含径向水力割缝钻孔导向压裂裂缝形态及影响要素

Fracture pattern and influencing factors of guided hydraulic fracturing with radial slot and well borehole

随着煤矿开采深度向千米级发展以及大采高特大采高技术的研发应用,大空间远场坚硬顶板问题突出,其结构失稳显著影响工作面矿压以及工作面临空侧巷道变形。地面钻井水力压裂是治理远场坚硬顶板的防治措施之一,但是水压裂缝形貌受地应力控制、形态单一,难以达到顶板“横切纵断”目标,限制了其弱化远场坚硬顶板的效能。因此,基于试验和理论分析开展了含径向水力割缝钻孔导向压裂形成“横切纵断”裂缝的研究,揭示了水力压裂裂缝和导向裂缝的竞争扩展规律,建立了基于Iwrin等效模型的割缝导向起裂扩展模型、钻孔裸眼起裂扩展模型以及钻孔割缝压裂裂缝起裂模式判定准则,探究了塑性区临界特征长度、割缝半径、钻孔半径、埋深对于裂缝起裂模式的影响。研究结果表明:含径向水力割缝钻孔导向压裂存在3种裂缝形貌:“横切”型单一主裂缝、“纵断”型单一主裂缝、“横切纵断”复合型裂缝网络;割缝塑性区特征长度和钻孔塑性区特征长度与流体压力均呈正相关关系,形成“横切纵断”复合型裂缝网络对应的塑性区临界特征长度随割缝半径的增大而增大,但是随钻孔半径的增大而减小;储层埋深对含径向水力割缝钻孔导向压裂裂缝形貌存在显著影响,随着埋深增加,裂缝形貌逐渐由“横切”型单一主裂缝向“横切纵断”复合型裂缝网络和“纵断”型单一主裂缝演变。特定埋深下,可以通过设计调整钻孔半径和割缝半径以使得钻孔塑性区长度和割缝塑性区长度同时达到塑性区临界长度,形成“横切纵断”复合型裂缝模式。

With the development of coal mining depth to KM level and the research and application of large mining height and super high mining technology,the far-field hard roof problem in overlying large space during coal mining is very serious.It has significant impact on the strong mining pressure in working face and the deformation of the roadway on the free face side of working face.Ground fracturing is one of the control measures for the hard roof in overlying large space,however,it always forms fractures with single shape that controlled by in-situ stress and has a limited effect on weakening far-field hard roof.For this purpose,the experimental and theoretical researches on the fracture pattern of slot guided borehole fracturing were conducted.The competitive law of hydraulic fracture and slot guided fracture was revealed.The slot guided fracture initiation model and well borehole initiation model were built and fracture initiation pattern criteria were formed.The influence of different critical characteristic lengths of plastic zone,different radius of slot,different radius of well and different depth on fracture initiation pattern were explored.The results show that three hydraulic fracturing modes exist in hydraulic slotted ground fracturing(transversely type,longitudinally type,composite type).The characteristic lengths of slot plastic zone and borehole plastic zone show positive correlations with fluid pressure.The corresponding critical length of plastic zone that forming composite type fracture shows positive correlation with the radius of slot while negative correlation with the radius of borehole.The bury depth of coal seam has significant impact on fracture initiation pattern that transient from longitudinally type to transversely type and composite type as bury depth increases.Under specific buried depth,the composite type fracture can be formed by adjusting both characteristic lengths of borehole plastic zone and slot plastic zone equal to the critical length of plastic zone through an appropriate design of the radius of well borehole and slot.