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SPE Members Abstract Due to the resolution limitation of seismic techniques faults shorter than the distance between seismic lines, or faults with very small throws are 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:calculation of the effect of stochastic shales on permeabilities (to obtain effective permeabilities for the reservoir blocks),generation of stochastic faults and specifying the faults encountering the block under consideration,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. Introduction Since the transmissibility across the formations is highly affected by the presence of faults, the distribution, geometry, and effect of these undetected faults on reservoir permeability must be evaluated before characterising and permeability must be evaluated before characterising and simulating the reservoir's performance. Brand modelled the stochastic faults in one of the very first studies in this area. According to the number of new faults detected by 30 new wells in a specific area, he used Monte Carlo techniques to find the expected number of stochastic faults for that area (to produce the same number of intersections by 30 vertical lines or wells). Brand investigated the effect of stochastic faults on transmissibility assuming the fault planes to create no restriction to flow if sands are juxtaposed across the fault. However, in the presence of sand/shale sequence the observations on fault planes showed how different the situation is. Model experiments and outcrop studies have indicated that the shear zones of normal faults in sand/shales sequences usually consist of smeared in laminae of shale and wedges of sands. Shi designed a special ringshear apparatus to study the development of shear zones under continuous shearing. A series of tests were done with sand/shale sequences under various simulated overburden pressures along a median slip plane. pressures along a median slip plane. The clay bands sheared-off in this fashion formed one continuous multi-layered clay gouge along the median slip plane of the shear zone. The clay gouge was somewhat contaiminated with sand grains derived from boundary zones, but proved to be an effective seal to water flow when tested. Sandy materials between two different clay bands were found in the fault planes when two or more clay beds were sheared. If the fault throw is substantial, the sand wedges out and two clay gouges on both sides amalgamate to form one continuous gouge. Studies on the Niger delta faults in Nigeria showed the fault zones to vary from less than one foot to about two feet. The continuity of the day gouge was observed over about 400 metre of exposure. If the fault throw is large, and the shales are relatively thin, the clay gouge becomes very thin and may disappear at the central part of the fault plane. The sealing capacity of the fault depends on the displacement pressure of the formation present in the fault zone. P. 497 |
