Representation of Fault Sealing in a Reservoir Simulation: Cormorant Block IV, UK North Sea

Abstract

Abstract The representation of structural complexities in numerical reservoir simulations is cumbersome and often over-simplified. The introduction of leak paths across faults is particularly paths across faults is particularly subjective, because patterns of fault sealing are generally not described by the geologist. A procedure is presented here which models fault sealing capacity, and allows potential leak paths to be located, ranked potential leak paths to be located, ranked and incorporated into the simulation model. The procedure involves the semi-quantitative modelling of fault sealing assuming clay smear and shale juxtaposition to be the primary sealing mechanisms. The capacity for sealing along faults cutting representative reservoir sequences is described on clay smear ‘ type ’. Information from the type panels is used to map out variations in fault sealing capacity along individual fault planes, allowing potential leak paths to be located and ranked. paths to be located and ranked. The reservoir simulation employs a coarse, curvilinear grid, designed to honour the structural configuration of the reservoir but facilitate fast processing times. Ranked leak paths are introduced into the simulation model during history matching to constrain cross-flow between reservoir sub compartments. This leads to a matched simulation model, and also provides a calibration of the fault seal model against production data. The procedure of combining the reservoir simulation and fault seal models illustrates the benefits of an integrated production geology/reservoir engineering production geology/reservoir engineering approach to reservoir modelling. Introduction The Cormorant Field lies in the prolific Middle to Upper Jurassic Brent province of the East Shetland Basin, in the U.K. sector of the Northern North Sea (Figure 1). The Field comprises four discrete accumulations, Blocks I to IV, the largest of which is Block IV. Since 1982, 98 MM stb out of an estimated ultimate recovery of 244 MM stb from Block IV have been produced. produced. Production from Block IV has been hampered Production from Block IV has been hampered by structural complexities in the reservoir which were not known prior to development. After discovery and subsequent appraisal, the Block was viewed as a simple, westerly-dipping fault block. Development of the Block was planned using rows of crestal oil producers supported by down-flank water injection (Figure 2). Subsequent development drilling and a new 3D seismic survey showed the Block to be substantially more complex, with widespread in-field faulting (Figure 3). P. 119