Getting to grips with gas

Transcription

1 yy y y Getting to grips with gas y yy y yy yy yy

2 For many years, reservoir engineers have striven to obtain an accurate throughtubing measurement of gas saturation in their reservoirs. The recent introduction of Thermal Decay Time tools combined with multi-well data base analysis has led to a major breakthrough. Tests on over 1 wells in the Middle East have proved that reliable through-tubing measurements are possible - allowing engineers to accurately identify problems such as gas fingering and secondary gas cap development. In this article, Patrick Crossouard and Mohamed Watfa use field examples to highlight the kinds of results that can be obtained with the new technology. Contributions by Mario Petricola

3 Gas saturation monitoring has posed a thorny problem for reservoir managers for many years. Attempts to determine the proportions of oil and gas in hydrocarbonbearing formations have met with little success. The main problem is one of resolution. Both open and cased hole logs are not sufficiently sensitive to allow the oil and gas fractions in the rock to be differentiated. At best, the techniques can provide only a qualitative assessment. However, as gas-associated problems in the region s major reservoirs began to emerge in mature fields (see box opposite), the demand for an accurate method of measuring gas saturation intensified. Until recently, the most widely used open-hole gas saturation monitoring technique involved analysing density and neutron logs. The apparent porosity, shown in a neutron log, should decrease when gas is present because of the lower hydrogen index. The density measurement should also show a corresponding reduction as the fluid density decreases. Therefore, if the lithology and clay volumes are known, the difference in the density/neutron curves will indicate the presence of gas. This is the theory, but in practice the method has several limitations. One of the main problems is that density and neutron tools have different depths of investigation and their readings may vary depending not only on the presence of gas but also on the volume of formation which they see. In addition, before gas saturation can be quantified, a value of water saturation in the invaded zone (S xo ) is required. The downhole gas density and accurate estimates for lithology and clay volumes also have to be obtained. In most wells, the invaded zone contains less gas than in the virgin reservoir. This can reduce the resolution of density/neutron readings and increase statistical errors in the measurement. To make matters worse, the neutron/density technique is even more restricted in oil-based mud as the effects of the invading oil filtrate dominate the readings. Open hole measurements only represent the reservoir at the time of drilling. It is far more crucial to take gas saturation readings after the well has been in production in order to check for gas fingering and the like. Because of this, three types of through tubing measurements have been developed, all of which involve some use of neutron tools. Fig. 3.1: GAS GUZZLER: The left diagram shows an oil reservoir and gas cap at the start of development. The wells are perforated in the oil zone. As the field matures (right), gas fingering (left well) and gas coning (right well) develops and this seriously affects production. If these problems can be spotted, remedial action can be taken. One technique is based on measuring two or more neutron counts, obtained using two or more source-detector spacings. The long-spacing counts have a deeper depth of investigation and will be more affected by gas than the readings taken with the short spacing detector. To use this technique it is necessary to find a non-gas bearing interval for comparison. A second method relies on measuring neutron porosity using the TDT or Slim Hole Compensated Neutron (CNT*) tool. This works on the same principle as the open-hole technique. The reduction in the hydrogen index in the gas produces a corresponding decrease in apparent neutron porosity. The third cased hole technique is based on the differences in the neutron capture cross-section (Σ) values of oil (2) and gas (8). 34 Middle East Well Evaluation Review

4 Unfortunately, all these cased hole measurements can be inaccurate as they are affected by the well environment - borehole fluids, casings, tubings, cement and fluid behind the casing (see page 38). Environmental effects become particularly pronounced when gas is in the borehole and no corrections can be made to these readings to compensate for the errors. However, over the past few years, two new developments have made gas saturation monitoring much more accurate. The introduction of the Dual-Burst Thermal Decay Time (TDT-P*) and the development of multi-well data base techniques. The TDT-P tool has now been tested in over 1 gas wells in the Middle East where it has been used successfully to detect gas coning and monitor changes in the gas/oil contact over time. The tool s principle of operation is described in detail in the next pages. Basically, the TDT-P tool provides an accurate formation capture cross-section measurement which is corrected insitu for borehole and diffusion effects. Because corrections are made in the well, there is no longer any need to gather information about the borehole fluid, casing or cement configurations. Tests in the Middle East have shown that the TDT-P tool s Σ measurement is accurate to within ±.3 capture units (cu). This translates into an accuracy in the computed gas saturation of about 1 saturation units (su) in a rock with a porosity of 2 porosity units (pu). The accuracy decreases as porosity declines. By using statistical correlations from a multi-well data base, it is possible to improve the accuracy of the TDT-P tool s readings so that they can be used in reservoirs with less than 15pu porosity. In pages 39 to 41, Middle East Well Evaluation Review looks at five examples from the region which highlight how the combined TDT-P tool/data base approach is helping to identify various gas-related problems. STEPPING ON THE GAS Fig. 3.2: EARLY WARNING SYSTEM: Regular gas saturation monitoring gives advance warning of gas chanelling and coning behind the casing. Gas Cap Gas/oil Channelling If your reservoir has a gas cap, you need to carry out gas saturation monitoring on a regular basis. So say the experts. Gas measurements help to optimize field production and prevent well and field problems escalating to the point where the reservoir may be irreparably damaged. The first priority is to keep an eye on gas cap expansion. This is essential for optimizing production and for determining the perforating interval to prevent excess free gas entering the wells. Regular gas saturation readings can also give an early warning of gas channelling and coning behind the casing. This can develop due to a poor cementing job or because of gas migration through fractures or high permeability zones. A large drawdown could lead to disastrous coning and gas fingering problems (figure 3.2). Without saturation monitoring, these localized problems might be assumed to be changes in the gas/oil contact. TDT-P Coning Fingering In reservoirs with large permeability variations, unmonitored gas injection can lead to streaks of gas passing rapidly through the formation and entering the well. The cost of gas saturation monitoring is minimal in comparison to the remedial action necessary to combat such fingering problems. Gas saturation monitoring also helps to reveal the growth of secondary gas caps which can develop due to declining reservoir pressure. These have been observed in reservoirs which originally did not have a gas cap. Finally, gas saturation can help to explain the reason for high gas/oil ratio production. Number 1,

5 Sorting out sigmas Fig. 3.3: The increased accuracy of the Dual- Burst TDT tool, together with measurements of borehole and formation capture cross-sections and hence in-situ borehole compensations, has extended the range of TDT tool monitoring applications. In essence, the improved readings allow us to distinguish between gas and oil saturation - previously impossible with any degree of confidence (see figure 3.3). For example, in a typical Middle East reservoir, the difference in readings between a formation with a 1% oil and 1% gas saturation may only be 4cu. Previous TDT tools provided readings which were only accurate to within 2cu, which introduced large errors in the computed gas saturation values. The oil-gas resolution of formation ( for )is in the range of 3 to 4cu in most of the Middle East s gas-bearing reservoirs. Errors in the Dual-Burst TDT tool s for measurements are now within.5cu and this has enabled gas saturation to be computed in rocks where the porosity is as low as 15pu. Previous generations of TDT tools offered limited scope for gas saturation monitoring because of their limited accuracy. As the volume of hydrocarbon reduced, so did the accuracy of the gas saturation measurements. DISTINGUISHING FEATURES: matrix from the TDT-P tool is independent of borehole conditions and does not need environmental corrections as illustrated by this limestone plot. This accuracy enables us to distinguish between oil and gas. Intrinsic limestone Intrinsic dolomite 1 Figure 3.4 shows how errors in for readings affect the value of the computed gas saturation, S gas. The improvement in the accuracy of the measured for from the Dual-Burst TDT tool (estimated in the range cu), allows accurate values of S gas to be estimated for volumes of hydrocarbon greater than 15pu Σ limestone TDT-K (uncorrected) TDT-M in 9 5/8" csg +tbg TDT-M in 9 5/8" csg TDT-M in 7" csg+tbg TDT-M in 7" csg TDT-K (corrected) TDT-P in 7" or 9 5/8"csgs with or without tbgs When only oil and gas movements are taking place in the reservoir (S w = original value estimated from open hole data), the for time-lapse equation can be defined as follows for changes in gas saturation S gas : S gas = ( 2-1)/ø( oil - gas)...(1) 2 2 Errors in computed gas saturation (%) cu 1.cu.5cu.25cu pu 6pu 4pu 2pu ø Hydrocarbon volume (%) Fig. 3.4 (Left): Errors in the computed gas saturation as a function of the total hydrocarbon volume, for different errors in the measured value of Σ. Fig. 3.5 (Right): The effects of errors in the Dual-Burst TDT tool derived porosity on the computed gas saturation Hydrocarbon volume (%) 36 Middle East Well Evaluation Review

6 TDT derived porosity (ø TDT ) Effective porosity (ø) water oil gas Σ for water oil 2 gas Effective porosity (ø) When water, oil and gas movements occur, three equations relating the three fluid saturations are necessary in order to solve for S w, S oil and S gas. The capture cross-section equation and the fluid balance equation are two obvious relationships which can be used: for = w.s w + oil.s oil + gas.s gas. + cl.v cl + mat (l - V cl - ø)...(2) S w + S oil + S gas = 1...(3) S gas Water Fig. 3.6 (Above): Two crossplots of effective porosity versus formation capture cross-section and calibrated Dual- Burst TDT measured porosity. From the results of data-base work undertaken over a large number of wells (see reference below), it was found that the Dual-Burst TDT tool porosity (ø TDT ), can be calibrated and used as the third equation. A linearized equation for ø TDT can be used if the range of porosity variations is narrow. TPHI 2 15 ø V quartz 15 PHIT Oil Gas 3 DOLO LIME SAND 3 Fig. 3.7a (Left): Validation of quartz, water oil and gas on a multiwell crossplot. Fig. 3.7b (Below): The TDT tool s porosity response to quartz, calcite and dolomite is crosschecked against open hole porosity in a complex reservoir. ø TDT = A.ø.S w + B.ø.S oil + C.ø.S gas + D.V cl + E.(l - V cl - ø)...(4) Parameters A, B, C, D and E are zoned constants representing the Dual- Burst TDT tool s porosity response to water, oil, gas, clay and matrix respectively. These are obtained from database evaluations using cross-plots and histograms over intervals where the formation fluid contents are determined. The ø TDT channel is calibrated in the data-base - in both water and oil zones - to match the effective porosity obtained from open hole evaluations. This approach is very similar to the use of CNT open hole porosity (NPHI) as a linear equation in ELAN evaluations. An additional advantage of equation (4) is the good oil-gas resolution of the ø TDT channel. Figure 3.6 shows how the ø TDT log compares with for for gas resolution. One drawback, however, is that the ø TDT values are affected by relatively larger percentage errors than are the for values. Figure 3.5 also shows plots which estimate the errors in the computed gas saturation from the ø TDT values. Number 1,

7 Calibration of TDT porosity measurements, essential for evaluating gas saturation, is achieved using multi-well data base te ch n i ques. The TDT poro s i t y, being related to the ratio of the near-tofar detector counts, is influenced by the a p p a rent borehole salinity and lith o l o- gy. One technique used to calibrate TDT porosity for changes in borehole salinity i nvo lves the ge n e ration of multi-we l l crossplots. T DT porosity is plot ted aga i n st th e total porosity deri ved from open hole i n te rp retation (ie, including the bound wa ter associated with shales) acro s s i n te rvals below and above the pro d u c- ing perfo rations in order to deri ve th e e ffect re l a ted to salinity. These cro s s- p l ots are also used to va l i d a te specifi c i n te rp retation para m ete rs such as th e to o l s response to diffe rent lithologies or fl u i d s. In fi g u re 3.7a, the capt u re cro s s - s e c- tion is plotted against the intergranular porosity across clastics hori z o n s, with a z-axis dividing five classes of gas s a t u rations (from red to black). This allows us to verify the choice of the foll owing para m ete rs - qu a rt z, wa te r, o i l and gas. Fi g u re 3.7b was ge n e ra ted to ch e ck the to o l s response to qu a rtz, calcite and dolomite in a comp l ex re s e rvo i r. The y-axis represents TDT porosity corre c ted for shaliness and hy d ro c a r b o n e ffects (gas and oil) and was deri ve d from the initial interpretation of selected key wells over a wide area. This inte r- p retation is based on an ite ra t i ve process where some of the results are used to certify the para m ete rs. Ite ration is needed in cases where insufficient 1% clean wa ter zones are available in a given field. The z-axis shows the volume of qu a rtz inte rp reted in the lith o l o g y. Points of decreasing qu a rtz c o n te n t m ove from the sand line towa rd th e l i m e stone and dolomite lines. When comp l eted, the TDT inte rp retation results are inte gra ted and ave r- aged, using other data base techniques, to generate saturation maps at different time slices. Further reading A Dynamic Multi-Well Data-Base, by A.F. Abed and M. Watfa. SPE paper number 15763, Time Lapse Saturation Monitoring and its Applications to Time Lapse Mapping, by R.L. Nutt and M. Watfa, SPE paper number 17975, presented at the 1989 Middle East Oil Show, Bahrain. BEATING THE BOREHOLE Combined Σ/ T DT porosity measurements are sometimes difficult to use in dual casing st rings set in enlarge d b o reholes. This is because the TDT n e a r- d ete c tor measurement is seve rely affected by the borehole signal and this in turn, affects the ra t i o - d e ri ve d p o rosity (see fi g u re 3.8). Howeve r, even in these difficult conditions, two other te ch n i ques can be used to Fig. 3.8: During TDT tool logging the near detector (N) measures the response from close around the borehole while the far detector (F) measures the response from the formation. M is the neutron source. determine gas saturation. The first option relies solely on the Σ m e a s u rement but can only be applied when no wa ter movement is s u s p e c ted - which is fre qu e n t ly th e case when monito ring gas caps. In such circumstances, by using the cpu i n c e n t i ve full diffusion algori thms of the dual-burst TDT (pre fe rred to th e field acquisition simpler model), the Σ measurement is derived only from the far detector reading. When th e re is wa ter move m e n t w i thin the re s e rvo i r, an alte rn a t i ve a p p ro a ch is necessary because a small rise in wa ter saturation re s u l t s in a large increase in Σ - masking the effect of gas replacing the oil. An extra m e a s u rement is needed to solve th e problem and the best option is to run a cased hole neutron log. When only a single TDT log has been recorded, a qualitative identification of gas can be obtained by calib rating the TDT far count re a d i n g against the open hole neutron porosity across inte rvals where the saturation is not expected to change. Figure 3.9 shows how this calibration te chnique works. In the crossplot, the TDT to o l s far count readings are plot te d a ga i n st the NPHI open hole re s u l t s. The far count deri ved TDT poro s i t y, d e fined by the best fit line th ro u g h these points, can be comp a red to th e o riginal open hole neutron porosity to reveal any gas move m e n t s. Fig. 3.9: CROSS REFERENCE: Gas movements in the formation can be seen be comparing crossplots such as this - made with data obtained after a period of production - with original open hole neutron porosity measurements. Dual-Burst Thermal Decay Time Logging Principles, by D.K. Steinman, R.A. Adolph, M. Mahadavi, E. Mrienbach, W.E. Preeg and P.D. Wraight, SPE paper number 15437, Dual-Burst Thermal Decay Time Processing and Examples, J-R. Olesen, M. Mahadavi and D.K. Steinman, SPWLA annual symposium, Middle East Well Evaluation Review

8 Gas Saturation : TDT-3 TDT-P TDT-P TDT-P 3 2 hours after TDT-P 2. (PU) 1. Water (TDT-1) Water (TDT-2) Water (TDT-3) Gas Saturation : TDT-2 Gas (TDT-1) Gas (TDT-2) Gas (TDT-3). (PU) 1. Oil (TDT-1) Oil (TDT-2) Oil (TDT-3) Gas Saturation : TDT-1 Sand Sand Sand. (PU) 1. Shale (TDT-1) Shale (TDT-2) Shale (TDT-3) Fig. 3.1: TIME-LAPSE MONITORING REVEALS GAS CONING: These three TDT-P tool logs show a large change in the gas/oil contact over a three-and-a-half year period in this Middle East reservoir. The third log (far right) was run two hours after the second log and the computed gas saturation has shown a small decline over this short time interval. This suggests the presence of gas coning - a theory supported by the gas saturation trend which shows a transition over a 45ft interval. Number 1,

9 65 WATER SIGMA 25. (CU) 5. GAS(TDT-P) OIL (TDT-P) GAS SATURATION POROSITY % CARBONATE GLOBAL RESULTS 1. (PU). 5. (PU). SHALE. (PU). Fig. 3.11: SECONDARY GAS CAP DEVELOPMENT: This TDT-P log (second track from the right) reveals the possible development of a secondary gas cap. The gas is not seen in the original open hole density neutron log (far right). The computed TDT-P results show that gas is present over the top 3ft of the reservoir between 6527ft and 6497ft. Note that the value of decreases (left track) over the same interval. WATER (TDT-1) HYDROCARBONS (TDT-1) SIGMA LOG BEFORE GAS 15. (CU) 5. SIGMA LOG AFTER GAS 15. (US) 5. WATER (TDT-2) GAS (TDT-2) OIL (TDT-2) Fig. 3.12: POINTING A FINGER: The porous section of this carbonate reservoir has a relatively high permeability and seems to contain gas even though the gas/oil contact lies above this level. The tight porosity intervals are know to be extensively fractured and all these factors indicate that gas fingering may be occurring Middle East Well Evaluation Review

10 Fig. 3.13: CHANNEL CHECK: In this carbonate reservoir, and ø TDT decrease across the interval of high gas saturation above 582ft. The well also has a transition zone over 6ft long and this may indicate that gas coning or chanelling is taking place. Open Hole POROSITY 5. (PU). GAS SATURATION SIGMA TDT-P TDT POROSITY Sgas 15. (CU) (PU) WATER GAS OIL DOLOMITE LIMESTONE 58 Sw from TDT GAS SATURATION 1. (PU). Sgas Sw Open Hole. (PU) (PU). Water (TDT-P) Gas (TDT-P) Oil (TDT-P) Sand Shale Water Hydrocarbons Sand Shale Fig. 3.14: A CUT ABOVE THE REST: This is an example of gas monitoring in a shaly-sand reservoir which has equally high vertical and horizontal permeabilities of approximately 7md. Over the interval between 57ft and 569ft there are changes in oil, water and gas saturation - a possible explanation of why the well was producing oil with a high water cut and gas/oil ratio. Number 1,