可控源微波辐射强化页岩气解吸与改善储层渗透性的作用机理

Shale gas stimulation is a significant technical problem that must be solved in the field of shale gas development. Enhancing shale gas desorption and permeability of reservoir is an effective way to solve this problem. The preliminary studies accomplished by the applicant had shown that the controlled microwave irradiation can promote the desorption of coalbed methane and improve the permeability of coal seam. Hereby, based on the similarity of gas occurrence, pore structure and reservoir properties in shale gas reservoir and coalbed methane reservoir, this project proposes a novel idea of using controlled microwave irradiation to enhance the desorption of shale gas and improve the permeability of shale gas reservoir. By means of experimental study, numerical calculation and theoretical analysis, this project are proposed to study the characteristics of shale gas desorption, damage and fracture, enhancing effect of shale gas reservoir under the controlled microwave irradiation. Firstly, the relationship between microwave irradiation parameters (loading method, power and irradiation time), methane desorption rate, pore-fissure structure, permeability and electromagnetic emission thermal effect would be acquired. And then desorption diffusion dynamics model for shale gas in shale microstructure under controlled microwave irradiation would be established. Thus, the dynamic characteristics of the desorption-diffusion and the rule of the improvement of permeability of shale gas reservoir under the controlled microwave irradiation would be revealed. Consequently, the mechanism of enhancing shale gas desorption and improving permeability of shale gas reservoir using controlled microwave irradiation is explored. This project aims to provide a theoretical basis for the establishment of a new kind of technology for shale gas stimulation using microwave irradiation.

页岩气增产是页岩气开发领域亟待解决的重大技术难题，而提高页岩气解吸速率与储层渗透性是其有效解决途径。申请者的前期研究证实，可控源微波辐射能够促进煤层气解吸、提高煤储层渗透性。据此，基于页岩气藏在气体赋存状态、孔隙结构和储层性质等方面与煤层气藏的相似性，本项目按照“可控源微波辐射促进页岩气解吸与储层致裂增渗”的新思路，采用实验研究、数值计算与理论分析的方法，研究可控源微波辐射下页岩气储层的甲烷解吸特性、损伤致裂特性和增渗效果，获取可控源微波参量(加载方式、功率、辐射时间)与页岩岩样的甲烷解吸速率、孔裂隙结构特征、渗透率和电磁辐射热效应之间的相互关系，建立可控源微波辐射下页岩气在页岩基质微孔中的解吸-扩散动力学模型，揭示可控源微波辐射下页岩气解吸-扩散的动力学特征与页岩气储层的致裂增渗规律，探寻可控源微波辐射强化页岩气解吸与改善储层渗透性的作用机理，为研发页岩气微波激励增产新技术奠定理论基础。

微波激励增产技术以其特有的加热特性、能源转换效率高和环保性，被认为是一种极具发展前景的页岩气开发新方法。为探索页岩气微波激励增产的可行性与实效性，本项目自主研制了复合馈入式微波辐射致裂与吸附解吸实验装置，开展了不同微波参量条件下页岩样的气体吸附-解吸试验与微波致裂实验，研究了可控源微波辐射下页岩的气体解吸特性、损伤致裂特征、孔裂隙结构及渗透性演化规律，获取了微波参量与页岩的气体解吸速率、电磁热效应之间的相互关系，确定了符合可控源微波辐射下页岩基质微孔中气体解吸特性的解吸-扩散动力学模型；建立了微波参量分别与页岩的P波速度、损伤因子、孔裂隙结构特征参数和渗透率之间的变化关系，获取了可控源微波辐射下页岩的气体解吸-扩散的动力学规律与损伤致裂效应，揭示了可控源微波辐射强化页岩气解吸与改善储层渗透性的作用机理，初步建立了页岩气储层微波激励增渗的基础理论。研究结果表明：(1) 在微波辐射下，页岩温度快速升高且呈非均匀性分布，微波功率越大，非均匀性越强。(2) 微波辐射能增大页岩内气体分子活性与扩散通道，显著提高页岩的气体解吸速度与解吸量，且累计气体解吸量随微波功率的增大而增加。(3) 微波辐射后页岩的中大孔和微裂缝的数量明显增大，而微孔的数量略有减少，孔隙连通性增加，且渗透率随页岩含水饱和度的增加而增大。(4) 微波辐射下页岩微结构损伤与微波功率、辐射次数、能量施加顺序以及页岩含水情况等因素相关，且存在微波能量阈值现象。(5) 微波对页岩中各造岩矿物的热效应，既引起页岩孔内水分汽化而产生蒸气压，又造成页岩基质非均匀膨胀而产生热应力。由于页岩升温的快速性，页岩基质颗粒在较高的蒸气压与热应力加载速率下，易表现出较强脆性，造成页岩的含水矿物晶片断裂、刚性颗粒内部与颗粒界面之间产生大量微裂缝，引起页岩宏观裂缝发育与破裂，呈现出微波对页岩的致裂效应，从而改善页岩气储层的渗透性。