Caracterização petrofísica de reservatórios argilosos com perfis avançados: aplicação a um poço na Bacia de Maracaibo, Venezuela

Abstract

The petrophysical reservoir characterization is the process of analysis and interpretation of geophysical data performed by depth of a well through the description of the processes that determine the viability of formation hydrocarbons producing. Basic and advanced logs data were available from well located at Maracaibo Basin, Venezuela. The workflow of the present study consists of analyze reservoir petrophysical characteristics in well logs, such as quality control curves, reservoir zoning and quick-look evaluation, calculating volume of clay, calculate the porosity and permeability, calculation of fluid saturation, identify facies and finally evaluate the net-pay of the reservoir. Clays are significant components in almost all kinds of sandstone reservoirs when present in large volumes make the petrophysical evaluation of complex profiles, for this reason, this study characterizes its major effects in the reservoir. The VNZ-A well show mostly the Misoa formation, they are interbedded sands and shales fluvial-deltaic origin of the Eocene of the Maracaibo Basin. Were identified seven stratigrafic zones, C-1 to C-7, among which were described four intervals reservoirs between C-6 and C-7 zones. By concept, shaly sand reservoirs have the clay matrix supported with quartz and other minerals, the large contact surface of the clays are able to bind to a substantial fraction of water on the surface. The intrinsic water in the clay contributes to the electrical conductivity of the reservoir, which attenuates resistivity. For this reason, an integrated petrophysical evaluation using basic and advanced logs is necessary. The model of Archie siliciclastic reservoirs becomes ineffective in reservoirs influenced by shale content, resulting in pessimistic values. Saturation models applied to this goal were divided into two: based on the volume of clay (Simandoux) and based on the electrical conductance (Waxman-Smits, Juahz and Dual-Water), this last model, showed the best oil saturation results. The use advanced data integrated with basic logs enabled better assess the shaly sand reservoir zones, seen by the significant increase in the reservoir thickness and hydrocarbons volum.