Predicting Gas Reserves in Abnormally Pressured Reservoirs

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. This paper was prepared for the 46th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, held in New Orleans, Oct. 3–6, 1971. 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 presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is 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 This paper is a study of abnormally high-pressured depletion-type gas reservoirs. In these reservoirs two distinct slopes are evident when a plot of shut-in bottom-hole pressures vs cumulative production (P/Z plot) pressures vs cumulative production (P/Z plot) is used to predict reserves. The final slope of the P/Z plot is steeper than the initial slope; consequently, reserve estimates based on the early life portion of the curve are erroneously high. The initial slope is due to gas expansion and significant pressure maintenance brought about by formation compaction, crystal expansion and water expansion. The early performance of the reservoir, as it appears on a P/Z plot, is due to the effective compressibility of the system. Effective compressibility varies from gas compressibility (theoretical P/Z performance) by the effects of the above mentioned pressure maintenance. At approximately normal pressure gradient the formation compaction is essentially complete and the reservoir assumes the characteristics of a normal gas expansion reservoir. At this point effective compressibility is about equal to gas compressibility, and this accounts for the second slope. Many costly decisions are based on the early life extrapolation of the P/Z plot. Therefore, it is necessary that we plot. Therefore, it is necessary that we understand the effects of hydrocarbon pore volume change on reserve estimates, productivity and abandonment pressure. Two methods have been derived for correcting reserve estimates from the early life data using formation compressibility, gas production and shut-in bottom-hole pressures. The first method uses the ratio of the average effective compressibility to the average gas compressibility, while the second method includes compressibility in the expansion term of the material balance equation. The developed corrections are accurate in theory, but such required information as formation compressibility is based on limited empirical data. This must be kept in mind when using the family of curves provided in this paper. An effort has been made to help establish abandonment pressure and to reconcile other related compaction problems. Introduction This paper is a study of abnormally high-pressured depletion-type gas reservoirs. In these reservoirs two distinct slopes are evident when a plot of shut-in bottom-hole pressures vs cumulative production (P/Z plot) pressures vs cumulative production (P/Z plot) is used to predict reserves. The final slope of the P/Z plot is steeper than the initial slope; consequently, reserve estimates based on the early life portion of the curve are erroneously high. The initial slope is due to gas expansion and significant pressure maintenance brought about by formation compaction, crystal expansion and water expansion.