The Outlook for Geopressured/Geothermal Energy and Associated Natural Gas
Authors: Dorfman Myron H. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Introduction

Worldwide energy shortages over the past decade have focusedattention on alternative past decade have focused attention on alternativeenergy sources that might reduce dependence on imported petroleum. Geothermalenergy in the form of dry steam or hot water has become a significantalternative energy source in many countries for electric power generation andvarious process heat applications. power generation and various process heatapplications. Intensive exploration for geothermal reservoirs is under way bymajor energy producer worldwide. Since the early 1970's, an unconventional formof geothermal energy-geopressured/geothermal energy-has been underinvestigation by the U.S. DOE and its predecessor agencies, in cooperation withindustry and various research units and universities. This effort has beenconcentrated in Tertiary deposits of the northern Gulf of Mexico basin alone, the Texas and Louisiana coastal region. This basin, one of several geopressuredbasins within the U.S., has given us the most information from the manythousands of wells drilled for hydrocarbons over the past 80 years, othergeopressured basins of similar type are found throughout the world. It has beenestimated that more than 60 such basins are known and others may exist. Thus, these investigations may have applicability not only in the U.S. gulf coastarea, but also in similar geologic environments worldwide.

Geology and Fluid Characteristics of the Gulf Coast Basin

In the northernGulf Coast basin. sedimentary deposits exhibit a maximum thickness of 50,000 ft(15 240 m), with the upper 25,000 ft (7620 m) composed primarily of alternatingseries of rock layers broadly classified as sandstones and shales. The lowerlayers consist almost entirely of shales, believed to be the origin of methanein the geopressured formations. The geopressure phenomenon in this regionresulted from rapid sediment loading from riverborne systems and their deltas, ending as a "leapfrogged" sediment distribution that has extended the coastlinefrom about 100 miles (160 km) inland to its present location, The penetrationof sands into underlying muds resulted in isolation of large sand members fromthe overlying strata. Above the isolated intervals, pressures throughout thebasin are approximately 0.465 psi/ft (10.5 kPa/m), which is considered normalhydrostatic pressure based on the fluid pressure of a column of saline water.However, beneath these normally pressured zones, the isolated units of sandsand muds contain pressures far greater than normal, and approach lithostaticpressure of 1.0 psi/ft (22.6 kPa/m). This is the result of compaction psi/ft(22.6 kPa/m). This is the result of compaction of sediments. As the overburdenon locked-in sands increases, the fluid within the sands must support a portionof the overburden, resulting in dramatic portion of the overburden, resultingin dramatic increases in pressure. In addition, temperatures at the top ofgeopressure [approximately 200 to 225F (93 to 107C)] cause conversion ofmontmorillonite shales to illite or chlorite, resulting in expulsion of anadditional 15% of the water contained in the shales into overlying and adjacentgeopressured sands. The additional free water forced into the sands causesadditional overpressure and decreases salinity within the zones withfine-grained mud particles acting as semipermeable membranes, permittingselective water escape and concentrating dissolved components in the remainingpore fluids.

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