Harris, Jerry M.¹, Olusoga Martins Akintunde¹, Tapan Mukerji¹, Jaime Urban¹
(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.