Injection and Thermal Breakthrough in Fractured Geothermal Reservoirs

In this study, the problem of cold water injection into a fractured geothermal reservoir is considered. During injection, the cold water will advance along the fractures, gradually extract heat from the adjacent rock matrix, and eventually arrive at the production wells. If the injected water has not fully heated (up) by then, detrimental effects on energy production from decreasing fluid enthalpies may result. This indicates the need to establish criteria for designing an injection/production scheme for fractured geothermal reservoirs. The model considered in this work consists of an injection well fully penetrating a fractured geothermal reservoir containing equally spaced horizontal fractures. A constant‐temperature liquid water is injected into the fractures, and with the rock matrix assumed to be impermeable, the effect of heat conduction on the advancement of the ‘cold’ water along the fractures is observed. Key dimensionless parameters that describe the physical system are identified and type curves are generated. These are intended to be used in the design of injection/production systems, mainly for determining the appropriate locations and the flow rates of the injection wells. Also, temperature contour maps are given, and these can be used to estimate the amount of recoverable energy from the geothermal system, based on a given injection/production scheme. Numerical studies are also made to investigate the importance of the assumptions in the analytical work and to extend the work to cases where the rock matrix is permeable. The results are intended to be used as a constraint on results calculated using the type curves. Finally, two simple numerical examples using a hypothetical doublet and the Cerro Prieto geological model are given. In the doublet case, a reservoir with two to five fractures is considered, and breakthrough times of 13 and 65 years, respectively, are calculated. Using typical injection rate and the Cerro Prieto geological model, our calculations indicate that the thermal front will have advanced only 240 m away from the injection well after 30 years of injection. It is furthermore illustrated that this estimate is independent of fractures in the reservoirs.