Heterogeneity in the Pennsylvanian SACROC Unit, Scurry County, Texas

Michael A. Raines
Kinder Morgan CO2 Co., L.P., 500 North Loraine, Suite 1175, Midland, Texas 79701
[email protected]

SACROC Unit covers the majority of the Kelly-Snyder field and a portion of the Dia-mond “M” field of Scurry County, Texas. It is located in the eastern half of the Midland Basin (the eastern sub-basin of the overall Permian Basin of western Texas and southeastern New Mexico). This 2.8 Billion Stock Tank Barrel (Original Oil in Place) Unit produces from the Pennsylvanian-aged Cisco and Canyon Formations on the Horseshoe Atoll. As is the case with many such Pennsylvanian reef complexes, the SACROC Unit exhibits a great deal of vertical relief and laterally complicated geometries. 

This highly heterogeneous reservoir is currently the focus of tertiary (CO₂) recovery efforts. Due to the sensitive nature of mechanical, physical, and economic constraints on tertiary recovery operations, it is important to have a good understanding of the reservoir and fluid migration within it. Therefore, even as tertiary flooding efforts continue in one portion of the field, a renewed focus on gaining an understanding of internal complexities continues in the most complicated portion of the field. In order to develop that understanding, Kinder Morgan CO₂ Co., L.P. (KMCO₂) recently acquired a 4-D (Time Lapse) cross-well seismic data set in the active project area using Core Labs / Tomoseis (now Z-Seis) equipment and processing. KMCO₂ is participating with the Bureau of Economic Geology at the University of Texas at Austin and other regional operators to gain a better understanding of internal architecture as seen through wireline log-correlations. Down-hole video has been run in one open-hole completion, and Injector-to-Producer pressure tests have displayed some very interesting (fast) results. 

The root causes of heterogeneity at SACROC are tectonics, icehouse conditions, and post-burial geochemical processes. Tectonics control growth initiation points and stress con-ditions. Sea level fluctuations force architecture by setting carbonate growth in a “keep-up” mode. This develops overly steep sides and associated erosional potential (truncation and gravity deposits), as well as sudden lateral facies variations during initial deposition. Geo-chemical processes further modify the reservoir through karst overprints, dissolution, and precip-itation. Production operations and treatment techniques further modify reservoir response, primarily through reservoir fracturing and localized pressure modification.

The complexity at SACROC Unit is seen on a variety of scales, is the result of a variety of natural and man-assisted processes, and manifests itself in a variety of ways during field operations. Therefore, a variety of techniques and tools must be focused on the problem in order to gain a reasonable expectation of fluid behavior in the reservoir. Reservoir development plans can not be optimized until an adequate understanding of heterogeneity is available.