Joint rock physics inversion of well log and 3D VSP data to model CO2 saturation and porosity at Cranfield Field, Cranfield, MS.

摘要

Analysis of the effects of injected CO_2 on the seismic response of reservoirs is important because it can provide improved characterization and monitoring of sites undergoing CO_2 injection. We completed a joint inversion of the contact cement model to better understand the effect of CO_2 saturation on the relationship between elastic parameters and reservoir properties of the Cranfield reservoir. We used p-impedance and Vp/Vs from well logs and 3D VSP data to invert jointly for porosity and fluid saturation. We calibrated a rock physics model to different depth intervals in well data from the Cranfield reservoir interval. In the two reservoir intervals Vp coefficients of 1.14 and 1.08 were for the shallow and deep portion, respectively. Vs coefficients used to correctly model the two intervals were 1.04 and .93. These coefficients correct for pressure and grain shape. We then performed fluid substitution to model density and velocity logs for different in situ CO_2 saturations. The logs, calculated to have a uniform pore fluid composition for all depth points, were input into the inverted model to generate modeled logs of saturation and porosity. Results indicated that the model was relatively accurate for porosity and fluid saturation when upscaled to represent seismic resolution. Lastly, a crossline from a 3D VSP was inverted for Ip, and a synthetic Is volume was generated. Results from the crossline at the well locations showed a relatively accurate porosity estimation. Inverting for pore fluid did not return the same degree of accuracy at this time.