Carbonate outcrops were taken from Ma ₅¹ sub-member in the Lower Paleozoic in the Yan’an gas field to conduct true tri-axial hydraulic fracturing experiments with water, liquid CO₂ and supercritical CO₂. CT scan was applied to analyze initiation and propagation laws of hydraulic fractures in carbonate rocks. The experiments show that supercritical CO₂ has low viscosity, strong diffusivity and large filtration during fracturing, which is more liable to increase pore pressure of rocks around wellbore and decrease breakdown pressure of carbonate rocks. However, it would cost much more volume of supercritical CO₂ than water to fracture rocks since the former increases the wellbore pressure more slowly during fracturing. For carbonate rocks with few natural fractures, tensional fractures are generated by fracturing with water and liquid CO₂, and these fractures propagate along the maximum horizontal principal stress direction; while fracturing with supercritical CO₂ can form shear fractures, whose morphology is rarely influenced by horizontal stress difference. Besides, the angle between propagation direction of these shear fractures near the wellbore and the maximum horizontal principal stress is 45°, and the fractures would gradually turn to propagate along the maximum horizontal principal stress when they extend to a certain distance from the wellbore, leading to an increase of fracture tortuosity compared with the former. For carbonate rocks with well-developed natural fractures, fracturing with fresh water is conducive to connect natural fractures with low approaching angle and form stepped fractures with simple morphology. The key to forming complex fractures after fracturing carbonate rocks is to connect the natural fractures with high approaching angle. It is easier for liquid CO₂ with low viscosity to realize such connection. Multi-directional fractures with relatively complex morphology would be formed after fracturing with liquid CO₂.

在延安气田下古生界马₅ ¹亚段碳酸盐岩露头进行水、液态CO ₂和超临界CO ₂真三轴水力压裂实验。应用CT扫描分析碳酸盐岩水力裂缝的起裂和扩展规律。实验表明，超临界CO ₂压裂时黏度低、扩散性强、滤失量大，更容易增加井筒周围岩石的孔隙压力，降低碳酸盐岩的击穿压力。然而，这将花费更多的超临界 CO ₂比水压裂岩石，因为前者在压裂过程中增加井筒压力的速度更慢。对于天然裂缝较少的碳酸盐岩，用水和液态CO ₂压裂产生张性裂缝，这些裂缝沿最大水平主应力方向扩展；用超临界 CO ₂压裂时可形成剪切裂缝，其形态很少受水平应力差的影响。此外，这些剪切裂缝在井筒附近的传播方向与最大水平主应力的夹角为45°，当裂缝延伸到井筒一定距离时，会逐渐转向沿最大水平主应力传播，导致与前者相比增加了骨折的曲折度。对于天然裂缝发育良好的碳酸盐岩，淡水压裂有利于连接小接近角的天然裂缝，形成形态简单的阶梯状裂缝。碳酸盐岩压裂后形成复杂裂缝的关键是连接大接近角的天然裂缝。液态二氧化碳更容易₂用低粘度实现这种连接。液态CO ₂压裂后会形成形态较为复杂的多向裂缝。