Hydraulic Fractures Induced by Water-/Carbon Dioxide-Based Fluids in Tight Sandstones

To investigate the impact of fluid viscosity and rock heterogeneity on the characteristics (such as geometric complexity, roughness, and micro-morphology) of hydraulic fractures (HFs), laboratory triaxial fracturing experiments were conducted on homogeneous and layered tight sandstones using water- and carbon dioxide (CO₂)-based fluids. The fractal dimension of HFs (D_f) and the stimulated fracture area were quantitatively analyzed based on computerized tomography scanning and three-dimensional reconstruction methods. The surface roughness and micro-characteristics of HFs were then evaluated. Experimental results showed that CO₂-based fracturing tended to create more HFs with a higher degree of geometric complexity (D_f ranging 2.1113–2.2271) than that of HFs created by water-based fracturing (D_f ranging 2.0952–2.1081) in a homogeneous tight sandstone. A complex HF network (D_f = 2.4092) was generated in the layered tight sandstone using supercritical CO₂ (SC-CO₂) fracturing. The SC-CO₂-induced fracture area in the layered tight sandstone was approximately three times larger than that of the homogeneous tight sandstone. Both the fluid viscosity and particle size of the host rock have a great effect on the roughness of the HF surface. CO₂-based fracturing tends to generate curved HFs with uneven and cracked HF surfaces and massive microfractures, whereas water-based fracturing is more likely to induce straightforward HFs with smooth and clean HF surfaces and few microfractures. These induced microfractures enhance the micro-complexity of HFs, which may contribute to additional conductivity in the production stage. The obtained results provide a laboratory research basis for fracturing fluid selection and fracturing treatment design in tight sandstone reservoirs.