SELF INDUCED CRUDE OIL CONTAMINATION OF A PUBLIC WATER WELL IN CENTRAL TEXAS

Many water wells have been drilled into the Edwards Aquifer in Central Texas along the Balcones fault system with some later abandoned because of natural petroleum contamination or high sulfur content. One well, drilled in 1928 to a depth of 700 ft, has a steel well casing consisting of 78 ft of 15.5-inch pipe followed by a 12-inch pipe that extends to 475 ft. The intake for a 450-gpm pump was set at 248 ft. The static water level is about 100 ft and normal production drawdown at 450-gpm was about 81 ft. In May 2001, a 700-gpm pump was installed with the intake set at 345 ft. When pumped at 700-gpm the drawdown was 233 ft. In August 2001, the well produced oil, which was pumped into the water distribution system, contaminating the water supply. A video survey, conducted on August 13, 2001, revealed an obstruction in the well at a depth of 477 ft, about 2 ft below the steel casing, that appears to be a pipe clamp. The size of this obstruction prevented any further inspection of the well. The formation exposed below the steel casing is a vuggy and highly porous limestone that was producing oil. Within a year over 100 ft of free product had accumulated in the well casing. Geochemical analysis of the oil determined that it is in the Austin Chalk family of oils, which had collected in vugs at the top of the Edwards aquifer. I believe that the increased water production and drawdown resulted in significantly increased flow velocities in the upper portion of the formation which effectively "hydroflushed" the oil reservoir. The presence of the obstruction limited the effective aquifer thickness to about 2 to 3 ft. Because the obstruction is larger than the well casing, it will be necessary to push it to the bottom of the well if the well is to be rehabilitated and put back in service.