Energised CO2 stimulation for mineral carbonation and corresponding ground deformation

Mafic and ultramafic rocks such as basalts and gabbro have reactive minerals such as olivine, pyroxene, and plagioclase to trap CO₂ into stable carbonates. When the carbonic acid (CO₂ dissolved in water) is injected to these rocks, stable carbonates such as calcite, dolomite, magnesite, and siderite are created and precipitated. However, such rocks suffer ultra-low permeability of the matrix which makes the reach of CO₂ to minerals a cumbersome task. Novel stimulation techniques as well as natural fractures are required to enhance the injectivity of CO₂ fluid into these rocks and create new storage opportunities. Energized fracturing with CO₂ is a promising method to enhance the injectivity of low-permeable target rocks, thanks to the unique thermodynamic and transport properties of CO₂. In order to ensure the safety and efficacy of storage medium, it is crucial to possess a comprehensive understanding of the movement of pressure plumes within geological features by monitoring the potential impact on the deformation of geological layers as well as the ground surface.

In this work, an extensive numerical simulation of energised fracturing with CO₂ is performed utilising a robust fracturing simulator and the Span-Wagner equation of state for CO₂. The simulation results show that CO₂ phase (liquid, gas or super-critical) plays an important role in fracture propagation speed, injection time and stimulated volume, However, the CO₂ under the supercritical state appears to be the favourable state for the purpose of stimulation. We compare opening versus shearing behaviour of fractures invaded by a fluid pressure plume. Combination of the two creates a mixed-mode deformation at the ground surface detectable via an array of tiltmeters. We present a novel inversion model that distinguishes the opening and shear modes of deformation and identifies the contribution of each mode in the observed tilt data.