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.

Temperature is an important factor for evaluating the seismic response of deep reservoirs. We develop an amplitude variation with offset (AVO) approximation based on the Lord-Shulman (LS) thermoelasticity theory. The model predicts two compressional (P and T) waves (the second is a thermal mode) and a shear (S) wave. The T mode is due to the coupling between the elastic and heat equations. In the thermoelastic case, the approximation is more accurate than in the elastic case. Its accuracy is veried by comparison with the exact equations calculated in terms of potential functions. We examine two reservoir models with high temperatures and compute synthetic seismograms that illustrate the reliability of the approximation. Moreover, we consider real data to build a model, and show that the approximate equation not only simplies the calculations, but is accurate enough and can be used to evaluate the temperature-dependent elastic properties, providing a basis for further application of the thermoelasticity theory, such as geothermal exploration, thermal enhanced oil recovery, and ultra-deep oil and gas resources subject to high temperatures. Temperature is an important factor for evaluating the seismic response of deep reservoirs. We develop an amplitude variation with offset (AVO) approximation based on the Lord-Shulman (LS) thermoelasticity theory. The model predicts two compressional (P and T) waves (the second is a thermal mode) and a shear (S) wave. The T mode is due to the coupling between the elastic and heat equations. In the thermoelastic case, the approximation is more accurate than in the elastic case. Its accuracy is veried by comparison with the exact equations calculated in terms of potential functions. We examine two reservoir models with high temperatures and compute synthetic seismograms that illustrate the reliability of the approximation. Moreover, we consider real data to build a model, and show that the approximate equation not only simplies the calculations, but is accurate enough and can be used to evaluate the temperature-dependent elastic properties, providing a basis for further application of the thermoelasticity theory, such as geothermal exploration, thermal enhanced oil recovery, and ultra-deep oil and gas resources subject to high temperatures.