Case Histories of a Practical Method for Determining Deliverabilities of West Texas Gas Wells
Authors: Mays J.T. 
Published: December 2007

American Institute of Mining, Metallurgical and Petroleum Engineers, Inc.
This paper was prepared for the 42nd Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Houston, Tex., Oct. 1–4, 1967. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
Case histories are presented which illustrate a practical procedure for determining stabilized back-pressure deliverability curves for West Texas gas wells. The method presented by Tek, Grove and Poettmann was used to calculate the performance coefficient C, while the slope n of the back-pressure curve was determined by isochronal methods. The calculated deliverabilities compare reliably with actual deliverabilities, based upon field data, for gas wells operating under various conditions and with different formation characteristics. The required data may be obtained routinely in reasonable time periods even for wells of relatively low permeability. The use of gas well format sheets and routine field measurements of rate and pressure permits both the rapid evaluation of gas well reserves and deliverability and the prediction of future performance on an individual well basis.

Introduction
Gas well deliverability is normally described by back-pressure curves relating flow rate and pressure drawdown performance. The general form of the equation expressing this relationship is:
(1)
where C is the performance coefficient and the exponent n is the reciprocal of the normal slope of the straight line when Q and [Pf2 - Ps2] are plotted on log-log paper. The performance coefficient C depends mainly upon the characteristics of the reservoir [permeability, thickness, drainage radius, etc.] and of the flowing fluid, while the exponent n depends primarily upon the type of flow and the manner in which the flow test is conducted. The procedure for determining the back-pressure performance of a gas well is described in Monograph 7 and the Manual prepared by the Railroad Commission of Texas. This type of test is often referred to as four-point tests.
To obtain stabilized back-pressure curves from the method described in Monograph 7, the rate and pressure data must be measured under stabilized flow conditions when the coefficient C is constant with time. The drainage radius is also constant and equal to either the reservoir boundary or the point of interference between offset producing wells when the flow is stabilized.