低温压裂液注入促进缝网形成的力学机理研究

Unconventional reservoir can be developed economically only through a large scale stimulated fracture network. Fracability and the difficulty of shear slip decide that whether geological dessert can be economically developed and the efficiency of stimulated fracture network. Previous studies have focused on the rock brittleness and the interaction between the natural and hydraulic fractures and the purpose are maximizing the fracturing efficiency under existing geological conditions, however, less study has focus on how to improve the conditions for forming fracture network. Existing methods of improving the fracturing effect are reducing the fracture initiation pressure and controlling the single fracture extension through the methods such as acidification and perforation, which has the limit improvement action on large scale volume fracture, so the problems that the internals with low fracability can’t be developed and the low efficiency induced by large displacement slick water can’t be solved. The following function may be produced to improve the efficiency of forming fracture network through low temperature fracturing fluids injection: (1) reservoir is cooled and contracted which may improve the brittleness, fracture development and fracability; (2) Thermal elastic expansion of fracturing fluids may accelerate the fluid infiltration into fracture which can reduce the friction coefficient, and form excess pore pressure, consequently the shear slip can be promoted. So, the temperature effect of shale and low temperature fracturing fluids will be tested with indoor experiment, then the calculation model coupled thermo-hydro-mechanical are established to analyze the extension of fracture network, and mechanics mechanism of promoting fracture network forming through low temperature fracturing fluids injection are revealed, finally the new theory and new methods for improving fracability and the stimulated efficiency of fracture network such as “cool or thermo cycling treatment through low temperature fracturing fluids injection” and so on are explored.

非常规储层须经大规模缝网改造才能形成产能。可压裂性和剪切滑移的难易是决定地质甜点是否经济可采和缝网改造效率的关键。以往的研究集中在岩石脆性、天然与人工裂缝相互作用等方面，目的是追求现有地质条件下最大的压裂效率，对改善缝网形成条件的研究较少。已有的改善方法是通过酸化、射孔等方式降低起裂压力和引导裂缝延伸，对体积压裂的改善作用有限，无法解决低可压性层段不可采和大排量滑溜水作业的低效率问题。低温压裂液注入很可能产生以下作用提高缝网形成效率：①储层冷却收缩，脆性、裂缝发育程度及可压裂性提高；②压裂液热弹性膨胀，加速流体渗入缝面降低摩擦系数并形成超孔隙压力，促进剪切滑移。因此，本研究拟通过实验研究页岩与压裂液的温度效应，建立缝网扩展的热-流-固耦合计算模型，揭示低温压裂液注入提高缝网形成能力和改造效率的力学机制，探索注入低温压裂液进行冷或冷热循环处理等提高储层可压裂性和缝网改造效率的新理论和新方法。

非常规储层须经大规模缝网改造才能形成产能。可压裂性和剪切滑移的难易是决定地质“甜点”是否经济可采和缝网改造效率的关键。以往的研究对改善缝网形成条件的研究较少。本研究通过实验测试分析了低温压裂液处理对页岩原始力学性质及孔缝结构的影响，揭示了低温压裂液处理对页岩可压裂性的改善机制与效果，建立了低温压裂液注入过程中流动及传热模型，揭示了超孔隙压力的形成机制，研究了压裂液注入对缝面摩擦性质的影响规律，建立了热-流-固耦合作用下缝网形成的计算模型及低温处理对缝网形成能力和缝网改造效果影响的评价方法，并开展了分析评价。研究结果表明，低温压裂液注入会造成页岩基质脆性增加，内部裂缝发育，同时，低温压裂液注入会提高储层孔隙压力，降低页岩黏聚力和内摩擦角，降低页岩抗拉强度及断裂韧性，产生低温诱导应力，强化页岩储层的可压裂性，促进缝网形成。项目研究成果对于提高非常规储层的动用程度及缝网改造效率等有具有重要的意义。研究过程中，出版专著2部，发表学术论文10篇，申请/授权发明专利5项，培养硕士研究生3人。