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

Abstract

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.

Virtual Special Issue of Recent Research Advances in China: Unconventional Gas

“…A review by Lu et al presents a summary and perspective on efficient exploitation of CBM in China. The original research articles on this topic cover several aspects related to CBM, including gas adsorption, methane production, , numerical modeling and simulation, , pore-scale measurements, , characterization studies, , etc.…”

“…A review by Lu et al 62 presents a summary and perspective on efficient exploitation of CBM in China. The original research articles on this topic cover several aspects related to CBM, including gas adsorption, 63 methane production, 68,71 numerical modeling and simulation, 64,66 porescale measurements, 65,69 characterization studies, 67,70 etc. This VSI is part of the series of VSIs on recent advances in selected energy research areas, launched by Energy & Fuels in January 2021.…”

Numerical Simulation of Multicrack Propagation Dynamics in Supercritical CO2 Fracturing of Tight Reservoirs

“…Fracturing is the most effective reservoir modification method in the process of developing low-permeability and ultralowpermeability tight oil and gas reservoirs. 1,2 Currently, the most commonly used fracturing working fluid is a water-based fracturing fluid system represented by slick water. 3,4 However, water-based fracturing fluids have problems such as high water consumption, easily cause environmental pollution, and are not suitable for water-sensitive reservoirs.…”

“…14−16 SC-CO 2 has good flow, penetration, and transfer properties and has advantages in reducing crack initiation pressure, connecting microcracks, forming complex fracture networks, and providing the possibility of carbon dioxide sequestration (Figure 1). 2,17,18 As a waterless fracturing method with broad prospects, SC-CO 2 fracturing is not yet clear about its complex mechanism and needs further theoretical support.…”

“…Fracturing is the most effective reservoir modification method in the process of developing low-permeability and ultralow-permeability tight oil and gas reservoirs. , Currently, the most commonly used fracturing working fluid is a water-based fracturing fluid system represented by slick water. , However, water-based fracturing fluids have problems such as high water consumption, easily cause environmental pollution, and are not suitable for water-sensitive reservoirs. − In this respect, fracturing of tight reservoirs using supercritical carbon dioxide (SC-CO 2 ) seems to give better results. , As a fracturing fluid, SC-CO 2 is not introduced into the water phase during the fracturing process, which saves water resources and does not produce water-sensitive phenomena. , SC-CO 2 has the advantages of a high diffusion coefficient and low interfacial tension, allowing SC-CO 2 to penetrate into microcracks and better connect the reservoir. − SC-CO 2 has good flow, penetration, and transfer properties and has advantages in reducing crack initiation pressure, connecting microcracks, forming complex fracture networks, and providing the possibility of carbon dioxide sequestration (Figure ). ,, As a waterless fracturing method with broad prospects, SC-CO 2 fracturing is not yet clear about its complex mechanism and needs further theoretical support.…”