AUTHORS Xing Zhang  Vector International Process- ing Systems (VIPS) Ltd., 10 The Courtyard, Eastern Road, Bracknell, Berkshire RG12 2XB, United Kingdom; xing.zhang.uk@gmail.com Xing Zhang received his first Ph.D. (mining en- gineering) from the University of Science and Technology, Beijing, China, in 1988 and his second Ph.D. (geosciences) from Imperial Col- lege, London, in 2002. Before joining Vector International Processing Systems as a senior technical consultant, he was an associate pro- fessor at the University of Science and Technol- ogy, 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 numer- ical analysis and modeling of fractured and faulted subsurface systems, in particular coupled stress and flow modeling for reservoir man- agement, such as prediction of top surface sub- sidence 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  Vector Internation- al Processing Systems (VIPS) Ltd., 10 The Courtyard, Eastern Road, Bracknell, Berkshire RG12 2XB, United Kingdom Nick Koutsabeloulis received his B.Sc. degree in civil engineering at the University of Thessalo- niki, Greece, in 1981 and a Ph.D. at the Uni- versity 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 world- wide oil companies. His current main research interest is the role of geomechanics in reser- voir 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 Hydromechanical modeling of critically stressed and faulted reservoirs Xing Zhang, Nick Koutsabeloulis, and Kes Heffer ABSTRACT A critical stress state around a faulted reservoir prior to production and injection is an important factor in the hydromechanical re- sponses 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 model- ing of a faulted reservoir subjected to a critical stress state prior to production operations. The modeling implies that the perme- ability distribution in a reservoir that is in a critical stress state is time dependent. A finite-element model with fully coupled geome- chanics 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, in- duced deformation evolution, and permeability changes. With a per- meability 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 lo- cally. In this case, the deformation is mainly elastic, and no perme- ability enhancements occur. Therefore, the impact of inelastic geo- mechanical 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 GEOHORIZONS AAPG Bulletin, v. 91, no. 1 (January 2007), pp. 31–50 31 Copyright #2007. The American Association of Petroleum Geologists. All rights reserved. Manuscript received August 22, 2005; provisional acceptance November 11, 2005; revised manuscript received July 17, 2006; final acceptance August 3, 2006. DOI:10.1306/08030605136