Publications

Seismic time-lapse is a proven tool to monitor changes due to production and injection in reservoirs. Today, there are approximately 120 enhanced oil recovery projects worldwide that involve CO2 injection. However, only two of these projects are currently being monitored. One of them is in the Postle Field in Texas County, Oklahoma, which is being led by the Reservoir Characterization Project (RCP) of the Colorado School of Mines. We have used P-wave seismic data to dynamically characterize the reservoir at Postle Field.RCP shot a 6.25 square miles 4D, 9C seismic survey in March 2008 and repeated another survey in December 2009 after 19 BCF of CO2 had been injected into the reservoir from 18 wells. In addition, an 8-square mile P-wave vintage survey was shot for Mobil Oil Corporation (herein called the Mobil survey) in 1995 during the dormant stage of the field. Fluid saturation and reservoir pressure changed between 1995 and December 2008.Cross equalization was performed to increase the repeatability of seismic volumes by eliminating non-repeatable noise in the data. This process corrects phase, time-shift, frequency, and amplitude mismatch between the data, which might occur due to different acquisition parameters, soil condition during the survey, and processing workflows. As a result, the Normalized Root Mean Square (NRMS) difference of the Mobil and the RCP baseline has been improved from an average of 1.4 to 0.4. For the pair of the RCP surveys, the NRMS value has been lowered from an average of 0.3 to 0.15. Quadrature attribute and seismic inversion-ofdifference provide qualitative and quantitative timelapse interpretation. A maximum P-impedance change of -7% was obtained from the inversion process, which is consistent with the fluid substitution model. The anomalies confirm the existence of faults in the area as pathways for fluid flow in the reservoir. Moreover, the anomalies show high permeability trends associated with three main fairways of sandstone deposition in the field.4D seismic has identified the main reservoir heterogeneities in the RCP study area. Well pattern performance confirms the existence of these heterogeneities. Dynamic reservoir characterization will assist future reservoir management efforts at Postle Field and is a technology that will likely grow in use for other CO2 floods worldwide.