Impact of CO2 injection through horizontal and vertical wells on the caprock mechanical stability

• CO2 pressure evolution differs significantly in vertical and horizontal wells.
• In vertical wells, CO2 pressure drops after an initial sharp increase.
• In horizontal wells, CO2 pressure builds up continuously.
• The most critical situation is at the beginning of injection in vertical wells.
• For the considered conditions caprock stability is slightly affected for both cases.

Subsurface injection of carbon dioxide (CO2) may induce large overpressures that may put caprock integrity at risk. This overpressure may induce microseismic events if the rock yields. Yielding of the caprock could open up a leakage path for CO2. Rock mechanical integrity is strongly related to fluid pressure evolution, which is significantly different for vertical and horizontal wells. While CO2 pressure builds up sharply at the beginning of injection, but afterwards drops, when injecting a constant CO2 mass flow rate through a vertical well, a horizontal well leads to a continuous CO2 pressure increase. Thus, for a vertical well, the less stable situation in the saline aquifer occurs at the beginning of injection. However, the changes induced in the effective stress field are small, so unstable conditions are unlikely both in a normal faulting and a reverse faulting stress regimes in extensive saline aquifers such as the one considered in this study. By contrast, fluid pressure becomes larger than that of a vertical well for a common length of horizontal wells (around 2 km), which causes a significant increase in horizontal total stresses that improves the reservoir and caprock mechanical stability in a NF stress regime, but worsens it in a RF stress regime. Though in general the caprock integrity is unlikely to be compromised, fluid pressure evolution should be always monitored and mitigation measures should be carried out if it deviates from its expected evolution.