Time-Lapse Seismic Monitoring and Dynamic Reservoir Characterization, Central Vacuum Unit, Lea County, New Mexico
Authors: Benson R.D. , Davis T.L. 
Published: December 2007

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 travel time 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. These anomalies arise in P-wave amplitude difference maps and variations in S-wave velocity anisotropy. An interpretation of these 4-D, 3-C seismic anomalies indicates that a CO2 miscible bank formed south of well CVU-97 near well CVU-200, and that the bank is contained by a permeability barrier near well CVU-200. P- wave and Si 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 ability to sense bulk rock/fluid properties with 4-D, 3-C seismology enables characterization of the most important transport property of a reservoir, namely permeability. Because of the high volume resolution of the 4-D, 3-C seismic, we can monitor the sweep efficiency of a production process to see if reserves are bypassed by channeling around lower permeability parts of the reservoir and the rate at which the channeling occurs. In doing so, we can change production processes to sweep the reservoir more efficiently.
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