The goal of this Central Vacuum Unit Project is to demonstrate the CO2 Huff-n-Puff process in a light oil, shallow shelf carbonate reservoir within the Permian Basin. The CO2 Huff-n-Puff process is a proven enhanced oil recovery (EOR) technology for Louisiana­ Texas gulf coast sandstone reservoirs. The reader is referred to three Society of Petroleum Engineers (SPE) papers, No. 15502, 16720 & 20208 for a review of the theory, mechanics and case histories of the process. The process has even been shown to be moderately effective in conjunction with steam on heavy California crude oils. Although the technology is proven in sandstones, it continues to be a very underutilized EOR option for carbonates.

The goal of this Central Vacuum Unit Project is to demonstrate the CO2 Huff-n-Puff process in a light oil, shallow shelf carbonate reservoir within the Permian Basin. The CO2 Huff-n-Puff process is a proven enhanced oil recovery (EOR) technology for Louisiana­ Texas gulf coast sandstone reservoirs. The reader is referred to three Society of Petroleum Engineers (SPE) papers, No. 15502, 16720 & 20208 for a review of the theory, mechanics and case histories of the process. The process has even been shown to be moderately effective in conjunction with steam on heavy California crude oils. Although the technology is proven in sandstones, it continues to be a very underutilized EOR option for carbonates.

The application of CO2 technologies in Permian Basin carbonates may do for the decade of the 1990's and beyond, what waterflood ing d id for this region beginning in the 1950's. With an infrastructure for CO2 deliveries already in place, a successful demonstration of the CO2 Huff-n-Puff process will have wide application. Profitability of marginal properties will be maintained until such time as pricing justifies a full­ scale CO2 miscible project. It could maximize recoveries from smaller isolated leases which could never economically support a miscible CO2 project.

The process, when applied during the installation of a full-scale CO2 miscible project could mitigate up-front negative cash-flows, possibly to the point of allowing a project to be self-funding and increase horizontal sweep efficiency at the same time. Since most full-scale CO2 miscible projects are focused on the 11 sweet spots 11 of a property, the CO2 Huff-n-Puff process could concurrently maximize recoveries from non-targeted acreage. An added incentive for the early application of the CO2 Huff-n-Puff process is that it could provide an early measure of CO2 injectivity of future full-scale CO2 miscible projects and improve real-time recovery estimates--reducing economic risk. By virtue of the very same blocking abilities that hamper miscible CO2 project water injection rates, water production may measurably be reduced--reducing lease operating expense. The CO2 Huff-n-Puff process could bridge the near-term needs of maintaining this large domestic resource base until the mid-term economic conditions support the widespread implementation of the more efficient full-scale miscible CO2 projects.

The goal of this technology demonstration is to gain an overall understanding of the reservoir qualities that influence CO2 Huff-n-Puff production responses within a heterogeneous reservoir such as the shallow shelf carbonate environment of the Central Vacuum Unit. A generalized reservoir model will be developed and used to determine the importance of various geological and operational influences upon the CO2 Huff-n-Puff process. Once the macro reservoir characterization is available, eight producing wells with varying reservoir parameters will be selected for the field demonstration project. One of these locations will be selected for detailed reservoir characterization. This detailed geologic model will be used for numerical compositional simulation to finalize the specific design parameters of the field demonstrations, and continued history matching and refinements to the project.

The reservoir characterization and numerical simulation will define the specific volumes of CO2 required and expected oil recoveries for each of the demonstration sites. The typical process cycle will involve the injection of an estimated 500 tons CO2 in a producing well. The CO2 will be injected in an immiscible cond ition, displacing the majority of the water within the wellbore vicinity, while bypassing the oil-in-place. The CO2 will be absorbed into both the oil and remaining water. The water will absorb CO2 quickly, but only a relatively limited quantity. Conversely, the oil can absorb a significant volume of CO2, although it is a much slower process. For this reason the prod ucing well will be shut-in for what is termed a soak period. This soak period normally lasts 1-4 weeks depending upon fluid and reservoir properties. The pressure in the near-wellbore vicinity will continue to increase to near minimum miscibility conditions during the soak. The no-flow pressure boundary of the waterflood pattern will serve to confine the CO2, reducing leak-off concerns. During the soak period the oil will experience significant swelling, viscosity and interfacial tensions will be reduced, and the relative mobility of the oil will increase as the pressure approaches miscible conditions. When the well is returned to production the mobilized oil will be swept to the wellbore by the waterflood. Incremental production is expected to return to its base level within 6-7 months. As shown in SPE papers No. 15497 and No. 20268 with actual field data, d iminishing returns are expected with each successive cycle, thus this proposal is to expose each of the producers to no more than three cycles of the CO2 Huff-n-Puff process over a two year period.

It is anticipated that this project will show that the application of the CO2 Huff-n-Puff process in shallow shelf carbonates can be economically implemented to recover appreciable volumes of light oil. The goals of the project are the development of guidelines for cost­ effective selection of cand idate reservoirs and wells, and estimating recovery potential.

By applying the CO2 Huff-n-Puff process in the near­ term, it is expected that the hydrocarbon resource base within the shallow shelf carbonate class of reservoirs can be extended to the mid-term timeframe where economic conditions would justify full-scale miscible CO2 development.

The Grayburg/San Andres formations produce a 38.0° API oil from an average depth of 4550' within the Central Vacuum Unit.Total oil column reaches as much as 600'. Porosity and permeability in the gross pay interval can reach a maximum of 23.7 %, and 530 md, respectively. The porosity and permeability over the gross pay interval averages 6.9 % and 10.0 md, respectively. The net pay interval averages will be determined during the reservoir characterization phase of the project. Although the residual oil saturation to waterflooding within the near wellbore vicinity has not yet been determined in detail, carbonate reservoirs typically leave behind a high residual oil saturation in the range of 30-35 % in the waterflood swept zones. Oil saturations in unswept zones within the heterogeneous reservoir will likely approach initial conditions. This is a significant volume of recoverable oil which is the target of this CO2 Huff-n-Puff process.

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