超临界CO_2致裂页岩实验研究

Experimental study on the supercritical CO_2 fracturing of shale

超临界CO_2压裂技术作为一项环保的无水压裂开采技术,在页岩油气的商业开采中,越来越受到重视;但目前关于页岩气超临界CO_2压裂的裂纹扩展和延伸机理的认识还不够深入。采用实验室自主研制的超临界CO_2致裂增渗实验装置,对页岩进行了超临界CO_2压裂实验研究,结合声发射监测技术、CT扫描技术分析了压裂过程中的裂纹扩展规律,以及对压裂前后的渗透率进行比较,得到如下结论:页岩的裂纹扩展可以分为容腔充填区段、孔隙压力累积区段、压裂破坏区段等3个区段,在达到起裂压力点时声发射信号最为强烈,超临界CO_2在压裂页岩过程存在相态的变化;压裂后页岩渗透率可提高3~4个数量级,压裂增渗效果显著;CT图像显示裂纹延伸方向整体上与最大主应力方向趋于一致,最先在井壁弱面处发生起裂,超临界CO_2压裂能够形成复杂裂隙网络。研究结果可为页岩气储层超临界CO_2压裂施工参数的选择提供依据。

Supercritical CO2 fracturing technology,as an environmental friendly non-aqueous fracturing technology for shale gas extraction,has attracted more and more attention in the commercial exploitation of shale gas and shale oil.But the mechanism of crack propagation and the extension of supercritical CO2 in shale are still not well understood.In this paper,the experimental study of supercritical CO2 fracturing in shale was carried out by using the self-developed instrument for simulating the supercritical CO2 fracturing and the increase of shale permeability. The acoustic emission monitoring technology,combined with CT scanning technology,was employed to investigate the crack propagation law of the fracturing process,and the comparison of the permeability of shale before and after fracturing was conducted to evaluate the fracturing efficiency.The main conclusions are as follows：the fracturing and crack-propagation process can be divided into three stages,such as cavity filling stage,pore pressure accumulation stage and fracture propagation stage.The maximum counts of the acoustic emission is reached at the initiation pressure point,and the phase change occurs during the supercritical CO2 fracturing shale process;after fracturing,the permeability of the shale increases 3-4 orders of magnitude compared to that before fracturing,so the permeability increase is noticeable by fracturing;CT images show that the whole direction of the crack propagation tends to be parallel to the direction of the maximum principal stress,and the cracks are firstly initiated in the weak planes of wellbore,supercritical CO2 fracturing can induce complex fracture networks.This study can provide a scientific basis for the selection of the engineering parameters of the supercritical CO2 fracturing for shale gas exploitation.