RESERVOIR MODELING OF GEOTHERMAL ENERGY PRODUCTION FROM STRATIGRAPHIC RESERVOIRS IN THE GREAT BASIN

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1 PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February -3, 23 SGP-TR-98 RESERVOIR MODELING OF GEOTHERMAL ENERGY PRODUCTION FROM STRATIGRAPHIC RESERVOIRS IN THE GREAT BASIN Milind Deo *, Richard Roehner *, Rick Allis #, Joseph Moore % * University of Utah, Chemical Engineering Department, # Utah Geological Survey, % University of Utah, Energy and Geoscience Institute 5 S. Central Campus Drive, Merrill Engineering Bldg. Salt Lake City, Utah 82 milind.deo@utah.edu ABSTRACT Large, potentially commercial geothermal resources exist in sedimentary rocks beneath high heat-flow basins of the United States. Geothermal reservoir modeling was performed to explore the available power density (MWe/km 2 ) attributable to two general classes of reservoir: a multi-layered sandwich and single high permeability layer. Variations in reservoir temperature (i.e. conductive heat flow), permeability, and layer thickness were evaluated. The high permeability layers were assumed to be horizontal and laterally extensive. Production wells were assumed to be pumped at a constant rate, and all produced water was injected at 75 o C after being cooled in a power plant. Modeling was undertaken using the STARS Advanced Process and Thermal Reservoir Simulator, Version 2, by Computer Modeling Group. Five reservoir models were simulated:. Sandwich (base) reservoir model to test heat sweep for a reservoir-seal configuration with an average reservoir temperature of 2 o C at 3 km depth; the reservoir comprised four 25 m thick layers with a permeability of millidarcy. 2. Single layer reservoir with the same initial temperature and transmissivity of the sandwich reservoir. 3. Low temperature (5 o C) sandwich reservoir model.. Low permeability sandwich reservoir model, involving lower permeability layers than the sandwich base model. 5. Short-circuit sandwich reservoir model where a high permeability layer results in a higher transmissivity than the base sandwich model. All models assumed isotropic permeability, uniform porosity (%), and an initial thermally conductive vertical temperature gradient and hydrostatic pressure gradient. All models utilized a five spot pattern with a 5 m well spacing, with the flow rate in producer and injector wells being gallons per minute. The base sandwich model, which may be representative of stratigraphic bedrock reservoirs beneath some basins of the Great Basin, has a power density 3 to MWe/km 2 over a 3 year period. INTRODUCTION Deep basins within the high heat flow parts of the western U.S. may have stratigraphic reservoirs between about 3 - km depth with temperatures of 5-25 C (Allis et al., 2; 22) and may be attractive for geothermal power production. These reservoirs are sub-horizontal and are much larger in area (> 2 km 2 ) than the traditional fault-hosted hydrothermal reservoirs that have been developed in the past within the Great Basin (typically < km 2 ). Blackwell et al. (22) have suggested that the subvertical up-flow zones of many of these hydrothermal systems that have been the target of production wells may be less than km 2 in plan-view Although the sub-horizontal bedrock layers considered by Allis et al. (22) are at slightly greater depths than traditional geothermal production wells, if these layers have sufficiently high transmissivity they may present a much easier drilling target. The large reservoir areas of these basins implies a power potential of hundreds of megawatts (MWe). The purpose of this study is to investigate the rate of heat extraction from sub-horizontal reservoirs consisting of a realistic range of permeabilities, layer thicknesses, and thermal regimes. Environmental considerations and reservoir management constraints to limit the pressure decline due to production, require all production water to be injected after being cooled in the power plant. Injection wells are open to the same units as production wells, and the heat sweep process is therefore analogous to water floods commonly used in secondary oil production. Constraints to the numerical simulation model of the heat sweep process have been based on findings from other components of this project (Moore and Allis, 23).

2 A review of existing databases on permeabilities encountered in oil and gas production wells in the Great Basin and adjacent Colorado Plateau basins (Kirby, 22), suggests that at 3 km depth, permeabilities of to millidarcy (md) are not uncommon. There is no characteristic thickness of high permeability bedrock layers, which may range from less than m to hundreds of meters in the Great Basin. The modeling therefore started with a base case model which had a 3 m thick sequence comprising four relatively permeable layers (the reservoir ), separated by intervening seal layers with relatively low permeabilities.(sandwich model) The reservoir layers have a combined transmissivity of Darcy-meters (D-m), corresponding to the low end of the transmissivities simulated by Sanyal and Butler (29, 2) in their economic modeling of lower-temperature basins such as the Gulf Coast sedimentary system. Our modeling then considers four other models with results from variations in reservoir characteristics compared to those from the Sandwich model.. MODEL INPUTS AND ASSUMPTIONS Modeling was undertaken using the STARS Advanced Process and Thermal Reservoir Simulator, Version 2 (Computer Modeling Group, Ltd., 2). Five reservoir models were simulated and are summarized below. Sandwich (Base Case) Reservoir Model This base case used an average reservoir temperature of 2 o C at 3 km depth. The reservoir comprised four 25 m thick layers with a permeability of md and an overall transmissivity of D-m. The seal layers between the high permeability layers have various thicknesses and a permeability of md (Tables, 2, and Fig. ). The model also has 5 m of low permeability ( md) rocks above and below the reservoir sequence A constant temperature on the upper and lower surface of the model to simulate an initial, thermally conductive regime was assumed. The initial pressure is assumed to be hydrostatic, with a pressure of 3 bar at 3 km depth. Single Layer Reservoir Model This model has the same general temperatures and transmissivity as the Sandwich model, but with no seal layers separating reservoir layers. Low Temperature Model This is the same as the Sandwich model, but with the average reservoir temperature being only 5 o C at 3 km depth. Low Permeability Model This is the same model as the Sandwich model, but with reservoir layers having 33 md, and a total reservoir transmissivity of 3 D-m. Short Circuit Model This model has one sandwich layer with a high permeability (3 md), while the other three reservoir layers have permeabilities similar to the Low Permeability model layer. The overall reservoir transmissivity is the same as for the Sandwich model.. Table : STARS Input for Sandwich and Single Layer reservoir models. Properties Sandwich Single Dimension 5 m x 5 m x 3 m No. of Layers 9 Grid (i, j, k) Permeability md-layers 2,,6,8 Transmissivity 5 m 5m variable m k: - 2 = 25 m, 5 m total (layer ) 2-25 = 5 m, 25 m total (layer 2) = 5 m, 35 m total (layer 3) = 5 m, 25 m total (layer ) = 5 m, m total (layer 5) = 5 m, 25 m total (layer 6) = 5 m, 65 m total (layer 7) 76-8 = 5 m, 25 m total (layer 8) 8 = 25 m, 5 m total (layer 9) D-m Porosity. Rock Therm. Cond. 2.5 W/m o C Water saturation.99 Initial Temperature Initial Pressure md-layer 5 2 o C at 3 km depth Gradient 35 o C/km 3 Bar at 3 km depth Gradient Bar/km md-layers,3,,6,7,8,9 Well Pattern 2 x ¼ Injector located at, m and,m Water inj. (Total) 2 x ¼ Producer located at, m and,m gallons/minute, (US); 32 bar BHP, 75 o C (Each of 2 injectors has ¼ of gpm, or 5.7 l/s) Table 2: Variation in STARS Input for the Sandwich, Low Temperature, low Permeability, and Short-Circuit Models. Properties Sandwich Low Temp. Low Perm. Short-Circuit. Permeability md-layers 2,,6,8 md-layers 2,,6,8 3 md-layers 2,,6,8 3 md-layer 33 md-layers 2, 6,8 Transmissivity D-m D-m 3 D-m D-m Initial Temp. 2 o C 5 o C 2 o C 2 o C

3 Injector Producer ,2,, ,2,, ,,,2, ,, ,, Sandwich (red = md units) Single Layer (red = md) Short Circuit (red = 3 md; Low Perm (3 md units) Geothermal Sedimentary Basin Geothermal Sedimentary Basin Permeability I (md). day J layer: Permeability I (md). day J layer: ,, 2 3File: single 5layer 6model.irf 7 8 9,, Injector Producer 2 Injector Producer 2 Date: 9/2/22 Scale: :27 Z/X:.: Low Temp ( md units; 5 C) ,,,2, feet. 2.. meters Light blue = 33 md) Figure : Permeability layering for the five models. Assumptions The assumptions made for the modeling work include: isotropic permeability, uniform porosity (%), ability to ignore wellbore heat losses, well perforations in producing layers only and that boundary effects are negligible. The upper and lower boundaries of the model were assumed to be 5 m above and below the reservoir seal zone. These boundary zones had a permeability of md and were sufficiently thick to not influence conditions within the reservoir-seal zone for the duration of the model lifetime (5 years). All models utilized a common five spot pattern with a 5 m well spacing as shown in Figure 2. This well spacing was chosen after some preliminary modeling to ensure that large changes in temperature would be seen on a time scale of the economic life of a power plant (~ 3 years) feet. 2.. meters File: sandwich short circuit model final.irf Date: 9/2/22 Scale: :27 Z/X:.: assumption for all models is that production wells were pumped at a constant rate, and all produced water was reinjected at 75 o C after being cooled in the power plant. Negligible wellbore heat loss was assumed, similar to that in the earlier modeling work of Sanyal and Butler (2). 9 6To improve the general performance of the models in 3terms of computation time, the computational grid was manipulated so that depth dimension (k) of the i, j, k grid for the central layers of the stratigraphic reservoir was reduced to 5 m, while the cap layers with low permeability were reduced to 25 m. This did result in some un-natural blocking of calculated temperature results at the cap layer boundaries, but was inconsequential in terms of predicted temperature breakthrough times. The model domain took advantage of flow symmetry and was a 5 m x 5 square in plan-view with one quarter production and injection wells on the corners as shown in Figure 2. The results are easily scaled to calculate the efficiency of the heat sweep process, and after allowing for the power plant conversion efficiency of the produced water to power generation, the power generation density (MWe/km 2 ) can be calculated. The 5 m well spacing consists of two production and two injection wells per square kilometer, or 8 times the output from one quarter wells in the model domain. The relationship between the initial conditions for the modeling and the assumed pressure and temperature conditions for a hypothetical high heat flow basin are shown in Figure. 3. Figure 2: Well spacing used in the modeling. The design flow rate in the production and injection wells were gallons per minute (63 liters/second), about half the maximum rate normally achievable for geothermal pumps. A critical Figure 3: Relationship of model assumptions to the temperature and pressure boundary conditions. The change in the temperature gradient at the top of the model represents the increase in thermal conductivity between the overlying sedimentary cover and the underlying bedrock containing the

4 Pressure Change (bar),2,, 9,2,, ,2,, 9 8 7,2,, ,2,, -,2,, Wellhead Temperature (oc) ,,,2,3 reservoir. This thermal regime is consistent with that in high heat flow sedimentary basins in the western U.S. (8 to mw/m 2 ; Allis et al., 22). Also shown is the maximum range of pressures that were calculated from the models during production and injection. These pressure changes are small compared to a lithostatic pressure gradient and an inferred fracture gradient equivelant to 9% of the lithostatic pressure. RESULTS The most obvious differences in the response of each model to production and injection are shown in the temperature changes after 3 years (Figure. ). Geothermal Sedimentary Basin Geothermal Sedimentary Basin Temperature (C) 957. day J layer: Temperature (C) 957. day J layer: File: single layer model.irf ,, ,, Producer 2 Injector Producer 2 Injector Producer 2 Injector Date: 9/2/22 Scale: :27 Z/X:.: ,,,2, ,,,2, ,,,2, ,,,2, ,,,2,3 Geothermal Geothermal Sedimentary Sedimentary Basin Basin Temperature Temperature (C) 957. (C) day 957. J layer: day J layer: File: sandwich short circuit model final.irf Date: 9/2/22 Scale: :27 Z/X:.: between the reservoir and seal units (33:), so a greater proportion of heat is being swept from the Sandwich Single Layer Low Temperature Low Permeability Short Circuit Figure 5: Trends in production wellhead temperature with time. adjacent seal units on a time scale of decades. As depicted in Figure 6, the pumped production wells are flowing as in the other models, but the lower total transmissivity causes a greater lateral pressure gradient between the injection and production wells (about 6 bars compared to about 3 bars with the other models). In contrast, the seal units above and below the Single Layer model contribute little to the heat sweep process. 3 2 Producers Injectors Sandwich (Inj) Sandwich (Prod) Single Layer (Inj) Single Layer (Prod) Low Temp. (Inj) Figure : Cross-sections of temperature ( C) after 3 years of injection and production for all five models Low Temp. (Prod) Low Perm. (Inj) Low Perm. (Prod) Sht. Circ. (Inj) Sht. Circ. (Prod) Temperatures of less than C have broken through in both the Single Layer model and high permeability layer of the Short Circuit model within 3 years. Although the Sandwich, Single Layer and Short Circuit models have the same total reservoir transmissivity ( D-m), large differences in the extent and amplitude of cooling are apparent. These differences are highlighted by plotting the wellhead production temperatures with time (Figure 5) calculated by adding the heat inflow to production wells occurring from each of the reservoir layers. Perhaps surprisingly, the best thermal performance comes from the Low Permeability model. The main reason for this is there is less permeability contrast Figure 6: Pressure changes with time at reservoir level in all five models. The most rapid thermal decline occurs in the high permeability layer (3 md) of the Short Circuit model, with a decrease in production wellhead temperature from 2 to about 5 C within years. However at longer times, this model performs substantially better than the Single Layer model, and at 5 years, the wellhead temperature is not far below that for the Sandwich model. Figure 7 shows what happens to the inflowing hot water at the production wells for each of the reservoir units in the Short Circuit model. The initial rapid thermal decline is

5 Heat Output/Quarter Well (MWth) Heat Output/qtr well (MWth) Heat Output/qtr. well (MWth) due to the focused ingress of cool injection water within the high permeability layer, which is initially about times the flow in each of the other three reservoir layers. However, the flow rate in the high permeability layer diminishes by about 2% due to the effects of increased dynamic viscosity of the cool water whereas the flow rate through the other three lower-permeability layers increases by between 5 and %. After years, the high permeability layer is providing only half the total heat inflow to the well, and by 5 years this fraction has decreased to 5%. well. These heat outputs are equivalent to 33 and 2 MWth per full production well Sandwich Layer 2 ( md) Layer ( md) Layer 6 ( md) Short Circuit Layer 2 (33 md) Layer (3 md) Layer 6 (33 md) Layer 8 (33 md). Layer 8 ( md) Figure 8: Thermal contributions from the four reservoir layers in the Sandwich model Figure 7: Thermal contributions from the four reservoir layers in the Short Circuit model. The subtle effects of increasing water viscosity with decreasing temperature can also be seen in the Sandwich model (Figure 8). Here, the layers all have the same permeability and thickness, so the only variable is temperature and its effect on the physical properties of water. In the early production years the rate of heat inflow to the production wells increases with increasing depth and initial layer temperature. However, as these deeper layers preferentially cool, the injection flow into them decreases and the flows in the two upper layers, which are now warmer than the deeper layers, increases. After about 3 years, the pattern of heat inflow with depth has reversed, with the highest inflow occurring in the uppermost layer, and the lowest inflow in the deepest layer. In reality, natural variability in permeability on a local scale within layers would likely obscure this effect. 3 2 Sandwich Single Layer Low Temperature Low Permeability Short Circuit Figure 9: Trends in heat output with time from the five models. The electric power output per production well, and therefore the power density, has been calculated using the conversion efficiency relationship established by Allis and Larsen (23) for air-cooled binary power plants. Figure shows trends that are similar to those for heat output trends, except that the decline in the power output with time is more severe. This is due to the decreasing conversion efficiency as production temperature declines. The implications of these trends for the scale and economics of geothermal power generation from bedrock-hosted reservoir sequences beneath high heat flow basins are being studied separately. When the production wellhead temperatures (Figure 5) are combined with the fixed flow rate of 25 gallons per minute per quarter well (5.7 l/s), the heat output from the quarter wells in the models is a scaled version of Figure 5 shown in Figure 9. The heat output from the four models with 2 C as the initial reservoir temperature decreases from about 8 MWth/quarter well, and that for the 5 C initial temperature it decreases from 5 MWth per quarter

6 Power Density (MWe/km2) 2 8 Sandwich ( D-m) Single Layer ( D-m) Low Temperature ( D-m) Low Permeability (3 D-m) Short Circuit ( D-m) ACKNOWLEDGEMENTS The project was partially funded by the Geothermal Technologies Program of the U.S. Dept. of Energy (Award DE-EE528) Figure : Trends in power density with time, a measure of the efficiency of the heat sweep process of injected water within the reservoir-seal zone between injection and production wells. CONCLUSIONS Investigation of the responses of stratigraphic reservoir-seal sequences to production and injection for geothermal power generation shows that the best reservoirs may be those that have a range of permeabilities on the order of to md distributed over a depth range of hundreds of meters. The greatest differences in the long-term thermal output of production wells occurs between the Low Permeability model and the Single Layer and Short Circuit models. Models with a single, relatively high permeability layer have a significantly worse thermal response over several decades than lower permeability model with reservoir layers separated by seal layers. During 3 years of production and injection, production wells in the Low Permeability model each generated MWe-years of power compared to 65 9 MWe-years per production well in the Single Layer and Short Circuit models. A consistent result from all the models was that vertically distributed reservoir layers allow a much greater fraction of heat to be swept from lower permeability seal units. In all models, the lateral pressure gradient induced between injectors ranged between 3 and 6 bars, which is not unusual for geothermal developments. In addition to the strong influence of the vertical permeability distribution of high permeability layers, these results are sensitive to the assumptions about the injector-producer well spacing of 5 m and the pump rate of gpm (63 l/s per well). Increasing the well spacing is a common mitigation response in geothermal developments where injection water break-through occurs at nearby production wells. Changes in the pump rate will also inflow the rate of cross-flow between injectors and producers. These are factors that will be considered in future modeling. REFERENCES Allis, R., Moore, J., Blackett, R., Gwynn, M., Kirby, S., and Sprinkel, D. S. 2. The potential for basin-centered geothermal resources in the Great Basin. GRC Transactions, 35, Allis, R., Blackett, R., Gwynn, M., Hardwick, C., Moore, J., Morgan, C., Schelling, D., and Sprinkel, D. 22. Stratigraphic Reservoirs in the Great Basin the Bridge to Development of Enhanced Geothermal Systems in the U.S. Transactions, Geothermal Resources Council, 36, Allis, R. and Larsen, G., 23. Performance of aircooled binary plants: an analysis using the Pacificorp Blundell unit near Milford, Utah. Draft Report to Geothermal Technologies Program, Project DE-EE528 Novel Geothermal Development of Deep Sedimentary Systems in the U.S. pp. 2. Computer Modeling Group Ltd., 2. Advanced Process and Thermal Reservoir Simulator, Version 2, by Computer Modeling Group, Ltd., Calgary, CA. Kirby, S. M., 22. Summary of compiled permeability with depth measurements for basin fill, igneous, carbonate, and siliciclastic rocks in Utah and adjoining states. Report to Geothermal Technologies Program, Project DE-EE528 Novel Geothermal Development of Deep Sedimentary Systems in the U.S., Open File Report 62, Utah Geological Survey, pp. 3. Moore J. N. and Allis, R., 23. Phase Stage-Gate Report to DOE Technologies Program, December 22: Project DE-EE528 Novel Geothermal Development of Deep Sedimentary Systems in the U.S. Sanyal, S., and Butler, 29. Geothermal power from wells in non-convective sedimentary formations an engineering economic analysis. Transactions, Geothermal Resources Council, 33, Sanyal, S., Butler, S., 2. Geothermal Power Capacity from Petroleum Wells Some Case Histories of Assessment, Proceedings World Geothermal Congress, Bali, Indonesia, April 2.

7 Smith, J. S., Bloggs, R. T. and Jones, E. R. (97). "Magnetic Anomalies in Geothermal Systems, Journal of Fluid Mechanics, 25,