Hydro-geomechanical modelling of seal behaviour in overpressured basins using discontinuous deformation analysis M. Rouainia a, * , H. Lewis c , C. Pearce b , N. Bicanic b , G.D. Couples c , M.A. Reynolds c a School of Civil Engineering and Geosciences, University of Newcastle, Newcastle NE1 7RU, UK b Department of Civil Engineering, University of Glasgow, Glasgow G12 8T, UK c Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UK Received 1 December 2004; received in revised form 2 November 2005; accepted 4 November 2005 Abstract A coupled hydro-geomechanical modelling environment, developed to evaluate the coupled responses of fluid flow in deforming discontinuous media, is described. A staggered computational framework is presented, where the two simulations tools, HYDRO and DDA, communicate via the mapping of an equivalent porosity (and related permeabilities) from the rock system to the fluid phase and an inverse mapping of the pressure field. Several algorithmic and modelling issues are discussed, in particular the computational procedure to map the current geometry of the discontinuous rock blocks assembly into an equivalent porosity (and permeability) field. A generic, geometrically simple, overpressured reservoir/seal system is analysed for illustration. Further examples investigate discontinuous, fractured configurations in flexure causing a degree of spatial variability in the induced stresses. Model predictions show that the combination of hydraulic and mechanical loads causes a dilational opening of some pre- existing fractures and closure of others, with strong localisation of the modified flow pattern along wider fracture openings. D 2005 Elsevier B.V. All rights reserved. Keywords: Discontinuous deformation analysis; Porous media; Hydro-geomechanical modelling; Overpressured basins; Seal failure 1. Introduction The existence of overpressure in hydrocarbon reser- voirs is a major reservoir management concern from both a safety and an economic perspective. Hydrocarbons are generated in the deep organic-rich mudrocks and tend to migrate to porous and permeable reservoir rocks. In locations where low permeability sealing rocks overlie the reservoir rocks a hydrocarbon trap is created (see Fig. 1). In most cases the sealing layer actually acts as a leaky seal with some flux of hydrocarbons out of the reservoir (Couples, 1999). This sealing layer is often characterised by some matrix permeability and small-aperture frac- tures and, when overpressures are created in the reser- voir, has a higher fluid pressure below the seal than above it: this is the scenario investigated here. Such a leaky seal system operates at considerable depths and pressures, with small, but complex, block interactions, and is quite impractical for reproduction in a testing laboratory. This paper describes a numerical method of investigating the behaviour of such systems, illustrating the use of this approach in some very simple reservoir- seal scenarios. On the computational front, continuum approaches have been used successfully for many subsurface geo- 0013-7952/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2005.11.004 Engineering Geology 82 (2006) 222 – 233 www.elsevier.com/locate/enggeo * Corresponding author. Tel.: +44 191 222 3608; fax: +44 191 222 5322. E-mail address: m.rouainia@ncl.ac.uk (M. Rouainia).