Natural Gas From Geopressured Aquifers?
Authors: Randolph Philip L. 
Published: December 2007

This paper examines potential for natural gas production from geopressured Gulf Coast reservoirs production from geopressured Gulf Coast reservoirs whose thermal and mechanical energy production would be marginal or sub-marginal in relation to requirements for electricity generation. A base case evaluation with reservoir characteristics similar to those previously published for Frio Formation reservoirs in South Texas revealed reservoir criteria for producing natural gas to be much less stringent than for electricity generation. Parametric studies of cost for producing natural gas as the value of individual reservoir parameters is varied reveal maximum sensitivity to those parameters most difficult to quantify. These are effective in-situ permeability, pay zone thickness, reservoir drive and permeability, pay zone thickness, reservoir drive and drainage area.

Potential significance of reservoir characteristics not reflected in the calculations are qualitatively discussed. These are the pressure dependence of compaction drive and permeability, the possibility of a gas drive due to gas trapped in pores as a result of relative permeability and the possibility of an increasing gas/water ratio due to expansion of the trapped gas resulting in finite permeability to natural gas.

Introduction

Several recent publications have suggested that an enormous natural gas resource base may exist in geopressured aquifers that parallel the Texas and Louisiana Gulf Coasts at depths below 800 feet. Early studies produced estimates for quantities of natural gas in-place as high as 26,000 trillion CF onshore and 49,000 trillion CF when offshore sandstone reservoirs were included. A substantial fraction of this natural gas was projected to be dissolved in water having a temperature in excess of 300 degrees F.

Studies funded by the Geothermal Division of ERDA have concentrated primarily upon potential for electricity generation using the hydraulic and thermal energy produced. Natural gas production was considered to be a by-product.

More recent publications have questioned whether the primary emphasis should be upon producing natural gas rather than electricity. producing natural gas rather than electricity. This report examines that issue in the context of the relationship between reservoir characteristics and wellhead cost for natural gas production. The range of quantitative values for input parameters was chosen by reviewing current literature and interviewing professionals now engaged either in relevant research professionals now engaged either in relevant research or in gas production from geopressured reservoirs.

Interviews revealed a striking contrast in opinion regarding potential for resource development between individuals experienced in natural gas exploration in geopressured areas and individuals projecting production by computer simulation. In projecting production by computer simulation. In general, experienced natural gas exploration and production personnel were very pessimistic regarding production personnel were very pessimistic regarding the potential for geopressured reservoir development for geothermal energy and somewhat less pessimistic regarding natural gas production by stripping from a geopressured brine. This contrast with optimistic computer projections motivated examination of whether the quantitative values of reservoir characteristics and other inputs to prior computer simulations were truly descriptive of nature.

COMPUTATION PROCEDURE

Computations to examine the cost significance of various parameters employed an extension of the reservoir engineering techniques published by W.J. Bernard for a bounded, circular aquifer.