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 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.