The Isochronal Testing of Oil Wells

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.

Analysis and Evaluation of Perforating and Perforation Cleanup Methods

“…The results were found to be acceptable. The Fetkovich option of the program used these production and pressure data to calculate the best least-squares curve fit of the data on a log Q vs. log !l.p2 plot and the values of C and n for the backpressure equation 6 where Q = oil flow rate, STB/D, or gas flow rate, 10 3 scflD, Pwj = wellbore flowing pressure, psi, and P r = average reservoir pressure, psi.…”

Nodal Systems Analysis of Oil and Gas Wells

“…The following procedures can be used: (1) Fetkovich 13 procedure, (2) IPR Methods For Gas Wells. Generally, a three-or four-flow-rate test is required for a gas well from which a plot is made on log-log paper and the appropriate equation derived.…”

Inflow Performance Relationships for Solution-Gas-Drive Reservoirs

“…Field tests presented in Ref. 3 indicate that the exponent n, should be in the range 0.5 ~ n ~ 1. Based on our knowledge of gas well behavior, Fetkovich attributes exponent values less than unity to non Darcy flow, although he does acknowledge that the exponent, n, can be less than unity strictly as a result of variation in fluid properties.…”