Analysis of CO2 effects on porosity and permeability of shale reservoirs under different water content conditions

CO2 technology has been the key technology to enhance oil and gas recovery for shale oil and gas reservoir development. After injection, CO2 can react with water and generate carbonic acid, which then can react with various minerals in shale, leading to changes of porosity and permeability. However the specific research conclusions remain controversial on whether the effect of CO2 is improvement or damage for porosity and permeability. In CO2 hybrid fracturing, The areas with varied water content are formed due to the difference in mobility. So in this article, we carried out systematic research to clarify the interaction between CO2 and shale in different water content conditions (sufficient water, deficient water and water-free) and draw a pertinent conclusion. And the results show that the effects of CO2 treatment on shale have significant differences among different water content. Under sufficient water conditions, CO2 fully dissolves into water to generate carbonic acid, and large dissolved pores are generated by the massive dissolution of minerals such as feldspar and carbonates in the shale matrix, leading to the increase of porosity and permeability. Under deficient water conditions, only a small amount of carbonic acid is formed after the reaction of CO2 and water, leading to a significant decrease of porosity and permeability. Under water-free conditions, no dissolution is observed on the shale matrix, while some soluble components of organic matter may be extracted by CO2 to form organic pores, leading to slight changes in porosity and permeability. In large scales, although both porosity and permeability in the near-wellbore water-sufficient area is improved, the CO2-affected region presents the overall decline of porosity and permeability, due to the negative effects on the water-deficient area. So, it is necessary to enlarge the volume of water-sufficient areas to get a better effect of enhanced oil and gas recovery.