二氧化碳对页岩力学性质劣化规律的影响

针对目前缺少不同相态CO_2对页岩微观结构及力学参数影响规律研究的现状,通过室内岩石-压裂液浸泡测试系统,对不同相态CO_2浸泡后岩石矿物组分、微观结构及力学参数进行了测试。研究结果表明:液态、超临界态CO_2相比较滑溜水对矿物颗粒微观结构的影响更为明显,矿物颗粒之间的孔隙增大4%～15%,易导致页岩宏观力学性质受到劣化影响;不同介质浸泡后页岩力学参数劣化受影响程度排序为:抗拉强度泊松比峰值强度弹性模量,滑溜水浸泡后页岩脆性特征变化不明显,而液态、超临界态CO_2浸泡后页岩脆性增强40%～50%。液态CO_2有利于改善岩石微观孔隙和降低岩石抗拉强度,超临界态CO_2有利于提高微孔隙/裂隙穿透性,提高岩石脆性和降低破裂压力。研究结果为页岩气CO_2干法压裂工艺优化提供了一定的实验支撑和理论依据。

Study on the effect of carbon dioxide on the degradation of shale mechanical properties

Carbon dioxide, as a fracturing medium instead of conventional water-based fracturing, has great potential in shale gas fracturing. Through the laboratory rock fracturing fluid immersion test system, the mineral composition, microstructure and mechanical parameters of the shale rock after immersion in different phase of carbon dioxide are tested, which have been compared with the results of conventional slick-water fluid immersion, the effect of carbon dioxide on the mechanical property degradation of shale is revealed. The results show that the immersion process of different fracturing medium will not change the mineral composition of shale; compared with liquid carbon dioxide and supercritical carbon dioxide, the influence of slick-water on the microstructure of mineral particles is more obvious, and the porosity between mineral particles increases by about 4-15%, which leads to the deterioration of the macroscopic mechanical properties of shale; the order of the degradation degree of mechanical parameters of shale after the immersion of different media are as follows: tensile strength > Poisson''s ratio > peak strength > modulus of elasticity, the brittleness change of shale after soaking in slick-water is not obvious, but the brittleness of shale after soaking in liquid and supercritical carbon dioxide increases by about 40-50%. Liquid carbon dioxide can improve the micro porosity and reduce the tensile strength of rock, while supercritical carbon dioxide can improve the permeability of micro porosity / fracture, improve the brittleness of rock and reduce the fracture initiation pressure. Through the research, it provides some experimental support and theoretical basis for the optimization of carbon dioxide fracturing to the shale gas.