Abstract: A Methodology to Evaluate Regional Hydraulic Controls on Flow from Hydrocarbon Reservoirs into Overlying Aquifers

Alan E. Fryar, C. W. Kreitler, M. C. Akhter, A. J. Avakian, W. F. Mullican, III

Because drilling, completion, and abandonment practices for oil and gas wells have improved over the past century, some older abandoned wells may be mechanically deficient or inadequately plugged, thus posing a risk of contamination to underground sources of drinking water. The risk of saltwater contamination of freshwater aquifers through inadequately plugged, abandoned wells increases if the hydraulic potential of the oil-and-gas-bearing brine formations is higher than that in the overlying freshwater aquifers.

A screening method to prioritize this risk involves three steps using, primarily, computerized databases and computer mapping software. First, average regional potentiometric surfaces of aquifers and reservoirs are generated from aquifer water-level measurements and the conversion of bottom-hole pressure measurements from oil and gas reservoirs to hydraulic heads. Next, differences in hydraulic heads between aquifers and reservoirs are calculated to delineate regional residual areas of upward (positive) or downward (negative) hydraulic gradients. Third, locations of abandoned wells and class II injection wells are plotted relative to residuals to examine where waterflooding, pressure maintenance, and saltwater disposal may cause or exacerbate the potential for upward flow.

Three areas were used as case studies for testing the method. Positive residuals in the South Texas basin (informally defined to include the Val Verde basin, Maverick basin, part of the Rio Grande Salt basin, and the Austin Chalk trend) result from natural geopressuring in formations deeper than 6000 ft, which are negligibly affected by class II injection wells. Positive residuals in the greater Permian basin (including the northwestern shelf, Delaware basin, part of the Palo Duro basin, Central Basin platform, Midland basin, southern shelf, and Fort Worth basin) may reflect injection for enhanced recovery in the west and natural hydrologic processes in the eastern shelf region. Residual surfaces for the San Juan basin indicate several areas with a natural potential for upward migrati n of brine. Even though the methodology is limited by the scarcity of computerized databases of bottom-hole pressure measurements and other data from formations with water salinities greater than 3000 mg/L, it does provide a regional screen that identifies areas with a greater potential for upward brine migration.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994