致密砂岩脆性控制机理与声发射评价方法研究

Brittleness is an essential and fundamental parameter of unconventional reservoir evaluation and stimulated reservoir volume fracturing design. While there are a variety of empirical evaluation methods, the theoretical bases are diverse and the results vary considerably. Based on laboratory experiments and starting with rock fabric characteristics and brittle failure acoustic emission response respectively, this project planned to use mathematical methods such as relativity and single factor sensitivity to research inner control mechanism and apparent calculation method of porous and micro-fractured rock brittleness in the tight sandstones of Yanchang Formation in Ordos Basin. Acoustic emission can feed back rock compaction, elasticoplastic deformation and the whole process of fissure formation, evolution and destruction. By analyzing count, energy, frequency, wave form and other parameters acquired from core acoustic emission experiment as well as the effects of stimulated reservoir volume fracturing of corresponding well and the experimental results of single mineral rock acoustic emission, we explored a calculation method of brittleness index based on acoustic emission. Furthermore, we investigated the relationship between brittleness index and other conventional mechanical parameters. By means of core description, mercury intrusion, thin section identification, scanning electron microscope and X-ray diffraction, the project analyzed structural characteristics of internal defects such as rock pore throats and micro fractures and cementation, component characteristics of rock minerals and clay types and contents. Based on nonlinear failure process from micro to macro scale，we also researched brittle failure mechanism of reservoir rock. The project aims to clarify brittleness control mechanism of porous rock, establish a uniform calculation method of brittleness index and lay a theoretical foundation of unconventional reservoir evaluation and stimulation.

脆性是岩石破裂时未出现非弹性应变的力学性质，作为非常规储层的必要基础参数，现有评价方法的理论基础不一、表达形式多样、结果差异大。本项目拟针对鄂尔多斯盆地延长组致密砂岩，引入岩心声发射实验，研究计数、频率、声波尤其是能量等声发射参数在形变破坏全过程中的信息反馈，结合岩心物性改善程度和对应油井体积压裂效果，建立基于声发射参数的脆性指数计算方法，并探讨脆性指数与其他常规力学参数间的关系。应用非平衡条件下微裂纹成核、扩展并演化至宏观破裂的非线性跨尺度破坏理论，基于致密砂岩内部微裂缝较发育的现实，通过室内实验分析岩石的矿物组成特征和孔喉、胶结、微裂缝等结构特征，运用相关性分析和裂缝扩展有限元计算等手段，重新评估矿物成分决定脆性的现有认识，探究致密砂岩脆性的主控因素。项目旨在明确多孔多缝岩石脆性的控制机理，建立统一的脆性指数计算方法，为非常规储层评价和体积压裂设计提供可靠的脆性参数。

本项目建立了一体化的具有理论与实用价值的致密砂岩脆性评价方法。在实验方面，利用铸体薄片鉴定、扫描电镜分析和x衍射技术对样品进行了相应的分析，确定了矿物与粘土成分、颗粒大小、孔隙类型。运用高压与恒速压汞，确定了孔喉大小与分布，进而掌握了致密砂岩储层的物性与孔喉特征。随后, 通过岩心声发射实验所获得的事件数、能量数等声发射参数对岩心的破坏过程进行分析。在理论方面，基于损伤力学原理，建立了岩石脆性与声发射事件数的关系，从而获得岩石的脆性指数，分析了声发射脆性指数与此前建立的岩石力学脆性指数和矿物含量脆性指数之间的关系。通过系统分析储层评价、岩石力学与声发射等实验结果，探讨了致密砂岩岩石脆性的控制机理，并结合压裂分析方法论证了声发射脆性指数的合理性。研究结果表明，岩石的声发射具有突发性，并存在初始区、剧烈区、下降区和沉寂区等4个不同的阶段，声发射事件的数量反映了微裂纹产生的多少。声发射能量率与声发射事件率不一定存在很好的对应关系，这与岩石的组成和物性有关。声发射脆性与岩石力学脆性和矿物含量脆性之间存在一定的相关性；三轴压缩实验后的岩心渗透率的大幅提高，并与岩石脆性成正相关关系；压裂分析发现，在相同施工参数的情况下，脆性越大所形成的SRV区体积也越大，相应的储层改造和开发效果会越好。