Question

Explain the use of GOC, GAS CAP, OWC in reservoir fluid contacts

Answer 1

The identification of GOC, GAS CAP, OWC in fluid contacts is essential for field reserve estimates and field development and, also, for detailed formation evaluation. For the accurate calculation of some petrophysical parameters, such as porosity, the reservoir interval has to be zoned by fluid type, to account for differences in fluid saturations and fluid properties (e.g., hydrogen index, density, sonic transit time) in the various intervals: gas cap, oil column and aquifer zone. The fluid contacts may vary over a reservoir either because of faults, semipermeable barriers, rock quality variations / reservoir heterogeneity, hydrocarbon-filling history or a hydrodynamic activity. Horizontal contacts are typically taken into consideration, although irregular or tilted contacts occur in some reservoirs.For example, for more-accurate porosity calculations, the reservoir’s vertical interval needs to be subdivided by fluid type to account for differences in the average fluid saturation and, hence, differences in fluid density or sonic travel time in the various fluid intervals: gas cap, oil column, or aquifer. For the water saturation (Sw) calculations, the depth of the OWC or GWC, or more particularly the related free water level (FWL), is a required input for any Sw calculations using capillary pressure, Pc, data. These depths need to be defined in every wellbore, to the extent that they occur.