Long Term Deliverability Forecasts for Dual Porosity Gas Reservoirs

Abstract

This paper was prepared for presentation at the 47th Annual Fall Meeting of the Society of Petroleum Engineers held in San Antonio, Tex., Oct. 8–11, 1972. 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 who the paper is presented. Publication elsewhere after publication in the JOURNAL 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. 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 It is well recognized that the field deliverability of dual porosity reservoirs does not follow the projections made using standard analytical or two dimensional modelling techniques. Although the literature reports a number of methods to analyze the pressure buildups for such reservoirs, these methods do not provide a means of computing long term deliverability. This paper discusses the use of three dimensional modelling techniques to determine reserves and to project the deliverability of dual porosity gas reservoirs. The two field applications presented include:a naturally fractured reservoir, anda reservoir where severe stratification makes it behave like two or more independent reservoirs communicating through shale-breaks or "windows" between them. The pressure buildups recorded in these fields are unusual. Wells with core permeability of 20 to 40 md continue to buildup for more than fifteen months and exhibit an extraordinary rise near the end of the shut-in period. A history match using the above model resulted in the determination of the pore volume of each porosity system and the "flow capacity" between these systems. The model, for the prediction of deliverability, simultaneously solves the equations describing flux flow through the reservoir, the wellbores, the tubings and the surface system. Results show that the effect on deliverability of changes in the surface gathering system, number of wells or the operational strategy will be seriously miscalculated if the dual porosity effect is not considered. Introduction The ability to accurately forecast gas field deliverability under various operating conditions is a necessity for the optimum exploitation of any gas reservoir.