Abstract

The completion of a number of recent infill wells, and restimulations of a number of older wells has resulted in a significant increase in both oil and water production in the Grayburg and San Andres in the Foster - South Cowden Field. Collection and analyses of formation flood waters has succeeded in enhancing the understanding of the results of these completions. A database of 530 water analyses, dating from 1962 (pre-waterflood) to present, were used to develop baselines for the flood and formation waters and determine the source of the produced water. Recent isolated completions provided sufficient data to determine the chemistry of "virgin" lower Grayburg and San Andres waters. The composition of upper and lower Grayburg, San Andres, and flood waters differ significantly in the concentration of calcium, magnesium, sodium, sulfate, bicarbonate, and chloride. The upper Grayburg water originally contained 27,000 PPM total dissolved solids (TDS), the lower Grayburg 37,500 PPM TDS, and the San Andres 62,000 PPM TDS, indicating that there was no vertical communication between the reservoirs despite a lack of significant permeability barriers.

Some of the problems that can be addressed using water analyses included:

• Is the produced water from the San Andres, or upper Grayburg, or lower Grayburg?

• Did a frac stay in zone?

• Is the produced water a mix of Grayburg and floodwater?

• Was a rapid change in production accompanied by a change in composition of produced water?

• Was scale the potential cause of a production drop?

Water has been inefficiently injected into the Grayburg in this portion of the Foster - South Cowden Field since 1961, but only now is an effort being made to systematically flood the upper Grayburg. The decision to abandoned the San Andres, and limit development of the lower Grayburg was based in part on water chemistry analyses. Produced water analyses have indicated significant changes in the concentration of the common anions and cations from pre- to post-frac analyses, indicating that the fracs were successful in contacting previously undrained, but flooded, reservoir. The success of `pipeline' fracs in staying in-zone in the lower Grayburg was confirmed by water analyses. Scaling was identified as the cause of a rapid production decline in one well, and eliminated as the cause in another.

Analysis of produced water has been demonstrated to provide critical information about the source of water and downhole conditions in a mature reservoir. This information, available from no other source, has been successful in reducing uncertainty in the decision making process.

We would like to acknowledge James J. Reeves and Hoxie W. Smith for conceiving and managing the DOE study and for being responsible for the geophysical study. Since April 1996, William C. Robinson has been responsible for reprocessing and reinterpreting the seismic data and for the geophysical study. Also since that date, Robert C. Trentham has been responsible for project management.