Geothermal Energy Reservoir Research Articles

Hot dry rock geothermal reservoir testing: 1978 to 1980

The Phase I Hot Dry Rock Geothermal Energy reservoirs at the Fenton Hill field site grew continuously during Run Segments 2 through 5 (January 1978 to December 1980). Reservoir growth was caused not only by pressurization and hydraulic fracturing, but also by heat-extraction and thermal-contraction effects. Reservoir heat-transfer area grew from 8000 to 50,000 m 2 and reservoir fracture volume grew from 11 to 266 m 3. Despite this reservoir growth, the water loss rate increased only 30%, under similar pressure environments. For comparable temperature and pressure conditions, the flow impedance ( a measure of the resistance to circulation of water through the reservoir) remained essentially unchanged, and if reproduced in the Phase II reservoir under development, could result in “self pumping”. Geochemical and seismic hazards have been nonexistent in the Phase I reservoirs. The produced water is relatively low in total dissolved solids and shows little tendency for corrosion or scaling. The largest microearthquake associated with heat extraction measures less than minus one on the extrapolated Richter scale.

Using dual‐well seismic measurements to infer the mechanical properties of a Hot Dry Rock Geothermal System

High‐frequency (8–15 kHz) seismic waves that travel between two boreholes are used to probe the detailed structure of rock in the Los Alamos National Laboratory Fenton Hill hot dry rock geothermal energy reservoir. We study travel times of direct P waves and spectral characteristics of waveforms in an effort to determine whether or not large fluid‐filled fractures are present in the rock between the two boreholes. Data from two experiments, separated in time by 1 year, are studied. We find dramatic differences between the two experiments in P wave velocity and in spectral characteristics of waveforms that traversed the same region. We also find variations in P wave velocity and spectra as a function of location during one experiment.Loss of high‐frequency content of direct‐arriving P waves during the second dual‐well seismic experiment is compared with Q, found by using the coda wave technique (Aki and Chouet, 1975), and also with theoretical results on frequency dependence of transmission through a water‐filled fracture to determine the minimum number of large fractures in the system. Two to four fractures with thickness about 4 mm and vertical extent of as much as 200 m are required to explain observations during the second experiment. Efficient transmission of high frequencies observed during the first experiment is interpreted as meaning that fracture thickness was less than 1 mm at that time. Increased fracture thickness during the second experiment is probably due to a 75‐day period of heat extraction during which in situ temperature dropped by as much as 100° below the virgin temperature. Since the coefficient of linear thermal expansion of granite is approximately 10−5/°C, a decrease in temperature of 100°C in a region on the order of 1 m around the fracture is required to explain the increased fracture thickness.

State-of-the-Art of Models for Geothermal Recovery Processes

Recent interest in geothermal energy development has contributed to the advance in the modeling of nonisothermal flows, especially of the two-phase, steam-water phenomena. In this paper, the key processes associated with a geothermal energy reservoir are described and the current approaches are pointed out. The state-of-the-art of geothermal modeling is reviewed by comparing the governing equations, numerical methods, code availability, validations, and applications of several selected major existing models. The needs for further studies are discussed.

Energy extraction from fractured geothermal reservoirs in low‐permeability crystalline rock

Two hot dry rock geothermal energy reservoirs were created by hydraulic fracturing of Precambrian granitic rock on the west flank of the Valles Caldera, a dormant volcanic complex, in the Jemez Mountains of northern New Mexico. Heat was extracted in a closed‐loop mode of operation, injecting water into one well and extracting the heated water from a separate production well. The first reservoir was produced by fracturing the injection well at a depth of 2.75 km, where the indigenous rock temperature was 185°C. The relatively rapid decline in temperature of the water produced from the first reservoir, 100°C in 74 days, indicated an effective fracture radius of about 60 m with an average thermal power extracted of 4 MW. A second, larger reservoir was created by refracturing the injection well 180 m deeper. Downhole measurements of water temperature at the reservoir outlet as well as temperatures inferred from chemical geothermometry showed that the thermal decline of this reservoir was negligible; the effective heat transfer area of the new reservoir must be at least 45,000 m2, nearly 6 times larger than the first reservoir. In addition, reservoir residence time studies employing visible dye tracers indicated that the mean volume of the second reservoir is 9 times larger. Other measurements showed that flow impedances were low and that downhole water losses from these reservoirs should be manageable. The geochemistry of the produced water was essentially benign, with no scaling problems apparent. Moreover, the level of induced seismic activity was insignificantly small.

Changes in P wave velocity during operation of a hot dry rock geothermal system

P wave arrival times have been picked on seismograms recorded during two dual well seismic experiments, separated in time by one year, that were carried out in the Los Alamos Scientific Laboratory Hot Dry Rock Geothermal Energy Reservoir. Average in situ P wave velocity is computed from arrival times measured along segments of the borehole. Velocity decreases as temperature drops owing to heat extraction from the reservoir. The velocity changes linearly with the change in temperature at a rate of 1.07 (km/s)/(100°C). The linear decrease in velocity leads to the important conclusion that P wave velocity can be used as an effective measure of change in in situ temperature during operation of the geothermal system. The change in velocity is attributed to an increase in the microcrack porosity which is a result of the heat extraction from the rock.

P/Z Behavior for Geothermal Steam Reservoirs

Abstract Certain of the natural geothermal-energy reservoirs are of the type called "vapor dominated." These reservoirs contain steam in the top of the reservoir and may contain boiling water below. Some simplifying assumptions were made to predict the pressure and temperature vs production history of pressure and temperature vs production history of such reservoirs. These predictions are compared with normal hydrocarbon gas reservoirs using the standard p/Z plots.The results show that the presence of a boiling water phase will have a considerable effect on the pressure behavior of such systems. Further, the pressure behavior of such systems. Further, the porosity of the system will have a marked effect. porosity of the system will have a marked effect. Extrapolations of early data will be optimistic if the porosity is low and pessimistic if the porosity is high. In all cases, the steam zone will remain at the original temperature, though the temperature of the boiling water drops as the pressure declines. Introduction Two basic types of geothermal reservoirs are being used commercially worldwide to produce electric power. One type produces hot water from the reservoir. This water is partially flashed at the surface, producing steam to drive the turbo-generators. The two largest installations of this type are at Wairekei in New Zealand and Cerro Prieto in Mexico, just south of the Imperial Valley of California. The other type of reservoir has been called vapor dominated. The fluid is slightly superheated steam at reservoir conditions and nearly all the produced fluid is steam, with small amounts of inert gases. The two major installations of this type are at the Geysers in northern California and at Lardarello in Tuscany, Italy.Although these latter reservoirs contain large volumes of steam as vapor, there is a possibility that they also contain boiling water at great depths. The purpose of this paper is to investigate the behavior of steam and steam-water systems as they are produced. We would like to know whether the pressure vs production characteristics of these pressure vs production characteristics of these systems differ from each other enough to give clues as to the original nature of the reservoir. Such information might be extremely useful in predicting the reserves of such systems.We will look at three basic systems. The first is a system completely filled with steam but no water present. The second is a system where there is present. The second is a system where there is water on the bottom and steam on top. As steam is produced, some of the water will boil and the liquid produced, some of the water will boil and the liquid level will drop with production. The third system also will contain liquid water on the bottom, but we will assume that the liquid boils throughout the liquid system and that the liquid level will not drop. In this system a steam saturation builds up within the boiling liquid zone.We will assume that for all these reservoirs the fluid influx is negligible. We recognize that in an actual reservoir system, the fluid influx rate might be important compared with the production rate, but this simplifying no-influx assumption usually should be used in first-step analyses of reservoirs. Further, there is evidence that this assumption is valid for at least the Geysers and the Wairekei reservoirs. MATERIAL AND ENERGY BALANCES Normally in oil and gas reservoirs, only a material balance is necessary. Although boiling and condensation occur in such reservoirs, the heat effects are so small that an energy balance is not necessary. The reservoirs remain essentially isothermal. STEAM ONLY If only steam is in the reservoir (with no water), the same isothermal characteristics hold as for oil and gas reservoirs. This is because the heat capacity of the rock is so large compared with that of steam. Thus, a steam reservoir can be treated in the same way as an ordinary gas reservoir; we can plot p/Z vs cumulative production and get a plot p/Z vs cumulative production and get a straight line. The intercept on the abscissa is equal to the original fluid in place. The equation is (1) SPEJ P. 407

Preliminary Assessment of a Geothermal Energy Reservoir Formed by Hydraulic Fracturing

Abstract Two 3-km-deep boreholes were drilled into hot granite in northern New Mexico to extract geothermal energy from hot, dry rock. Both boreholes were hydraulically fractured to establish a flow connection. Fracture-to-borehole intersection locations and in-situ thermal conductivity were determined from flowing temperature logs. In-situ measurements of rock permeability and compressibility show a strong dependence on pore pressure. An estimate of the minimum horizontal earth stress was derived from fracture extension pressures and found to be one-half the overburden stress. It was found that fracture growth was remarkably simple to achieve in the low-permeability granite and that these factures appear to be "self-propped." The present connection has a large flow impedance that probably cannot be reduced to a useful level. Establishment of a prototype heat exchange system will require redrilling prototype heat exchange system will require redrilling to intersect directly one of the fractures. Introduction A program designed to demonstrate the feasibility of extracting energy from hot, dry rock has been initiated at the Los Alamos Scientific Laboratory. Basically, it is proposed that man-made geothermal energy reservoirs can be created by drilling into relatively impermeable rock to a depth where the temperature is high enough to be useful, creating a reservoir by hydraulic fracturing, and then completing the circulation loop by drilling a second hole to intersect the hydraulically fractured region, or by drilling into the immediate vicinity of the first fracture and then creating a second fracture that intersects the first one. Thermal power would be extracted from this system by injecting cold water down the first hole, forcing the water to sweep by the freshly exposed hot rock surface in the reservoir-fracture system, and then returning the hot water to the surface where the thermal energy would be converted to electrical energy or used for other purposes. System pressures would be maintained such that only one phase, liquid water, would be present in the reservoir and the drilled holes. The concept is described in more detail by Smith et al., and estimates of heat extraction rates have been reported by Harlow and Pracht and McFarland and Murphy. Briefly, it was Pracht and McFarland and Murphy. Briefly, it was found that a large fracture about 1 km in radius, in hot, 200 to 250C rock, is sufficient to provide an average of 60 MW of thermal power for 20 years. In addition, the thermal contraction associated with the cooling of the rock will result in thermal stress cracking, crating additional porosity and heat-transfer surfaces, so that hundreds of megawatts of thermal power may be provided ultimately. DRILLING AND COMPLETIONS The hot, dry rock concept is being investigated in a series of field experiments at Fenton Hill, located on the west flank of a dormant volcano, the Valles Caldera, in the Jemez mountains of northern New Mexico. In Dec. 1974, the first deep borehole, GT-2, was drilled to a depth of 2.929 km (9,609 ft) in granite, where the temperature was 197C (386F), cased to 2.917 km (9,571 ft), and then fractured in the open hole below the casing. The variation of equilibrium temperature and geology with depth is shown in Fig. 1. The Precambrian crystalline rocks were encountered at 730 m (2,400 ft). From this depth to the bottom the geothermal gradient is reasonably constant, although it is somewhat lower in the gneiss as compared with the deeper formations (44C/km vs 57C/km). The steeper gradient in the sediments above 730 m is caused by their lower thermal conductivity. Based on the conductivity measurements at 2.77 km (9,100 ft), the geothermal heat flow at this site is 0.16 W/sq m, 2 times the worldwide average. SPEJ p. 317

Study of heat conduction models of geothermal energy reservoirs

Temperature anomalies in a rock formation are considered as direct evidence of the existence of geothermal reservoirs. Prospecting for geothermal energy by measurement of the temperature gradient in the rock formation overlying a geothermal reservoir could, theoretically, be effective. Thermal conduction models of different presumed geothermal reservoir shapes are discussed. A geothermal reservoir is considered to be a constant hot body overlain by physically uniform rock formations. The temperature distribution and conduction heat flow in the overlying formation could be expressed theoretically with the equation: ∂ 2 T ∂ x 2 + ∂ 2 T ∂ y 2 + ∂ 2 T ∂ z 2 = 0 Temperature distribution in the overlying rock formation may be represented by the surfaces of constant temperature whose shapes are closely related to the shape of the hot body. A constant temperature surface resembles more closely the shape of the hot body the nearer it is to the hot body. The shape of temperature curves at equal depth in a vertical cross-section reflects also the depth, shape and size of the geothermal energy reservoir. The differences in material and inflow of ground water in the overlying formation would seriously affect the temperature distribution, the shape of the constant temperature surfaces, and the shape of the temperature curve for the equal depths, consequently making them differ from the presumed models.

Geothermal Energy Reservoirs: ABSTRACT

potential of a geothermal area is dependent primarily on volume and temperature of the reservoir and adequacy of fluid supply. Inadequate fluid supply may be a more common limiting factor than inadequate heat supply. Except in very porous reservoirs, most of the heat is stored in rocks rather than in pore fluids. Geothermal fields can be classified as hot-spring systems or as deep insulated reservoirs with little surface expression; gradations also exist. Hot-spring systems have high near-surface permeability, at least locally, on faults and fractures, permitting fluids to escape at high rates. Deep reservoirs with little surface expression require the presence of permeable reservoir rocks capped by insulating rocks of low permeability. Liquid water is generally the dominant fluid, but steam can form by boiling as hot water rises to levels of lower pressure. Dry steam areas probably are rare. About 30 areas in the United States have been explored for geothermal energy, but the existence of dry steam has been proved only at The Geysers. Extensive utilization of geothermal energy therefore must depend largely on steam flashed from hot water with decrease in pressure. Problems that confront broad utilization of geothermal energy include: (1) discovery of reservoirs with adequate supply of energy and natural fluids; (2) deposition of CaCO3 or SiO2, (3) chemical corrosion, (4) objectionable chemicals in some effluents, and (5) inapplicability of existing public laws. optimum environment for a geothermal reservoir includes (1) potent source of heat, such as a magma chamber; such heat sources are most likely to occur in regions of late Cenozoic volcanism; (2) reservoirs of adequate volume, permeability, and porosity; and (3) capping of rock of low permeability that inhibits convective loss of both fluids and heat. A deep well-insulated reservoir may have at least 10 times the energy content of an otherwise similar, shallow, uninsulated reservoir. End_of_Article - Last_Page 568------------