A novel water saturation well logging evaluation model for tight carbonate gas reservoir based on pore-throat radius ratio from SEM images

Traditionally, resistivity well logs have been used to determine water saturation (Sw) in oil and gas reservoirs based on Archie's Law. However, the intricate pore structure of tight carbonates poses challenges, as their electric conductivity does not adhere to this law, thereby complicating the assessment of water/gas saturation in gas reservoirs. To address this, the study analyzed SEM images of carbonate samples to understand the pore structures. Utilizing the Slice-Gans model, 3D digital cores were constructed and resistivity was simulated.

The findings reveal that as porosity increases, the cementation exponent m in Archie's formula also increases. Theoretical derivations indicate that this is primarily due to changes in the PTRR as porosity varies. Dolomite and calcite are the primary minerals in the carbonates studied, and the PTRR differs between dolomite and calcite pores. Consequently, the m value varies depending on the mineral composition. By applying a parallel conductive model, the m value for cores with different dolomite and calcite volume fractions can be calculated.

Additionally, the study analyzed rock resistivity experiment results and found that the saturation exponent n's value also varies with porosity, with a fitted relationship established. This enabled the creation of a final variable m and n model. When applied to practical logging data, the calculated Sw values were found to be consistent with the bounded water saturation obtained from nuclear magnetic resonance (NMR) experiments in gas layers, validating the accuracy of the new model.