Full-Sample X-ray Microcomputed Tomography Analysis of Supercritical CO2 Fracturing in Tight Sandstone: Effect of Stress on Fracture Dynamics

The commercial production from tight oil and gas reservoirs has been facilitated by the multistage hydraulic fracturing of horizontal wells. This process typically requires the pumping of large amounts of slick water into the subsurface, and this could be challenging in areas with a limited supply of water. Despite the commercial success of hydraulic fracturing with water, it still faces the problem of clay swelling and potential contamination of underground water. This has led to research studies and field applications of liquid or supercritical carbon dioxide (SC-CO2) fracturing in unconventional oil and gas resources. Considering that the propagation and characteristics of these man-made fractures are controlled by the fracturing fluid and mechanical state of the reservoir, we performed a series of fracturing experiments on tight sandstones using water and SC-CO2 at different stress magnitudes. To explore the morphology of the fractures and quantify their attributes, we proposed a novel full-sample fracture analysis approach, which is based on microcomputed tomography (CT) imaging. The results of this study indicate that the breakdown pressure is a linear function of the minimum principal stress and tensile strength. We observe that the pattern and geometry of the fractures created from SC-CO2 fracturing is more complex than those of water fracturing under the same stress conditions. Our experimental results also indicate that smaller differential stresses lead to the creation of more fracture branches and that fracture propagation is significantly affected by the presence of initial bedding planes. Furthermore, our quantification of the fracture attributes (based on fracture extraction and digitization) indicates that SC-CO2 fracturing leads to the creation of more complex fractures with rougher surfaces than water fracturing. This experimental study proposes a new full-sample fracture quantification approach, which can be implemented to analyze fracture attributes precisely and effectively. The results from this work could provide insights and guidance for the field application of SC-CO2 fracturing in unconventional oil and gas resources.

Automated summary

The study compared the fracturing of tight sandstones using water and SC-CO2 at different stress levels, finding that SC-CO2 fracturing resulted in more complex fracture patterns and geometry with rougher surfaces than water fracturing.