Unconventional modelling of faulted reservoirs: a case study

An example is presented of detailed unconventional gridding of the North Rankin Field, which is a large, structurally complex gas-condensate field offshore Western Australia. A non-Cartesian areal grid was used with corner point geometry to approximate a generalized curvilinear coordinate system for the surface and interior of each reservoir unit. Coordinate lines in the vertical plane at any node in the grid were tilted where necessary to define sloping edges and sides of grid blocks. Thus, any sloping twisted surface could be modelled. To investigate possible communication across faults between different geological units, transmissibilities at faults were automatically calculated for any over-lapping cells and sensitivities made of the effect of varying these transmissibilities on well production, recovery factors, pressure decline and water encroachment. The model was solved with a fully implicit simulator using a Newton-Raphson iteration method for the non-linear equations and a variant of the Conjugate Gradient procedure with a preconditioning matrix for the linear equations.