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Abstract

Pre-mature water breakthrough can be a common problem for carbonate reservoirs. The field concerned here has producing intervals from the Late Triassic Baldonnel Formation, dominantly by porous dolostone and limestone. Water was produced from some wells, raising a question whether this implied the end of field-life or was attributed to fractures or local high permeability streaks. A multidiscipline study involving geological, petrophysical, petrographical, geophysical, reservoir-engineering analysis, and field management provided an integrated solution to the problem. Structural grids based on 2D seismic mapping were tied with well picks. Detailed core and thin-section analyses allowed us to identify petrophysical facies and rock types, which were then linked to a sequence stratigraphic framework. A geocellular model was constructed to delineate 3D variation of petrophysical properties. It was difficult to quantify the original gas-water contact (GWC) using log data alone due to the presence of bitumen in the non-reservoir zones. Therefore, pressure data were analyzed to derive P/Z plots and obtain original reserves in place. The result of P/Z analysis agrees with the geological model, suggesting that the original GWC was at 775 meters subsea. By modeling the remaining reserve based on step-wise variations of current GWC and comparing the result of decline curve analysis, the current GWC was identified at between 758 and 762 meters subsea, giving a remaining reserve of 89 BCF (70% recoverable). This study resulted in a higher degree of confidence in resource capture, leading to a better strategy for field management and development.

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