Experimental Study on the Impact of CO2 Treatment on Different Lithofacies in Shale Oil Reservoirs

CO2 technology has been progressively used in the development of shale oil. After injection, CO2 can react with formation water to form carbonic acid, which then reacts with carbonate and silicate minerals, resulting in changes in porosity and permeability. However, there are some debates as to whether the effect of CO2 improves or damages porosity and permeability. So, in this paper, systematic experiments were carried out to clarify the interaction between CO2 and shale in different lithofacies and to draw a pertinent conclusion. The results showed that the shale in Qingshankou Formation could be divided into three main lithofacial types: foliaceous shale, laminated feisic shale and laminated diamictic shale. There were relatively more pores, some natural microfractures and small mineral particles in foliaceous shale, a few micropores and large mineral particles in laminated feisic shale, some biogenic calcium carbonate minerals and hardly any micropores in laminated diamictic shale. Due to the diversity of micromorphology and mineral composition, the effects of CO2 treatment had significant differences. For foliaceous shale, CO2 treatment had both improving and damaging effects on porosity and permeability; for laminated shale, both porosity and permeability improved significantly. So, it is necessary to identify the main lithofacies of target formation before the application of CO2 technology in shale oil reservoirs.