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Subsidence of geologic surface structures due to withdrawal of fluids from aquifers and petroleum reservoirs is a phenomenon experienced throughout the world as the demand for water and hydrocarbons increases with increasing population growth. This book addresses the definition and theories of subsidence, and the influences of unique conditions on subsidence; it includes discussions of specific field cases and a basic mathematical model of reservoir compaction and accompanying loss of porosity and permeability. The book is designed as a reference for readers giving immediate access to the geological events that establish conditions for compaction, the mathematical theories of compaction and subsidence, and practical considerations of field case histories in various regions of the world.
Land subsidence is the loss of surface elevation as a result of the removal of subsurface support. The mechanisms by which this can occur may be natural in origin or induced by human activities. Common causes of land subsidence include the removal of oil, gas, and water from underground reservoirs; dissolution of limestone aquifers (sinkholes); underground mining activities; drainage of organic soils; and hydrocompaction (the initial wetting of dry soils). Overdraft of aquifers is the major cause of a really extensive land subsidence, and as ground-water pumping increases, land subsidence also will increase. The U.S. Geological Survey (USGS) has a long-standing history of describing, mapping, and conducting process-oriented research in land subsidence. In 1955, the Geological Survey formed the Mechanics of Aquifers Project under the direction of Joseph F. Poland to study the processes that result in land subsidence due to the withdrawal of ground water. From 1955 to 1984, this research team gained international renown as they advanced the understanding of aquifer mechanics and land-subsidence theory. In addition to conducting pioneering research, this group also provided a focal point within the USGS for the dissemination of technology and scientific understanding in aquifer mechanics.
This study estimates the magnitude of geothermal energy from fifteen major known US sedimentary basins and ranks these basins relative to their potential. Because most sedimentary basins have been explored for oil and gas, well logs, temperatures at depth, and reservoir properties are known. This reduces exploration risk and allows development of geologic exploration models for each basin as well as a relative assessment of geologic risk elements for each play. The total available thermal resource for each basin was estimated using the volumetric heat-in-place method originally proposed by Muffler (USGS). Total sedimentary thickness maps, stratigraphic columns, cross sections, and temperature gradient Information were gathered for each basin from published articles, USGS reports, and state geological survey reports. When published data was insufficient, thermal gradients and reservoir properties were derived from oil and gas well logs obtained on oil and gas commission websites. Basin stratigraphy, structural history, and groundwater circulation patterns were studied in order to develop a model that estimates resource size and temperature distribution, and to qualitatively assess reservoir productivity.
Single-phase and two-phase geothermal reservoirs are currently being exploited for power production in Italy, Mexico, New Zealand, the U.S. and elsewhere. Vertical ground displacements of upto 4.5 m and horizontal ground displacements of up t o 0.5 m have been observed at Wairakei, New Zealand that are clearly attributable to the resource exploitation. Similarly, vertical displacements of about 0.13 m have been recorded at The Geysers, California. No significant ground displacements that are attributable to large-scale fluid production have been observed at Larderello, Italy and Cerro Prieto, Mexico. Observations show that subsidence due to geothermal fluid production is characterized by such features as an offset of the subsidence bowl from the main area of production, time-lag between production and subsidence and nonlinear stress-strain relationships. Several plausible conceptual models, of varying degrees of sophistication, have been proposed to explain the observed features. At present, relatively more is known about the physical mechanisms that govern subsidence than the relevant therma mechanisms. Although attempts have been made to simulate observed geothermal subsidence, the modeling efforts have been seriously limited by a lack of relevant field data needed to sufficiently characterize the complex field system.