| Title | An Innovative 3D Thermo-Poroelastic Model for Studying the Long Term Behavior of Geothermal Systems |
|---|---|
| Authors | Koh, Joshua; Shaik, Abdul Ravoof; Rahman, Sheik S. |
| Year | 2011 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Geothermal; EGS; stimulation; shear dilation; thermal stresses; heat extraction |
| Abstract | A thermo-poroelastic numerical geothermal reservoir model has been developed for investigating shear dilation effects during hydraulic stimulation and thermal drawdown of geothermal reservoirs. The numerical model simulates the shearing of fractures and the permeability change by coupling thermo-poroelastic fluid flow and shear dilation analyses. The simulation is based on a discrete fracture network model in which dynamic treatment of the characteristic properties (aperture, length and orientation) of individual fractures is adopted and used to evaluate fluid flow in a heterogeneous thermo-poroelastic reservoir. The numerical reservoir model is applied to a naturally fractured geothermal system subjected to different degrees of stimulation by induced pressure to determine the degree of permeability enhancement required to maximize hot water production. The results of this study show that for a nominal stimulation bottomhole pressure of 12,000 psi, the maximum level of shear dilation is observed after a period of 42 weeks for a medium density fracture system (0.5 m-1). This stimulation produced and economic flow rate of 100 L/s at an acceptable pressure loss of 1000. We have also observed that the major factor responsible for heat recovery is the degree of interconnection of the fractures in the reservoir. The average matrix temperature drawdown is in the order of 20° C for an initial reservoir temperature of 200°C over a 16 year production period. For the case of a highly interconnected injector-producer pair, the produced fluid temperature drops from an initial temperature of 200°C to about 160°C after 16 years of production. Thermal stresses due to matrix cooling results in a gradual increase in the production rate. |