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2 Geothermal Resources CounCi 1, TRAiJSACTIOiJS Vol. 4, September 198 RAFT RIVER GEOTHERMAL SITE: A REINJECTION STUDY Jl. Ahmeq, K. M. olgemuth, A. S. Abou-Sayed, J. F. Schatz, A. H. Jones Terra Tek, Inc. Salt Lake City, UT 8418 ABSTRACT Analysis of 'transient pressure tests combined with spinner surveys and a knowledge of the geology of the area have allowed us to define the injectivi ty potential of the Raft River geothermal site. The present two injection wells will allow approximately two to four months of injection at the required rate of 25 gpm without fracturing the formation. A numerical simulator has been used to model the injection portion of the reservoir to investigate options such as hydraulic fracturing or the drilling of an additional well to provide sufficient subsurface disposal. Lake Formation, target for the reinjected fluid, is composed predominantly of tuffaceous si1 stone and sandstone with minor sections of gravel and sand or poorly consol i dated sand (Covington, 1979). Below the casing, both holes are open in the Salt Lake Formation reaching a depth of 3888 and 3858 feet respectively. The casing in RRG6 is completed down to 1695 feet, in RRG7 it extends down to 244 feet. (See Fig. 5). INTRODUCTION The Raft River geothermal resource in southern Idaho is being developed for electric production utilizing a medium temperature (145OC) resource. For the projected 5 Me pilot plant, a supply of 25 gpm of the geothermal fluid is needed. It is preferred that the spent brine be injected into zones deeper than the known agricultural aquifers and ells RRG6 and 7 are earmarked for this purpose. This paper presents our analysis of several injectivity tests performed by EG&G Idaho on RRG7 to characterize the injection capability of the formation. The available geological information about the area and preliminary results of a spinner survey done by the U.S. Geological Survey (Schimschal and Keys, 1979, personal communication) have been included in the injectivity test analysis. A wellhead pressure limit of 5 psi has been imposed to prevent injection formation fracturing. Our approach to analysis is to use a two dimensional radial numerical simulator with parameters determined by the test results and from geological data. ap of the Raft River Val ley Area. Fig* 7 From Mabey et al., 1978). GEOLOGY The Raft River Valley is located at the northern edge of the Basin and Range province just south of the Snake River plain (Fig. 1.). The U.S. Geological Survey has carried out a comprehensive program to elucidate the geology of the valley (Mabey et a1., 1978; Keys and Sull ivan,1979). It is a north-trending Cenozoic depression bounded on the east, south, and west by-mountains. The Salt Fig. 2. Raft River ell Locations. (En1 arged square section of Fig. 385

3 ~~~~~~ Ahrped et. a INJECTION TESTS RRG7. During August-September, 1979, EG&G Idaho, Inc. conducted three injection tests in RRG7 at constant rates of 75, 62 and 45 gpm for five and one-half, eight and ninety-six hours respectively. Bottom hole pressure and temperature were recorded with a Hewlett-Packard (HP) probe and we1 1 head pressures and temperatures were recorded with a Paroscientific Digiquartt system. Measurements were monitored during injection and following shut-in (falloff). During the pressure falloff, the condition of uniform mobility is more clearly achieved than during injection because the fluid is partially isothermal for a considerable distance around the we1 1 bore (Earl ougher, 1977). Therefore, emphasis has been put on interpreting the falloff data. All three injectivity tests provided similar results. For brevity, only the 75 gpm data is discussed in detail here. Fig. 3 is the semi-log plot of pressure and temperature falloff with time as recorded downhole. From the semi-log straight line the average formation permeability of the open hole was calculated as 37 md and the well had a skin factor of +.1. hen the bottom hole temperature started dropping significantly, the pressure decay rate reduced. At that time, the wellbore was cooling at a much faster rate than the surrounding reservoir and a back pressure on the sandface was created causing a reduction in bottom hole pressure drop. Type curve match of the data estimates the formation compressibility to be 1.4 x 1-6 psi-l. Similar falloff data analysis of the 62 and 45 gpm tests have provided consistant results. The flow properties around RRG7 as calculated from the three tests are listed in Table 1, along with the well and fluid properties. Average properties are 37. f 1.3 md permeability, 1.5 k.1 x 1-6 psi-l total system compressibility, and negligible (+.7 to -.3) skin factor. These reservoir properties correspond to a 58 foot radius of investigation OPU BOTTOM HOLE FALLOFF n I72 - OATA PLOT - I32 6 o PRESSURE O K 3 &TEMPERATURE 5 I $ n pt I7 - I24 E a -I I2 5 P K.37 md z S Fig. 3. EFFECT I I I 75 GPM Bottom Hole Falloff Data. TOTAL RAOIUS OF TEST PERMEM I LI TV SKIN COMPRESS I 81 L I TY I NVESTI TION md FACTOR psi-' ft gp 36.9 * x gm x gpn x t 1.3 TABLE 1 ell and Reservoir Properties Around RRGI No. 7 Casing Depth Bottom Hole Depth Fonnation Thickness Average Open hle Radius Fonnation Porosity Fluid Viscosity Initial Reservoir Pressure Initial Reservoir Tenpeature ellbore Storage Coefficient Dimension1 6s ellbore Storage Coefficient 244 feet 3858 feet 1814 feet.58 feet.2 fraction.285 cp 1677 psi 97OC.226 res. bbls./psi 22.4 Negligible 1.5 t.1 x 1-6 UP to 58oO ft. I12 E rn 18 During the 45 gpm injection test at RRG7, the wellhead interference pvssure at RRG6 was monitored. Fig. 4 is the log-log plot of the wellhead pressure changes versus time. Using an expon- ' ential integral solution, the formation capacity between the RRG6 and 7 has been calculated to be 2.1 x lo5 md-ft. For a formation thickness of 2193 feet (open hole in RRG6) the average formation permeability is 96 md. Since this is substantially higher than the 37 md measured around RRG7, a zone of high permeability is implied to exist within the vicinity of the two wells. A spinner survey performed on RRG7 by the USGS (Schimschal and Keys, 1979, personal communi - cation) during the 45 gpm test indicated a nearly uniform fluid intake over the entire open hole. A previously performed spinner survey in RRG6 has indicated that 5 percent of the fluid was being taken by the first 3 feet below the casing (a zone that is cased in RRG7). The remaining water was accepted uniformly throughout the lower part of the well, similar to RRG7. This can possibly i./ LOG - LOG PLOT INTERFERENCE AT RRGI ELL NO GPM TEST AT RRGI ELL NO. 7 I IO 1 ATIME. HRS. Fig. 4. OO 1 Interference Pressure Change Variation at RRG6. 386

4 explain the high 96 md permeability calculated from the interference test. Assuming the open hole in RRG6 to have the same 37 md permeability (as seen around RRG7) except the uppermost 3 feet, the effective permeability of this high fluid intake zone can be calculated as 47 md. This permeable zone (from hereon called the 'thief zone') detected in RRG6 did not have any effect on pressure measurements at RRG7. Though the thief zone is cased off at RRG7, any direct communication (within reasonable distance from the wellbore) between the injection zone and the thief zone, should have resulted in a high positive skin factor due to partial penetration effects (Kazemi and Seith, 1969). Careful investigation of geophysical logs indicates that there are twenty feet of a high resistivity zone in RRG7 between 214 feet and 216 feet. This zone of high density rock is an indication of a tighter formation and may well be acting as a barrier between the uniform open hole and the thief zone. This anomaly is also * seen in RRG6 at 19 feet depth but it is weak. From the injectivity test, spinner surveys and log evaluation, a picture of the injection zone has been hypothesized and is illustrated in Fig. 5. INJECTION PERFORMANCE PREDICTION A two dimensional radial numerical simulator has been used to history match the injectivity tests. The average reservoir flow properties listed in Table 1 and the hypothesized reservoir as illustrated in Fig. 5 were used in the simulation. INJECTION FORMATION Ahmed et. a1. (2) Uncased Thief Zone (perforating the casing and letting fluid flow into the thief zone of both the we.l.1 s ) In view of the uncertainty of the reservoir geology, several boundary condi ti ons have been considered. For simplicity and because the conclusions are not substantially altered, only an infinite reservoir is presented in- detail here. Cased Thief Zone: Fig. 6 illustrates the wellhead pressure rise with time at RRG7 with simultaneous injection of 125 gpm into each RRG6 and 7. It approaches the 5 psi limit within a month. Even the presence of a possible sink to the north would have not effect at early times (prior to about four years of injection) because of its location at a great distance. The location of the Bridge Fault, whether it is a barrier or sink, will not alter this conclusion. Two options were considered to alleviate the injection problem - hydraulic fracturing of the well and drilling a new well. - v) eso?so 65 K v) w g SI n a w R 7 I \BARRIER SALT LAKE FORMATION (Se4lmmtory) K 3?m4 c,m l.5lllo-@psl-~ 2% 1 I I I I I.1 1. IO 12 IO' 1' lo5 Fig. 6. INJECTION TIME, HRS. Cased Thief Zone. TD. Fig. 5. ~21 C llu8' PRE-CAMBRIAN BASEMENT (Ianwum) Hypothesized Injection Formation. The reservoir model has been used to predict pressure behavior at RRG6 and 7. The properties of the injected fluid are 71 C brine with less than 4 percent NaCl (Viscosity =.4 cp). Two possibilities for injection into the formation have been considered: (1) Cased Thief Zone (water not allowed to enter the thief zone in both RRG6 and 7). Inducing a Massive Hydraulic Fracture (MHF) 3 feet high with each wing 26 feet long would not significantly improve the injection potential. The relatively high formation permeability neces'sitates' a highly conductive fracture that would need to be inches in width, a feat impossible to achieve. Even such a fracture would provide only 2 to 25 percent increase in the injectivity capability (McGuire and Sikora, 196). Drilling a new injection well with the intentions of minimizing pipeline length and meeting the wellhead pressure '. requirement was also investigated. Provided a sink is present on the northeast corner of the valley, a well drilled about two miles north of RRG7 would satisfy the pressure requirements when 84 gpm is injected into each of the three wells for thirty years. ithout definite evidence for the presence of such a sink, it is premature to plan' for a new well at this time. 387

5 Ahmed et. a1. Uncased Thief Zone: Injection of fluid into the thief zone introduces the possibility that fluid wi 11 migrate into overlying agricultural aquifers. The State of Idaho wishes to prevent this, although the thief zone may well be in poor contact with these aquifers. A numerical study has been performed to assess the pressure behavior when allowing the fluid to enter the thief zone in both the wells. Fig. 7 illustrates the rise in wellhead pressure at RRG7 for simultaneous injection of 125 gpm into each of the wells, The raidal extent of the thief zone is analogous to that of the lower part of the injection formation. An infinite reservoir will permit adquate injectivity for thirty years. However, any significant reservoir barrier would cause adverse pressure increases at earlier times, possibly within one to two years. 1 I I I I I I.t I 1 IO* IO' IO' 18 Id' Fig. 7. INJECTION TIME. HRS. Uncased Thief Zone. CONCLUSIONS The present study warrants the following remarks : Injectivity potential bf the Raft River geothermal s i te has been adequafely def i ned, hen fluid is not allowed to enter the thief zone, only two to four months of injection at 125 gpm into each well can be entertained. A massive hydraulic fracture will not substantially improve injectivity, and success by drilling a new well is heavily dependent on the existence of a large sink near the site. Allowing the thief zone to accept fluidalong with the present open. hole will satisfy the injection program with an infinite reservoir. The present wells drilled to the producing horizons could solve the injection problem, but this may introduce the chances of early thermal and/or hydrological breakthrough to the producing formation. It is evident that detailed geological considerations (size and continuity of thief zone, location and potential of a sink or sinks, location and effectiveness of barriers) are critical to the injection design of the site. ACKNOLEDGEMENTS Appreciation and thanks are due to Roy Mink and Susan Prestwich of the Department of Energy (DOE) and Max Dolenc, Dennis Goldman and Bob Hope of EG&G Idaho, Inc. for making the transient.pressure data available to use. e also want to thank Scott Keys, Ulrich Schimschal and Richard Hodges of the U.S. Geological Survey for providing a preliminary interpretation of their spinner surveys. This work was supported by DOE Contract number DE-AC7-77ET2831. REFERENCES Covington, H. R., "Deep Drilling - Raft River Geothermal Area, Idaho, Raft River Geothermal Injection ell No. 6, "U. s. Geological Survey Open- Fi 1 e Report ,1979. Earlougher, Jr. R. C., "Advances in ell Analysis", SPE monograph Vol. 5 Dallas, Texas, Kazemi, H., and Seith, M. S., "Effect of Anistropy and Stratification on Pressure Transient Analysis of ells with Restricted Flow EntrY," Journal of Petroleum Techno1 ogy, May 11969, pp Keys,. S. and Sullivan, J. K., "Role of Borehole Geophysics in Defining the Physical Characteristics of the Raft River Geothermal Reservoir, Idaho," Geophysics, Vol. 44, pp , Mabey, D. R., Hoover, D. B., ' Donne1, J. E. and ilson, C.., "Reconnaissance Geophysical Studies of the Geothermal Svstem in Southern Raft River Valley, Idaho," keophysics, Vol. 43, pp , McGuire,. J. and Sikora, U. L., "The Effect of Vertical Fractures on ell Productivity," trans., AIME, 219, 16, pp