The Impact of Low Throw Faults on Fluid Flow Along a Layercake Reservoir

This paper presents the results of a numerical modelling study on the impact of low throw faults on flow along reservoir units. We investigate specifically the case of a permeable reservoir unit of uniform thickness sandwiched between two impermeable seal units, that is cut by a fault with throw less than the thickness of the reservoir. Situations like this are often observed in outcrop at a range of scales, and are also seen on seismic data from producing fields, although often such faults will be below the level of seismic resolution. We use a series of 2D single phase finite difference models to calculate transmissibility along a 100m long section of reservoir offset by a vertical fault. We vary the fault throw, kv/kh ratio and reservoir permeability profile, and also run models incorporating low permeability fault rocks. We use the results to derive quantitative empirical equations for fault transmissibility multipliers and flow resistance as a function of these variables. The results show that the area reduction index, used in many reservoir simulators, will overestimate the impact of these faults on horizontal transmissibility, except where low permeability fault rocks are present, when it will underestimate the impact of the faults.