Unveiling the Optimal Water Saturation Equation for Unconsolidated Reservoirs: A Case Study From the Tambaredjo Oil Field, Suriname

Selecting the optimal water saturation (Sw) equation for reservoir characterization can be an intricate task. Numerous equations exist, each with inherent strengths and limitations. The Tambaredjo Field in Suriname has historically employed the Indonesian equation. While recent research suggests the Modified Indonesian equation, originally developed for Athabasca tar sands, offers a better fit for initial water production in multiple wells, its applicability to the Tambaredjo Field’s unique characteristics (unconsolidated and heavy oil) raises concerns. This study proposes a comprehensive methodology to assess Sw equations and identify the most suitable candidate for the Tambaredjo Field. Two crucial steps were undertaken: 1. Comprehensive core data inventory: All core data relevant to Sw equation calibration were compiled, including direct and indirect measurements of irreducible water saturation (Swir). Twenty-seven (27) samples underwent various tests like capillary pressure, mercury injection, resistivity index, nuclear magnetic resonance (NMR), and relative permeability, serving as calibration references for diverse Sw equations (e.g., Indonesian, Dual Water, etc.). 2. Development of the Water Saturation Equation Assessment Tool (WSEAT): This tool facilitates the evaluation and selection of the best-fit equation. WSEAT directly compares individual equation outputs against core sample data while analyzing all sensitive input variations. Evaluation via WSEAT revealed the Modified Indonesian equation as the best initial fit. However, its origin in tar sand applications necessitated further investigation. Modifications, assumption reviews, and solutions for challenging input estimations were explored. Consequently, a novel approach was developed to integrate clay and silt corrections into Sw calculations. Instead of a single term for shale, clay and silt were treated separately. The derivation included formation water resistivity (Rw) for each component (sand, clay, and silt). Additionally, a laminar approach was considered, as this approach has shown better results in some areas of the field. The proposed equations were evaluated well by well to verify their alignment with field production performance. Results in the Tambaredjo Central Area (TAMC) demonstrate a significant improvement in Sw, from 41.4 to 24.5%. This translates to an actual recovery value of 11.2%, which aligns more closely with the anticipated primary recovery for the field. This study presents a comprehensive approach for selecting the optimal Sw equation for unconsolidated reservoirs and exemplifies its successful application in the Tambaredjo Field. The proposed Suriname Clay Sw equation, tailored to the specific reservoir characteristics, offers enhanced accuracy and aligns better with production data.