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Pronounced changes of upward natural gas migration as precursors of major seismic events
Abstract
Pronounced changes in the rate of natural gas migration to the surface of the Earth in the vicinity of faults and fractured zones can be used as precursors of major seismic events. The authors propose to use them in conjunction with other integral parameters characterizing the energetic state of a tectonic system for the development of a multicomponent earthquake prediction technique. Thus, it is necessary to develop and implement a stationary natural gas mobility monitoring system covering the vicinity of major faults and fractured zones at the area of interest.
This chapter contains sections titled: Introduction Geochemical Exploration for Petroleum Primary and Secondary Migration of Hydrocarbons Origin of Migrating Hydrocarbon Gases Driving Force of Gas Movement Types of Gas Migration Paths of Gas Migration Associated with Oilwells Wells Leaking Due to Cementing Failure Environmental Hazards of Gas Migration Migration of Gas from Petroleum Wellbores Case Histories of Gas Migration Problems Conclusions
Subsidence due to fluid(s) withdrawal from oil/gas reservoirs can give rise to faults and fractures in the overlying strata above the reservoir, through which gas and fluids can migrate from the reservoir to the surface. Prevention/mitigation of this hazard is also discussed.
Environmental hazards associated with oil and gas production mainly include (1) subsidence due to fluid withdrawal and (2) gas migration to the surface. The former aspect is discussed in this article. Subsidence and differential subsidence can crack concrete highways, curbs and sidewalks, weaken foundations and structures of buildings, and also generate faults and fractures through which gas and/or fluids can migrate to the surface.
Much has been written in the petroleum geology literature on the geochemical evolution of pore liquids and gases associated with fluid flow systems in recent and ancient sedimentary basins. The dialogues include observations about the origin of interstitial fluids, measurements of the active chemical diagenetic processes, and resulting mass-transport properties, which arise during the development of sedimentary basins. Effects of thermal and chemical factors and the dynamic transfer of fluids within the basins leave imprints on the pore–fluid chemistry and generation of abnormally high- (AHFP) or abnormally low-formation pressures (ALFP). This chapter presents and validates a hypothetical model that explains the differences between the salinities of pore water in sandstones and shales in the gravitationally compacted sedimentary basins of Tertiary age. The explanation presented here is based on two diverse, relative scales of resolution—microscopic (10–2 to 10–4 m) and gigascopic (> 105 m). The gigascopic scale presents evidence from field observations, whereas the microscopic scale focuses on laboratory experiments that dealt with the chemistry of fluids in the pore space. Mathematical and conceptual models are presented and discussed, which support these observations. Additionally, the relevance of the isotopic character of shale pore water is evaluated for this environment.
Urban encroachment into areas historically reserved for oil and gas field operations is an ever-present problem within the Los Angeles Basin. The recent frenzy in real estate development has only intensified what can be characterized as a conflict in land usage. Subsurface mineral rights are severed from surface ownership, often resulting in developments being approved without adequate consideration of the underlying oil and gas field consequences. Also, surface operations are frequently co-located within residential areas without consideration of the health and safety consequences of emissions of toxics to air. This paper presents a review of the environmental, health and safety hazards posed by urban oilfield operations, with an emphasis upon the lessons learned from the L.A. Basin: Original Urban Oilfield Legend (see Castle and Yerkes 1976; Denton and others 2001; Endres and others 2002; Kouznetsov and others 1994; Katz and others 1994; Schumacher and Abrams 1994; and Schoell 1983). The Los Angeles Basin has provided the authors with one of the largest natural laboratories in the world for studying the consequences of these issues. The results presented are part of a long-term research program based upon the application of geoscience and petroleum engineering principles in obtaining a fundamental understanding of the root causes of the environmental hazards posed. Topics addressed include: (1) vertical migration of gas to the surface along faults and improperly completed or abandoned wellbores (e.g., due to poor cementing practices), (2) subsidence caused by the fluid production and declining reservoir pressures, (3) soil and groundwater contamination resulting from historic oil and gas field operations, and (4) air toxics resulting from surface operations. A number of case histories are discussed that illustrate the seriousness of the problem. A clear case is made for the urgent need for closer coordination and education by the petroleum industry of the local government planning departments. These departments have the principal role in determining land use policies, acting as the lead agency in performing environmental site assessments (e.g., under the California Environmental Quality Act), and in establishing mitigation measures for dealing with the long-term environmental hazards. This paper establishes prudent practices on the part of oilfield operators for the monitoring and mitigation of these hazards.
Aseismic ground failure is associated with regional land subsidence caused by ground-water withdrawal in at least 14 areas in 6 States in the United States. Two types of ground failure—tensile failures (causing earth fissures) and shear failures (causing surface faults)— are recognized. Fissures forming straight to arcuate patterns are caused by stretching related to bending of strata above a localized differentially compacting zone. Fissures forming complex polygonal patterns probably are caused by tension induced by capillary stresses in the zone above a declining water table. Surface faults occur along preexisting faults, many of which behave as partial ground-water barriers. Man-induced differential water-level declines across the preexisting faults have been sufficient to account for the heights of the historical offsets by a differential compaction mechanism. In the area most affected by surface faulting, Houston-Galveston, Texas, significant differential water-level declines across faults have not been observed and the specific mechanism of faulting has not been
If only the depth and thickness of compressible beds can be estimated, a simple hand calculation is available as a predictive technique and for many purposes is adequate. An example of such a technique uses Schatz, Kasameyer, and Cheney's depth-porosity model. Their depth-porosity model for reservoir compaction is modified in the present paper and is combined with a modified form of Geertsma's nucleus of strain model to form a single predictive technique. The nucleus of strain model accounts for attenuation of vertical movement through the overburden. Based on this combined model, prediction of ultimate vertical subsidence are made at eleven specified locations. At five of these sites in California, Texas, and New Zealand, these predictions are compared to measured subsidence. Agreement between predicted and observed subsidence is good. If field measurements of water-level fluctuations and the resulting time-dependent compression and expansion of geologic strata are locally available, use of a more refined predictive technique is justified. Tolman and Poland's conceptual model and Riley's parameter evaluation method have been combined with Helm's one-dimensional finite difference computational scheme to form a powerful time-dependent predictive technique. This technique has been field verified successfully at more than two dozen sites in California and Texas. Use of such a computer code allows prediction of observed time lags in nonlinear compression and expansion of layered sedimentary materials in response to arbitrary sequences of rising and falling water levels. 79 references, 22 figures, 4 tables.
In seismically active regions, three groups of time-dependent phenomena influence each other and their interrelationship may be used to formulate a methodology for the prediction of variations in their characteristics. Upward gas mobility depends on the distribution and geometry of vertically and subvertically oriented fractures, faults and their permeability, which depends on their width, formation and rock pressure, and history of seismictectonic processes in the region of study. On the other hand, probability of earthquakes is also linked to the rock pressure and conditions at the faults' surfaces. In turn, earthquake occurrence may lead to changes in formation pressure and to fluctuation of conditions along surfaces of fractures and faults. Subsudence causes formation of vertically and subvertically oriented fractures in the geologic section above oil and gas fields. Formation of these fractures, in turn, may cause an increase in upward gas mobility. Joint monitoring of seismic activity, surface manifestations of upward gas mobility, and subsidence of the upper parts of the geologic section may lead to new methods for the control of possible upsurge of gas leakage. Measurements of subsidence and surface gas leakage may lead to new criteria for indication of increased earthquake possibility. Currently, a combined neural net based prediction of upward gas mobility, ground subsidence, and seismic activity in seismically active regions is under development by the authors.
Subsidence over producing oil and gas fields, caused by a reduction of formation fluid pressure and consequent compaction of reservoir rocks, enhances existing fractures and creates new ones. A theoretical prediction of the whole set of associated deformation can not be completely reliable because of physical obscurity and incompleteness of the effective-stress theory and associated models and their inadequacy to describe the three-dimensional processes in a geological environment. Empirically, incompleteness of the overburden weight transmission to the compacting reservoir is obvious from the existence of vertical tensile strain and elongation of formation overlying the reservoir. All this emphasizes the importance of empirical approach. The authors reviewed field observations of subsidence deformations made by many authors.
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Some characteristics of radon concentration in the subsoil layer in seismically active areas
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