Thermo-poroelastic AVO modeling of Olkaria geothermal reservoirs

Abstract

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 reservoir modeling 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, and the fluid properties under different temperature conditions have been considered. A plane-wave analysis illustrates the effects of thermal conductivity, specific heat, and porosity on velocity dispersion and attenuation of the fast-P, Biot P, and thermal P waves. AVO modeling by P-wave incidence is conducted to investigate the effects of temperature, porosity, and fluid type on the R/T coefficients. For trachyte reservoirs with a temperature less than 500 °C, limited changes in the thermophysical properties have negligible effects on wave propagation, whereas significant effects are due to temperature, porosity, and fluid type. Synthetic seismogram also indicates that the thermo-poroelastic effect noticeable changes the seismic responses compared with the traditional elastic and thermoelastic models. The thermo-poroelastic AVO method proposed in this paper reveals the variation of R/T coefficients of geothermal reservoirs under different conditions. The results can be used as a precursor to geothermal reservoir monitoring, fluid leakage or short circuits.