Liu Du, Shu Longyong, Wang Yanbin, Ni Xiaoming, Huo Zhonggang, Wang Weihua
China Coal Research Institute, Beijing 100013, China.
CCTEG China Coal Research Institute, Beijing 100013, China.
ACS Omega. 2023 Oct 3;8(41):38072-38082. doi: 10.1021/acsomega.3c03999. eCollection 2023 Oct 17.
Coal structure is one of the key geological factors that affects the effect of coal reservoir stimulation. Based on the geological spatial combination characteristics, thickness, and proportion of different coal structures, the coal reservoir is divided into different coal structure combination types. The hydraulic fracturing device is used to carry out indoor fracturing experiments and dissect the crack initiation and expansion characteristics with different coal structure combinations. The results show that the coal structure combination is of the binary type (undeformed coal + granulated coal or cataclastic coal + granulated coal), and the undeformed coal (cataclastic coal) can overcome the tensile strength and minimum principal stress when it is driven by the high-pressure fluid. The undeformed coal (cataclastic coal) ruptures and forms longitudinal cracks. The increasing proportion of granulated coal inhibits crack expansion and promotes the transverse deformation of coal. The interface contact point between the undeformed coal (cataclastic coal) and granulated coal easily fractures along the cross section of the specimen. When the coal structure combination is the triplex type (undeformed coal + granulated coal + cataclastic coal or cataclastic coal + granulated coal + cataclastic coal), the undeformed coal or cataclastic coal is transformed. The forming fractures in the undeformed coal (cataclastic coal) can cut through the soft coal when the thickness of the undeformed coal (cataclastic coal) is large and the thickness of granulated coal is thin. On the contrary, it is not easy to cut through. When the coal structure combination is granulated coal + cataclastic coal + granulated coal, the cataclastic coal fails under shear stress and forms the crack along the cross section of the coal sample. The granulated coal inhibits the crack expansion at both ends. The research results have an important indicative significance for further understanding the fracture initiation and propagation mechanism of hydraulic fracturing with complex coal structures in coal reservoirs.
煤体结构是影响煤储层压裂效果的关键地质因素之一。依据地质空间组合特征、不同煤体结构的厚度及比例，将煤储层划分为不同的煤体结构组合类型。利用水力压裂装置开展室内压裂实验，剖析不同煤体结构组合下的裂纹起裂与扩展特征。结果表明，煤体结构组合为二元型（原生结构煤+碎粒煤或碎裂煤+碎粒煤）时，原生结构煤（碎裂煤）在高压流体驱动下能克服抗拉强度和最小主应力，发生破裂并形成纵向裂纹。碎粒煤比例增加会抑制裂纹扩展，促进煤体横向变形。原生结构煤（碎裂煤）与碎粒煤的界面接触点易沿试样横截面发生断裂。当煤体结构组合为三元型（原生结构煤+碎粒煤+碎裂煤或碎裂煤+碎粒煤+碎裂煤）时，原生结构煤或碎裂煤发生转变。当原生结构煤（碎裂煤）厚度大、碎粒煤厚度薄时，原生结构煤（碎裂煤）中形成的裂缝能切割软煤；反之则不易切割。当煤体结构组合为碎粒煤+碎裂煤+碎粒煤时，碎裂煤在剪应力作用下破坏，沿煤样横截面形成裂缝，碎粒煤抑制裂缝两端扩展。研究成果对进一步认识煤储层复杂煤体结构水力压裂裂缝起裂与扩展机理具有重要指示意义。
