Due to the resolution limitation of seismic techniques, faults shorter than the distance between seismic lines, or faults with very small throws axe often not detected. The impact of such small (stochastic) faults on reservoir transmissibility can be extremely detrimental, especially in intensively faulted areas. Such faults are unlikely to be detected by well tests. In the presence of stochastic shales – which smear on the stochastic fault planes – the reservoir permeabilities can be considerably reduced as a result of the vertical barriers to horizontal flow created by the smeared fault planes. In this paper, a technique is presented for modelling such stochastic faults and for calculation of their effect on reservoir permeabilities (transmissibility). Continuity distribution functions (CDF) for stochastic fault dimensions are derived from published literature, and triangular distribution functions are used in modelling the faults.
The permeability computations are done in three stages: (i) calculation of the effect of stochastic shales on permeabilities (to obtain effective permeabilities for the reservoir blocks), (ii) generation of stochastic faults and specifying the faults encountering the block under consideration, (iii) and evaluation of the effect of the smeared areas of these faults on the reservoir permeabilities. These calculations are performed by a set of specially coded computer programs.
The calculations reveal the impact of stochastic faults on reservoir permeabilities (especially horizontal permeability) and suggest that the effect of stochastic faults should be assessed before simulating reservoir performance.