| Title | Asymptotic Analysis of Thermal Stimulation of Geothermal Reservoirs |
|---|---|
| Authors | Kyungjae IM, Derek ELSWORTH, Yi FANG |
| Year | 2015 |
| Conference | Stanford Geothermal Workshop |
| Keywords | asymptotic analysis, permeability, binary temperature |
| Abstract | The evolution of initially low permeability geothermal reservoirs is sensitive to changes in effective stress driven by injection fluid pressures, fluid temperature and chemical effects. These influencing factors typically act on different timescales – fluid pressures fastest and with greatest reach form the wellbore, followed by more slowly propagating temperature effects then by chemical effects. Each of these mechanisms act due to feedbacks on effective stresses. We explore the influence of thermal stimulation in a simplified geometry of spherical (or radial) injection into a reservoir of initially isotropic stress and of homogeneous permeability and temperature. Where the system is ubiquitously fractured and soft in dilation bounding behaviors may be recovered in closed-form to determine the change in injection rate and the evolution of the stimulated volume with time. The closed-form analysis requires that fluid transmission within the reservoir is steady (ratio of permeability to fluid storage is large) and that the injected fluid is in thermal equilibrium with the rock (product of fracture spacing and flow rate is small). Under these conditions the reservoir develops binary temperature, viscosity and permeability profiles with radius enabling the transient response to be define in terms of straightforward dimensionless parameters that constrain a unique response. These results provides a rapid and robust estimation of injection rate and the area of enhanced permeability zone with time. The dimensionless analysis suggests that such reservoir development can be simply characterized by the difference between transmissivity ratio between two zones and inner and outer boundary ratio. The utility and reliability of this method has been demonstrated by the comparison of this solution with results of a complex coupled thermo-hydro-mechanical (THM) model, defining the regions of validity of such an approach. |