液氮辅助页岩体积压裂增效机理研究

In order to obtain the desired production, shale formation should be fractured so as to improve the flow capacity of SRV. Whether a shale reservoir has the ability to be effectively fractured or not depends on many geological factors such as natural fractures, rock brittleness and in-situ stress difference coefficient. Shale rock with higher brittleness is one of the necessary conditions for the formation of the fracture network. Due to the liquefied gas usually has ultra-low temperature, the rock surfaces with great temperature gradient will shrink fast, when they get in touch with the liquefied gas. Therefore, abundant micro fractures are induced and the brittleness of rock are increased. This phenomenon may highlights a new potential technology of high efficiency SRV for shale gas. Our research plans to study the brittle damage mechanisms of shale under ultra-low temperature of the liquid nitrogen environment, and analyze the feasibility of the liquid nitrogen assisted network-fracturing technology for shale gas reservoir. First, the micro damage evolution characteristics, stress-strain constitutive features, brittle enhance mechanism and effective thermal conductive properties of shale under different freezing time and freezing-melting circulation are to be studied with comprehensive laboratory experiments such as NMR, SEM and macro rock mechanics tests. Then, the micro damage constitutive model of shale under ultra-low temperature, the effective thermal conductivity model and thermal-mechanical coupled fracture model of shale will be set up. The computational software of the finite element method will be programmed to solve this multi-physics coupling problem. Evolution process of complex fractures under liquid nitrogen will be analyzed, which will be verified by true triaxial fracturing simulation test. At last, based on the above research results, The appraisal diagrams of shale reservoir suitable for liquid nitrogen assisted fracturing will be given.

页岩储层只有通过体积压裂形成复杂缝网才能获得理想产能，而较高的脆性是形成缝网的必要条件之一。液氮等低温液化气体与岩石接触时，会使岩石表面发生高速收缩变形，形成大量微裂隙，岩石脆性显著提高，为高效页岩储层体积压裂提供了新的技术发展方向。由于液氮压裂国外尚未规模应用，国内则刚刚开始探索，其致裂机理及规律尚未掌握。为了深入揭示页岩超低温热-力耦合损伤破裂机制，探讨液氮辅助页岩体积压裂的可行性，本研究拟综合利用NMR、SEM、宏观岩石力学测试等多种手段，对不同冻结时间和冻融循环次数条件下页岩细观损伤演化特征、有效热传导特性、低温增脆机制加以研究，建立页岩超低温细观损伤本构模型、有效热传导特性演化模型以及超低温热-力耦合损伤断裂数学模型，开发多物理场耦合有限元计算程序，分析页岩在液氮超低温环境下复杂裂缝扩展规律，并利用室内真三轴压裂物理模拟实验进行验证。在此基础上建立液氮压裂地层适应性评价图版。

液氮无水压裂作为一项新的压裂技术，可在一定程度上解决页岩气藏水力压裂开采中环境污染和储层改造效果差的问题。该压裂技术还处于探索阶段，压裂增效机理尚不明确。鉴于此，本项目综合采用物理实验、理论分析和数值模拟等研究手段，开展了液氮辅助页岩体积压裂增效机理研究。主要取得以下结论和认识：（1）液氮处理后室温下页岩的声波波速降低，渗透率增加，核磁共振测试发现页岩中各级孔隙数量均增加，弹性模量、抗压强度、抗拉强度及脆性指数均呈现不同程度的降低，且液氮处理后页岩的物理力学变化幅度受页岩的饱和程度及层理发育方位影响较大。（2）液氮处理后室温下真三轴无水压裂实验表明，页岩的裂隙发育程度增加，水平应力差异系数减小，裂缝扩展阻力减小，弱面力学性能弱化，储层的可压性增强，起裂压力下降。（3）建立了页岩冻融细观损伤多场耦合本构模型，自主开发了岩石冻融细观损伤数值模拟软件，分析了液氮作用下不同敏感因素对页岩增脆-损伤和力学特性的影响，指出液氮作用下页岩主要经历冻胀和融缩两种变形机制，导致页岩产生随机离散性的细观张性破坏。液氮冻结引起的深冷冻缩、矿物晶体位错塞积和孔隙冰胶结增强效应，能够显著提高页岩的强度和脆性；而融化引起的融缩变形会增加页岩的损伤程度，导致页岩的强度和脆性明显降低。（4）揭示了液氮注井后井眼围岩起裂压力变化规律，指出井眼围岩的起裂压力随着液氮温度的降低呈现出先降低、后增加、再降低的趋势，随着水平应力差、地层温度、地层孔隙度和弹性模量的增加而大幅度降低。（5）分析了液氮冻融后水平井水力压裂过程中压裂方式与段间距均会影响多裂缝的扩展形态。指出相比顺序压裂，Texas两步法压裂方式下形成的缝网面积较大。本研究从机理上证实了液氮无水压裂的可行性，为后期压裂开发页岩气等致密油气藏提供了基础。