陕蒙地区厚硬砂岩顶板定向水力压裂预割缝倾角优化及防冲实践

Pre-existing crack angle optimization of thick sandstone roof during directional hydraulic fracturing and its application to anti-rockburst in Shannxi and Inner Mongolia areas

预割缝倾角是深孔顶板定向水压致裂重要施工技术参数之一。通过半圆盘试件3点弯(SCB)断裂力学试验以及水压致裂力学模型解析分析,研究预割缝倾角影响坚硬顶板在定向切槽诱导下水压致裂过程中的断裂特性。结果表明:水压裂纹的起裂方向明显受到预设裂纹倾角影响,但最大主应力对裂纹扩展方向起到控制作用。在地应力一定条件下,随着预割缝倾角增加,临界注水压力减小,Ⅰ型应力强度因子KⅠ逐渐增加,Ⅱ型应力强度因子KⅡ先增加后减小。为保证水压裂纹既受到较大的张拉作用,又能利用地应力的作用发生剪切扩展,KⅡ达到最大值时的预割缝倾角被选为施工水力压裂参数取值。该参数设计被应用到巴彦高勒煤矿厚层砂岩顶板水压致裂工况,对比压裂过程中顶板的煤炮强度和频次以及压裂前后辅回撤通道附近的微震、应力变化,证明了设计参数下水压致裂裂隙范围有效的起到了防冲效果。

Pre-existing crack angle is one of the important technical parameters for deep hole roof directional hydraulic fracturing. Through the fracture mechanics test of semi-circular bending and the analysis of hydraulic fracturing mechanics model, the fracture characteristics of the hard roof under the influence of pre-existing crack angle on the hydraulic fracturing induced by the directional grooving were studied. The research shows that the pre-existing crack inclination angle affects the crack initiation direction of the hydraulic crack, but the maximum principal stress plays a controlling role in the crack propagation direction. Under the certain condition of crustal stress, the critical water injection pressure decreases with the increase of the pre-existing inclination angle, the type I stress intensity factor K_Ⅰ increases gradually, and the type II stress intensity factor K_Ⅱ increases first and then decreases. In order to ensure that the hydraulic crack is subjected to a large tensile stress effect, and the shear expansion can be performed by the action of ground stress, the pre-existing crack inclination angle when the K_Ⅱ reaches the maximum value should be valued according to the actual construction parameter. The fracture mechanics test and theoretical results were applied to the actual hydraulic conditions of the thick sandstone roof of the Bayangole coal mine. The strength and frequency of the coal cannon in the roof during the fracturing process and the microseisms and the change of stress near the auxiliary retracement channel before and after fracturing were compared, which proves that the range of hydraulic fracturing crack under the designed parameters effectively promotes anti-rock burst.