Measurement of Rock Electrical Parameters and Analysis of Influencing Factors of Quaternary Mudstone Biogas Reservoirs in Qaidam Basin

Abstract

As a new and unconventional reservoir, Quaternary mudstone biogas reservoirs in the Qaidam Basin are widely distributed and thick. They have great potential for biogas exploration. However, the mudstone biogas reservoirs in this area characterized by weak cementation, high clay content, and easy hydrolysis. Using existing methods, it is difficult to obtain the rock electrical parameters and evaluate the saturation. This greatly affects the evaluation of oil and gas reserves. This study selected core samples from the Quaternary mudstone biogas reservoir in Qaidam Basin, prepared the samples through core conformation pretreatment, and conducted rock electrical experiments using the self-absorption water augmentation method to obtain the rock electrical parameters of the mudstone biogas reservoirs for the first time. Then, the factors influencing the rock electrical parameters of the mudstone biogas reservoir were analyzed. The results showed that: (1) the porosity of the mudstone core samples was generally greater than 20%, and the permeability was in the range of 0.05–5 mD. The rock mineral composition was dominated by clay minerals, followed by carbonate minerals, quartz, and feldspar. The clay mineral composition was mainly illite, followed by montmorillonite and chlorite, with a small amount of kaolinite. (2) The use of wire-electrode cutting and high-temperature, heat-shrinkable tube to wrap the core sample played a supporting and protecting role in the core. Using the self-absorption water augmentation method, information was obtained showing that the distribution of cementation index m of the reservoir core samples ranged between 1.89–2.08, with an average value of 1.99, and the distribution of saturation index n ranged between 1.872–2.270, with an average value of 2.09. (3) Organic matter content had no obvious effect on rock electrical parameters. With the decrease of clay mineral content, the quartz content increased, the permeability increased, and the cementation index increased gradually. The saturation index increased with the increase in clay mineral content and the decrease of quartz content and permeability. The above results laid a petrophysical basis for the evaluation of the saturation of mudstone biogas reservoirs and could provide technical support for the comprehensive evaluation of the reservoir and the calculation of reserves.