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ABSTRACT: Supercritical CO2, as a new type of non-aqueous working fluid, is considered to have a great potential and can be used as a drilling fluid or fracturing fluid for unconventional hydrocarbon development. The supercritical CO2 meets water to form carbonic acid, which has certain corrosiveness. During drilling and fracturing process, the supercritical CO2 will contact directly with the rock and have certain influence on the mechanical properties of the rock. In this paper, sandstone samples were soaked in supercritical CO2 under simulated formation conditions to study the effect of different time, temperature, and pressure on the mechanical properties of sandstone, such as rock compressive strength, tensile strength, wave velocity, and mineral composition, etc. The sandstone outcrop samples mined from Chang 6 block of Yanchang Oilfield (which is located in Yanchang County, Shaanxi Province) were put into the sample cell. Then the characterization techniques, such as scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, etc. were used to observe mechanical properties of the samples before and after the process of treatment. The results showed that compressive strength, tensile strength, and wave velocity of rock decrease with the increase of treatment time, temperature and pressure of supercritical CO2; the content of dolomite, calcite, etc. in the rock structure also gradually decreased. After the supercritical state of CO2 is reached, temperature has little influence on the mechanical properties of the rock and the pressure is the main influencing factor. As the time of treatment increases, the loss volume of mechanical strength parameters increases, but the amplitude decreases. Furthermore, the surface morphology of the rock after soaking is rougher and the porosity is more developed. The research results provide a theoretical foundation for the application of supercritical CO2 fluid in unconventional oil and gas development.
1. INTRODUCTION
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