不同燃爆载荷作用下页岩破裂特性及孔裂隙结构改性规律

Fracture characteristics and pore structure evolution law of shale under different methane explosive fracturing loads

页岩储层甲烷原位燃爆压裂是一项新型无水压裂技术,不同燃爆载荷下井周的破裂特性及孔隙结构改性规律尚不明确,无法为关键工艺参数的确定提供依据。自主搭建耐压100 MPa燃爆管道,对?55 mm×55 mm筛管完井页岩试样进行不同燃爆载荷的冲击压裂实验,实验过程中采集爆炸压力-时程曲线,取样表征页岩破裂形态随燃爆载荷的变化规律,通过典型裂缝面分析燃爆压裂致裂机理,通过?1 000 mm×1 000 mm页岩相似材料燃爆压裂实验,阐明燃爆载荷压裂储层的主控因素,基于压汞法表征不同燃爆载荷下页岩孔裂隙结构的演化规律,主要结论如下:(1)提升燃爆混合气体的初始压力能够显著提升燃爆峰值载荷和压力上升倍数,增强燃爆威力;(2)随着燃爆载荷的提高,井周由单一主裂缝转变为多条径向裂缝协同起裂,但燃爆载荷过大容易在井周产生粉碎性破坏,本研究中最佳爆炸压力为71 MPa;(3)燃爆冲击波和爆轰产物气楔扩缝效应是导致裂缝起裂-扩展的主控因素,燃爆压裂有利于构建粉碎区范围小、裂隙区范围大的裂缝体系;(4)燃爆冲击波主导页岩大孔和微裂隙体积随燃爆载荷升高而显著提升,动态冲击和高温热效应的双重控制机制导致微孔呈先降低后升高的变化规律。本研究通过实验室及大尺度燃爆压裂实验阐明了燃爆载荷对井周储层破裂特性和孔裂隙结构改性的控制机制。

Methane in-situ explosive fracturing technology is a revolutionary waterless fracturing technology.The fracturing characteristics and pore structure modification mechanisms of shale in near-wellbore region under varied explosive loads remain unclear,and some key parameters are unable to be determined for technological applications.In this study,a methane explosion tube with the maximum pressure resistance of 100 MPa was built and the explosion fracturing experiments were conducted on the ?55 mm×55 mm sieve tube completion shale samples.The methane explosion pressure-time curve in the pipeline was collected during the experiment.The morphology evolution of fractures with increasing loads was characterized by collecting fracture specimens.Large-scale fracturing experiments on the ?1 000 mm×1 000 mm shale analogs elucidated the controlling factors of reservoir fracturing by explosive loading.Mercury intrusion porosimetry revealed the evolutionary patterns of shale pore-fracture structures.The main findings are as follows:(1) Increasing the initial pressure of methene-oxygen mixture gas can significantly improve peak blasting loads and pressure rise multiples;(2) With the increase of the explosive loading,the near-wellbore region transits from a single dominant fracture to multiple radial fractures that initiate collaborated cracking.However,excessive loads may cause crushed zones near the wellbore,with the optimal fracturing pressure being 71 MPa in this study;(3) Deflagration shock waves and detonation product gas wedge effects are the controlling mechanisms for fracture initiation and propagation.Deflagration fracturing helps to construct stimulated zones with small crushed regions but extensive fracture networks;(4) The volumes of shale macropores and microcracks remarkably increase with rising deflagration loads,exhibiting a decreasing then increasing trend for micropores,jointly controlled by dynamic impacts and thermal effects.This study elucidates the control mechanisms of deflagration loads on reservoir fracturing behaviors and pore-fracture structure modifications through laboratory and largescale experiments.