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Abstract
Time-lapse (4-D) seismic data is often able to detect changes related to the injection of CO2. However, to quantify these changes is problematic. Understanding the CO2 injection process is critical to relating the seismic response to changes within the reservoir. In most CO2 injection projects, a pure liquid CO2 is initially injected into an oil reservoir, combining with the oil, resulting in the oil swelling and becoming more mobile. Reservoir changes include variations in fluid saturation, compressibility, phase and density along with changes in reservoir temperature and pressure.

Time-lapse (4-D), multi-component (9-C) seismic data has been acquired, processed and interpreted in three oil fields that have undergone CO2 injection. Vacuum field has been monitored with four seismic surveys during an EOR project in the San Andres formation with CO2 being injected over a 200 meter zone. Weyburn field has been monitored with three seismic surveys during an EOR project in the fractured Midale member of the Mission Canyon Formation with an approximately 20 meter thick zone. West Pearl Queen field has been monitored with two seismic surveys during a CO2 sequestration test in a 12 meter thick Queen Formation.

Combining the geologic, geophysical and petroleum engineering data allows an integrated approach to interpreting the time-lapse anomalies associated with CO2 injection. Integrating the multi-component seismic data allows the separation of the fluid changes from the pressure changes, allowing a much more definitive interpretation of the anomalies associated with the CO2 injection process. Multi-component seismology is critical in monitoring CO2 movement in the Vacuum, Weyburn, and West Pearl Queen fields.

Vacuum Field
The Vacuum field (Figure 1) was discovered in 1929 with the drilling of the Socony Vacuum State 1 well in Section 13-T17S-R34E, Lea County, New Mexico. Vacuum field produces predominately from the San Andres Formation, in a shallow-shelf carbonate depositional setting. Structurally, it is positioned on the shelf edge of the Permian Basin’s Northwest Shelf. The structurally high shelf crest is located just west of the RCP study area. Porosity and permeability within the productive zones average 11.8% and 22.0 md, respectively. The San Andres gross pay zone can reach 200 meters in thickness. It is divided into two main pay zones: Upper and Lower San Andres. The Lovington Sandstone, a silty interval, segregates the two zones.

Multicomponent seismic data has revealed the presence of faults with 3 to 7 meter of vertical offset at the reservoir level. These faults cause partial sealing conditions to occur in the immediate vicinity. This observation has been substantiated by reservoir simulation. These faults can act to bank hydrocarbons. This occurs mainly in the Upper San Andres because the flow units are thin and small amounts of vertical throw are sufficient to juxtapose flow units against flow barriers. As a result, hydraulic fracturing has been very attractive when confined to the Upper San Andres zones. Successful horizontal drilling efforts further support the faulting.

S-wave amplitude analysis provides a high-resolution measure of S-wave splitting, which can be useful for resolving major flow units and deriving reservoir parameters in areas containing secondary porosity development. The secondary porosity and vuggy nature of the reservoir at Vacuum field allows for S-wave characterization of the higher permeability conduits. Large S1 amplitudes correlate to high vuggy porosity in the San Andres reservoir. P-wave seismic attributes were uncorrelated with well productivities, but high-resolution S-wave splitting parameters derived from S-wave analysis provided an excellent correlation to well productivity.