Numerical Study on the Temperature Behavior in Naturally Fractured Geothermal Reservoirs and Analysis Methodology for Geothermal Reservoir Characterization and Development

Abstract An important way to develop geothermal energy is by producing low-medium temperature fluids from naturally fractured geothermal reservoirs. Pressure analysis is the most used to characterize such reservoirs for improving development efficiency. However, pressure inversion easily leads to non-uniqueness and cannot estimate thermal properties. Additionally, no reliable methods are proposed to evaluate the development potential of geothermal reservoirs. To narrow the gap, this study aims at studying the temperature behaviors and exploring suitable analysis method for characterizing geothermal reservoir and evaluating development potential. The numerical and analytical models are simultaneously established to analyze the temperature behaviors. Our models account for the J-T effect (μJT), adiabatic heat expansion/compression effect (η), reservoir damage, viscous dissipation, heat conduction and convection effects. The solution's development is dependent on the fact that the effects of reservoir temperature changes on transient pressure can be ignored so that the pressure and energy equations can be decoupled. We firstly compute reservoir pressure field based on Kazemi model, then use this obtained pressure field to solve the energy-balance equations. The numerical solution is verified and is found to be in good agreement with the proposed analytical solutions. This work shows that the most used constant μJT and η assumption will produce inaccurate temperature results when reservoir temperature changes significantly. Moreover, we find that temperature behaviors can exhibit three heat radial flow regimes (HRFR) and a heat inter-porosity regime with V-shape characteristic. Fracture thermal storativity ratio and matrix heat inter-porosity coefficient defined in this study can be estimated from this characteristic, which are further used to evaluate geothermal development potential. Our work also shows that temperature data can give information that would not be provided by conventional pressure analysis. The temperature derivative curve will show ‘hump’ characteristic if reservoir is damaged. The temperature data can characterize the skin-zone radius and permeability. More than that, the properties such as J-T coefficient, effective adiabatic heat expansion coefficient and porosity can be estimated. Eventually, an integrated workflow of using both temperature and pressure data analysis is presented to characterize naturally fractured geothermal reservoir for the first time. Simulated test examples were interpreted to demonstrate its applicability.