Harris, Jerry M.1, Olusoga Martins Akintunde1, Tapan Mukerji1,
(1) Stanford University, Stanford, CA
ABSTRACT: Predicting Pressure and Saturation Changes from Elastic Wave Velocities in a CO2-Flooded Coal Bed Methane: A Modeling Study
Understanding the effects of changing seismic signatures from fluid saturation and
pressure changes is germane to the problem of monitoring the injection and spread of CO2.
Recent papers on the estimation of changes in reservoir properties as functions of changes
in seismic properties have focused mostly on hydrocarbon reservoirs. It is important to
develop similar concepts to guide the feasibility of monitoring coal beds undergoing CO2
injection. Moreover, the basic coal physics and the interactions with CO2 and
methane are still poorly known. To address these issues, physics-based models are
developed to predict changes through the CO2-ECBM (Enhanced Coal Bed Methane)
scenario. We attempted to quantitatively relate the sensitivity of seismic properties to
variations in pore fluid saturation and pressure using forward modeling. We used
laboratory data on four (4) samples of Australian Permian coal from Yu etal 1993. Gassmann
equation and appropriate fluid properties were used to model the data sets.
The results of the analysis show that the Gassmann derived P- and S-wave velocities for the saturated coals are highly pressure dependent. The estimated changes in P-wave velocity, bulk modulus and density are largest at low differential pressures. The modeling study further reveals that the seismic velocities (Vp and Vs ) are not so dependent on changes in saturation. Coal, depending on its rank and geology, exhibits significant variations in porosity and structure. While our results are valid for the Permian coals examined, they may be representative of other coals as well.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.