Thie paper was preperad for presentation et the SPHJOE Ninth Symposium on Improved Oil Recovery held in Tulsa, Oklahoma, U. S.A.,17-20 April 1S94. Thie paper wee se!ectad for presentation by an SPE Program Cemmiftss fellowing review of information conteinsd in en abstraof aubmiffed by the fruiiroii~). COfitefiiS of % pai%i, ss PfeaentSd, have not @n reviewed by theSociety of PetroIaum Engineers and are aubjecf to corraotion by the author(a). The materiel, cc presented, does not neceeaarily reflect any poaRion of the EMety of Petroleum Engineers, its offiire, or members. Papers preaanted at SPE maafhge are aubjaot to publication review by Edtorial Committees of the Society of Petroleum Engineers Parmieaion to cc+y ia restricted to an ebafrecf of not more than SCklwerds. Illustretkma may not be cepied, The ebatrecf ehoufd contain cenapicueus acknowledgment of where and by whom the paper k presented. Write Librarian, SPE, P.O. Sex SSSSSS, Richardson, TX 750S33SW, U.S.A., Telex 1SS245 SPEUT.

The purpose of this project was to economically design an optimum carbon dioxide (CQ) flood for a mature waterflood nearing its economic abandonment. The original project utilized advanced reservoir characterization and C 0 2 horizontal injection wells as the primary methods to redevelop the South Cowden Unit (SCU). The development plans; project implementation and reservoir management techniques were to be transferred to the public domain to assist in preventing premature abandonment of similar fields. The Unit was a mature waterflood with water cut exceeding 95%. Oil must be mobilized through the use of a miscible or near-miscible fluid to recover significant additional reserves. Also, because the unit was relatively small, it did not have the benefit of economies of scale inherent in normal larger scale projects. Thus, new and innovative methods were required to reduce investment and operating costs. Two primary methods used to accomplish improved economics were use of reservoir characterization to restrict the flood to the higher quality rock in the unit and use of horizontal injection wells to cut investment and operating costs. The project consisted of two budget phases. Budget Phase I started in June 1994 and ended late June 1996. In this phase Reservoir Analysis, Characterization Tasks and Advanced Technology Definition Tasks were completed. Completion enabled the project to be designed, evaluated, and an Authority for Expenditure (AFE) for project implementation submitted to working interest owners for approval. Budget Phase I1 consisted of the implementation and execution of the project in the field. Phase I1 was completed in July 2001. Performance monitoring, during Phase 11, by mid 1998 identified the majority of producing wells which under performed their anticipated withdrawal rates. Newly drilled and reactivated wells had lower offfake rates than originally forecasted. As a result of poor offfake, higher reservoir pressure was a concern for the project as it limited C02 injectivity. To reduce voidage balance, and reservoir pressure, a disposal well was therefore drilled. Several injection surveys indicated the C02 injection wells had severe conformance issues. After close monitoring of the project to the end of 1999, it was evident the project would not recover the anticipated tertiary reserves. The main reasons for under-performance were poor in zone CO2 injection into the upper San Andres layers, poorer offtake rates from newly drilled replacement wells and a higher than required reservoir pressure. After discussion internally within Phillips, externally with the Department of Energy (DOE) and SCU partners, a redevelopment of South Cowden was agreed upon to commence in year 2000. The redevelopment essentially abandoned the original development for Budget Phase I1 in favor of a revised approach. This involved conformance techniques to resolve out of zone CQ injection and use of horizontal wells to improve in zone injectivity and productivity. V Phase 2 activities commenced in October 2000 w i t h ...

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The work reported here covers Budget Phase I of the project. The principal tasks in Budget Phase I are the Reservoir Analysis and Characterization Task and the Advanced Technology Definition Task. Completion of these tasks have enabled an optimum carbon dioxide (CO?) flood project to be designed and evaluated from an economic and risk analysis standpoint. Field implementation of the project has been recommended to the working interest owners of the South Cowden Unit (SCU) and approval has been obtained. The current project has focused on reducing initial investment cost by utilizing horizontal injection wells and concentrating the project in the best productivity area of the field. An innovative CO, purchase agreement (no take or pay requirements, C02 purchase price tied to West Texas Intermediate (WTI) crude oil price) and gas recycle agreements (expensing cost as opposed to large capital investments for compression) were negotiated to further improve project economics. A detailed reservoir characterization study was completed by an integrated team of geoscientists and engineers. The study corisisted of detailed core description, integration of log response to core descriptions, mapping of the major flow units, evaluation of porosity and permeability relationships, geostatistical analysis of permeability trends, and direct integration of reservoir performance with the geological interpretation. The study methodology fostered iterative bidirectional feedback between the reservoir characterization team and the reservoir engineeringlsirnulation team to allow simultaneous refinement and convergence of the geological interpretation with the reservoir model. The fundamental conclusion from the study is that South Cowden exhibits favorable enhanced oil recovery characteristics, particularly reservoir quality and continuity. Detailed core des.criptions were made of two full cores and several partial cores fiom the South Cowden Unit. Core information fiom the contiguous Emmons and Moss Units were also incorporated into the study. The core study concluded that reservoir quality in the South Cowden Unit is controlled primarily by the distribution of a bioturbated and diagenetically altered rock type with a distinctive "chaotic" texture. The "chaotic" modifier derives from the visual effect of pervasive, small-scale intermixing of tan oil-stained reservoir rock with tight gray non-reservoir rock. The Grayburg-San Andres section is divided into multiple zones based on the core study and gamma ray markers that correlate wells across the unit. The type log for South Cowden Unit Well No. 8-19 is shown in Figure 8. Each zone is mapped as continuous across the field. The "chaotic" reservoir rock extends from Zone C (4780'-4800') to the lower part of Zone F (4640'-4680'). Zones D (4755'-4780') and E (4680'-4755') are considered the main floodable zones, though Zone F is also productive and Zone C is productive above the oilwater contact.. I 1 Repeat Formation Tester (RTF) measurements indicate good vertical pressure communication ...

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