A Multi-Physics Fields Coupling Model for Supercritical CO₂ Seepage in Shale Formation

Supercritical carbon dioxide (ScCO₂) can protect shale formation from hydration damage. When ScCO₂ is utilized to develop shale oil and gas resources, it is necessary to establish the multi-physics fields coupling model to describe ScCO₂ seepage in shale formation. The multi-physics model describing water-based fluid seepage is not capable explaining ScCO₂ flow because of a great variation in physical properties of ScCO₂ with pressure and temperature. In this study, a thermal–hydro–mechanical coupling model is constructed to describe ScCO₂ seepage based on the transport and thermodynamic properties of CO₂, combined with the effect of ScCO₂ on mechanical properties of shale. The finite element method is used to solve the distribution of formation temperature, pore pressure and stress with time and position. The results show that compared with water seepage, the variation of formation temperature is greater, the pore pressure is lower, the stress difference near the wellbore in the direction of minimum in-situ stress is bigger, and the tangential stress in the direction of maximum in-situ stress is lower when adsorption-induced strain is neglected. The radial stress and tangential stress decrease with increasing time. This research can provide theoretical basis for wellbore stability analysis and fracturing evaluation when using ScCO₂ as the drilling fluid and fracturing fluid, separately.

As one of the unconventional resources, shale oil and gas resource is an important alternative to widely used conventional resources, which is attracting worldwide attention (Jin and Chen, 2019). Currently, the commercial exploitation technologies are massive hydraulic fracturing and horizontal drilling for developing shale gas resource. However, shale formation contains clay mineral, the hydration damage will be produced when clay mineral meets water. In the process of drilling or fracturing with water–based fluid, wellbore collapse or poor fracturing stimulation may be encountered due to hydration swelling. Meanwhile, hydraulic fracturing operation will consume a large amount of water resource, which limits the application of hydraulic fracturing in water–lacking area. In order to avoid formation damage and conserve water resource, the technology of drilling or fracturing with supercritical CO₂ is proposed (Wang et al., 2018; Wen et al., 2020).