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Accurate OGIP estimation in shale gas reservoirs is challenging due to strong heterogeneity, significant adsorbed-gas contribution, and high uncertainty in key petrophysical parameters. This study evaluates OGIP using both a conventional petrophysical volumetric model and an adsorption-enhanced shale gas formulation designed to incorporate shale-specific gas storage mechanisms, applying deterministic calculations and Monte Carlo Simulation (MCS) to quantify uncertainty. A ±20% probabilistic range was assigned to area, thickness, porosity, and water saturation, and three probability distributions—Uniform, Triangular, and Normal—were used to generate 3000 simulation runs, identified as the optimal threshold for stable and convergent output. Results show that deterministic OGIP values closely match the P50 probabilistic outcomes for all distributions, validating the deterministic approach as a median-case estimate. However, MCS reveals substantial variability between P10 and P90, highlighting the limitations of single-value estimates. The adsorption-enhanced formulation predicts approximately 10% higher OGIP than the conventional model, reflecting improved representation of adsorbed gas and shale-specific storage mechanisms. Overall, the findings demonstrate that Monte Carlo simulation provides a broader characterization of uncertainty, while the adsorption-enhanced shale gas formulation captures additional shale-specific storage mechanisms associated with adsorbed gas behavior.
Khan et al. (Mon,) studied this question.