Enhanced Gas Recovery From Geopressured Aquifers
Authors: Geer Ernest C. , Cook Harold L. 
Published: December 2007

Along the Gulf Coast of the United States, geopressured aquifers occur which contain natural gas and water. Natural gas may exist in geopressured aquifers as a continuous gas phase, as gas dissolved in the aquifer water, or as a dispersed gas phase (residual gas).

The aquifers exist at pressures great enough to flow water to the surface of the ground in artesian fashion, and some commercial gas reservoirs are found in them. Where the characteristics of the formation containing the aquifer will permit the production of large quantities of water, it may be possible to produce with the water, gas of all the three types produce with the water, gas of all the three types described. In the case of a watered-out geopressured gas reservoir, solution gas is produced with the water. Also residual gas expansion, in response to reduction in the aquifer pressure, increases pore volume saturation and results in gas mobility. Gas cap or attic gas not recovered during conventional production may be coned into a well as pressure is reduced in the aquifer, if the current gas/water contact is sufficiently close to the well.

Reservoir simulation has shown that a significant fraction of the three types of gas contained in a geopressured aquifer can be recovered, and a production method to accomplish this has been designed and patented. Results of projected production schedules patented. Results of projected production schedules are presented to demonstrate the recovery possible using this technique.

Introduction

In water-drive, geopressured gas reservoirs, there is always gas left in-place at the end of conventional production. As geopressured zones are drilled more production. As geopressured zones are drilled more extensively, an increasingly large number of these gas reservoirs will be abandoned after conventional production has ceased due to water invasion. An production has ceased due to water invasion. An investigation has been made of techniques and production processes to recover a considerable portion of this processes to recover a considerable portion of this gas. If the physical conditions of the reservoir and the aquifer are suitable, significant quantities of gas can be recovered from the remaining gas in-place. Both technical and economic considerations must be evaluated for each reservoir that is a candidate for production. Total gas produced must be sufficient to recover all costs and give an adequate return on investment, but such economic analyses are outside the scope of this paper. paper.

DISTRIBUTION OF GAS and WATER IN GEOPRESSURED AQUIFERS

Natural gas is contained in geopressured aquifers in three forms:a continuous gas phase pore volume saturation in the presence of connate water,gas dissolved in the aquifer water, anda dispersed gas phase pore volume saturation, small bubbles surrounded by water.

These three types of gas saturation will be called free gas saturation, solution gas, and residual gas saturation.

Free Gas Saturation

Of first interest to producers of geopressured gas reserves is free gas saturation. It has yielded production in gas wells with minimal technical problems production in gas wells with minimal technical problems fort for years. The primary consideration is that a gas well produce gas essentially water-free. Where water lies beneath the gas accumulation, well perforations are placed high in the free gas portion of perforations are placed high in the free gas portion of such a reservoir and gas production proceeds, essentially water-free, until with expansion, the aquifer water invades the gas reservoir and eventually floods the entire perforated well interval. After the invasion of the gas reservoir by water, some gas is trapped e rock pores after the mobile gas has been produced at the well(s). In many instances, some free gas saturation remains in the reservoir above the new water level after water has flooded the well(s).