The Isochronal Testing of Oil Wells

Abstract

Abstract This paper presents the results and methods of analyzing isochronal and flow after flow multipoint back-pressure tests conducted on oil wells. Tests were conducted in reservoirs with permeabilities ranging from 6 MD to >1000 MD. Reservoirs in which oil well multipoint back-pressure tests were obtained ranged from highly undersaturated, to saturated at initial reservoir pressure, to a partially depleted field with a gas saturation existing above the critical. Each of these three reservoir fluid states can result in different interpretation methods. Back-pressure tests were run to pseudo-steady state in the field where the saturation was above the critical gas saturation. In all cases, oil well back-pressure tests curves were found to follow the same general form as that used to express the rate-pressure relationship of a gas well:Equation From some 40 oil well back-pressure tests examined, the exponent n was found to lie between 0.568 and 1.000, very near the limits commonly accepted for gas well back-pressure curves. Flow point alignment to establish an oil well back-pressure curve on the customary log qo vs. log Î"(p2) plot is considered to be as good as that obtained on gas well back-pressure tests. This paper demonstrates that gas wells and oil wells behave very similarly and should be tested and analyzed using the same basic flow equations. Introduction Multipoint back-pressure testing of gas wells is an accepted procedure for establishing a gas well's performance curve. Flow after flow1 and isochronal2 testing are the two basic methods commonly used. In high permeability reservoirs, either method can be employed. In low permeability reservoirs, the Isochronal method of testing eliminates the transient effects that can severely distort the results obtained from a flow after flow test. Methods for analyzing and calculating gas well performance curves have been the subject of numerous investigations. The bulk of these investigations have examined non-Darcy flow behavior, the primary reason that multipoint tests are conducted.