Overview of the CO2 Pilot in the Spraberry Trend Area

The overall goal of this project is to assess the economic feasibility of CO2 flooding the naturally fractured Spraberry Trend Area in West Texas. This objective is being accomplished by conducting research in four areas and implementation of a field demonstration pilot. Research areas are as follows: 1) extensive characterization of the reservoirs, 2) experimental studies of crude oil/brine/rock (COBR) interaction in the reservoirs, 3) analytical and numerical simulation of Spraberry reservoirs, and, 4) experimental investigations on CO2 gravity drainage in Spraberry whole cores. This report provides initial results of the project for each of the four areas and outlines the field demonstration pilot progress. In the first area, reservoir characterization has been established based on petro physical and geological analysis combined with core-log integration. A shaly sand rock model for describing the Spraberry Trend Area Reservoir has been established, and as a result, a better log interpretation algorithm for identifying Spraberry pay zones has been developed. In the second area, COBR interaction in the Spraberry matrix has been analyzed based on results of laboratory experiments. Initial water saturation and historical water saturation in the Spraberry sands has been determined to be between 0.20 and 0.40 depending on permeability of the sand. Macroscopic displacement efficiency during water imbibition has been estimated to be about 50%. Wettability of the Spraberry sands has been determined. The Amott wettability index to water was estimated to be about 0.55 indicating that the Spraberry sands are weakly water wet. Water-oil capillary pressure has been established. The experimental capillary pressure curve confirms the rock wettability determined based imbibition test. Interfacial tension (IFT) between Spraberry oil and brine has been measured to be 32 mN/m. Experimental results have been used in analytical and numerical reservoir simulations. In the third area, performance of the Spraberry reservoirs has been explored based on reservoir characterization and laboratory investigations. Scaling of imbibition oil recovery results to reservoir geometry indicates that higher oil recovery should have been achieved during water flooding, although the Spraberry sands are weakly water wet. Reasons for the poor performance of water flood were analyzed. Inflow performance of Spraberry Trend wells has been analyzed using a new mathematical model developed for wells intersecting long fractures. Computer simulation of a Spraberry water flood pilot has been conducted using laboratory measured parameters to understand Spraberry water flood performance. In the fourth area, efficiency of CO2 gravity drainage has been investigated based on laboratory experiments. Minimum Miscibility Pressure (MMP) was measured to be 1,550 psig. IFT of the CO2 /Spraberry oil under reservoir conditions was determined. The IFT at the MMP is about 1.5 mN/m. Investigation of vaporization of oil fractions into CO was initiated. Preliminary 2 results show insignificance of the mechanism. CO2 gravity drainage experiments were carried out using Spraberry oil and whole cores. 51% of original oil in place was recovered from a 0.01 md Spraberry whole core within 200 days during CO2 gravity drainage. Experimental data were matched by a mathematical model. The field demonstration pilot is underway to test the results of the laboratory and modeling applications. This pilot consists of 6 WIW's, 3 producers, and 4 GIW's and the associated production/injection facilities. The GIW's will be drilled during the end of 1999 and the CO2 injection is slated to begin during the 1st quarter of 2000. Extensive field testing is ongoing to further characterize the reservoir. These tests include pressure buildups and falloffs, step-rate injection tests, injection profile logs, and the interference test currently in progress.

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