富有机质页岩氧化致裂增渗加速气体传输机理研究

The organic shale generally deposits under oxygen-free environment, and gas in tight shale matrix experiences a poor mass transfer rate. Multi-stage hydraulic fracturing is commonly used in shale gas wells, but the flowback rate was definitely low, which might intensify the sharp reduction of production rate after early stage. In this project, considering the oxidizable property of the shale organic matter and pyrite, we would explore the mechanism about how to convert the detrimental fracturing fluid retention to beneficial force which could stimulate the fragmentation of shale matrix and accelerate gas desorption under the guiding thought of contradiction transform and turning harm into benefit. The scientific issue of this project is the transformation of shale gas storing and flow mechanism before and after rock oxidation. The key works may include the following aspects. Firstly, conduct the experiments of the impact of matrix oxidation on physical properties and pore structure of differential shales to analyze the proper shale for oxidation stimulation. Secondly, analyze the range of oxidation action with imbibition experiments of varies fracturing fluid. Thirdly, simulate the reaction process of shale matrix with oxidizing fluids and analyze the effect of oxidation action on the adsorption/desorption, diffusion and flow , in order to explore the mechanisms of motivating adsorbed gas desorption, enhancing matrix permeability and increasing fracture density with oxidation method. Finally, propose proper evaluative methods of fracturing fluid in consideration of its influence on multi-scale mass transfer of shale gas. All of the above works would provide us a solid foundation for the formation of a new shale gas reservoir stimulation method which can stimulate a highly efficient and dense fracture network in shale gas reservoir and improve the gas recovery ultimately.

富有机质页岩沉积于缺氧的还原环境，基块致密，富含有机质，气体传输困难，气井压裂液返排率低，压力和产量递减快。项目针对页岩中有机质、黄铁矿等矿物易氧化的特征，利用“矛盾转化,变害为利”的思想，围绕“页岩氧化作用下气体赋存状态及储渗空间转化机制”这一科学问题，探索将压裂液滞留这一不利因素转化为改变气体赋存状态、激发页岩破裂的有利条件。实验研究氧化作用对不同性质页岩物性与孔隙结构的影响，分析适合氧化改造的页岩气层性质；基于页岩对不同性质压裂液渗吸实验，分析页岩气藏的氧化作用范围。模拟原地条件下页岩与氧化性液体作用过程及对页岩气吸附/解吸、扩散、渗流的影响，研究氧化激发页岩吸附气解吸、提高基块渗透率、增加裂缝密度的机理，促使页岩气跨越扩散阶段，实现跨尺度渗流。考虑压裂液对气体多尺度传输能力的作用，形成适合页岩气体传输的压裂液评价方法，为形成高效密集缝网的页岩气藏改造及提高气体采收率新方法提供基础。

考虑页岩致密和水力压裂后大量滞留压裂液储层这些不利的工程地质因素，结合页岩富含有机质与黄铁矿等特点，基于矛盾转化“变害为利”的思想，项目紧密围绕 “氧化作用下页岩气赋存状态及其储渗空间的转化机制”这一核心科学问题，开展了利用氧化性压裂液消耗页岩有机质与黄铁矿等组分，从而提高基质渗透率、诱发页岩岩石破裂、切割页岩基块和激发吸附气解吸的研究。通过揭示氧化作用下富有机质页岩组分演变机制及孔隙结构响应特征，探究岩心尺度页岩氧化绝对增渗的机理和建立适合氧化改造的页岩气层性质评价方法，明确了富有机质页岩物性与孔隙结构的氧化改造作用；通过评价有机质、孔隙结构和湿度对页岩甲烷吸附/解吸能力的影响，揭示了氧化溶蚀对页岩甲烷吸附/解吸能力的影响机理，证实了富有机质页岩氧化气体赋存状态的转换；通过对页岩氧化致裂现象的可视化表征，实验探究了富有机质页岩氧化溶蚀致裂和氧化结晶胀裂两种机理，进而从损伤力学和断裂力学的角度揭示了原地条件下富有机质页岩氧化致裂机理；通过揭示富有机质页岩渗吸氧化性流体特征，实验探究氧化作用下水相渗吸路径改善对自吸的影响，分析了氧化作用下页岩中水相分散机理并明确富有机质页岩渗吸氧化液行为，揭示了富有机质页岩氧化性流体渗吸分布行为；基于氧化缓解页岩储层损害潜力的评价、氧化液提升页岩气体传输能力的分析和适合页岩气层改造的压裂液评价方法的建立，论证了氧化提高页岩气体传输能力的可实践性。研究成果为油气储层氧敏性概念的提出、页岩气井氧化压裂液“零返排”增产提效和氧化“减量增渗”协同水力压裂提高页岩气藏采收率思路的形成奠定了重要理论基础，为富有机质页岩储层氧化改造的潜力评价、推广应用提供了理论依据，对实现富有机质页岩气藏压后长期改造、延缓页岩气体产量递减速率、绿色环保高效开发具有重要意义。