Evaluation of the fluids saturation in a multi-layered heterogeneous carbonate reservoir using the non-archie water saturation model

Abstract

The evaluation of fluids saturation represents a continuous challenge in all reservoirs specially the carbonate ones. The variation of the pore structure in the carbonate reservoirs is one of most important factors affecting the determination of the fluids’ saturation. Focke (Focke and Munn, 1987) et al. showed that the variations of the pore structure result in variations of the cementation exponent (m) in the Archie and the modified-Archie water saturation models. They showed that the difference between inter-particle and vuggy pore system results in a variation in (m) of 2 versus (m) of 4. They also found that the fractures porosity has an effect opposite to the vuggy porosity resulting in (m) to be lower than 2.A complex carbonate reservoir in the Middle East composed of twenty-nine layers and sub-layers has complex pore structure, varying laterally and vertically, across the reservoir. To capture this variability based on Focke's findings, the cementation exponent (m) is measured in the laboratory using multiple core plugs representing all layers and the sub-layers. The values of (m) in this reservoir varied between 1.8 and 2.2 confirming combination of fractures, inter-particle, and small vuggy porosity.To calculate the water saturation in this reservoir, the Archie-based Simandoux water saturation model is used where the cementation exponent (m) values are changed on layer-by-layer basis to reduce the uncertainties in the water saturation. This approach represents an economical burden and a technical challenge due to the need to have measured values of the exponent (m) for every drilled well to capture the complexity and the heterogeneity of the reservoir.Recently, Oraby, 2020) developed a non-Archie water saturation model to determine the formation water saturation in conventional reservoirs based on a modification and a generalization of the (Passey et al. 1990, 2010, 2012). Total Organic Carbon, TOC, model that is used in determining the organic richness of the unconventional reservoirs. This model has the advantages of not requiring any of the Archie electrical parameters (a, m, n) or the formation water resistivity Rw.The Oraby non-Archie water saturation model is applied and tested in this complex reservoir. To examine the validity of the Oraby model, the Oraby water saturation model is equated with the Simandoux Archie-based model to back calculate the cementation exponent (m). The calculated values of the exponent (m) are compared to the core measured. The model also pointed out to other variations in (m) that were not captured during the sampling of the core plugs.In this paper, the applications of the Oraby non-Archie model are discussed and comparisons between the calculated cementation exponent (m) with the core measured values are shown and proved the validity of the model.