超临界二氧化碳无水压裂新技术实验研究展望

EXPERIMENTAL STUDY ON SUPERCRITICAL CO_2 FRACTURING

超临界二氧化碳是一种介于气体和液体之间的流体,密度接近于液体,而黏度接近于气体;它的表面张力很低,扩散系数高,具有很强的渗透能力,能渗透到岩石中的天然微裂缝,压裂中有利于复杂网络裂缝的形成,而且能置换被岩石吸附的烃类,使其变成游离态而有利于油气的增产与产出;超低的表面张力有利于液体的返排,从而降低油气层的伤害。因此,将超临界二氧化碳流体作为压裂液进行压裂改造,是二氧化碳干法压裂的一种发展方向,不仅可以节约水资源,还具有返排率高,储层伤害小,增产潜力大等特点。实验研究表明,超临界二氧化碳压裂的破裂压力比清水和液态二氧化碳压裂的破裂压力低,并且其分形维数和声发射源到平面(源到该平面距离平方和最小)的平均距离大,更容易产生波状的裂缝和裂缝分支,有利于形成复杂的网络裂缝,特别适合页岩气的增产改造和有效开发。

As a kind of fluid between gas and liquid states, supercritical CO2 is characterized by low surface tension, high diffusion coefficient, and strong infiltrability. Its density is close to that of liquids, whereas the viscosity is similar to that of gases. During a fracturing, supercritical CO2 can infiltrate into natural fractures and help forming a complex fracture network, and replace hydrocarbons adsorbed by rocks in favor of oil and gas production. In addition, its extra-low surface tension benefits fluid flowback, thus reducing formation damage. As a development trend of CO2 fracturing, supercritical CO2 fracturing fluid not only can economize water resource, but also has some advantages such as high flowback rate, small reservoir damage, and large potential of production increase. Experimental study shows that compared with water and liquid CO2 fracturing, supercritical CO2 fracturing has a lower fracture pressure. Its large fractal dimension and long average distance from acoustic emission source to plane may easily cause wavelike fractures and fractures with branches. The obtained complex fractures are conducive to forming fracture network. Therefore, supercritical CO2 is especially suitable for the stimulation and effective development of shale gas.