Effects of Permeability Variation and Production Rate on Recovery from Partial Water Drive Gas Reservoirs
Authors: Cronquist Chapman 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the 48th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Las Vegas, Nev., Sept. 30-Oct. 3, 1973. 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.
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Abstract
A two dimensional reservoir simulator has been used to investigate the effect of areal permeability variation and production rate on gas recovery from partial water drive gas production rate on gas recovery from partial water drive gas reservoirs. The prototype reservoir is typical of those found on the Gulf Coast., i.e. it is low dip, highly porous and permeable and exhibits wide areal variation in permeability. permeable and exhibits wide areal variation in permeability. For the uniform well pattern and areal permeability variation studied recovery efficiency was essentially independent of the degree of permeability variation and the dimensionless gas production rate. Gas recovery from individual wells at equivalent production rate. Gas recovery from individual wells at equivalent structural positions, however, varied widely. It was concluded that areal permeability variation and production rate can influence the uniformity of aquifer production rate can influence the uniformity of aquifer encroachment in partial water drive gas reservoirs. Under conditions of irregular well pattern and the wide areal variation in permeability typically encountered in Gulf Coast reservoirs, it is suggested that not only well but also reservoir recovery will be influenced by degree of permeability variation and production rate.

Introduction
In recent years increasing attention has been paid to the behavior of gas reservoirs under varying degrees of water influx. Previous authors have noted that in this type of reservoir Previous authors have noted that in this type of reservoir a plot of p/Z versus Gp may exhibit a variety of shapes, ranging from curves decreasing monotonically to those having a downward inflection. The actual shape has been reported to be a function of production rate and aquifer properties. Reserve determination in production rate and aquifer properties. Reserve determination in this type of reservoir by extrapolation of an apparent straight line relationship between p/Z and Gp can lead to large error. The term "partial water drive", not previously defined, is used here to describe reservoir behavior in which the rate of aquifer influx is insufficient to stabilize reservoir pressure at prevailing production rates. Under these conditions, reservoir prevailing production rates. Under these conditions, reservoir pressure will continue to decrease over life. Abandonment of pressure will continue to decrease over life. Abandonment of wells, however, will be caused by watering out due to aquifer encroachment, rather than by low capacity due to declining pressure.