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Hydromechanical modeling of critically stressed and faulted reservoirs

¹ Vector International Processing Systems (VIPS) Ltd., 10 The Courtyard, Eastern Road, Bracknell, Berkshire RG12 2XB, United Kingdom; xing.zhang.uk{at}gmail.com
² Vector International Processing Systems (VIPS) Ltd., 10 The Courtyard, Eastern Road, Bracknell, Berkshire RG12 2XB, United Kingdom
³ Heriot Watt Institute of Petroleum Engineering, Riccavton Campus, Edinburgh, Scotland, EH14 4AS, United Kingdom

Xing Zhang received his first Ph.D. (mining engineering) from the University of Science and Technology, Beijing, China, in 1988 and his second Ph.D. (geosciences) from Imperial College, London, in 2002. Before joining Vector International Processing Systems as a senior technical consultant, he was an associate professor at the University of Science and Technology, Beijing, worked as a research fellow at Southampton University, and later became a senior research fellow at Imperial College, London. His research interests are mainly in geomechanics. His current research is numerical analysis and modeling of fractured and faulted subsurface systems, in particular coupled stress and flow modeling for reservoir management, such as prediction of top surface subsidence and reservoir compaction, estimate of fault reactivation, restoration of oil-field initial stress state, assessment of wellbore stability, and optimization of wellbore completion.

Nick Koutsabeloulis received his B.Sc. degree in civil engineering at the University of Thessaloniki, Greece, in 1981 and a Ph.D. at the University of Manchester, England, in 1985. He is the managing director and founder of Vector International Processing Systems Limited, which provides software and services in reservoir management through geomechanics for worldwide oil companies. His current main research interest is the role of geomechanics in reservoir management, particularly prediction of top surface subsidence and reservoir compaction, estimate of fault reactivation and assessment of wellbore stability. He has conducted research and consultancy work in various aspects in reservoir management, including establishing initial reservoir in-situ stress prior to production, such as in salt structure regions, predicting reservoir deformation caused by depletion and injection, evaluating fracturing caused by hydraulic and thermal impact, and assessing fault reactivation during production.

Kes Heffer received his M.A. degree in natural sciences from the University of Cambridge in 1970 and his M.Sc. degree in petroleum reservoir engineering from Imperial College, London, in 1971. He worked for BP for 29 years, initially in worldwide operations as a petroleum and reservoir engineer and later in research into issues of reservoir description. Since 1999, he has been an Honorary Fellow of the Institute of Petroleum Engineering at Heriot Watt University, Edinburgh, and has conducted research and consultancy work. His current main research interest is the role of geomechanics in the processes of fluid flow in reservoirs and the degree to which these approach a critical point.

A critical stress state around a faulted reservoir prior to production and injection is an important factor in the hydromechanical responses during production. The purpose of this article is to show how the long-range correlations of production rates observed in several oil fields can be reproduced with hydromechanical modeling of a faulted reservoir subjected to a critical stress state prior to production operations. The modeling implies that the permeability distribution in a reservoir that is in a critical stress state is time dependent. A finite-element model with fully coupled geomechanics and flow was used. The modeling has been applied to an approximation of the complex structure of the Gullfaks reservoir in the North Sea, including the far-field stress regimes and fault systems, although the model is considerably simplified in the search for generic, instead of field-specific, principles.

Under a critical stress state, a small change of the effective stress caused by fluid-pressure changes in the reservoir is likely to trigger reservoirwide hydromechanical reactions, irrespective of whether the change was at a local scale or a reservoir scale. Such responses include fault reactivations, volumetric and shear strain changes, induced deformation evolution, and permeability changes. With a permeability enhancement model, permeability increase is expected if fault reactivation and shear strain change occur. In contrast, if the in-situ stress is not at a critical state, the reservoir reacts locally. In this case, the deformation is mainly elastic, and no permeability enhancements occur. Therefore, the impact of inelastic geomechanical interactions (particularly shear deformation) at a critical point is likely to be very influential on reservoir fluid flow. This critical-point behavior gives explanation to the widespread field observations of long-range correlations in well rates, which are inferred to be manifestations of reservoir-scale mechanical responses involving faults, instead of the local hydraulic links that Darcy flow between wells implies. Permeability changes occur during inelastic deformation despite injection pressures being much lower than the confining stress (the minimum total principal stress). The increase in permeability in the reservoir rocks is caused by the dilation normal to the surface of the faults and/or fractures, which is caused by the shearing along the faults and/or fractures, instead of hydrofracturing. This confirms that dilational shearing can develop despite the effective stress regime being compressive. Dilational shearing has a major impact on the deformation of reservoir rock during production and is an important mechanism for generating conductivity on fractures under a fluid pressure that is lower than the confining stress, possibly even in reservoirs under depletion.

This article has been cited by other articles:

				X. Zhang, N. C. Koutsabeloulis, K. J. Heffer, I. G. Main, and L. Li Coupled geomechanics flow modelling at and below a critical stress state used to investigate common statistical properties of field production data Geological Society, London, Special Publications, January 1, 2007; 292(1): 453 - 468. [Abstract] [Full Text] [PDF]