used durng the GZB experment (schematcally shown n Fgure 1) utlzes a telemetrc lne wth 20 sesmc recever traces to record the mult-offset sesmc data durng the SWD experment. The sesmc traces are postoned n the proxmty of the rg, spaced wth nter-trace nterval of 10 m, mnmum and maxmum offset from the well head 20 m and 210 m, respectvely. Ths sesmc-lne extenson was evaluated and decded assumng an expected maxmum drllng depth of approxmately 200 m for ths ntal test. Each sesmc trace was recorded by 12 vertcal geophones (Sensor SM4 U-B 10Hz) dsposed n a lnear-array confguraton centered at the selected trace-recordng postons. The azmuth of the sesmc lne was selected n the drecton of an accessble feld wthn the GZB area wth good logstc and ground surface condtons. Some of the traces of the sesmc lne were postoned at close poston wth respect to a post of permanent GZB sesmometers, also used by GZB to montor drllng and for repeated geophyscal experments at ths test ste, wthn a local network of permanent sesmologcal montorng (Wttg, Bracke 2012). Other SWD traces where dedcated to record wth the same acquston parameters a certan number of plot traces from reference accelerometers nstalled on the rg s hydraulc top drve at the top of the DTH drll strng, as schematcally shown n Fgure 1, to measure the sgnal propagated through the drll strng and drllng plant. More sensors were nstalled at dfferent postons on the top drve, to verfy measurements wth dfferent sgnal to nose rato (S/N) local condtons, to select the optmal plot sgnals usable for correlaton wth the sgnals recorded by sesmc sensors located n the proxmty of the well ste. 3.2 SWD acquston parameters The SWD system performs by repeated contnuous sgnal recordng n sutable drll-bt depth ntervals (Poletto and Mranda, 2004). Ths task s acheved usng cascades of contguous records synchronzed at the begnnng of every mnute. Each record ncludes all the plot and sesmc traces. The SWD data tme samplng nterval was one mllsecond, and the total tme duraton of each ndvdual record was 50 s. The SWD data were synchronzed by acqurng a GPS channel, also recorded n a trace of the feld records, whch enables further processng and possble comparson wth external data acqured by the local sesmologcal network. In SWD applcatons by conventonal rotary systems the avalablty of mudloggng parameters drves automated data acquston and pre-processng. In the DTH SWD drll-bt tral at GBZ, the drll-parameters control system specfcally developed for the hydraulc flud hammer and avalable at the rg ste was ntally connected to the SWD system, and controlled by operators. The data communcaton was tuned durng the experment for qualty control purposes, wthout actvatng durng ths frst communcaton test the fully-automated data acquston and processng modalty. The whole dataset of recorded drllng parameters was downloaded and stored for deferred SWD data characterzaton, analyss and processng. The drect control of recordng bt depth and drll strng length was realzed by settng average acquston levels at bt depth ntervals of four m, correspondng to the begnnng of the jont of a new drll-ppe secton. The flud-hammer acquston n the shallow formatons was characterzed by fast drllng, wth expected ROP of the order of 20 m/h or more. Under these condtons, the total recordng tme for each SWD depth level of four m n whch to obtan the average bt sgnal was about 12 mnutes or less. The analyss shows that ths tme s suffcent to obtan energetc sgnals for SWD purposes. After ntal conductor casng, the SWD survey started at the bt depth of approxmately 20 m. Durng the survey, tests wth varyng drllng parameters,.e., partal changes of the drvng flud pressure resultng n varatons of the hammer frequency, were performed to obtan a wder emsson by the percussve bt source. The survey was completed at drllng depth of approxmately 136 m. In ths acquston nterval a total number of 30 averaged SWD depth levels were acqured, correspondng to a total data acquston tme of approxmately nne hours for drllng parameters and four hours for SWD data. 3
3.3 Drllng parameters Some man drllng parameters recorded durng the acquston ntervals are shown n Fgures 3 and 4. Fgures 3a,b,c,d show the drllng bt depths (m), averaged ROP (m/h), WOB (kn) and torque (Nm) parameters, respectvely, n the SWD test nterval durng SWD actve acquston, apart from a short perod around 2.3 10 4 s. Fgure 4 shows, a) a detal of the pump pressure (bar) and b) the revoluton speed (rev/mn) n a selected perod correspondng to one hour of actve acquston. The detaled plot of Fgure 4 makes t possble to observe the varatons n the nstantaneous pressure, wth a seres of steps programmed and utlzed durng the SWD test to ncrement the varablty of the drll-bt sesmc sgnal, and the correspondng varatons n RPM. a) b) c) d) Fgure 3: a) Drllng depth versus progressve acquston tme, correspondng to a total tme of approxmately nne hours of parameter acquston. In a short nterval at approxmately 2.3 10 4 s there s a short no-drllng phase due to rg mantenance. b) In the same horzontal tme scale of, plot of the average rate of penetraton (m/h). c) Plot of nstantaneous weght on bt and d) nstantaneous torque durng the same recordng ntervals of and. a) b) Fgure 4: a) Detal of pump pressure n a tme nterval correspondng to one hour of drllng data acquston. Each actvepressure zone corresponds to a new drll-ppe length. The varaton of pressure was programmed wth dfferent ncreasng and decreasng step values of few mnutes, for some selected depth levels, to ncrease the varaton n the drll-bt sesmc sgnal. Rght b) Plot of the revoluton speed (RPM) n the same acquston nterval. 4
4. SWD RESULTS The acquston of SWD data at regular drll-bt source depth ntervals provdes whle drllng reverse vertcal sesmc profle (RVSP) data,.e., borehole sesmc profles wth recprocal geometry wth respect to the conventonal wrelne ones (e.g., Poletto and Mranda, 2004). In ths ntal test, the recordng start and stop was partally automated wth the control of the acquston parameters managed drectly by n-feld operators. The acquston of the SWD data was performed startng at depth 20 m. The acquston depth nterval was set four m, followng the start of each new secton of drll ppe. Dependng on the dfferent rate of penetratons and drllng condtons a varable number of records, rangng between 4 and 14, was acqured for each ndvdual drllppe length. The deepest SWD level was at bt depth 136 m, the total number of source depth levels was 30. The total recordng tme durng drllng was approxmately four hours, wth average recordng tme of approxmately eght mnutes per depth level. 4.1 Raw feld data Fgure 5 shows an example of raw feld gathers obtaned by the geophones of the sesmc lne whle the bt was drllng at 96 m depth. Fgure 5a shows raw feld traces n a selected tme gate, wth pseudo-perodc events and also random sgnal components n tme, and coherent events n space. We can observe the coherent hyperbolc-trend move out of the sgnal along the offset, as expected for harmonc sgnals generated at the bt locaton. Fgure 5b shows the correspondng spectra wth perodc frequency peaks and dstrbuted broadband background energy of lower ampltude level. Durng the survey, the pseudo-perodc events change for ther frequency content at dfferent bt postons and drllng tmes characterzed by varable drllng parameters. Each panel of Fgure 6 shows the frequency spectra of the sgnals of a selected trace represented for the whole sesmc-whle-drllng acquston perod. The top panel shows the spectra of a top-drve plot sgnal. The bottom panel shows the spectra of the feld geophone sgnal wth trace number #208 at 90 m offset. We can observe smlar trends of the events n these spectra. a) b) Fgure 5: a) Raw SWD sgnals, offset traces recorded at drllng depth 96 m. b) Spectra of the traces n. We can observe the frequency peaks comb of the DTH water hammer source workng wth perodc and pseudo-perodc acton. a) b) Fgure 6: a) Spectrum of the plot trace durng the survey. b) Spectrum of the geophone trace #208 durng the survey. Both present a frequency peak at varable frequency around approxmately 30 35 Hz (arrows), and harmoncs. 5
4.2 Basc SWD sgnal processng The raw feld data were pre-processed whle drllng to obtan nterpretable sesmograms. We recall the common and well-known SWD approach by cross-correlaton wth a reference plot sgnal (e.g., Rector and Maron, 1991; Poletto and Mranda, 2004), n whch the sgnal of the geophone trace x(t) s cross-correlated wth the correspondng sgnal of a sutable-selected plot trace p(t). The plot trace can be obtaned n dfferent ways and, for some aspects, the process s smlar to sesmc nterferometry (e.g., Wapenaar et al., 2008). Ths crosscorrelaton process to get nterpretable sesmograms from raw drll-bt feld data can be smply expressed n the Fourer frequency doman as S( ) X ( ) * ( ), (1) P where s the angular frequency and * denotes complex conjugaton. The summaton ndex represents the feld-record ndex, and the summaton s extended over records all belongng to the same source-depth level, thus provdng an average result n the depth-level nterval. In our case the depth level ntervals were sampled every four meters, n correspondence of the drll-ppe lengths. The sesmogram s(t) s obtaned by nverse-fourer transformng Equaton (1). The cross-correlaton approach extracts the drll-bt correlated sgnal n the sesmc recordngs. It requres also correctng for propagaton and flterng effects contaned n the plot trace p(t), to obtan the sesmc sgnal n the trace s(t) wth correct delays, and preserve only the bt-to-recever sesmc transfer functon of x(t). In case of deal wdeband random source sgnals, say, as when usng a whte random source, ths process recovers a sesmc trace as from a transent source of mpulsve waveform actng at the bt locaton. Dependng on the nature of the drll and of the bt source dynamc behavor, the drll-bt SWD data typcally devate from these condtons, so that addtonal processng s necessary to mprove the RVSP result by removng the source sgnature and plot transfer functon. An effectve approach s to use the average plot autocorrelaton A( ) P( ) * ( ), (2) P to calculate a deconvoluton operator D( ), so that the deconvolved sesmogram becomes S D ( ) D( ) X ( ) P * ( ). (3) Prevous equatons assume n some way statonary transfer functons over the summaton doman. Dependng on the varablty and characterstcs of the source, Equatons (2) and (3) can be also used n a modfed form to calculate and apply, record per record (or tme gate per tme gate), non- averaged operators D ( ) derved from autocorrelatons A ( )=P ( )P * ( ) before the summaton of the data of a depth level, namely S ( ) D ( ) X ( ) P * ( ). (4) DI The deconvoluton operators are calculated and appled ether one-sded usng D( ), to remove ant-causal cross-correlaton events ntroduced by the plot sgnal, or two sded by ncludng and applyng a causal operator for the bt radated sgnal when the source perodc or pseudo-perodc effects are relevant and persstent also n the sesmc sgnal. A causal deconvoluton operator can be obtaned, as an approxmaton, from the complex conjugate of D( ) or by sgnal focusng methods. These dfferent deconvoluton approaches provde dfferent approxmated solutons, n terms of transent-sgnal focusng and presence of resdual nose after nverse flterng. The type and qualty of plot sgnal s mportant n ths process. In the DTH SWD tral we have nvestgated the drllng and recordng condtons, by expermentng dfferent plot sensor solutons, n terms of reference transducer (accelerometer) optmal locaton on the top drve on the rg, and the use of off-rg sensors (geophone), whch also detect a relevant component of drll bt sgnal on the ground, also at hgh sesmc frequences. Fgures 7 and 8 show two dfferent examples of cross-correlated and two-sded deconvolved data obtaned at close drllng depth postons usng the reference plot sgnal of an accelerometer nstalled on the top drve. These two levels are characterzed by dfferent source emsson, wth (Fgure 7) less relevant and (Fgure 8) more relevant perodc behavor of the hammer bt source, nterpreted as relatve to a percusson frequency of 31-33 Hz (see also the general trends of Fgure 6). 4.3 Plot sgnal analyss The qualty control of the drll-bt SWD sgnal typcally uses the axal plot sgnal analyss to characterze the bt source and the wave propagaton n the drll strng from bottom hole to surface. The average plot sgnal transfer functon per depth level can be represented, algned at zero tme, by the plot-sgnal auto correlatons after one-sded deconvoluton, whch s appled to remove the ant-causal events n the plot autocorrelaton. Ths approach makes t possble to dentfy and nterpret drll-strng events, such as long perod multples between bottom hole and top of the drll strng, as well as short perod bottom hole reverberatons, and other possble acoustc events developed durng drllng. It also may evdence possble presence of other nose sources. In the DTH SWD experment at GZB test ste the drllng nterval s qute short, from 20 to 136 m, and ths makes the sgnal analyss n depth less effectve. The sgnal analyss confrms that the observed events represent contrbutons of the plot sgnal from the bt. Fgure 9a shows the top-drve plot sgnals plotted versus drll-strng length. In ths example the data are fltered n a hgh-frequency bandwdth above 36 Hz, to better evdence short-range propagaton detals n a short-tme correlaton wndow. The drect arrval algned at zero tme s represented by negatve polarty, to better evdence reflecton events at approxmately 0.05 s at postve correlaton tmes n the deeper traces. At ths stage, the analyss s only partal, also due to the short length of the nvestgated nterval. Also n the presence of a sgnfcant amount of drll-bt sgnal energy n the plot sgnal, some nose s present due to local nosy condtons at the plot recordng poston on the top drve, and due to the lmted capablty to ncrease the amount of the stacked data to mprove S/N (random), due to hgh rate of penetraton wth ths type of system. 6
Fgure 7: Examples of crosscorrelaton and reference deconvoluton for the offset shot gather at 84 m bt depth level. In the sgnal s broadband, wth clear transent drect arrval (at approxmately 0.05 s n the near-offset traces) and coherent nose from the rg (at approxmately 0.15 s). Data are not corrected for plot sgnal delay. (c) (d) Fgure 8: Examples of crosscorrelaton and deconvoluton for the offset shot gather at 88 m bt depth level. There s a dfferent content of pseudo-perodc events n 8a and Fg.7a. After deconvoluton, the sgnal s broadband, wth clear transent drect arrval and coherent nose from the rg n, smlar to 7b. Data are not corrected for plot delay. A smlar result can be obtaned by observng the rg-plot sgnals by means of the geophone sgnal. In other words, snce the qualty of the geophone sgnals has demonstrated to be good, we have used the geophone traces to extract and observe the sgnal n the rg-plot data. Ths result obtaned usng two near-offset geophone traces for crosscorrelaton and deconvoluton wth the rg plot sgnal (causal response), s shown n Fgure 9b. Compared to the rg plot result of Fgure 9b, we can observe correspondng perodc events at the deeper traces, whch confrm the elongaton of the drll strng and consequently the deepenng of the bt sgnal. 7
Fgure 9: a) One-sded deconvolved rg-plot autocorrelatons plotted versus strng length, and b) rg-plot sgnal obtaned cross-correlatng and one-sded deconvolvng the rg sgnal by geophone traces as reference sensors. 5. ANALYSIS AND INTERPRETATION OF RESULTS Notwthstandng the prelmnary character of the methodologcal test performed at GZB usng the SWD adapted technology together wth the DTH flud hammer source, usng a surface sesmc lne wth lmted extenson and a lmted drllng-depth nterval, the measured SWD data can be used to obtan some nformaton about the surroundng area. From geologcal pont of vew, the subsurface s characterzed by shallow sedments and coal layers wth lttle structural varatons. There s a synclne flank dppng to North West and strkng SW-NE. There s another synclne SE of the borehole. The structural complexty of the area can be clearly observed n the deconvolved results of Fgures 7b and 8b, n whch the drect arrvals approxmately at 0.05 s n the short-offset traces present rregular varatons, and devaton from a hyperbolc move out expected wth horzontal layers. A smple and very prelmnary nterpretaton gves evdences of the subsurface complexty. On the bass of the analyss of the SWD data collected n offset and depth, we test some tral model to calculate synthetc sgnals, ncludng rg radated nose, and compare full waveforms n synthetc and real SWD data n offset and depth. In ths process, the SWD data obtaned by top-drve plot sgnals are corrected for the delays of the plot sgnal propagaton n the drll strng, from ts bottom to the surface where they are recorded. Compared to the exstng geologcal model of the area (Fgure 10a), a smplfed model of the shallow subsurface s shown n Fgure 10b, where the blue dashed lnes represent the zones of the measurements at the surface and n depth, and the well poston s at (0,0). The geologcal complexty s nterpreted by a varaton of the layer nterface, tunng ts shape and the velocty model. The medum s assumed to be a Posson medum. The model s used to calculate synthetc sesmc sgnals by a fnte dfference elastc code, dscretzed wth pxels of 2 2 m, usng a 30-Hz central frequency source wavelet. The comparson of the real SWD sgnals and of the synthetc results performed usng gathers at dfferent recever offsets and bt source depths confrms the general agreement of the nterpreted trends, even f the model of Fgure 10b s over smplfed at ths stage of analyss. Fgure 11 shows the comparson of synthetc and real common-source sgnals obtaned wth the drll bt at 116 m depth and the surface geophone traces at offsets from 20 to 210 m. Fgure 12 shows the comparson of common-recever reverse VSP sgnals obtaned wth the bt source between 20 and 136 m depth and a geophone trace at offset of approxmately 130 m (trace #212). Fgure 10: a) Overvew of the complex geologcal area around the well zone (red dashed lne). b) Smplfed local velocty model derved from analyss of SWD data and used to calculate synthetc sesmograms for comparson wth SWD results. Blue dashed lnes ndcate surface-recever and depth-drllng ntervals. 8
Tme (s) Tme (s) Tme (s) Tme (s) Wttg et al. Fgure 11: Common-source gathers at depth 116 m, traces versus offset. a) Synthetc sgnal, and b) real sgnal. The syntetc result smulates and partally reproduces the varaton n the drect arrvals at about 100 m offset. Depth (dm) Depth (dm) Fgure 12: Common-recever (geophone trace #212 at 130 m offset) gathers. a) Synthetc sgnal, and b) real sgnal. Source depth s represented n decmeters. In general, the trends of the events are n agreement, even f the real result shows more complex wavefelds. The synthetc result smulates the varaton n the arrvals at approxmately 110 m depth and 0.1 s n the real sgnals. 9
Tme (s) Tme (s) Wttg et al. 6. USE OF OTHER PILOT SIGNALS Fnally, as an addtonal potental applcaton to mprove the SWD results, we show some examples obtaned usng focused sesmc plot sgnals. Ths result s represented by Fgure 13, where the deconvolved data are obtaned usng a combnaton of rg-plot sgnal and of a focused sesmc sgnal for the source depth level 84 m, compared to the result obtaned usng only the rg plot sgnal. Ths example shows also that the SWD sesmc nterferometry approach,.e., data-based redatumng, can be a possble effectve approach when ntegrated wth the use of rg plot sgnals (Poletto et al., 2010). However the sesmc focusng approach may provde results that may depend on the choce of the sgnal event analyzed for focusng purposes, and requres some careful evaluaton. Fgure 13: Sgnals obtaned at common-source depth 84 m. a) Result by mxed rg plot and focusng sgnals and b) only rgplot sgnal for correlaton and deconvoluton wth geophone sgnals of the offset sesmc lne. 7. DISCUSSION AND PERSPECTIVES From ths prelmnary test, dfferent aspects and perspectve emerge. These are: The test allowed us to nvestgate the drll-bt SWD technology adapted for geothermal purposes together wth DTH water hammer drllng. The test was desgned and planned as a quck prelmnary test, thus performed wth a lmted number of sensors and a portable acquston system, and by n-feld adapton of the communcaton protocols of the SWD and DTH control unts. Ths made t possble to only partally perform automated acquston whle drllng. Ths s an mportant ssue, especally durng fast drllng condtons, typcal of ths type of drllng tool, to optmze the data recordng under sutable drllng wndows wthout loss of recordng tme. The physcal dmensons of the test were lmted to shallow drllng and short-offset ranges, of the order of 130 and 210 m, respectvely. The lmtaton to these ranges makes more dffcult to nvestgate the wavefelds and ther move out. Under these condtons, n the proxmty of the rg, drllng, sgnal and rg nose are supermposed. At shallow drllng depths relatve to geophone offset, the radaton condtons for compressonal sgnals, as those produced by the axal-percussve DTH flud hammer source are not favorable. Based also on prevous SWD experence, the evaluaton s that wth deeper drllng the sgnal s patterns are clearer and the drllng condtons more relevant for demonstraton of standard use of the method. An objectve of future applcatons s to perform a deeper test usng wder recever offsets. Next techncal steps and targets are to mprove the ntegrated drllng-parameter control, by optmzaton of the automated communcaton protocol, thus allowng fully-automated acquston smlar to that performed n the prevous SWD experence wth conventonal, full-sze rotary-drllng systems. As a general consderaton, the results of ths test at the GZB ste have shown that the method provdes rather hgh-qualty sgnals, usable wth cross-correlaton and deconvoluton methods. The percussve nature of the pseudo-perodc source adds frequency content that makes t possble to recover a wdeband sgnal, for whch focusng and nterferometry approaches are also consdered to ntegrate the nformaton obtaned by rg reference plot sgnals. The qualty of the plot sgnal s mportant. A man target for future applcatons s to optmze the nstallaton of plot sensors postoned onto the DTH drllng hammer rg, wth the purpose to mprove the recordng of the plot sgnal n the drll strng, and to mnmze the dsturbance from local-nose sources nvolved n the flud hammer hydraulc drver mechansm. Ths means and nvolves a dedcated ntegraton of the SWD system wth the DTH SWD technology. 10
8. CONCLUSION We show the results of a DTH flud hammer drll-bt SWD survey performed wth a hydraulc water hammer drllng system whle drllng a shallow well at GZB n Bochum. Ths hghly effcent, rapd drllng system performs through ntense downhole axal vbraton, whch provdes a sutable source for reverse VSP purposes. The test was performed by adaptng the automated SWD technology already used wth conventonal geothermal rotary drll systems. The results gve sgnfcant ndcatons about sgnal qualty, and show that sesmc frequency content s relevant also at hgh frequency. Improvement of the method and technology usable for fast geothermal drllng purposes, for nvestgaton of structures to be drlled and around of the well n 2D and 3D confguratons, are envsaged. The analyss shows the presence of structural varatons for the local geologcal condtons around the well and ahead of the drll bt. ACKNOWLEDGMENTS Authors thank Fabo Meneghn for the preparaton and management of the acquston system n the feld, and Massmo Lovo for the techncal assstance durng acquston and data analyss. Authors thank also Bancamara Farna and Aronne Cragletto (OGS) for the assstance n the preparaton of synthetc sgnals and software assstance. We thank the RUB sesmologsts, n partcular, Marc Boxberg, and Kasper Fsher for ther partcpatng and sharng. At GZB, we especally thank Mandy Duda, Thomas Andolfsson and Gregor Bussman for provdng auxlary data and helpful comments on geologcal model analyss; and, last but not least, we do thank the complete GZB feld staff and the drllng team for ther excellent work and support and for beng such good hosts. REFERENCES Maln, P.: Combnng sesmc and electromagnetc observatons n complex rock. Communcaton by Presentaton at AGIS meetng n Karlsruhe, Germany (2012). Poletto, F.: Energy balance of a drll-bt sesmc source. Part 1: Rotary energy and radaton propertes. Geophyscs 70, T13-T28 (2005a). Poletto, F.: Energy balance of a drll-bt sesmc source. Part 2: Drll-bt versus conventonal sesmc sources. Geophyscs 70, T29- T44 (2005b). Poletto, F., and Bellezza, C.: Drll-bt dsplacement-source model: Source performance and drllng parameters. Geophyscs 71, F121 (2006). Poletto, F., Corubolo, P., and Comell, P.: Drll-bt sesmc nterferometry wth and wthout plot sgnals. Geophyscal Prospectng 58, 257 265 (2010). Poletto, F., Corubolo, P., Schlefer, A., Farna, B., Pollard, J. S., and Grozdanch, B.: Sesmc whle Drllng for Geophyscal Exploraton n a Geothermal Well: Proceedngs of the GRC Conference San Dego, CA, US (2011). Poletto, F., and Mranda, F.: Sesmc whle drllng. Fundamentals of drll-bt sesmc for exploraton, Elsever, Pergamon vol 35 (2004). Poletto, F., Mranda, F., Corubolo, P., Schlefer, A., and Comell, P.: Drll-bt sesmc montorng whle drllng by downhole wred-ppe. Geophyscal Prospectng, n Press, do: 10.1111/1365-2478.12135 (2014). Rector III J. W., and Maron, B. P.: The use of drll-bt energy as a downhole sesmc source. Geophyscs 56, 628-634 (1991). Vasconcelos, I., and Sneder, R.: Interferometry by Deconvoluton: Part 2 Theory for elastc waves and applcaton to drll-bt sesmc magng. Geophyscs, 73, S129-141 (2008). Vollmar, D., Wttg, V., and Bracke, R.: Geothermal Drllng Best Practces: The Geothermal translaton of conventonal drllng recommendatons - man potental challenges; IGA Academy report, nternatonal Fnance Corporaton IFC; Bochum, Germany (2013). Wapenaar K., Draganov, D., and Robertsson, J.: Sesmc nterferometry: Hstory and present status. Socety of Exploraton Geophyscsts, Geophyscs Reprnt Seres No. 26, ISBN 978-1-56080-150-4 (2008). Wttg, V., and Bracke, R.: DTH Flud Hammer drllng developments and latest R&D actvtes at GZB n Bochum; Internatonal Geothermal conference GeoTherm n Offenburg, Germany (3-2012). Wttg, V., et al.: Hydraulc DTH Flud / Mud Hammers wth Recrculaton capabltes to mprove ROP and Hole Cleanng for deep, hard rock Geothermal Drllng; WGC 2015 abstract #21053, In Press (2015). 11