Water Production from Abnormally Pressured Gas Reservoirs in South Louisiana
Authors: Wallace W.E. 
Published: December 2007

Water Production from Abnormally Pressured Gas Reservoirs in Pressured Gas Reservoirs in South Louisiana

In these cases of abnormally pressured gas reservoirs that produce water or that give evidence of water influx, there is reason to think the water is being extracted from shale as a result of accelerated compaction.

Introduction

In this paper we are limiting our investigation to so-called "volumetric" reservoirs; those having sandstone porosity surrounded by shale, containing gas, condensate, and salt water, all under conditions that, at least in the beginning, exceeded a pressure-depth ratio of 0.465 psi/ft (normal hydrostatic pressure). All examples are taken from fields in South Louisiana and are of ages ranging from Oligocene (Frio) to Miocene. We have included reservoirs that produced gas, condensate, and varying amounts of water. Some produced 100 percent water, some less or none at all, produced 100 percent water, some less or none at all, and in some we only suspect that there is water influx.

Pressure-depth ratios greater than 0.465 psi/ft require special explanations. In most of the cases studied, the ratios have been 0.8 plus. Examples with much smaller ratios are included to illustrate certain points. We are particularly concerned with the gas points. We are particularly concerned with the gas well that watered out but still had substantial surface pressure. The very low topographic relief in Louisiana pressure. The very low topographic relief in Louisiana precludes any appreciable pressure head that could be precludes any appreciable pressure head that could be derived from the elevation of the surface outcrop of a sand. These greater pressure ratios that do not have obvious explanations require special investigation.

Early in this study we noted that in several reservoirs there was a fairly definite drop in pressure before salt water appeared with the gas. This pressure drop was often as great as 1,000 psi. This report is an expansion of that investigation in which we attempted to make a series of simple predictions and ended with a fist of complicated qualifications. The appearance of water or its failure to appear in wells follows a definite pattern of conditions that we will discuss in the following pages.

Craft and Hawkins define water influx in the following words. "The water which encroaches into a reservoir upon a decline in pressure may be due to one or a combination of the following:

artesian flow, where the water-bearing strata outcrops at the surface, (2) expansion of water in the aquifer, expansion of known or unknown accumulations of oil and/or gas in the aquifer, and compaction of the aquifer-rock."

The concept of the water coming from the surrounding shale was apparently not anticipated in Item 4. In 1962 we published a paper 2 in which we were primarily concerned with unusual reservoir space primarily concerned with unusual reservoir space arrangements of gas and water, and the expansion of dissolved methane. At that time we downgraded the effect of shale compaction as a source of any appreciable amount of water. Today we have reversed our stand and feel that shale compaction is the cause of most of this water with the mysterious origin.

Other references to shale as a source of limited water drive have been rare. Atwater and Miller questioned the possibility that water from shale had created a "partial[ly] effective water drive" in a gas field near New Orleans.

JPT

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