Experiments on the rock electricity and application research considering the influence of CO2 content; a case study of the Dongfang 13 gas field

A large number of CO2-bearing gas layers are distributed in the deep layers in the Huangliu and Meishan Formations of the Dongfang 13 gas field. The presence of CH4, CO2, and formation water in CO2-bearing gas layers complicates the change of logging response characteristics. In particular, the unclear understanding of the additional conductivity change law formed by the coupling of CO2 and formation water results in low accuracy of the water saturation results, and the aforementioned type of gas layer can easily be misidentified as the gas-water mixed layer. By conducting multiple rock electrical experiments under simulated formation temperature and pressure conditions, core flooding experiments were conducted with different contents of CO2 and CH4 mixed gas to determine the lithology coefficient b, saturation coefficient n, and cementation coefficient m of cores under different experimental conditions. Additionally, we also analyzed the relationship between the electrical parameters of rocks, CO2 content, and water saturation to reveal the influence of coupling of CO2 and formation water on the change of electrical parameters of rocks under high temperature and pressure conditions. The experimental results show that when the mixed gas with different CO2 contents is dissolved in the formation water, there is no difference between the measured lithology coefficient a and the cementation coefficient m; as the CO2 content increases, the lithology coefficient b increases, and the saturation coefficient n decreases. Based on the experimental results, we established a prediction model of the change between lithology coefficient b, saturation coefficient n, and CO2 content. This model improves the accuracy of water saturation calculations in the CO2-bearing gas layers along with that of fluid identification.