超临界二氧化碳脉冲致裂深部页岩动态损伤及渗流响应机理

China's southwest region is rich in shale gas reserves, but most favorable exploration and development areas are located at a depth of more than 3,500m. Deep shale is in a complex environment of ultra-high temperature, high in-situ stress and high osmotic pressure, and gradually changes from elastic-brittleness to ductility, which is not conducive to the formation of a complex fracture network for seepage channels. Hydraulic fracturing is the most important means of exploiting shale gas at present, but it has high initiation pressure, single fracture expansion. Also, it is prone to water sensitivity, water lock reaction, and induce earthquake or other secondary disasters. Based on the previous research, this project puts forward the idea of reforming and increasing permeability of deep shale by supercritical carbon dioxide pulse fracturing. The experiment of supercritical carbon dioxide pulse fluid shocking and soaking on shale will be carried out under different conditions. The influence of the micro-meso-macro analysis on the physicochemical structure and mechanical properties of deep shale will be analyzed to build a dynamic damage evolution model. The true triaxial experiments of supercritical carbon dioxide pulse fracturing shale under multi-field coupling are to be carried out, and the relevant criteria and judgment model for fracture initiation and propagation are to be established to reveal the formation mechanism of complex fracture network, which will illustrate the mechanism of supercritical carbon dioxide pulse fracturing effect on reservoir reconstruction and increasing permeability. The research results can provide a theoretical basis for enhancing the efficient development of deep shale gas with supercritical carbon dioxide.

我国西南地区页岩气储量丰富，但多数勘探开发有利区处于3500m以深。深部页岩处于超高温、高地应力、高渗透压的复杂环境，逐渐由弹脆性向延塑性转化，不利于形成大面积渗流通道的复杂缝网。采用水力压裂开采页岩气起裂压力高，裂缝扩展单一，且容易出现水敏、水锁反应，诱发地震等次生灾害。本项目在前期研究基础上，提出超临界二氧化碳脉冲致裂深部页岩改造及增渗思路：拟开展超临界二氧化碳脉冲流体冲击、浸泡页岩实验，从微观-细观-宏观分析对深部页岩物理化学结构、力学特性等影响规律，建立超临界二氧化碳脉冲作用下页岩动态损伤演化模型；开展多场耦合下超临界二氧化碳脉冲致裂页岩真三轴实验，建立相关起裂扩展准则及判断模型，揭示超临界二氧化碳脉冲致裂页岩复杂缝网形成机制，阐明超临界二氧化碳脉冲致裂深部页岩的改造及增渗机理。研究成果可为超临界二氧化碳高效开发深部页岩气提供理论支撑。

针对传统水力压裂技术在开采深部页岩气藏时存在裂缝单一、水敏、水锁等瓶颈效应，开展了超临界二氧化碳对深部原位页岩相互作用下的改造、增渗及封存机理研究。项目研究了不同温度下二氧化碳处理前后页岩的微观多尺度孔隙和表面化学结构等变化规律，阐明了超临界二氧化碳流体对页岩多尺度的结构损伤机制；通过分析非均匀孔隙压力梯度下的页岩导向压裂扩展，建立了相关起裂压力和扩展压力数学模型，并阐明了基于非均匀孔隙压力梯度与割缝导向压裂方法的裂缝扩展规律；同时，开展了三轴条件下不同侧向压力系数的超临界二氧化碳压裂页岩实验，对比了超临界二氧化碳压裂页岩时的应力场和裂缝场变化规律，获得了相应的起裂压力判断准则；此外，项目分析了二氧化碳、甲烷在页岩表面的选择性系数、吸附亲和力、表面势能和熵随二氧化碳浓度增加变化规律，并获得多种气体在蒙脱石等黏土矿物上的平衡吸附特性和吸附热力学特征变化规律，综合多种因素分析揭示了二氧化碳、甲烷以及混合气在页岩表面竞争吸附机制。项目研究成果为推进超临界二氧化碳在深部页岩气藏开采应用和地质封存方面提供了强有力的支撑作用。