Pressure Decay Measurement

This paper was prepared for the 41st Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Dallas, Tex., Oct. 2–5, 1966. 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. Measurements of permeability in core samples of inhomogeneous rock are susceptible to errors resulting from inadequate sample size and defects inherent in the existing techniques. This paper presents an experimental method of radial permeability determination which permits rapid measurements on whole core samples with minimum bias from sampling and measuring procedures. The technique is analogous in many respects to permeability calculations from pressure buildup data. Pressure in the sample cell is increased abruptly and the time required for gas in the pore space to reach pressure equilibrium with the chamber pressure is measured by electronically monitoring the decaying cell pressure. This time is proportional to permeability. In addition to permeability data, the configuration of the pressure decay curve may yield insight into such useful core characteristics as pore size distribution. Introduction Geological formations are not normally homogeneous with regard to lithology and structure. Since, in the past, we have dealt largely with oil and gas production, most of which is derived from the least variable lithologies, we are not well equipped to deal with the more inhomogeneous formations now becoming important in gas storage, waste disposal, and lower grade oil and gas reservoirs. Part of our problem is the immense computational complexity which arises from the addition of more independent variables to our reservoir performance equations. This, however, is susceptible to solution by use of the modern high speed computer. The real "bottleneck" is in the acquisition of enough reliable data upon which to base our more sophisticated approach to reservoir studies. The subject of the present paper is not a major "breakthrough" but serves, we hope, as an example of the kind of review and refinement of techniques which is required from those of us who are concerned with data gathering to support the efforts of those of you who are involved in the prediction and control of reservoir performance.