PERMEABILITY CHANGES ON WELLBORE CEMENT FRACTURES MODIFIED BY GEOCHEMICAL AND GEOMECHANICAL PROCESSES

Abstract:

Experimental studies were conducted using batch reactors, X-ray microtomography (XMT), and computational fluid dynamics (CFD) modeling to determine changes in cement fracture surfaces, fluid flow pathways and permeability, and cement fracture propagation with geochemical and geomechanical processes. Portland cement-basalt interface sample with artificial fractures was prepared to study the geochemical and geomechanical effects on the integrity of wellbores containing defects caused by subsurface activities. Cement-basalt interface sample was subjected to mechanical stress at 2.7 MPa before the chemical reaction. CFD modeling was performed to simulate flow of supercritical CO₂ within the fractures before and after the application of mechanical stress. The model results highlighted the complex flow characteristics within the fracture and also changes in flow patterns due to application of geomechanical stress. The CFD model predicted ~45% increase in permeability after the application of geomechanical force, which increases the fracture aperture. The same sample was reacted with CO₂-saturated groundwater with impurity H₂S (1 wt.%) at 50°C and 10 MPa for 3 to 3.5 months under static conditions. XMT provided three-dimensional (3-D) visualization of the opening and interconnection of cement fractures due to mechanical stress. Even after a 3.5-month reaction with CO₂-H₂S-saturated groundwater at 50°C and 10 MPa, CaCO₃ (s) precipitation occurred more extensively within the cement fracture rather than along the cement-basalt interfaces. Micro X-ray diffraction analysis also showed that major cement carbonation products of CO₂–saturated groundwater reacting with impurity H₂S were calcite, aragonite, and vaterite, consistent with cement carbonation by pure CO₂-saturated groundwater, while pyrite was not identified due to low H₂S content. The experimental results imply that the wellbore cement with fractures is likely to be healed during exposure to CO₂-saturated groundwater under static flow conditions, whereas fractures along the cement-caprock interface are likely to remain vulnerable to the leakage of CO₂. The study suggests that geochemical and geomechanical processes have coupled effects on the wellbore cement fracture evolution and fluid flow along the fracture surfaces.