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Casing deformation from fracture slip in hydraulic fracturing
Accepted Manuscript Casing deformation from fracture slip in hydraulic fracturing Fei Yin, Lihong Han, Shangyu Yang, Yong Deng, Yongming He, Xingru Wu PII:
S0920-4105(18)30194-3
DOI:
10.1016/j.petrol.2018.03.010
Reference:
PETROL 4751
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 20 August 2017 Revised Date:
1 March 2018
Accepted Date: 1 March 2018
Please cite this article as: Yin, F., Han, L., Yang, S., Deng, Y., He, Y., Wu, X., Casing deformation from fracture slip in hydraulic fracturing, Journal of Petroleum Science and Engineering (2018), doi: 10.1016/ j.petrol.2018.03.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fei Yina,b, Lihong Hanc, Shangyu Yangc , Yong Denga, Yongming Hea, Xingru Wub, ∗ a College of Energy, Chengdu University of Technology, Chengdu, Sichuan 610059, China; b The Mewbourne School of Petroleum & Geological Engineering, The University of Oklahoma, Norman, OK 73019, USA; c State Key Laboratory for Performance and Structure Safety of Petroleum Tubular Goods and Equipment Materials, CNPC Tubular Goods Research Institute, Xi’an 710077, China. ∗
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Corresponding author. E-mail addresses: [email protected] (F. Yin), [email protected] (X. Wu).
Abstract: Casing failure during hydraulic fracturing is infrequent, but its consequence and impact can be significant for the shale gas wells. It may result in inaccessible cased hole and environmental and safety concerns over wellbore integrity during the shale gas reservoir development. Based on theoretical analysis, numerical simulation and field measurement, the failure mechanisms of casings in shale gas wells are revealed. Through parametric sensitivity analysis, the countermeasures are recommended. Results indicate that one of the casing failure mechanisms in shale gas wells is shear deformation induced by the slip of shear fractures. The simulated casing deformation is consistent with the lead impression. Casing curvature is introduced to assess the traveling capability and casing integrity. The maximum curvature is significantly larger than that of normal directional borehole, which causes the resistance of downhole tools passing through casing. Decreasing the crossing angle and employing the cement with a low Young’s modulus or even no cementing can be the effective measures to prevent casing failure in this mode. The research findings provide new evaluation criterion and controlling methods for casing integrity in shale gas wells under hydraulic fracturing.
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Casing Deformation from Fracture Slip in Hydraulic Fracturing
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Key words: casing integrity; hydraulic fracturing; shale gas well; shear slip
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1. Introduction
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Massive hydraulic fracturing along a horizontal well is the key technology in developing low permeability resources such as shale gas and tight oil. Current researches about hydraulic fracturing mainly focus on fracture propagation and well performance (Abou-Sayed et al., 2005; Li et al. 2012; Yuan et al., 2015; Yuan et al., 2017). In practice, the rate of casing failure in different modes for tight formation is 1
ACCEPTED MANUSCRIPT higher than that of conventional reservoir development in the process of hydraulic fracturing. For example, there were 8 wells among 11 horizontal wells presented casing deformation in the ChangNing and WeiYuan shale gas fields in China. In these regions, the casing deformation or failures occur frequently during the multi-stage hydraulic fracturing process, which significant impacted later operations and production efficiency. Sustaining casing integrity is one of the challenges of shale gas wells. Daneshy (2005) attributed casing failure to a consequence of large formation deformation, including compressive, tensile and shear stresses in the formation. Furui et al. (2010) found that fracturing and acidizing can lead to compaction effect, borehole instability and casing deformation. He et al. (2014) simulated the reservoir deformation during hydraulic fracturing and found the fractures can lead to casing and subsurface deformation. Using downhole tilt data in an observation well, Wang et al. (2015) identified that the local buckling, connection failure and shear failure are the main failure modes. Lian et al. (2015) and Yu et al. (2016) concluded that some casing failures were the joint result of rock property change, asymmetric treatment zones and stress field redistribution using finite element modeling. The heterogeneity severity of stress field increases significantly. Yin et al. (2015) calculated the annulus pressure build-up during hydraulic fracturing and the maximum external pressure of casing. Liu et al. (2017) owned the main reason of casing failure to local loads. Gao et al. (2017) studied the rock slip along bedding planes and natural fractures induced by asymmetric fracturing causes casing deformation. Yan et al. (2017) thought the cement-void pressure decline might be the main reason of casing deformation. Xi et al. (2017) found the activation of weak plane in shale beddings caused casing shear deformation. The previous researches have involved different mechanisms and haven’t supplied measures to deal with casing failure in shale gas wells. Although the problem of casing deformation/failure in shale gas wells has been identified in last few years and many scholars and engineers have attempted to solve it, since there are so many variables attributing to this phenomenon, it is still a great challenge in the shale gas development process. In this paper, we further analyzed the casing failure mechanism by using numerical simulation and field measurement. The shear failure mechanism induced by fracture slip and a numerical modal are proposed. The mechanical behavior of casing in the slip fractures is analyzed, and casing curvature is newly introduced to assess casing integrity in shale gas wells under hydraulic fracturing. Furthermore, the effects of key factors on casing integrity are investigated. We recommend some effective measures to reduce and/or prevent casing failure for shale gas wells under hydraulic fracturing. The novelty of the research is that it supplies a new failure mechanism, a corresponding simple model and effective countermeasures for casing integrity under hydraulic fracturing.
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2. Failure mechanism
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The Mohr-Coulomb criterion is often used as a slip criterion for geo-material. It can be written linearly as:
τ max = c + σ ′ tan φ σ ′=σ − p
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Where, τmax is the shear strength, MPa; c is the cohesion, MPa; ϕ is the internal friction angle, °; σ' is the effective normal stress, MPa; σ is the normal stress, MPa; p is pore pressure, MPa. The tri-axial rock tests demonstrate the cohesion and the internal friction angle decrease with the increase of the moisture content. The Eq. 1 shows that the shear strength of rock is reduced when fracturing fluid is injected into rock as the effective normal stress decreases. The strain accumulation will continually increase the shear stress acting on the fractures until reaching the limit stress. At that point, the fractures or faults will slip and produce earthquakes (McGarr, 2014). Rock slip from unbalance crustal stress can be caused by high-pressure water injection (Maxwell et al., 2009; Ozan et al., 2011) in hydraulic fracturing in shale reservoir. Given the large fracturing volume, the excessive stimulated regions, and the high pumping rate, the fracture initiation and propagation not only generate tensile fractures, but also can cause mechanical behaviors such as shear failure, leap and slip around the well (Chipperfield et al., 2007). When the vertical fracture propagates and intersects the inclined fractures (could be natural fractures), shear failure occurs near the inclined fractures and shear fractures are generated (Zeng and Yao, 2015; Hou et al., 2016) as shown in Fig. 1. The red region in Fig. 1 represents tensile fractures and the blue region represents shear fractures. Shear failure results from convergence and correlation of microcracks (Yu et al., 2013). Hydraulic fracturing causes fracture dilation and permeability increase (Dusseautl, 2011). Shale gas reservoir usually contains numerous natural fractures. Shear failure of natural fractures provides the possibility of complex fracture network.
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Fig. 1. Tensile and shear fractures induced by hydraulic fracturing.
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Fig. 2. Micro-seismic event, vertical cross section and plan view.
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Fracture slip during hydraulic fracturing can cause casing shear failure (Liu et al., 2016; Haghshenas et al., 2017) as shown in Fig. 3. When a fracture slips through the wellbore, a shear load will impose on the casing. Casing is prone to damage under the action of formation shear slip. This failure mechanism can be validated by the detection of lead impression. Fig. 4 shows an example of this deformation by the lead impression in the H3-6 well. It displays the wedge shape on one side of casing, which indicates that casing is subjected to a lateral force induced by fracture slip. The deformed casing restricts tool access to the borehole down below. Thus, it is frequently seen that the milling shoe and bridge plug meet resistance after fracturing several stages.
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The natural fractures can cause asymmetric fracture propagation and proppant distribution (Shiozawa and McClure, 2016). Fig. 2 shows the asymmetric fracturing areas surrounding a well (Azad et al., 2017) detected by micro-seismic during hydraulic fracturing. It indicates that the fracture distribution is asymmetric about the wellbore. The asymmetric fracture and inferred nonuniform overpressure could cause formation deformation and even slip along fractures.
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Fig. 3. Diagram of casing deformation induced by fracture slip.
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3. Casing integrity assessment
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3.1. Modelling methodology
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Finite Element Method (FEM) and Distinct Element Method (DEM) are often used to study the complicated rock mechanics problems in hydraulic fracturing. The preset fracture plane is simulated by cohesive element in the ABAQUS-Soil module with which the stress, pore pressure and fracture geometry can be predicted (Zhang et al., 2012; Xue et al., 2008). The PFC of DEM can be used to simulate the initiation and propagation of complex network of fractures (Zeng and Yao, 2015; Yu et al., 2013; Hamidi and Mortazavi, 2012). The fractures are affected by principal stress components, rock property, fracturing fluid property and structural planes. Here, we focus on the mechanical behavior of casing under the fractures slip occurring during hydraulic fracturing. A 3D nonlinear finite element model is established to simulate the mechanical behavior of casing crossing slip formation. A bilinear model is used to model the constitutive relation of casing steel, and the Drucker-Prager model is used to simulate the cement sheath and formation. Fig. 5 shows the finite element model of casing in slip rock. The cuboid rock block has the dimensions of 4000 mm × 1200 mm× 1200 mm. A natural fracture with the dip angle α and width of ∆d is created in the rock block. The rock block is divided into a sitting part and a mobile part (or two mobile parts). The normal displacements on the settled part surface are zero. The slip displacement of the mobile part is represented as s. The y-axis and z-axis displacements on the front and back surfaces of the mobile part are s·sinϕ and −s·cosϕ. Ignoring the bonding strength, the interaction relations among borehole, casing and cement sheath are simplified to be frictional contact.
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Fig. 4. Lead impression of H3-6 well.
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We used a real case example to show the casing deformation from fracture slip. In the case, the initial size of the production casing is Φ127×12.14 mm with a grade of P110. The mechanical properties of shale, cement sheath and casing are listed in Table 1. Hydraulic fracturing causes shear fractures and their slip. The fracture width is assumed as ∆d=20 mm. The slip displacement of fracture plane is assumed as s=20 mm. The crossing angle between well axis and fracture planes θ=75°. The friction coefficients among borehole, casing and cement sheath are f=0.2. Table 1 Material properties of shale, cement sheath and casing. Material
Shale
Young’s modulus
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ratio
Yield
Tangent
strength
modulus
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(MPa)
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Frictional
(MPa)
angle (°)
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40.2
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0.23
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24.0
210000
0.3
758
2000
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Cement sheath
Casing
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Fig. 5. The finite element model of casing in slip rock.
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Numerical simulations were performed to analyze the casing behavior under the fracture slip. The diagram of casing transverse displacement is shown in Fig. 6, and the casing Mises stress is shown in Fig. 7. They indicate that the fracture slip leads to casing large deformation and stress concentration.
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Fig. 6. Contour diagram of casing transverse displacement.
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Fig. 7. Contour diagram of casing Mises stress.
Fig. 8 shows the casing transverse displacement distribution under various slip displacements. At the axial position from −600 mm to +600 mm from the fracture plane, the transverse displacement increases sharply. The transverse displacement of casing the axial position far away from the fracture plane changes relative slower. The maximum transverse displacement occurs at 1260 mm from the fracture plane.
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Fig. 8. Casing transverse displacement distribution under various slip displacements.
Casing von Mises stress distribution under various slip displacements is shown in Fig. 9. It displays the shape of double peaks. The peak von Mises stress occurs at ±420 mm from the fracture plane. But, this fracture slip does not cause yield because the von Mises stress is much less than yield strength.
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Fig. 9. Casing von Mises stress distribution under various slip displacements.
The predicted deformation is consistent with the lead impression (Fig. 4). The failure mode of casing is elastic shear deformation other than material yield. Therefore, the von Mises yield criterion isn’t applicable to assess casing integrity. Casing integrity assessment of shale gas wells under hydraulic fracturing should consider the traveling capability of downhole tools. 8
ACCEPTED MANUSCRIPT The curvature is introduced to represent the traveling capability. Casing curvature distribution under various slip displacements is shown in Fig. 10. The borehole curvature in the building up section of H3-1 is 4.4°/25 m. When slip displacement is 20 mm, the maximum casing curvature reaches to 29.2 °/25 m. The maximum curvature occurs at fracture plane, which is significantly larger than that of normal directional borehole.
Fig. 10. Casing curvature distribution under various slip displacements.
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4. Proposed solution to casing deformation
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Casing integrity is affected by many factors such as the properties of casing, cementing, and formation in the process of well drilling and completion. Through parametric sensitivity analysis, the countermeasures are recommended to prevent casing deformation and failure during hydraulic fracturing in shale gas wells.
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4.1. Casing design
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The conventional countermeasures for avoiding casing failure include adopting heavier casing and higher steel-grade casing. Assuming the other parameters as constants, the wall thickness of casing is a variable. Various wall thicknesses of casings such as 12, 14, 16, 18, 20 mm are chosen for the horizontal section, and the corresponding simulations are performed. The effect of casing wall thickness on the maximum curvature of casing is shown in Fig. 11.
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Fig. 11. Effect of casing wall thickness on maximum curvature of casing.
With the increase of casing wall thickness, the maximum curvature of casing decreases slightly. When the casing wall thickness varies from 12 mm to 20 mm, the maximum curvature of casing only decreases 8%. The maximum curvature of casing is still too large for later downhole operations. Furthermore, the effect of casing steel grade on the maximum curvature is analyzed. The von Mises stress and the deformation of casing are almost same for P110 to TP125V. In the shale gas fields, the casings with steel grades from TP95S to TP140V have been reported to casing deformation. Theory and practice indicate that the high steel-grade and thick wall casings can’t solve the problem of casing failure induced by hydraulic fracturing.
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4.2. Cementing operation
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Young’s modulus is a key parameter of the cement sheath. Various Young’s moduli of cement sheaths such as 0.1, 1, 10, 20, 30 GPa are chosen, and the corresponding simulations are performed with all other parameters remaining unchanged. The effect of Young’s modulus of cement sheath on the maximum curvature of casing is shown in Fig. 12.
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Fig. 12. Effect of Young’s modulus of cement sheath on maximum curvature of casing.
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With the decrease of Young’s modulus of cement sheath, the maximum curvature of casing decreases sharply. Under the condition of the Young’s modulus of cement sheath varying from 30 GPa to 0.1 GPa, the maximum curvature of casing decreases 74%. So, downhole tools can pass through the small deformed casing more easily when the low Young’s modulus cement is employed. We further suggest that no cementing operation is adopted in the interval containing natural fractures and faults. Besides, assuming the other parameters as constants and the Young’s modulus of cement sheath as 0.1 GPa, the Poisson’s ratio of cement sheath is a variable. Various Poisson’s ratios of cement sheaths such as 0.1, 0.2, 0.3, 0.4, 0.49 are chosen, and the corresponding simulations are performed. Results indicate that the effect of Poisson’s ratio of cement sheath on casing deformation is very slight.
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4.3. Well trajectory
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The Young’s modulus, Poisson’s ratio and strength of the formation are rock properties and usually assumed to be constants. Although the dip angle of the slip natural fracture can’t be changed, the crossing angle between the fracture and borehole can be changed by adjusting the deviation angle of borehole. Assuming the other parameters as constants, the crossing angle is a variable. Various crossing angles such as 30°, 45°, 60°, 75°, 90° are chosen, and the corresponding simulations are performed. The effect of crossing angle on the maximum curvature of casing is shown in Fig. 13.
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Fig. 13. Effect of crossing angle on maximum curvature of casing.
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With the decrease of crossing angle, the maximum curvature of casing decreases significantly. For example, the maximum curvature of casing averagely decreases 72% when the crossing angle varies from 90° to 30°. Therefore, well trajectory should be designed to decrease the crossing angle between well axis and fracture planes to reduce the risk of casing failure induced by hydraulic fracturing.
5. Conclusions
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The failure mechanism of casings induced by the slip of shear fractures during hydraulic fracturing is studied using a finite element model. The predicted deformation is consistent with the lead impression. The fracture slip leads to casing deformation and stress concentration. The transverse displacement increases sharply around the fracture plane and the peak von Mises stress occurs at ±420 mm from the fracture plane. Casing curvature is newly introduced to assess casing integrity in shale gas wells under hydraulic fracturing. The maximum curvature of casing occurs at fracture plane, which is significantly larger than that of normal directional borehole. Enhancing casing strength can’t reduce casing deformation under slip rock. Decreasing the crossing angle and employing the low Young’s modulus cement or even no cementing are the effective countermeasures to prevent casing failure during hydraulic fracturing in shale gas wells.
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Nomenclature
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c p
= the cohesion, MPa; = the pore pressure, MPa; 12
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= the normal stress, MPa; = the effective normal stress, MPa; = the shear strength, MPa; = the internal friction angle, °; = the dip angle of fracture, °; = the crossing angle between well axis and fracture planes, °; = the gap of fracture, mm; = the slip displacement of fracture, mm; = the friction coefficient, dimensionless.
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Acknowledgment
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The authors would like to thank the financial support from Youth Science Fund of Chengdu University of Technology (Grant no. 2017QJ12).
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by Hydraulic Fracture Complexity. SPE Reservoir Evaluation & Engineering 12(1), 48-52. McGarr, A., 2014. Maximum magnitude earthquakes induced by fluid injection, J. Geophys. Res. Solid Earth 119, 1008-1019.
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Highlights
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Casing failure mechanism in shale gas wells under hydraulic fracturing is revealed. A novel finite element model of casing in slip rock is proposed. Casing curvature is newly introduced to assess casing integrity. Countermeasures that prevent casing failure under fracturing are recommended.
S0920-4105(18)30194-3
DOI:
10.1016/j.petrol.2018.03.010
Reference:
PETROL 4751
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 20 August 2017 Revised Date:
1 March 2018
Accepted Date: 1 March 2018
Please cite this article as: Yin, F., Han, L., Yang, S., Deng, Y., He, Y., Wu, X., Casing deformation from fracture slip in hydraulic fracturing, Journal of Petroleum Science and Engineering (2018), doi: 10.1016/ j.petrol.2018.03.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fei Yina,b, Lihong Hanc, Shangyu Yangc , Yong Denga, Yongming Hea, Xingru Wub, ∗ a College of Energy, Chengdu University of Technology, Chengdu, Sichuan 610059, China; b The Mewbourne School of Petroleum & Geological Engineering, The University of Oklahoma, Norman, OK 73019, USA; c State Key Laboratory for Performance and Structure Safety of Petroleum Tubular Goods and Equipment Materials, CNPC Tubular Goods Research Institute, Xi’an 710077, China. ∗
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Corresponding author. E-mail addresses: [email protected] (F. Yin), [email protected] (X. Wu).
Abstract: Casing failure during hydraulic fracturing is infrequent, but its consequence and impact can be significant for the shale gas wells. It may result in inaccessible cased hole and environmental and safety concerns over wellbore integrity during the shale gas reservoir development. Based on theoretical analysis, numerical simulation and field measurement, the failure mechanisms of casings in shale gas wells are revealed. Through parametric sensitivity analysis, the countermeasures are recommended. Results indicate that one of the casing failure mechanisms in shale gas wells is shear deformation induced by the slip of shear fractures. The simulated casing deformation is consistent with the lead impression. Casing curvature is introduced to assess the traveling capability and casing integrity. The maximum curvature is significantly larger than that of normal directional borehole, which causes the resistance of downhole tools passing through casing. Decreasing the crossing angle and employing the cement with a low Young’s modulus or even no cementing can be the effective measures to prevent casing failure in this mode. The research findings provide new evaluation criterion and controlling methods for casing integrity in shale gas wells under hydraulic fracturing.
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Casing Deformation from Fracture Slip in Hydraulic Fracturing
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Key words: casing integrity; hydraulic fracturing; shale gas well; shear slip
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1. Introduction
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Massive hydraulic fracturing along a horizontal well is the key technology in developing low permeability resources such as shale gas and tight oil. Current researches about hydraulic fracturing mainly focus on fracture propagation and well performance (Abou-Sayed et al., 2005; Li et al. 2012; Yuan et al., 2015; Yuan et al., 2017). In practice, the rate of casing failure in different modes for tight formation is 1
ACCEPTED MANUSCRIPT higher than that of conventional reservoir development in the process of hydraulic fracturing. For example, there were 8 wells among 11 horizontal wells presented casing deformation in the ChangNing and WeiYuan shale gas fields in China. In these regions, the casing deformation or failures occur frequently during the multi-stage hydraulic fracturing process, which significant impacted later operations and production efficiency. Sustaining casing integrity is one of the challenges of shale gas wells. Daneshy (2005) attributed casing failure to a consequence of large formation deformation, including compressive, tensile and shear stresses in the formation. Furui et al. (2010) found that fracturing and acidizing can lead to compaction effect, borehole instability and casing deformation. He et al. (2014) simulated the reservoir deformation during hydraulic fracturing and found the fractures can lead to casing and subsurface deformation. Using downhole tilt data in an observation well, Wang et al. (2015) identified that the local buckling, connection failure and shear failure are the main failure modes. Lian et al. (2015) and Yu et al. (2016) concluded that some casing failures were the joint result of rock property change, asymmetric treatment zones and stress field redistribution using finite element modeling. The heterogeneity severity of stress field increases significantly. Yin et al. (2015) calculated the annulus pressure build-up during hydraulic fracturing and the maximum external pressure of casing. Liu et al. (2017) owned the main reason of casing failure to local loads. Gao et al. (2017) studied the rock slip along bedding planes and natural fractures induced by asymmetric fracturing causes casing deformation. Yan et al. (2017) thought the cement-void pressure decline might be the main reason of casing deformation. Xi et al. (2017) found the activation of weak plane in shale beddings caused casing shear deformation. The previous researches have involved different mechanisms and haven’t supplied measures to deal with casing failure in shale gas wells. Although the problem of casing deformation/failure in shale gas wells has been identified in last few years and many scholars and engineers have attempted to solve it, since there are so many variables attributing to this phenomenon, it is still a great challenge in the shale gas development process. In this paper, we further analyzed the casing failure mechanism by using numerical simulation and field measurement. The shear failure mechanism induced by fracture slip and a numerical modal are proposed. The mechanical behavior of casing in the slip fractures is analyzed, and casing curvature is newly introduced to assess casing integrity in shale gas wells under hydraulic fracturing. Furthermore, the effects of key factors on casing integrity are investigated. We recommend some effective measures to reduce and/or prevent casing failure for shale gas wells under hydraulic fracturing. The novelty of the research is that it supplies a new failure mechanism, a corresponding simple model and effective countermeasures for casing integrity under hydraulic fracturing.
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2. Failure mechanism
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The Mohr-Coulomb criterion is often used as a slip criterion for geo-material. It can be written linearly as:
τ max = c + σ ′ tan φ σ ′=σ − p
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Where, τmax is the shear strength, MPa; c is the cohesion, MPa; ϕ is the internal friction angle, °; σ' is the effective normal stress, MPa; σ is the normal stress, MPa; p is pore pressure, MPa. The tri-axial rock tests demonstrate the cohesion and the internal friction angle decrease with the increase of the moisture content. The Eq. 1 shows that the shear strength of rock is reduced when fracturing fluid is injected into rock as the effective normal stress decreases. The strain accumulation will continually increase the shear stress acting on the fractures until reaching the limit stress. At that point, the fractures or faults will slip and produce earthquakes (McGarr, 2014). Rock slip from unbalance crustal stress can be caused by high-pressure water injection (Maxwell et al., 2009; Ozan et al., 2011) in hydraulic fracturing in shale reservoir. Given the large fracturing volume, the excessive stimulated regions, and the high pumping rate, the fracture initiation and propagation not only generate tensile fractures, but also can cause mechanical behaviors such as shear failure, leap and slip around the well (Chipperfield et al., 2007). When the vertical fracture propagates and intersects the inclined fractures (could be natural fractures), shear failure occurs near the inclined fractures and shear fractures are generated (Zeng and Yao, 2015; Hou et al., 2016) as shown in Fig. 1. The red region in Fig. 1 represents tensile fractures and the blue region represents shear fractures. Shear failure results from convergence and correlation of microcracks (Yu et al., 2013). Hydraulic fracturing causes fracture dilation and permeability increase (Dusseautl, 2011). Shale gas reservoir usually contains numerous natural fractures. Shear failure of natural fractures provides the possibility of complex fracture network.
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Fig. 1. Tensile and shear fractures induced by hydraulic fracturing.
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Fig. 2. Micro-seismic event, vertical cross section and plan view.
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Fracture slip during hydraulic fracturing can cause casing shear failure (Liu et al., 2016; Haghshenas et al., 2017) as shown in Fig. 3. When a fracture slips through the wellbore, a shear load will impose on the casing. Casing is prone to damage under the action of formation shear slip. This failure mechanism can be validated by the detection of lead impression. Fig. 4 shows an example of this deformation by the lead impression in the H3-6 well. It displays the wedge shape on one side of casing, which indicates that casing is subjected to a lateral force induced by fracture slip. The deformed casing restricts tool access to the borehole down below. Thus, it is frequently seen that the milling shoe and bridge plug meet resistance after fracturing several stages.
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The natural fractures can cause asymmetric fracture propagation and proppant distribution (Shiozawa and McClure, 2016). Fig. 2 shows the asymmetric fracturing areas surrounding a well (Azad et al., 2017) detected by micro-seismic during hydraulic fracturing. It indicates that the fracture distribution is asymmetric about the wellbore. The asymmetric fracture and inferred nonuniform overpressure could cause formation deformation and even slip along fractures.
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Fig. 3. Diagram of casing deformation induced by fracture slip.
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3. Casing integrity assessment
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3.1. Modelling methodology
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Finite Element Method (FEM) and Distinct Element Method (DEM) are often used to study the complicated rock mechanics problems in hydraulic fracturing. The preset fracture plane is simulated by cohesive element in the ABAQUS-Soil module with which the stress, pore pressure and fracture geometry can be predicted (Zhang et al., 2012; Xue et al., 2008). The PFC of DEM can be used to simulate the initiation and propagation of complex network of fractures (Zeng and Yao, 2015; Yu et al., 2013; Hamidi and Mortazavi, 2012). The fractures are affected by principal stress components, rock property, fracturing fluid property and structural planes. Here, we focus on the mechanical behavior of casing under the fractures slip occurring during hydraulic fracturing. A 3D nonlinear finite element model is established to simulate the mechanical behavior of casing crossing slip formation. A bilinear model is used to model the constitutive relation of casing steel, and the Drucker-Prager model is used to simulate the cement sheath and formation. Fig. 5 shows the finite element model of casing in slip rock. The cuboid rock block has the dimensions of 4000 mm × 1200 mm× 1200 mm. A natural fracture with the dip angle α and width of ∆d is created in the rock block. The rock block is divided into a sitting part and a mobile part (or two mobile parts). The normal displacements on the settled part surface are zero. The slip displacement of the mobile part is represented as s. The y-axis and z-axis displacements on the front and back surfaces of the mobile part are s·sinϕ and −s·cosϕ. Ignoring the bonding strength, the interaction relations among borehole, casing and cement sheath are simplified to be frictional contact.
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Fig. 4. Lead impression of H3-6 well.
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We used a real case example to show the casing deformation from fracture slip. In the case, the initial size of the production casing is Φ127×12.14 mm with a grade of P110. The mechanical properties of shale, cement sheath and casing are listed in Table 1. Hydraulic fracturing causes shear fractures and their slip. The fracture width is assumed as ∆d=20 mm. The slip displacement of fracture plane is assumed as s=20 mm. The crossing angle between well axis and fracture planes θ=75°. The friction coefficients among borehole, casing and cement sheath are f=0.2. Table 1 Material properties of shale, cement sheath and casing. Material
Shale
Young’s modulus
Poisson’s
(MPa)
ratio
Yield
Tangent
strength
modulus
(MPa)
(MPa)
Cohesion
Frictional
(MPa)
angle (°)
20900
0.18
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-
10.3
40.2
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0.23
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9.0
24.0
210000
0.3
758
2000
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-
Cement sheath
Casing
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3.2. Case analysis
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Fig. 5. The finite element model of casing in slip rock.
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Numerical simulations were performed to analyze the casing behavior under the fracture slip. The diagram of casing transverse displacement is shown in Fig. 6, and the casing Mises stress is shown in Fig. 7. They indicate that the fracture slip leads to casing large deformation and stress concentration.
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Fig. 6. Contour diagram of casing transverse displacement.
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Fig. 7. Contour diagram of casing Mises stress.
Fig. 8 shows the casing transverse displacement distribution under various slip displacements. At the axial position from −600 mm to +600 mm from the fracture plane, the transverse displacement increases sharply. The transverse displacement of casing the axial position far away from the fracture plane changes relative slower. The maximum transverse displacement occurs at 1260 mm from the fracture plane.
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Fig. 8. Casing transverse displacement distribution under various slip displacements.
Casing von Mises stress distribution under various slip displacements is shown in Fig. 9. It displays the shape of double peaks. The peak von Mises stress occurs at ±420 mm from the fracture plane. But, this fracture slip does not cause yield because the von Mises stress is much less than yield strength.
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Fig. 9. Casing von Mises stress distribution under various slip displacements.
The predicted deformation is consistent with the lead impression (Fig. 4). The failure mode of casing is elastic shear deformation other than material yield. Therefore, the von Mises yield criterion isn’t applicable to assess casing integrity. Casing integrity assessment of shale gas wells under hydraulic fracturing should consider the traveling capability of downhole tools. 8
ACCEPTED MANUSCRIPT The curvature is introduced to represent the traveling capability. Casing curvature distribution under various slip displacements is shown in Fig. 10. The borehole curvature in the building up section of H3-1 is 4.4°/25 m. When slip displacement is 20 mm, the maximum casing curvature reaches to 29.2 °/25 m. The maximum curvature occurs at fracture plane, which is significantly larger than that of normal directional borehole.
Fig. 10. Casing curvature distribution under various slip displacements.
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4. Proposed solution to casing deformation
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Casing integrity is affected by many factors such as the properties of casing, cementing, and formation in the process of well drilling and completion. Through parametric sensitivity analysis, the countermeasures are recommended to prevent casing deformation and failure during hydraulic fracturing in shale gas wells.
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4.1. Casing design
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The conventional countermeasures for avoiding casing failure include adopting heavier casing and higher steel-grade casing. Assuming the other parameters as constants, the wall thickness of casing is a variable. Various wall thicknesses of casings such as 12, 14, 16, 18, 20 mm are chosen for the horizontal section, and the corresponding simulations are performed. The effect of casing wall thickness on the maximum curvature of casing is shown in Fig. 11.
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Fig. 11. Effect of casing wall thickness on maximum curvature of casing.
With the increase of casing wall thickness, the maximum curvature of casing decreases slightly. When the casing wall thickness varies from 12 mm to 20 mm, the maximum curvature of casing only decreases 8%. The maximum curvature of casing is still too large for later downhole operations. Furthermore, the effect of casing steel grade on the maximum curvature is analyzed. The von Mises stress and the deformation of casing are almost same for P110 to TP125V. In the shale gas fields, the casings with steel grades from TP95S to TP140V have been reported to casing deformation. Theory and practice indicate that the high steel-grade and thick wall casings can’t solve the problem of casing failure induced by hydraulic fracturing.
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4.2. Cementing operation
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Young’s modulus is a key parameter of the cement sheath. Various Young’s moduli of cement sheaths such as 0.1, 1, 10, 20, 30 GPa are chosen, and the corresponding simulations are performed with all other parameters remaining unchanged. The effect of Young’s modulus of cement sheath on the maximum curvature of casing is shown in Fig. 12.
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Fig. 12. Effect of Young’s modulus of cement sheath on maximum curvature of casing.
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With the decrease of Young’s modulus of cement sheath, the maximum curvature of casing decreases sharply. Under the condition of the Young’s modulus of cement sheath varying from 30 GPa to 0.1 GPa, the maximum curvature of casing decreases 74%. So, downhole tools can pass through the small deformed casing more easily when the low Young’s modulus cement is employed. We further suggest that no cementing operation is adopted in the interval containing natural fractures and faults. Besides, assuming the other parameters as constants and the Young’s modulus of cement sheath as 0.1 GPa, the Poisson’s ratio of cement sheath is a variable. Various Poisson’s ratios of cement sheaths such as 0.1, 0.2, 0.3, 0.4, 0.49 are chosen, and the corresponding simulations are performed. Results indicate that the effect of Poisson’s ratio of cement sheath on casing deformation is very slight.
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4.3. Well trajectory
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The Young’s modulus, Poisson’s ratio and strength of the formation are rock properties and usually assumed to be constants. Although the dip angle of the slip natural fracture can’t be changed, the crossing angle between the fracture and borehole can be changed by adjusting the deviation angle of borehole. Assuming the other parameters as constants, the crossing angle is a variable. Various crossing angles such as 30°, 45°, 60°, 75°, 90° are chosen, and the corresponding simulations are performed. The effect of crossing angle on the maximum curvature of casing is shown in Fig. 13.
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Fig. 13. Effect of crossing angle on maximum curvature of casing.
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With the decrease of crossing angle, the maximum curvature of casing decreases significantly. For example, the maximum curvature of casing averagely decreases 72% when the crossing angle varies from 90° to 30°. Therefore, well trajectory should be designed to decrease the crossing angle between well axis and fracture planes to reduce the risk of casing failure induced by hydraulic fracturing.
5. Conclusions
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The failure mechanism of casings induced by the slip of shear fractures during hydraulic fracturing is studied using a finite element model. The predicted deformation is consistent with the lead impression. The fracture slip leads to casing deformation and stress concentration. The transverse displacement increases sharply around the fracture plane and the peak von Mises stress occurs at ±420 mm from the fracture plane. Casing curvature is newly introduced to assess casing integrity in shale gas wells under hydraulic fracturing. The maximum curvature of casing occurs at fracture plane, which is significantly larger than that of normal directional borehole. Enhancing casing strength can’t reduce casing deformation under slip rock. Decreasing the crossing angle and employing the low Young’s modulus cement or even no cementing are the effective countermeasures to prevent casing failure during hydraulic fracturing in shale gas wells.
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Nomenclature
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c p
= the cohesion, MPa; = the pore pressure, MPa; 12
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= the normal stress, MPa; = the effective normal stress, MPa; = the shear strength, MPa; = the internal friction angle, °; = the dip angle of fracture, °; = the crossing angle between well axis and fracture planes, °; = the gap of fracture, mm; = the slip displacement of fracture, mm; = the friction coefficient, dimensionless.
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Acknowledgment
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The authors would like to thank the financial support from Youth Science Fund of Chengdu University of Technology (Grant no. 2017QJ12).
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Highlights
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Casing failure mechanism in shale gas wells under hydraulic fracturing is revealed. A novel finite element model of casing in slip rock is proposed. Casing curvature is newly introduced to assess casing integrity. Countermeasures that prevent casing failure under fracturing are recommended.