Geothermal Energy Recovery for Urban Heating Applications: Risks and Rewards

Abstract It is possible to recover large quantities of heat from modestly hot reservoirs (90 °C). The harvested energy can be used to heat homes and businesses in the winter months. To accomplish this, water is circulated through the hot formation using injection and production wells. The water flows through the rock matrix and any natural or induced fractures. This paper discusses a field study to determine the feasibility of recovering heat from the subsurface by water injection into a high permeability sandstone reservoir. The heat recovery rate is computed and the risks associated with large scale water injection are evaluated. It is shown that it is difficult to avoid the formation of fractures in the injection well. The increase in pore pressure can also result in the slippage of natural faults. No previous study has systematically investigated the influence of both heat conduction and convection and the associated stress alteration and fracture height growth during long-term water injection. In this paper a general poro-thermo-elastic model is used to model the process of long-term water injection for geothermal heat recovery. The model is based on mass and energy balances for fluid flow, for the reservoir temperature, and a stress balance for the reservoir stress/deformation calculation. As the reservoir stress condition evolves over time, we apply a fracture propagation criterion to predict fracture initiation and growth. A Newton-Raphson formulation and fully implicit algorithms are used to ensure tight coupling between multiple physical components in simulations and are optimized to predict water injection-induced fracturing. A modified Mohr-Coulomb failure criterion is used to detect the possibility of inducing natural fault slippage during geothermal energy recovery. The results demonstrate that it is possible to induce fracture propagation and natural fault slippage during long-term water injection for geothermal energy recovery. A comprehensive sensitivity study is conducted to investigate the effect of solids content, injection rate, stress contrast, containment, and injection temperature. The results indicate that (1) reducing the injection rate is a possible way to delay fracture initiation; (2) the fractures can potentially breach the shale above the injection zone. However, vertical migration of the fractures will be limited to a few meters; (3) improving the water quality delays the onset of fracturing but fractures still propagate after a few months of injection; and (4) increasing the injection water temperature also reduces the fracture length, but it is not possible to completely avoid injection induced fractures. Furthermore, the possibility of slip of natural faults is evaluated. Stresses and pore pressures computed at the location of the fault indicate that vertical faults are unlikely to slip; and (2) a fault is likely to slip if it has a dip angle of over 20 degrees. The fully 3-D poro-thermo-elastic flow and fracture propagation model presented in this paper provides a valuable tool to evaluate the rewards and risks associated with geothermal energy extraction. No such study has been undertaken in the past to our knowledge. Methods to reduce the risks of fracturing and fault activation are suggested based on the results.