The behavior of abnormally high-pressured gas reservoirs in sand formations with randomly interbedded, thin, noncontinuous shale layers is examined using numerical simulation. The relevance of the internal water drive resulting from shale compaction and the influence of mobile water generation on reservoir and well behavior are discussed.

Introduction

An increasing number of hydrocarbon reservoirs discovered at great depths have pressures that are significantly higher than the hydrostatic pressure. Because of the high temperatures found at great depths, a large percentage of these superpressured reservoirs are gas or percentage of these superpressured reservoirs are gas or gas-condensate types.

Several authors discussed the conditions that help create superpressured reservoirs. Such reservoirs often are found in thick sedimentary basins characterized by high-velocity sedimentation. Tightly interbedded shale and sand layers (under impermeable layers) are typical of superpressured reservoirs. The compressibility of abnormally high-pressured formations is higher than normal resulting from their understressed condition.

When the abnormally high-pressured formation is a hydrocarbon reservoir, the high rock compressibility and even higher compressibility of the shale layers, either interbedded or boundary, may contribute appreciably to reservoir energy.

Purpose Purpose It long has been recognized that reservoir rock compaction as well as interstitial water expansion, both resulting from pressure depletion, can contribute appreciably to the energy supporting production in abnormally high-pressured gas reservoirs.

Several authors pointed out that in reservoirs embedded between thick shale layers, a sensible contribution to the production mechanism may originate by water expelled from the confining shale layers. Because these shale layers are very thick, the time required for the decreasing reservoir pressure to diffuse throughout the shale body may be more than the productive life of the reservoir.

When reservoir rock contains very thin, noncontinuous shale layers, randomly distributed within the sand body, the time delay for water release from interbedded shale compaction can be disregarded. Wallace presented a list of actual gas reservoirs where water released presented a list of actual gas reservoirs where water released from the compaction of interbedded shale layers did contribute relevantly to reservoir drive, thus creating what might be termed an "internal water drive."

In this study, production behavior of abnormally high-pressured gas reservoirs with thin shale interbeddings was examined. The study aims

  1. to evaluate the energy contribution to reservoir drive resulting from water released from interbedded shale compaction; and

  2. to gain information about the adverse influence that diffused generation of mobile water (resulting from shale compaction) may have on well performance, considering that water production in gas wells poses disposal and often difficult lifting problems .

JPT

P. 937

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