Simultaneous inversion of five physical parameters of submarine gas reservoir from synthetic elastic impedance for high-efficiency reserve evaluation

Efficient and accurate estimations of submarine gas reservoir porosity and gas saturation are essential for successful exploration and development. Traditional methods often rely on cumbersome processes and empirical porosity relationships that lack clear physical significance in rock physics modeling. Additionally, these methods often overlook variations in the water-gas mixture law used for fluid bulk modulus modeling, which can lead to less reliable results. To address this, a new method is proposed based on the simplified two-phase Biot-type equations and the weighted average equation of mixed fluid bulk modulus. In this method, gas saturation, porosity, clay and carbonate contents of the sediment matrix, and the water-gas mixture calibration factor are treated as five independent variables in a non-linear function, with elastic impedance as the dependent variable. The approach simultaneously inverts these five parameters using an interior-point optimization algorithm, minimizing the squared misfit between the elastic impedances synthesized from well log data and those modeled from rock physics theory. Testing this method at Site 1245E, Hydrate Ridge on the Cascadia Margin, produced acceptable root mean square errors for gas saturation (0.0546) and porosity (0.0275), with mineral constituent proportions closely matching smear slide analyses. Inversion tests at three additional gas-bearing sites also yielded promising results, demonstrating that this new method surpasses previous approaches in porosity and gas saturation estimations accuracy.