A Study Of Simultaneous Production Of Water And Gas From Aquifers Containing Initial Immobile Free Gas
Authors: Martin John C. 
Published: December 2007

Results are presented of a study of the performance of single wells producing from high pressure aquifers containing initial immobile free gas. It is not known to what extent such aquifers exist; however, evidence of immobile free gas has been found in some deep high pressure Gulf Coast water sands. The operation envisioned involved mobilizing the initial free gas by pressure depletion of the aquifer and, in some cases, increasing the produced gas-water ratio by employing large bottom hole pressure drawdowns. In all cases large volumes of water are produced and must be disposed of either on the surface or by injection. It is hoped that sufficient free gas can be produced to make the operation economical.

Unfortunately, most of the gas production comes late in the pressure decline after large amounts of water have been produced. Under proper conditions calculations indicate that this effect can be partially offset by increased individual well production rates since the producing gas-water ratio was found to be a function of the producing bottom hole pressure under certain conditions. Results also indicate that single well variable rate tests are capable of yielding indications of the presence of free gas and of the difficulties involved in mobilizing the gas.

Introduction

Recent interest in geopressured-geothermal aquifers has led to a number of economic studies. Some of these studies have concluded that production of hot water and its dissolved gas would be economic provided suitable aquifers can be found and that provided suitable aquifers can be found and that the gas sold for $1.50 to $2.00 per MCF. Other studies are less optimistic and Reference 5 concludes that the price needed for economic operations range from $4 to $15 per MCF and possibly higher. Unfortunately, the economics appear rather marginal even for the favorable aquifer conditions assumed in some of the studies. There is also the question if such favorable aquifers exist.

If one assumes that water and gas production from geopressured-geothermal aquifers is not currently economic, one may ask, "Are there productive formations which are intermediate between gas reservoirs and geopressured-geothermal aquifers which are economical to produce?" Such formations would produce water and gas simultaneously. An example might be a sequence of water sands with a thin gas zone on top of some or all sands. Another possibility consists of geopressured-geothermal aquifers with initial immobile free gas. At this point it is not clear that such aquifers exist; however, some sands in geopressured formations have been interpreted from logs as containing free gas and have produced water on short term tests. If immobile free gas is found in geopressured-geothermal aquifers it would be similar to water sands with immobile oil saturation or similar to a flooded-out gas reservoir with trapped free gas.

The operation envisioned in this paper involves mobilizing the initial free gas by pressure depletion of the aquifer and in some cases large pressure drawdown in the wells. In all cases, large volumes of water are produced and must be disposed of either on the surface or by injection. It is hoped that sufficient free gas can be produced to make the operation economical. The heat in the water may be of some value for electrical power production or other uses; however, under current conditions it appears that the produced gas would be considerably more valuable than produced gas would be considerably more valuable than the heat in the water or the mechanical energy available resulting flout high wellhead pressures.

The effects of immobile free gas in water sands have been studied by others. These references present results of reservoir simulator calculations which indicate that immobile gas can be mobilized by pressure reduction to allow significant amounts of pressure reduction to allow significant amounts of gas to be recovered. Unfortunately, the necessary pressure reduction in large aquifers may require pressure reduction in large aquifers may require the drilling of many expensive wells and the production and disposal of large volumes of water. production and disposal of large volumes of water. Results presented herein of reservoir simulation calculations of variable rate tests indicate that such tests can give a clear indication of the presence of free gas provided only a small increase in presence of free gas provided only a small increase in gas saturation is needed to allow the gas to flow.