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West Texas carbonate fields

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San Andres and Clearfork are two carbonate reservoir intervals that are present over a considerable area of the Permian Basin in west Texas. These reservoirs (e.g., Wasson, Slaughter, Seminole) contain several billion bbl of approximately 30°API oil. They are very-layered, heterogeneous carbonates and dolomites that have large variation in permeability from layer to layer. Interestingly, because of the complex hydrocarbon-accumulation history of this basin, much of this area has an underlying interval that contains residual oil saturation.

Most of these reservoirs were discovered in the late 1930s and the 1940s. Even where both the San Andres and the Clearfork were found to be oil-bearing, surface-lease and unitization considerations required the reservoirs to be developed by drilling separate sets of wells. These reservoirs had approximately two decades of primary production.


From the 1960s through the 1980s, almost all of these reservoirs underwent waterflooding. During this period, many SPE technical papers addressed aspects of these waterfloods. Of particular interest was the continuity, or discontinuity, of the pay intervals and the extent to which infill drilling from 40-acre well spacing to 20 acres, and possibly to 10 acres, could be economically justified by increased oil recovery.[1][2][3] Another issue was what pattern style was the best to use.[4] The recovery factors for the sum of primary production and waterflooding ranges from 25 to 60% of original oil in place (OOIP); the variation is caused mainly by geological factors in these different geographical areas and by differences between the San Andres and Clearfork reservoirs.[2]

Pressure coring and sponge coring are technologies that were developed specifically for these waterfloods to determine the residual oil saturation to waterflooding in these carbonate reservoirs.[5] These techniques were used to determine the variation in oil saturation from well-swept higher-permeability layers to poorly swept lower-permeability layers.

The west Texas carbonate waterfloods have proved quite successful and have recovered significant additional oil. Also, the San Andres reservoirs’ waterfloods have provided much of the data and related technical studies that were needed to justify implementation of enhanced oil recovery (EOR) by using CO2 injection. During the tertiary EOR projects, water injection in the water-alternating-gas (WAG) mode has continued to control the mobility of the low-viscosity CO2.


  1. Barber, A.H. Jr., George, C.J., Stiles, L.H. et al. 1983. Infill Drilling To Increase Reserves—Actual Experience in Nine Fields in Texas, Oklahoma, and Illinois. J Pet Technol 35 (8): 1530–1538. SPE-11023-PA.
  2. 2.0 2.1 Lu, G.F., Brimhall, R.M., and Wu, C.H. 1993. Geographical Distribution and Forecast Models of Infill Drilling Oil Recovery for Permian Basin Carbonate Reservoirs. Presented at the SPE Annual Technical Conference and Exhibition, Houston, 3–6 October. SPE-26503-MS.
  3. Maguson, W.L. and Knowles, J.D. 1977. Denver Unit 10-Acre Infill Pilot Test and Residual Oil Testing. Presented at the SPE Permian Basin Oil and Gas Recovery Conference, Midland, Texas, 10-11 March 1977. SPE-6385-MS.
  4. Christman, P.G. 1995. Modeling the Effects of Infill Drilling and Pattern Modification in Discontinuous Reservoirs. SPE Res Eng 10 (1): 4-9. SPE-27747-PA.
  5. Bilhartz Jr., H.L. and Charlson, G.S. 1978. Coring for In-Situ Saturations in the Willard Unit CO2 Flood Mini-Test. Presented at the SPE Symposium on Improved Methods of Oil Recovery, Tulsa, Oklahoma, 16-17 April 1978. SPE-7050-MS.

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