Effect of CO2 soaking time on replacement efficiency and reservoir properties of tight oil and gas reservoirs

In dry fracturing, CO2 enters a tight reservoir as the fracturing fluid and replaces the oil and gas therein by soaking. This enhances the oil and gas recovery and enables CO2 storage, thus reducing CO2 emissions and yielding the dual benefits of environmental protection and improved oil field production. The macroscopic benefits of soaking oil and gas reservoirs with CO2 are evident; however, there are few studies on the micro-erosion effect of CO2 on the pore throats of reservoirs during the replacement process. In this context, this study analyzed the influences of soaking time and permeability on the CO2 displacement efficiency in terms of the macroscopic displacement. The effects of CO2 displacement on the pore throat morphology, mineral composition, and fluid distribution of oil and gas reservoirs were qualitatively and quantitatively studied by scanning electron microcopy (SEM), mineral composition testing by X-ray diffraction, and fluid distribution scanning by computed tomography. The corrosion reaction between CO2 and rocks during the replacement process was chemically analyzed. The results showed that: (1) The CO2 replacement efficiency in the tight oil reservoirs was <20% under the same soaking time. The displacement efficiency increased by less than 3% with increasing soaking time. As the core permeability increased, the displacement efficiency increased as much as 15%-20%. In the gas reservoir, the replacement efficiency was >75%, and the weight of influence of the soaking time on the CO2 replacement efficiency was slightly greater than that of the core permeability; however, their difference was not significant. (2) The SEM, XRD, and CT scanning results showed that the pore morphology of the core is dissolved and blurred after CO2 replacement. The main eroded mineral component in the core was calcite. The porosity of the core increased after erosion, particularly near the fracture. The greater the distance from the crack, the lower the efficiency of CO2 dissolution and replacement to the pores by diffusion. (3) The analysis of the reaction mechanism between CO2 and rocks showed that CO2 dissolves and diffuses into the pore fluid in the post-pressure soaking stage, thus expanding the crude oil and acidifying the formation water. The generated carbonated fluid dissolves in the reservoir rock to some extent, which can appropriately increase the porosity and permeability of the reservoir rock. This paper provides a reference for the qualitative and quantitative study on the mechanism of macrocosm and microcosm replacement after CO2 dry fracturing.