Abstract
The primary challenge posed by carbonates in petrophysics lies in the complexity of their pore systems, which exhibit a wide range of pore sizes within a small section of a single carbonate. In some cases, even the grains possess microporosity, distinct from the porosity between them, resulting in two different pore systems. Understanding the contribution of these pore systems to reservoir productivity is crucial for accurately assessing current and future water saturation levels. Conventional saturation equations like Archie's are not recommended for complex carbonate formations due to uncertainties related to wettability and cementation factors (m and n), which can vary significantly due to diagenesis and fractures. Additionally, the presence of vugs (whether connected or isolated) can substantially impact production and permeability over the reservoir's lifespan.
To address these challenges, this paper introduces an integrated solution that employs nuclear magnetic resonance factor analysis (NMR-FA) in a carbonate reservoir in the Gulf of Suez, Egypt. This approach provides a precise understanding of reservoir quality, enables quantification of hydrocarbon in place, and optimizes oil production. The demonstrated methodology combines NMR-FA with NMR and spectroscopy data, focusing on the characterization of carbonates. By computing poro-fluid facies and integrating them with core data, it becomes possible to accurately determine the volumes of bound and free fluids, thus providing insights into the porosity and quantifying producible hydrocarbons—an influential factor in production. The new bound/free fluid volumes are utilized to calculate continuous NMR permeability, which is then calibrated using core data.
The classification of fluid facies based on pore-fluid porosities enables the evaluation of reservoir quality and identification of the sweet spot—the zones characterized by high producible hydrocarbon porosity and permeability, indicating the best reservoir quality for production.
Considering the complexity of carbonate reservoirs and the presence of secondary porosity is crucial when performing direct volumetric calculations for each constituent poro-facies within the T2 distribution. The results of this workflow successfully characterize the reservoir quality and producibility of the complex carbonate reservoir in the Gulf of Suez, Egypt, pinpointing its sweet spot. Furthermore, this workflow can be applied as a cost-effective solution in various scenarios and formation types.
