Generally, hydrocarbon reserves are determined by the irreducible water saturation using the volumetric equation of the original oil in place (OOIP), regardless of the initial water saturation in the transition zone and as a result overestimate hydrocarbon reserves. The objective of this work is to determine a water saturation model that allows modeling the initial distribution of fluids in the reservoir knowing the original oil-water contact and rock quality. The Leverett J function was used as a variable independent from initial water saturation model; it controls the flow of fluid and depends on the height above the free water level, the interfacial tension, Oil-water fluid density, absolute permeability and effective porosity. In order to define this function, capillary pressure data from oil-water system of drain cycle from special core analysis were used. The core data were modeled and averaged for each rock type with the capillary pressure function from "JNM Jing XD Van Wunnik SCA-9807 (1998)". Then, these curves were converted to values of "J" and these were related with water saturation from capillary pressure curves to obtain functions type SWINIT: f (J). This model gave excellent results in the fluid initial behavior, it means, knowing the original oil-water contact and rock quality, the initial water saturation was quantified in in wells completed in the reservoir. The aforementioned was validated with production / pressure tests. Finally, two sensitivities were performed to calculate OOIP, the first was determined with the irreducible water saturation and the second with the initial water saturation, resulting in a error difference about 5%, in this case, obtaining higher reserves were the calculated with irreducible water saturation. The initial water saturation profile of the wells generated from the model, it is useful to initialize the numerical simulation from reservoir "A", because this model takes as an input data the original oil water contact, which represents the limit reservoir fluid. This model does not depend on the date of drilling wells to determine the initial water saturation of the reservoir.
La saturación de agua inicial (Swi) a partir de registros está influenciada por el drenaje de fluidos de los pozos productores, generando subestimación del petróleo original en sitio (POES). Para restaurar las condiciones iniciales del yacimiento, es necesario utilizar pruebas de presión capilar (Pc) de drenaje, que determinan la distribución de Swi previa a cualquier producción de hidrocarburos. Esta investigación tuvo como objetivo determinar el modelo de Swi, basado en curvas de Pc por tipo de roca, para una mejor estimación del POES del yacimiento LUZ de la cuenca de Maracaibo. El procedimiento metodológico incluyó: recopilación de datos (registros y núcleos, con 15 muestras de Pc), descripción de tipos de roca, determinación del modelo de Swi, y estimación del POES. Entre los resultados, destacan: el modelo J-Leverett se ajustó mejor a las curvas de Pc del yacimiento para todos los tipos de roca; el POES estimado utilizando la saturación de agua (Sw) del modelo propuesto basado en presión capilar y la calculada usando registros, mostró un 19,8 % de discrepancia, evidenciando la importancia de un modelo robusto para incrementar la certidumbre en el cálculo de reservas.
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