Digital quantification of fracture in full-scale rock using micro-CT images: A fracturing experiment with N2 and CO2

How to effectively and cleanly exploit unconventional resources is currently being widely concerned. Due to the unavoidable limitations of water-based fracturing, CO2 and N2 etc., are regarded as alternative fluids for some specific reservoirs such as the arid area or high clay swelling formation. To investigate the characteristics of fracture induced by nonaqueous fluid in unconventional tight formation, fracturing experiments on tight sandstone using different fluid of water, N2, liquid CO2(L-CO2) and supercritical CO2(SC-CO2) were conducted. A novel fracture analysis approach based on CT scanning is also proposed to quantitatively analyze the micromorphology of fractures including fracture network pattern, fractal dimension (FD), aperture, area ratio (AR) and fracture volume etc. Results show that water and N2 can only induce a relatively simple fracture pattern. Whereas, fractures induced by CO2-based fluid have more complex fracture geometry with many fracture branches and various propagation orientations. Quantitative analysis based on image processing and fracture reconstruction algorithm indicates that the CO2-based fluid has up to 4.4 times larger fracture-induced capacity and can create fracture with up to 2.6 times higher complexity and 23.4% higher roughness than that generated by water-based fracturing. Comparing to SC-CO2, L-CO2 and water, N2 has the least fracture-induced capacity (83% of water) and produces more smooth and uniform fractures. On the other hand, the distributions of deviation angle suggest that fracture propagation is not only influenced by stress state, but also significantly affected by the original bedding plane. This experimental study proposes a precise and effective approach for fracture characteristic quantification, which can be further implemented into fracture simulation model. It also provides theoretical reference and guidance for field operation of waterless fracturing in development of unconventional oil and gas.