Abstract: Impact of Fault Permeability on Oil Production from an Upper Albian Turbidite Reservoir, Campos Basin, Brazil

Maciel, W. B. - Petrobras/E&P and G. Pickup - Heriot-Watt University, UK

In a porous hydrocarbon reservoir, faults can form barriers or baffles to fluid flow by the juxtaposition effect, when a lower permeability lithology is placed against a higher permeability one, or by lithological transformations occurring in the host rock. A fault rock can be classified as hydraulically passive or hydraulically active. A hydraulically passive fault rock occurs when the original host rock petrophysical properties (i.e. K, Phi and Pc) were not altered, and the effect on the hydraulic properties of the reservoir is just the juxtaposition effect of layers that were not originally juxtaposed. Fault zones with hydraulically active faults may act as either conduits or barriers to flow. Hydraulically active fault rocks can be formed mainly by three mechanisms: (i) cataclasis of sandstones, (ii) shale/clay smearing; and (iii) cement percolation through the fault plane.

In order to assess the potential impact of different fault petrophysical properties on oil production in a Brazilian turbidite reservoir, a 2D multiphase-flow simulation sensitivity study (Fig. 1) with explicit representation of the faults was carried out over a geologic cross-section. Two faults with relatively low offsets, 10 - 20 m, were modeled. The fault system is related to the gravitational gliding of an Aptian salt succession over a Pre-Aptian detachment surface. The reservoir (50 md average horizontal permeability) comprises two lobate turbidite successions composed of poorly-sorted, heterogeneously calcite-cemented, fine to very fine grained sandstones. These are frequently interbedded with finer-grained, non-reservoir facies (debris flows and hemipelagic sediments).

Great attention was focused on the petrophysical characterization of the faults. To obtain plausible realizations of the fault properties, a methodology which used the shale gauge ratio was applied. It is based on the triangle plot and on empirical data correlation. Furthermore, mechanisms to reduce original host rock permeabilities, such as cataclasis and cementation were also taken into account. Thus, the faults plane grid blocks were allocated different petrophysical properties along the planes, depending on the lithology of the host rock involved in the displacement. Doing this, we attempted to observe, in a multiphase flow simulation model, possible situations likely to occur in the real field life due to the interaction of the heterogeneous reservoir geological framework and the fault petrophysical properties. With these fault properties and relevant geological properties established, the multiphase flow simulation model was built (Fig. 2) with explicit representation of the faults. The different petrophysical models for the faults were used and the results compared. The main conclusions were as following: (1) the reservoir showed a trend to accumulate high pressure differentials in the upper parts of the reservoir and in the aquifer; (2) oil recovery showed a much higher sensitivity for variations in the permeability within lower values; and (3) variations of faults Kv/Kh ratio can be important in production.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil