Research on the brittle characteristics of shale under long-term CO2-H2O-shale coupling effects

Abstract

Hybrid fracturing focuses on optimizing the extraction of shale gas. In order to solve the issues of shale reservoir fracturing and CO₂ storage stability. Research was conducted to examine the mechanical properties of shale under long-term CO₂-H₂O-shale coupling effects. The results reveal that the SC-CO₂-H₂O-shale coupling effects increase shale porosity and pore size due to the dissolution of minerals. Concurrently, Shale cohesion and internal friction angle decrease. Following prolonged SC-CO₂-H₂O-shale coupling effects, there is a decline in shale strength and elastic modulus decline, alongside a decrease in axial strain (such as the growth of the compaction segment, but the shortening of the elastic and strain hardening segments). lateral strain increase, resulting in a higher Poisson's ratio. Additionally, the indices BI₁ rises by 47.44 %, whereas BI₂ drops by 66.85 %, improving the shale's drillability and cuttability. the indices BI₃, BI₄, BI₅, and BI₆ increase by 11.90 %, 45.10 %, 15.19 %, and 8.99 %, respectively, highlighting the shale's brittle characteristics. However, the indices BI₇, BI₈, and BI₉ decrease by 35.05 %, 38.20 %, and 46.67 %, indicating a reduction in the shale's fracturability. Confining pressure reduces lateral strain and increases axial strain, result in an increase in shale Poisson's ratio and strength. Following enhanced confining pressure, the indices BI₃ drops by 1.19–10.61 %, BI₄ decreases by 43.14–61.29 %, BI₅ falls by 1.27–14.29 %, and BI₆ declines by 1.12–8.42 %. Consequently, the failure characteristics transition from brittle to plastic. The SC-CO₂-H₂O-shale coupling effects facilitate the growth of fractures in unfractured areas. However, the elevated ground stress and reduced fracturability of the shale reservoirs restricts the growth of fractures in fractured zone, ensuring the stability of CO₂ storage.