Application of Variable Rate Analysis Technique to Gas Wells
Authors: Gupta Krishna C. , Andsager Richard L. 
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.

The pressure behavior of a gas well during the last of a series of flow rates is described with the help of Earner's point-source solution of the radial diffasivity equation and the principle of superposition. The resulting expression is used to analyze 24- to 72-hour drawdown and buildup tests conducted along with conventional regulatory-type multi-point back-pressure tests. The practical aspects of variable rate testing and analysis technique are discussed with the aid of actual field examples. The analysis of variable rate data generated by a one-dimensional numerical reservoir model and the actual field data show that this method gives results comparable with those obtained by long duration pressure buildup tests. A detailed calculation for an example problem is given to illustrate the application of this technique.

The technique provides a method for calculating reservoir parameters such as permeability-thickness product, skin factor, and extrapolated pressure prior to connecting the gas well to a gathering system and with minimum costs. With the help of this information, decisions regarding stimulation treatments can be made. Deliverabilities can be better predicted and, once the well is connected, a specific testing program can be designed to keep up to date on well performance.

Introduction

For over a quarter of a century, the multi-point back-pressure method of testing, as developed by Rawlins and Schellhardt, has been used to test the capacities of natural gas wells prior to and after connection to a sales outlet. The deliverability information obtained from the tests is used, among other things, to design heating, separation, and gathering facilities for the well and to forecast gas availability.

The worth and impact of the multi-point back-pressure method of testing can be measured by the fact that even today, after some 30 years, it forms the basis of all natural gas well testing manuals. The primary reason for its continued use is the simplicity with which tests can be conducted and interpreted. The method does have some shortcomings, however, when related to present-day reservoir engineering data needs. It does not provide any quantitative information about the producing formation itself, nor does it accurately predict the well's stabilized ability to produce.