Reservoir Characterization Correctly Identifies Pay Intervals and Improves Production from Grayburg and San Andres Formations in Portions of the Foster/South Cowden Fields, Ector County, Texas

Robert C. Trentham, Emily L. Stoudt
University of Texas of the Permian Basin, Odessa, TX

The Foster and South Cowden fields were discovered and initially developed in the late 1930’s. Production was established from a 550’ thick interval of Grayburg and San Andres carbonates. For 55 years, it was “common knowledge” (accepted without question) that both the upper Grayburg and upper San Andres were major contributors to both primary and waterflood production. A DOE supported study by Laguna Petroleum on a portion of that field initially assumed that the high porosity upper San Andres was responsible for 40% of the historic production, the lower Grayburg 5% and the upper Grayburg 55% of the production.

As part of the study in Laguna’s portion of the field, the entire Grayburg and San Andres interval was cored for the first time and isolated production testing of the San Andres was undertaken. Full modern log suites were run in the newly drilled and cored wells, 480’ of new core was recovered and analyzed, a geologic reservoir characterization generated, and a 3D seismic survey with inversion modeling interpretation of the porosity distribution completed, In addition, a program of bottom hole pressure tests, production tests and water chemistry analyses of all wells was initiated. Results indicated that the San Andres reservoir had low permeabilities, was not experiencing waterflood support, had low bottom-hole pressures, and suffered from large decline rates. Thus the San Andres formation had actually contributed less that 15% of the historic production from the field. The revised history match and reservoir simulation confirmed this conclusion.

Bottom-hole pressure tests indicated that the upper Grayburg was well supported by the existing waterflood, but that a large number of wellbores were not in communication with the flood. Water analyses indicated that the upper Grayburg, lower Grayburg and San Andres all had different connate water chemistries and were not in vertical communication. Additionally, the water chemistries of the upper Grayburg indicated variable communication with historic or present day floodwaters.

A program of setting bridge plugs to isolate the upper Grayburg flood, and stimulations of wells with high pressures and low produced volumes was undertaken. The highly successful program resulted in large increases in the volume of fluids produced. Water chemistry analyses indicated that the waters produced after the stimulations were different from those previously produced, and that flood-supported intervals were contacted. Production at the inception of the project was 5,500 BOPM and was expected to decline to ~2,500 BOPB by 2003. The recommended well work resulted in production at the beginning of 2002 reaching ~8,000 BOPM.

The Laguna study area is shelfward of the Glorieta platform margin. Other studies in the area had described Grayburg and San Andres reservoirs basinward of that margin. From core and log correlation, the San Andres – Grayburg boundary in this study area is placed +/- 400’ below the Queen - Grayburg boundary.