An Analysis Of Production From Geopressured Geothermal Aquifers
Authors: Knapp R.M. , Isokrari O.F. , Garg S.K. , Pritchett J.W. 
Published: December 2007

The Gulf Coast region of the United States is underlain by deeply buried sandstone reservoirs containing low salinity water at abnormal pressures and elevated temperatures. In addition, the water is believed to contain significant amounts of dissolved natural gas. This geothermal resource, if present in sufficient recoverable quantities, has great energy producing potential and is located in one of the nation's major energy consuming areas. Dorfman and Kehle estimate the energy contained within the geopressured aquifers of Texas may be as much as 20,000 MW centuries excluding the natural gas.

A comprehensive theoretical formulation is presented for the important thermomechanical processes presented for the important thermomechanical processes operative in a geopressured geothermal reservoir. The formulation includes the effects of four major drive mechanisms (pore fluid compressibility, reservoir rock compaction, the evolution of dissolved natural gas and the influx of water from adjacent shale formations) expected to be operative during the productive life of the reservoir. Finite difference productive life of the reservoir. Finite difference techniques were used to solve the governing equations describing mass conservation, momentum and energy transport for two flowing phases in a multidimensional heterogeneous reservoir. Constitutive equations were used to describe the changes of fluid properties and reservoir parameters with changes in reservoir pressure and temperature. A series of test calculations pressure and temperature. A series of test calculations were performed to assess the sensitivity of reservoir performance to the gas in solution, sediment compaction performance to the gas in solution, sediment compaction and the reinjection of waste fluids. Based on these calculations, it is concluded that sediment compaction and water from interbedded shales can be significant depletion drive mechanisms in geopressured aquifers. The natural gas drive will probably not exceed the water expansion unless there is a significant initial gas saturation. Costs permitting, reinjection of produced fluids into geopressured geothermal aquifers will be desirable to both increase the recovery of thermal energy and natural gas.

Introduction

Geothermal energy is presently a small but viable contributor to the United States' energy supply. In addition to known areas of geothermal reserves in the western United States, a unique form of potential geothermal energy exists at moderate to great depths in geopressured aquifers underlying the United States Gulf Coast. Water from such geopressured aquifers often contains natural gas in solution. The geothermal water may be converted into electrical energy after lowering the pressure to extract the natural gas content. Moreover, the fact that temperatures in geothermal reservoirs change very little with time makes geothermal geopressured reservoirs a potentially attractive source of geothermal energy, as there may exist sufficient pressure in these reservoirs to deliver substantial amounts of fluid.

The northern shoreline of the Gulf of Mexico extends more than 1,000 miles, from the Rio Grande River to the Florida panhandle. Underlying a large portion of this shoreline area, both onshore and offshore, in a strip 200 to 300 miles wide axe clastic sedimentary deposits of great thickness.

The penetration of sands into underlying muds as a result of continuing deposition or faulting resulted in isolation of large sand members from continuous permeability channels to the overlying strata. Above the intervals thus isolated, pressure throughout the basin approximate 0.465 psi/ft. This is considered normal hydrostatic pressure based on the fluid pressures exerted by a column of saline water. However, beneath the normally pressured zones the isolated units of shales and sands contain pressures far greater than normal. These abnormally pressures far greater than normal. These abnormally pressured zones are now commonly referred to as pressured zones are now commonly referred to as geopressured zones. Dickey and Dorfman and Kehle state that the generation of geopressure is primarily the result of compaction phenomena. Newly deposited sediments have high porosities and axe saturated with water.