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.

当前，如何有效和清洁地开发非常规资源是人们广泛关注的问题。由于水基压裂的不可避免的局限性，CO ₂和N ₂等被认为是某些特定油藏（如干旱地区或高粘土膨胀地层）的替代流体。为了研究非常规致密地层中非水流体引起的裂缝特征，采用不同的水，N ₂，液态CO ₂（L-CO ₂）和超临界CO ₂（SC-CO ₂）流体对致密砂岩进行压裂实验。） 进行了。提出了一种基于CT扫描的新型断口分析方法，定量分析了断口的网状结构，分形维数（FD），孔径，面积比（AR）和断口体积等微观形貌。结果表明，水和N ₂可以仅引起相对简单的断裂模式。然而，基于CO ₂的流体引起的裂缝具有更复杂的裂缝几何形状，具有许多裂缝分支和不同的传播方向。基于图像处理和裂缝重建算法的定量分析表明CO ₂基压裂液的压裂能力最多可达4.4倍，与水基压裂压裂相比，压裂液的复杂度高2.6倍，粗糙度提高23.4％。与SC-CO ₂，L-CO ₂和水N _2相比具有最小的裂缝诱发能力（83％的水），并产生更平滑和均匀的裂缝。另一方面，偏角的分布表明，裂缝的扩展不仅受应力状态的影响，而且还受到原始层理面的显着影响。该实验研究提出了一种精确，有效的裂缝特征量化方法，可以将其进一步应用到裂缝模拟模型中。也为非常规油气开发中的无水压裂现场作业提供理论参考和指导。