Dynamic Reservoir Characterization of a CO2 Huff'n'Puff, Central Vacuum Unit, Lea County, New Mexico

Abstract

Abstract The Reservoir Characterization Project (RCP) is an industry sponsored consortium whose mission is to develop and apply 3-D and 4-D ("time-lapse"), 3-C seismology and associated technologies to improve reservoir performance and hydrocarbon recovery while reducing environmental impact. RCP Phase VI is the multidisciplinary, 4-D, 3-C study of a CO2 injection project in Vacuum field, a shallow shelf carbonate reservoir located on the Northwestern Shelf of the Permian Basin of West Texas and Southeastern New Mexico. The CO2 "huff-n-puff" in well Texaco CVU-97 at Vacuum Field and the repeated 3-D, 3-C seismic surveys were performed from October 30, 1995 to December 27, 1995. The initial 3-D, 3-C survey was acquired from October 28 through November 13. CO2 injection began November 13, 1995 and lasted until December 8, 1995. The "soak" period extended from December 8 through December 28, after which Texaco CVU-97 was returned to production. The second 3-D, 3-C survey was acquired during the "soak" period, from December 21 to December 28. The compressional data provides a measure of the bulk rock compressibility, rigidity and density, while shear wave data is sensitive to rigidity and density. The combined use of P-wave, S1 shear and S2 shear seismic data allow different views of the bulk rock properties in the subsurface. Our interpretation methodology strives to use these volumes to delineate spatial variations in the subsurface related to lithology, porosity, pore structure variations related to preferred permeability directions, and variations in pore fluid pressure and properties. Interval traveltime comparisons between the P- and shear volumes are a robust and sensitive indicator of lithology, porosity and pore geometry, and the intensity of fracturing. In particular, the TS1/TP and TS2/TP measures show regions of lower VP/VS ratio in the southwest portion of the survey area. This portion of the reservoir produces less fluid with a lower water cut than do areas of the field exhibiting a higher VP/VS ratio. The VP/VS measure appears strongly correlated with the reservoir production characteristics. Anomalies between the two seismic surveys are readily visible on both the P-wave and S-wave seismic surveys. During the CO2 injection program both the fluid composition and reservoir pore pressure were altered. An interpretation of these seismic anomalies indicates the P-wave and S1 shear data are delineating reservoir zones where fluid compressibility and/or viscosity have changed due to CO2 injection and the subsequent migration of lighter hydrocarbons. The P-wave and S1 shear data indicate fluid movement to the northwest from CVU-97 along the direction of maximum horizontal stress towards the portion of the reservoir with better effective permeability. The S2 shear data, on the other hand, appears to be more sensitive to changes in pore pressure or effective stress. As more fractures and low aspect ratio pore structure are opened with increasing pore pressure, the attenuation of the S2 data increases. Mechanisms for the increased attenuation could be related to decreasing the effective stress and local fluid flow processes. The shear-wave polarization is seen to exhibit variation between the initial and repeat surveys consistent with changes in anisotropy due to reservoir processes. Introduction Improving reservoir performance and enhancing hydrocarbon recovery while reducing environmental impact are critical to the future of the petroleum industry. P. 427^