Mechanisms of short brittle creep deformation and failure of Wufeng shale in Changning

doi:10.3969/j.issn.2096-1693.2025.03.027

Abstract

Abstract:

To investigate the triaxial compression creep deformation behavior and failure mechanisms of shale, this study conducted stepwise incremental loading creep tests on Wufeng Formation shale of Changning under confining pressures of 10 MPa, 30 MPa, and 50 MPa. The characteristics of axial, radial, and volumetric creep curves were analyzed. Combining computed tomography (CT) and scanning electron microscopy (SEM) techniques, the macroscopic and microscopic creep failure mechanisms of shale were systematically examined. A damage model was employed to fit and predict the long-term creep behavior of shale. The results indicate that: Volumetric creep effectively characterizes the evolutionary features of shale creep, with creep crack initiation stress and damage stress threshold corresponding to approximately 60% and 80% of peak strength, respectively. Shale creep failure is predominantly governed by microcrack initiation and coalescence, forming spatially distributed shear fracture networks characterized by shear-compaction bands and branched cracks. Multiple subparallel fractures develop within the shear-compaction bands, while extensive branching cracks readily form in surrounding regions. The damage model accurately predicts the long-term creep behavior of shale. Parameter A, associated with loading stress and elastic modulus, serves as the primary controlling factor for creep. This research can be applied to the analysis of the influence of shale creep on wellbore stability and artificial fracture closure during the drilling and fracturing processes of shale oil and gas reservoirs.

Key words: shale, creep mechanism, accelerated creep, shear fracture network, damage model

摘要：

为了揭示页岩三轴压缩蠕变变形规律及破坏机制，本研究采用长宁五峰组页岩露头开展了围压10 MPa，30 MPa，50 MPa条件下的逐级增量加载蠕变实验，分析轴向、径向和体积蠕变曲线特征，结合计算机断层扫描(CT)和扫描电镜(SEM)技术系统探讨页岩宏微观蠕变破坏机制，同时采用损伤模型预测页岩长期蠕变行为，结果表明：体积蠕变能有效表征页岩蠕变演化特征，其蠕变起裂应力与损伤应力阈值分别为峰值强度的60%和80%；页岩蠕变破坏以微裂纹形成-贯通为主，形成剪切压实带与分支裂纹的空间裂缝体，剪切压实带内发育多条近平行裂缝，其周围易于形成多分支裂纹；损伤模型能准确预测页岩长期蠕变行为，模型参数A为蠕变的主要控制参数，主要由应力水平及弹性模量决定。该研究可应用于页岩油气储层钻井、压裂过程中页岩蠕变对井壁稳定、人工裂缝闭合等影响分析。

关键词: 页岩, 蠕变机制, 加速蠕变, 剪切裂缝体, 损伤模型

CLC Number:

ZHOU Dawei, ZHANG Guangqing, LI Shiyuan, XU Quansheng, CAO Hu, ZHAO Chuyang, WANG Chen. Mechanisms of short brittle creep deformation and failure of Wufeng shale in Changning[J]. Petroleum Science Bulletin, 2025, 10(6): 1240-1251.

周大伟, 张广清, 李世远, 徐全胜, 曹虎, 赵楚阳, 王晨. 长宁五峰组页岩短期脆性蠕变变形与破坏机理研究[J]. 石油科学通报, 2025, 10(6): 1240-1251.

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URL: https://sykxtb.cup.edu.cn/EN/10.3969/j.issn.2096-1693.2025.03.027

https://sykxtb.cup.edu.cn/EN/Y2025/V10/I6/1240

Figures/Tables 12

Fig. 1 Macro and micro characteristics of shale samples from the Wufeng Formation

Fig. 2 Experimental setup and axial stress-strain curves of shale

Table 1 Scheme of Triaxial Stress-Stepping Loading Test

编号	P_c/MPa	σ_p/MPa	σ_ac/MPa
编号	P_c/MPa	σ_p/MPa	1级	2级	3级	4级	5级	6级	7级	8级	9级	10级	11级	12级
1#	10	102	50	60	65	70	75	80	85	90	95
2#	30	187	90	108	117	126	135	144	153	162	171	180	189	198
3#	50	260	125	150	163	175	188	200	212

Fig. 3 The axial, radial, and effective Poisson’s ratio curves under P_c=30 MPa

Fig. 4 Creep curves of shale under three confining pressures

Fig.5 Relationship between creep variation and stress under different confining pressures

Fig. 6 Curves of accelerated volumetric creep and its slope

Fig. 7 Macroscopic failure characteristics of shale

Fig. 8 CT scanning characteristics of shale failure(P_c=30 MPa)

Fig. 9 Micro-features of shale creep failure

Fig. 10 Four models fitting experimental curve and prediction of creep after 32 years (P_c=50 MPa，σ_ac=188 MPa)

Table 2 Fitting parameters and predicted creep values under various σ_ac

拟合模型	参数	数值(σ_ac=188 MPa)	32年后预测值	数值(σ_ac=200 MPa)	32年后预测值
幂率模型	A	8.03×10^-6	0.049	8.06×10^-6	0.065
	m	0.42		0.43
	R²	0.9998		0.9998
对数模型	A	-0.000 67	0.0017	-0.000 82	0.0021
	B	0.000 12		0.00014
	R²	0.9948		0.9945
指数模型	A	0.000 57	0.00057	0.000 66	0.00066
	B	0.835		0.841
		9891		10482
	R²	0.9993		0.9995
损伤模型	A	0.20	0.0056	0.29	0.007
	B	97.0		100.1
	k	555.89		531.01
	R²	0.9999		0.9999

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