The Desert Peak Reservoir

In this report the term reservoir is considered to be that volume of rock above a depth of 10,000 feet which gives an isothermal temperature profile with a temperature near 400°F. Wells deeper than 10,000 feet in this area are unlikely to be profitable at the present time. The isothermal profile is assumed to be the result of geothermal fluids convecting in fractures. Of course, large fractures, which are referred to as producing zones within the reservoir, must be intersected if a well is to be successful. This nomenclature is essentially the same as the term "geothermal resource" proposed by Muffler and Cataldi (1977). The producing zones in the Desert Peak reservoir consist of fractures in competent pre-Tertiary rocks.

Facca and Tonani (1967) have proposed that a caprock is necessary for a geothermal reservoir to exist. If so, the caprock at Desert Peak is variable. Near wells B21-1 and B21-2 the Tertiary rhyolitic unit would be the caprock, and drilling has demonstrated that the rhyolitic unit is generally impermeable. Of the eight drill holes which have penetrated this unit, severe lost-circulation problems were encountered only once, in strat. test no. 2. In well B23-1 the rhyolitic unit and the pre-Tertiary phyllite, which has escaped significant thermal metamorphism, are likely caprocks.

The producing zones of the reservoir in well B21-1 are well documented. They were characterized by severe lost-circulation problems and other difficulties while drilling. Circulation was lost at depths of 3638, 3891, 3965-70, and 4000 feet. After circulation was lost at 3891 feet, the drilling fluid was displaced with nitrogen and the well flowed without further assistance for 41 hours before being shut in. Below 3891 feet circulation was accomplished by use of aerated water as the drilling fluid. The driller reported the first 8 feet below 3891 feet to be highly fractured. Below 3891 feet the well produced large amounts of water and steam during drilling, so it is possible that any deeper fractures were not detected by the driller. The data obtained during drilling show that fractures above 3899 feet are capable of supplying large amounts of fluid to the surface. Because very little data were obtained during the 41-hour flow test nothing can be said about the relative productivity of fractures above and below 3899 feet.

A suite of logs was run to total depth in well B21-1. No cores were taken. The two most definitive and useful available logs for locating fractures are the spontaneous-potential (SP) and sonic logs (fig. 27). The SP log shows relatively large positive deflections between 15 and 75 millivolts at depths of 3641, 3780, 3850, 3890, 3938, 3975, 4020, 4055, and 4090 feet. These deflections are interpreted as the result of moving water creating a streaming potential (Dakhnov, 1962). The sonic log shows the known fracture zones as irregular, low-velocity intervals. Cycle skipping is common within and between the known fracture zones. Below 3638 feet the sonic log obviously changes character. On the sonic log the largest fractures or fracture zones are present at 3641, 3888-3900, 3940, and 4020 feet. Several smaller or less clearly defined fractures are present at 3780, 3850, and 3982 feet. Other fractures are suggested by this log but require corroborating evidence to be clearly recognized. Examples occur at 4055 and 4085 feet.

The correlation between the driller's log, the sonic log, and the SP logs leave no doubt as to the location of the major fractures encountered by well B21-1. The con-

lost-circulation zones i---------------------(------1 I

Desert Peak Geothermal
FIGURE 27. Selected geophysical logs showing the producing zones in Desert Peak well B21-1.

ductivity and resistivity logs do show a few of the largest fractures, such as those at 3641, 3855-3860, and 3898 feet, but without supporting data most of the fractures do not show up clearly on these logs.

The producing interval in the reservoir at well B21-1 is at least 449 feet thick (between 3641 and 4090 feet deep). The total thickness of the reservoir below 4150 feet cannot be presently estimated. The logs suggest that the larger fractures or fracture zones comprise less than approximately 20% of the total thickness of the known reservoir. The cuttings from the reservoir usually lack any evidence of shearing and faulting. Two notable exceptions occur at 3895 and 3940 feet, where fault gouge and slickensides are abundant. These depths compare well with known fractures. The cuttings also suggest a lack of secondary minerals such as quartz or calcite filling large veins. Most of the known fractures in well B21-1 are within the greenstone; however, the two fractures at 3780 and 3850 feet appear to be in metasedimentary rocks.

The reservoir geology in well B21-2 appears to be similar to that in well B21-1, although it is not as well documented. Circulation was lost while drilling at depths of 2871 and 2889 feet. At 2894 feet attempts were made to flow the well. Some fluid was produced, but the well did not continue to flow without assistance, suggesting that a large share of the fluids produced by well B21-2 enter the well bore below 2894 feet. Below 2889 feet circulation was never regained, with the exception of cement chips while drilling out a plug. A core was taken between depths of 2937 and 2949 feet and is the only reliable information obtained below 2900 feet. Well B21-2 was logged to 2900 feet, and after logging the hole was deepened. However, when the hole was terminated at 3192 feet the mobilization of a logging truck for only 292 feet of new data could not be justified.

The response of the sonic log to the two known fractures at 2871 and 2889 feet in well B21-2 is similar to the response shown in well B21-1 (fig. 28). The fractures in well B21-2 show up well on the conductivity log, moderately well on the resistivity log, and poorly on the SP log. The fracture at 2871 feet has virtually no expression on the SP log and the fracture at 2889 feet is characterized by a 10-millivolt negative deflection, which is the reverse of the deflections shown in well B21-1. A caliper log from well B21-2 suggests a large fracture at 2871 feet but none at 2889 feet. These fractures also show up well on the density log as pronounced low-density spikes.

The 4-inch diameter core taken between 2937 and 2949 feet consists of angular fragments of greenstone which were poured out of the core barrel as irregular broken blocks. The largest fragment is the size of a fist and is the only fragment resembling a cylindrical core (fig. 29). This core fragment contains numerous small fractures, most of which are sealed with chalcedony and (or) calcite (Morris, 1978). The small fractures have at least one strongly preferred orientation with a dip of near 30° from horizontal. Several smooth faces on this fragment (which probably represent larger fractures) also dip approximately 30°. The original size of the larger fractures separating the angular fragments is unknown. The faces on the angular fragments are not coated with secondary minerals. Other less developed, near-vertical fractures are visible on this fragment.

The core is a metamorphosed andesitic breccia of variable green color and texture. The greenstone is pro-pylitically altered and contains abundant, finely disseminated pyrite. The core fragments show no evidence of shearing. Slickensides are conspicuously absent; however, slickensides observed in cuttings demonstrate that faulting has played a role in creating some of the fractures at Desert Peak.

An interesting aspect of the Desert Peak geothermal reservoir is that some of the fractures in well B21-2 apparently slowly leak fluids into the well bore, while others apparently produce large quantities of fluids. Figure 30 shows the bottom part of six temperature profiles from well B21-2 which were run over a period of five months between December 12, 1976, and May 15, 1977. All of these profiles show a temperature reversal near the bottom of the well. The depth of the reversal varies from 2900 to 3020 feet. This variation is probably due to operator errors in depth during the continuous wireline temperature surveys. The earliest profile on December 12, 1976, shows a reversal with a temperature decrease greater than 42°F. The last available profile, obtained five months later, shows a temperature decrease of 5°F. During drilling, over 2000 barrels of drilling fluid and a large volume of air were lost to the formation. The temperature profiles show that a large percentage of this fluid entered fractures but did not move very far away from the well bore. During the six months from December 1976 to May 1977, the well was flowed at rates from 288,000 to 456,000 lbs per hour for ten days. A graph (fig. 31) of the temperature decrease below the reversal versus the time after the last large amount of fluid was lost in the well shows that during the flow tests some of the lost drilling fluid gradually returned to the well bore. The temperature reversal decreased more rapidly when the well was flowed than when the well was static. However, even after six months not all of the lost drilling fluid had been removed. An additional complicating factor in analyzing this temperature rebound is that the temperature decrease was greater on May 15 than on March 18; this may be the result of operator or equipment errors in the temperature surveys.

Well B23-1 intersected the geothermal reservoir, but to date it has yielded little information about the reservoir. A detailed discussion of the logging and log interpretation of well B23-1 can be found in Sethi and Fertl (1979) and Benoit and others (1980). As this well was being drilled with water and aerated water, no complete losses of circulation occurred. At depths where the reservoir exists, small fluid losses occurred at 6285 and 6330 feet. At 8485 feet the well began taking 1500 barrels per day of drilling water, and this increased to 2600 barrels per day at 9215 feet when the use of aerated water began. It is possible that all the large circulation losses were in a small interval near 8400 or 8500 feet.

The logs run in well B23-1 have not clearly located any fractures in the reservoir. The sonic log was run, but due to high attenuation caused by aerated drilling fluid, it is of little value. The resistivity and SP logs show no obvious conductive zones or SP shifts where the reservoir is believed to exist.

lost-circulation zones

FIGURE 28. Selected geophysical logs sliowing the producing zones in Desert Peak well B21-2.

depth in feet

FIGURE 28. Selected geophysical logs sliowing the producing zones in Desert Peak well B21-2.

Geophysical Field Planet
FIGURE 29. Photograph of greenstone core from Desert Peak well B21-2. Note the prominent fracture set which dips about 30°.

The best available evidence concerning the reservoir in well B23-1 comes from the temperature log. Between 5200 and 8800 feet the temperature is quite constant (pi. 6) and close to reservoir temperatures present in wells B21-1 and B21-2. The reservoir is therefore believed to exist in this interval, suggesting a reservoir thickness of 3500 feet. Between 5200 and 7300 feet about half the rock is chlorite schist and hornfels (fig. 25) and the other half is granite. Between 7300 and 9641 feet granite is the dominant rock type.

The estimated flow rate of 100,000 to 140,000 lbs per hour in well B23-1 is quite low. There is presently an obstruction in the well bore near the bottom of the casing; the severity of this obstruction is unknown. If the obstruction is major, the flow rate is probably much higher and well B23-1 could be a commercial producer. If the obstruction is minimal, then well B23-1 is probably a subcommerical well. Whether the geothermal reservoir near well B23-1 is capable of only limited production or whether it was simply a matter of bad luck that the well never intersected fractures large enough to produce commercial quantities of fluid is unknown.

Wells B21-1, B21-2, and B23-1 have intersected the same geothermal reservoir. Prior to drilling well B23-1, a flow test of well B21-2 resulted in water level and pressure changes in B21-1 within one hour of the beginning of the flow test. This indicates very high permeability between the two wells, which are 4100 feet apart. When well B23-1 was flowed at 100,000 to 140,000 lbs per hour for nine days there were detectable water level changes in wells B21-1 and B21-2 (Yeamans, 1980).

The three wells are relatively widely spaced and were not located with the intention of intersecting known geologic structures such as faults. It is extremely unlikely that all three wells intersect a relatively minor geologic structure or were simply lucky. The reservoir at Desert Peak is believed to represent a large volume of fractured rock. If so, production holes can probably be drilled on a regular grid with the intention of intersecting an extensive horizontal target. The fracture set which dips about 30° from the horizontal in the one large core fragment (fig. 29) tends to substantiate the other evidence.

Any hard, competent, pre-Tertiary rock at Desert Peak should be capable of maintaining fractures and therefore would be a part of the reservoir. The only pre-Tertiary rock currently believed to be incapable of producing commercial quantities of geothermal fluids at Desert Peak is the phyllite, a soft rock which deforms plastically and is unlikely to maintain fractures for any length of time. Additionally, deep drilling at three other geothermal prospects in northwest Nevada has demonstrated that phyllite probably is not a viable reservoir rock. The amount and distribution of phyllite in the vicinity of the Desert Peak reservoir is largely unknown.

The configuration of the top of the reservoir is indicated by the contours of the estimated depth to 400°F (pi. 12). The configuration of the top of the producing zone or zones within the reservoir cannot be contoured with the two available data points and is likely to be irregular. Reliable data on the top of the producing zone are available only from wells B21-1 and B21-2. The top of the producing zone in well B21-1 at 3638 feet is 1908 feet below the top of the reservoir (400°F isotherm) and 500 feet below the top of the pre-Tertiary rocks. In well B21-2 the top of the producing zone is at 2871 feet, which is 331 feet below the top of the reservoir and only 63 feet below the top of the pre-Tertiary rocks. The depth of the producing zone in well B23-1 is not yet known. The top of the reservoir in well B23-1 is 975 feet below the top of the pre-Tertiary rocks.

To date, the producing zones in the Desert Peak geothermal reservoir have been confined to the pre-Tertiary rocks. The 400°F isotherm, which has been arbitrarily chosen to represent the upper boundary of the reservoir, is generally confined to pre-Tertiary rocks. In well B21-1 the 400°F isotherm rises high into the rhyolitic unit, but it is interpreted to be a minor feature related to a single fault or small fault zone. It is relatively simple to estimate the depth to the top of the reservoir using temperature data. At the present time there is no way to accurately estimate the depth to the top producing zone within the reservoir or to guarantee that all wells in the reservoir will intersect commercial producing zones. The producing zones are fractures. The pattern of fractures interpreted from well B21-1 data indicate that the reservoir is not pervasively fractured; rather, the fractures are irregularly concentrated in what may be discrete faults. Questions as to orientation, age, and distribution of these fractures remain unanswered.

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