正交节理与应力比值系数对水力裂缝扩展影响规律研究

doi:10.13809/j.cnki.cn32-1825/te.2020.05.008

摘要/Abstract

摘要：

地层中的节理对水力裂缝扩展延伸形成复杂缝网影响较大。为了研究水力裂缝遇到正交节理时的扩展行为,采用块体离散元方法对三维正交裂缝的水力压裂进行了模拟,分析了应力比值系数k（水平地应力/竖向地应力）对裂缝缝内压力和宽度的影响规律。研究结果表明,水力裂缝扩展到正交节理相交处时会受到阻碍,使相交节理面产生滑移,裂缝尖端发生钝化,阻碍裂缝扩展,而剪应力是产生阻碍作用的主要因素。当水力裂缝沿着宽度方向延伸后,剪应力发生变化,阻碍作用减小,裂缝突破阻碍作用而继续扩展。延伸方向受k值影响较大,当k为0.8和1.0时,竖向裂缝贯穿水平节理沿原方向扩展,但当k为1.2时,竖向裂缝将会转向,沿着水平节理方向扩展。水力裂缝的最大宽度和缝内压力受到k值影响也较大。当k为1.0时,裂缝最大宽度趋于稳定,裂缝内压力基本不变;当k为1.2时,水力裂缝转向水平节理扩展,最大宽度下降,缝内压力突降,随后才逐步稳定。水力压裂过程中水力裂缝穿层与否会影响压裂效果,因此可以选择合适的应力比值,实现对水力裂缝扩展方向的控制,从而对页岩水力裂缝扩展形态判定和压裂方案制定进行很好的指导。

关键词: 水力裂缝, 正交节理, 应力比值系数, 阻碍作用, 裂缝扩展, 离散元法

Abstract:

The joints in the stratum have a greater impact on the formation of complex fracture networks with hydraulic fractures. In order to study the propagation behavior of hydraulic fractures when they encounter orthogonal joints, the hydraulic fracturing of 3D orthogonal fractures is simulated based on the discrete element method, and the influence of the stress ratio coefficient k on the pressure and width of fractures is analyzed. The research results show that when the hydraulic fracture expands to the intersection of orthogonal joints, it will be hindered, so that the intersecting joint plane will slip and the fracture tip will be passivated, which will hinder the fracture expansion. Shear stress is the main factor that causes the obstruction. When the hydraulic fracture extends along the width direction, the shear stress changes and the hindrance decreases, so the crack can break through the hindrance and continues to extend. The extension direction is greatly affected by the value of k. When k is 0.8 and 1.0, vertical cracks extend through the horizontal joints and expand in the original direction. When k is 1.2, the vertical cracks will turn and expand along the horizontal joints. The maximum width of the hydraulic fracture and the pressure in the fracture are also greatly affected by the value of k. When k is 1.0, the maximum width of the fracture is stable, and the pressure in the fracture is unchanged. When k is 1.2, the hydraulic fracture expands along the horizontal joint after turning, the maximum width decreases, and the pressure in the fracture drops suddenly, and then gradually stabilizes. Whether the hydraulic fracture penetrates will affect the fracturing effect, a suitable stress ratio to control the direction of hydraulic fracture propagation can be chosen. Therefore, it can be a good guide for the determination of the shale hydraulic fracture propagation form and the formulation of the construction plan.

Key words: hydraulic crack, orthogonal jointing, stress ratio coefficient, obstruction, crack propagation, discrete element method

中图分类号:

TE357.1

张伯虎,周昌满,郑永香,刘建军. 正交节理与应力比值系数对水力裂缝扩展影响规律研究[J]. 油气藏评价与开发, 2020, 10(5): 55-62.

ZHANG Bohu,ZHOU Changman,ZHENG Yongxiang,LIU Jianjun. Influence of orthogonal joint and stress ratio coefficients on hydraulic fracture propagation[J]. Reservoir Evaluation and Development, 2020, 10(5): 55-62.

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参数	取值	参数	取值
弹性模量/GPa	20	注入流体黏度/（Pa·s）	0.001 5
泊松比	0.25	注入流体密度/（kg·m^-3）	1 000
岩体密度/（kg·m^-3）	2 600	注入点埋深/m	3 000
节理面内摩擦角/（° ）	20	重力加速度/（m·s^-2）	9.81
节理面黏聚力/MPa	0	注入点竖向地应力/MPa	76.52

方案	p	σ_x=σ_y	σ_z	长边S_z	短边S_z
缝内压力p变化	70	75	75	-158.347	-104.652
	75	75	75	-74.999	-75
	80	75	75	8.347	-45.348
	85	75	75	91.694	-15.696
	90	75	75	175.042	13.956
竖向应力σ_z变化	82	75	60	307.216	77.231
	82	75	65	218.706	40.325
	82	75	70	130.196	3.419
	82	75	75	41.143	-35.547
	82	75	80	-46.824	-70.393
	82	75	85	-136.006	-106.315
	82	75	90	-223.844	-144.205