Numerical study on the heat transfer behavior in naturally fractured reservoirs and applications for reservoir characterization and geothermal energy development

Naturally fractured reservoir characterization has always been a challenging problem worldwide due to highly heterogeneous. Pressure transient analysis (PTA) is the most used technique for reservoir characterization. However, the pressure inversion inevitably produces non-uniqueness. Additionally, naturally fractured reservoirs are also some of the most promising geothermal systems. A novel concept is to harness heat by producing water from abandoned hydrocarbon reservoirs. However, few literatures have discussed the geothermal development issues in such way by far. To narrow the gap, we present a numerical model to study heat transfer behaviors when fluids are produced from naturally fractured reservoirs. The J-T coefficient, adiabatic expansion, viscous dissipation, conduction, convection and reservoir damage are all considered in proposed model. Two advices for geothermal development are presented according to heat-transfer behavior study. Furthermore, an integrated workflow of combining temperature transient analysis (TTA) and PTA is proposed to determine the reservoir properties for the first time, which help overcome the shortcomings of conventional PTA. Results indicate that: (a) Two parameters proposed in this work (reservoir heat storage ratio and heat inter-porosity coefficient) can evaluate the feasibility of geothermal development, which are characterized by ‘V-shape’ feature on temperature derivative curve. (b) The reservoir temperature will drop when heat transfers to closed boundary, so the geothermal development should choose the reservoir with larger size. (c) TTA can characterize the radius and permeability of reservoir damage zone, which are not readily accessible from PTA. Meanwhile, TTA can distinguish the composite reservoirs and closed reservoirs, which helps address the non-uniqueness problem of PTA. Therefore, TTA can serve as a companion way with PTA to more clearly characterize reservoir with reasonable workflow.