Modeling of gas saturation based on acoustic wave and pore structure classification and application in the low-porosity and low-permeability reservoir

Gas saturation is an essential parameter in natural gas reservoir evaluation. As complex pore structure reduces the resistivity contrast between gas and water layers, gas saturation is difficult to calculate accurately by rock resistivity in low-porosity and low-permeability reservoir. The gas saturation is positively correlated with the relative value of compressional wave (P wave) slowness’ square based on the rock acoustic wave theory. In this study, rock samples are divided into four types of pore structures according to nuclear magnetic resonance (NMR) data. Relation models between the gas saturation and the relative value of P wave slowness’ square are established for each type. New models are further applied in field logging data in low-porosity and low-permeability gas-bearing sandstone reservoirs. The result shows that the new models calculated gas saturation is in good agreement with the gas-producing test. This paper provides a new way of qualitative evaluation of gas saturation. It enriches effective evaluation methods in low-porosity and low-permeability gas-bearing reservoirs with non-electrical logging data.