A numerical study on the long term thermo-poroelastic effects of cold water injection into naturally fractured geothermal reservoirs

摘要

The residing fracture system and the prevailing in situ stresses have a significant impact on fluid flow and heat transfer in crystalline rocks. The long term response of fracture systems to changes in effective stresses, in particular the long term geo-mechanical effects of thermal stresses on reservoir characteristics is of particular interest to the geothermal industry. In this paper, a geothermal reservoir model is presented, in which a thermo-poroelastic finite element module is coupled to a fracture geomechanical module. This describes fracture closure as a function of effective stress and the changes in parameters, such as effective permeability, porosity and discrete fracture apertures. The novelty of this approach lies in its dynamic treatment of the characteristic properties of individual fractures in simulating fluid flow and the pervasive response of the natural fracture system to cold fluid injection. The model uses well-established empirical elastic deformation solutions to calculate the response of natural fractures to changes in effective stress. The reservoir model is applied to a typical naturally fractured geothermal system with a fracture density of 0.25 m~(-1) and an injector-producer separation of 565 m. Fluid flow rates and corresponding pressure losses, produced water temperature, thermal drawdown, effective stresses and reservoir permeability were calculated over a production period of 10 years, to evaluate the long term effects of cold water injection on geothermal reservoir properties. Results of this study have shown that tensile thermal stresses normal to the fracture surfaces are induced as heat is extracted from hot reservoir rock by pervading cold fluid. The gradual decrease of the normal effective stresses in the fractured reservoir due to thermal drawdown of the rock matrix allows the natural fractures to open and thus, increases reservoir permeability. The thermal stress contribution to the long term permeability of the geothermal reservoir was shown to be significant. Large increases in injectivity for a given pressure drop were observed over the course of the production period. Also observed was the variation in produced fluid temperature due to the dynamic and heterogeneous distribution of permeability towards the production well. Short term thermal effects (<1 year) were only observed in the vicinity of the injection well and were relatively insignificant.