Thermoporoelastic AVO modeling of Olkaria geothermal reservoirs

Seismic AVO has a significant potential for fluid identification in time-lapse monitoring of the cyclic recovery of geothermal reservoirs. With this goal, we develop an AVO method based on the reflection and transmission (R/T) of elastic waves at an interface between two fluid-saturated thermo-poroelastic media. The method is applied to the Olkaria geothermal reservoirs in Kenya. This system is characteristic of a natural cyclic recovery, where cyclic meteoric water undergoes complex phase transition and thermo-hydro-mechanical coupling process. Conceptual models are built based on petrophysical and thermophysical properties of trachyte thermal reservoirs in the eastern field, with an attempt to model the shallow steam and deep boiling water zones. A plane-wave analysis illustrates the effects of thermal conductivity, specific heat, and porosity on velocity dispersion and attenuation of the fast-P, Biot slow-P, and thermal slow-P waves. AVO modeling by P-wave incidence is conducted to investigate the effects of temperature, porosity, specific heat, and fluid type on the R/T coefficients. For trachyte reservoirs with a temperature less than 400°C, limited changes in the thermophysical properties (e.g., thermal conductivity and specific heat) have negligible effects on wave propagation, whereas significant effects are due to temperature, porosity, and fluid type. Particularly, comparisons of cyclic recovery using water, supercritical CO2, and gas (dry case) as the heat transfer fluid, demonstrate that the crossplot of fluid factors and intercept gradient (PG) can be used as a precursor to hydrofracturing-induced permeability, fluid leakage or short circuits.