Abstract

When the Underground Injection Control Regulations were promulgated in 1980, existing Class II injection wells (saltwater disposal and secondary recovery injection wells) operating at the time were excluded from Area of Review (AOR) requirements. The U.S. EPA has expressed its intent to revise the regulations to include AOR requirements for such wells, but it is expected that oil- and gas-producing states will be allowed to adopt a variance strategy for these wells.

An AOR variance methodology has been developed by the authors under sponsorship of the American Petroleum Institute. The general concept of the variance methodology is a systematic evaluation of basic variance criteria that were agreed to by a Federal Advisory Committee. These criteria include absence of an underground source of drinking water (USDW), lack of positive flow potential from the petroleum reservoir to the overlying USDW, mitigating geological factors, and other compelling evidence.

To demonstrate lack of positive flow potential from the petroleum reservoir to the overlying USDW, the procedure that has been developed requires that reservoir pressures be converted to freshwater hydraulic head equivalents using the base of the USDW as the datum. These heads are then compared against USDW heads, and residuals are generated either graphically (using map overlays) or by subtracting grids on a computer and contouring. A negative residual implies that the hydraulic gradient between the oil reservoir and the USDW is such that there is no potential for flow of saline water from the oil reservoir to the USDW. Examples from the San Juan basin of New Mexico indicate that the procedure is simple in concept but complicated in practice.