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Unconventional oil and gas reservoirs, characterized by low porosity and permeability, often require multistage fracturing techniques for development. The high-pressure fracturing fluids with large volumes can easily cause alternating changes in both temperature and pressure within the casing. Using a theoretical model and field data from hydraulic fracturing operations, this paper calculated the alternating ranges of axial loads and temperatures in the reservoir section. Based on the calculation results, the temperature–load alternating coupling test of the P110 casing was carried out, and the tensile test was performed to analyze the yield strength variation law of the casing material. The results indicate that the yield strength, ultimate strength, and elastic modulus of casing materials are decreased under alternating thermal–mechanical coupling conditions. As the number of alternating cycles increases, there is an initial rapid decrease followed by a slower declining trend. Moreover, the tension–tension (T–T) cycles induce greater reductions in yield strength and ultimate strength than tension–compression (T–C) cycles. Meanwhile, under the same axial load condition, the higher the circulating temperature, the more significant the reduction in yield strength and ultimate strength. In essence, this is the result of the coupling effect of low-cycle fatigue and temperature aging. Finally, based on the experimental data, a yield strength prediction model of the P110 casing under the alternating thermal–mechanical coupling condition was established. The research results provide theoretical guidance for the safe design and material selection of a casing string under multistage volumetric fracturing conditions of shale gas exploration. [ABSTRACT FROM AUTHOR]

During the development of shale gas, one of the major challenges is the casing deformation (CD) in the horizontal section due to the geological activity. Recently, the casing deformation before hydraulic fracturing (CDBF) occurred in multiple shale gas wells in L block in China. In this paper, based on the theory of tubular mechanics, the relationship between casing buckling and CDBF caused by casing running in is analyzed qualitatively and quantitatively. It is found that the buckling deformation caused by running the casing string process is not sufficient to prevent the tool (Φ99 mm bridge plug). On the other hand, the mechanism of CDBF is systematically analyzed based on the actual field data and finite element (FE) method. In order to analyze the CDBF mechanism, the comparison between the actual casing trajectory and the reservoir rock (S#1, S#2, S#3 and M#1) in horizontal section in H2 platform is carried out, and the preliminary CDBF mechanism is proposed. Then, two groups of FE models are established to reduce the CD process to verify the preliminary mechanism. It is found that the numerical simulation results (high stress section length from FE model) are in good agreement with the actual CD logging results. Based on the analytical results, the corresponding mitigation measures are proposed based on the analysis of the CDBF mechanism. Our work could offer a detailed theoretical basis and reference of CDBF for shale gas well application. [ABSTRACT FROM AUTHOR]

In the complex fracturing process of shale gas wells, casing is subjected to serious deformation, which can easily to the failure of wellbore integrity and the reduction of well construction productivity. It is particularly important to clarify the casing deformation mechanism and carry out effective control. Based on the logging data of casing deformation from well and full-scale indoor tests, the casing deformation mechanism is mainly considered to be shear and non-uniform extrusion deformation caused by formation slip and displacement control, i.e., the ultimate working conditions. The slip displacement boundary (<40 mm) under complex fracturing conditions is quantified to provide the design and optimization basis. Then, the influence laws of steel grade and wall thickness on the shear and non-uniform extrusion bearing characteristics are studied, using unconventional oil and gas well casing simulation test systems for 110 ksi (φ139.7 × 10.54 mm) and 125 ksi (φ139.7 × 12.7 mm) casings. Furthermore, combined with the full-scale simulation tests and finite-element simulation, the effects of elastic and modified cement slurry with hollow glass beads on casing deformation are compared and studied. The results show that the deformation capacity mitigation of casing is limited by reducing the cement elastic modulus and increasing the elastic cement thickness. By reasonably adding hollow glass beads of modified cement slurry, the maximum geological movement absorption of cement slurry is up to 27 mm. This new method can obviously decrease casing deformation and have an excellent control effect. Combined with the cementing technology of Luzhou blocks, the formula of modified cement slurry is optimized, and the optimization window of the casing deformation control process is formed, which can ensure the smooth progress of engineering fracturing. [ABSTRACT FROM AUTHOR]

Abstract Hydraulic fracturing induces the shear failure of natural fracture which contributes to complex fracture network. The magnitude of fracture slip potentially causes some undesirable consequences including wellbore instability, casing deformation and fault reactivation. Therefore, it is important to predict the fracture slip induced by hydraulic fracturing for a safe and efficient stimulation. In this paper, we used a 2D hydro-mechanical coupled model to predict the injection-induced slip. The hydraulic and natural fractures are embedded in formations with cohesive zone model. The fracture propagation, rock deformation and wellbore slip displacement are captured in the 2D model. The wellbore slip displacement is input into a small-scale 3D mechanical model of casing in slip rock to simulate casing behavior. Casing curvature is also introduced to assess casing integrity. Results indicate that the rock deforms asymmetrically with respect to the wellbore after the shear failure of natural fracture. Particularly, there is a shear slip along the natural fracture. The simulation result shows that the casing failure mechanism is shear deformation induced by the fracture slip during hydraulic fracturing. The curvature of deformed casing is larger than that of directional well trajectory. The predicted results are validated by the logging and operation data from the field. This work provides a novel quantitative method for predicting fracture slip and evaluating well integrity during hydraulic fracturing. Highlights • Fracture slip and casing deformation induced by fracturing is firstly quantified. • Hydraulic fracturing could cause fracture slip and casing shear deformation. • Casing curvature is newly introduced to assess casing integrity. • The relative error of predicted and measured casing diameters is less than 3.7%. • The perforations should be set far away from the weak interfaces. [ABSTRACT FROM AUTHOR]

Shale gas development in the Sichuan-Chongqing region in China has experienced a high level of casing deformation or failure during hydraulic fracturing treatment, which has resulted in significant productivity decline and economic losses. This problem has motivated many studies in the literature but focusing on the shearing deformation using the finite element method. This paper categorizes the casing failure modes and mechanisms in the region and presents casing collapse laboratory results. The designed laboratory tests rendered us critical data points in casing deformation under different loading scenarios. For multiple stage hydraulic wells in the region, we propose to use a strain-based protocol for casing design instead of the current design based on maximum stress. • Present laboratory experiments on casing deformation due to shearing stress. • Propose a model to predict the maximum allowable stress for a casing tubular. • Update statistics on casing deformations and failures in the Changning-Weiyuan region. • Introduce one of a kind laboratory experiment setup for full size tubular testing. [ABSTRACT FROM AUTHOR]

The wall of perforation hole was directly pressed by the high-pressure fracturing fluid, making cement integrity more challenged in fracturing process. However, the current calculation of perforated cement sheath stress neglects the influence of transient temperature variation during fracturing process. In this work, a fully coupled thermo-hydro-mechanical (THM) model incorporating the transient heat transfer process of fracturing fluid was set up to describe the transient stress variation of cement sheath during fracturing. The maximum tension stress criterion and Mohr-Coulomb criterion were chosen as the criterions for the cement failure. The proposed model was successfully verified through its excellent agreement with experimental observations. In addition, based on the proposed model, the influence of fracturing parameters and cement mechanical parameters on cement integrity was analyzed. The results illustrate that a spindle-shaped tensile-shear failure area is formed near the cement hole. The casing pressure and the cement mechanical parameters are the main controlling factors for the failure of the perforated cement sheath. The extremely low casing pressure are beneficial for maintaining cement sheath integrity. Besides that, only optimizing the Poisson's ratio or elastic modulus of cement cannot maintain the perforated cement integrity unless improving both simultaneously. • A coupled thermo-hydro-mechanical model was used to evaluate the cement integrity in fracturing. • The effect of temperature increases the risk of cement failure at the perforation hole. • There are both tensile failure and shear failure at the inner wall of the perforated hole. • The tensile failure of the perforated cement sheath precedes the shear failure. • Cement failure can not be diminished by only decreasing the casing pressure or adjusting mechanical parameters. [ABSTRACT FROM AUTHOR]

• This paper uncovered the failure mechanism of crankshaft in fracturing pump during factory fracturing. The insufficient fatigue strength was due to absence of surface hardening layer for failed 1st crank pin. Moreover, fillet radius was a main factor for fatigue crack initiation. • This paper provided specific suggestions to prevent the crankshaft rupture through optimization the fillet radius of thread root in radial oil-hole. Factory fracturing is characterized by high pressure, large discharge capacity and long-time service for shale gas well, then unite of fracturing pump operates under more severe condition with complex load and corrosion. Failure analysis was performed for crankshaft via theoretical calculation, finite element modelling and fractographic observation in fracturing pump with model QWS-2800. The results indicated that fatigue fracture was the dominant failure mechanism, with evidence of beach and striation marks and multiple fatigue cracks. The fracture plane had nearly 45° inclination with respect to the shaft axis. The main cause of insufficient fatigue strength was due to absence of surface hardening treatment. Fillet radius was a main factor for fatigue crack initiation, the fatigue crack mostly located in the thread root of radial oil-hole. Finite element analysis results indicated that the stress increased remarkably as fillet radius decreased. Then stress concentration became much higher with the transition of thread root from smooth to sharp fillet. It provides some suggestions to prevent the crankshaft rupture in terms of fatigue strength and thread structure. [ABSTRACT FROM AUTHOR]

• Fault slip causes casing deformation in shale gas wells. • The casing shear strength is obtained through the shear deformation experiment. • A three-dimensional finite element model of fault slip is established. • Positive correlation between fault slip and casing deformation. • Casing deformation prevention methods include reducing displacement, using low elastic modulus cement and small-sized casing. Horizontal wells and multi-stage hydraulic fracturing technology make it possible for large-scale commercial exploitation of shale gas. However, the problem of casing deformation of horizontal wells in Sichuan shale gas field has become increasingly prominent, and the Fracturing tools can not be smoothly put to the designed positions. These problems have caused many sections of horizontal wells to be unable to carry out normal fracturing, which has seriously affected the efficient development of shale gas. Field statistics and previous literature studies show that the casing failure mode of Weiyuan shale gas field is shear deformation, and the shear load caused by fault slip is the main factor causing casing deformation. However, the deformation of casing under displacement shear load is still uncertain. The shear deformation experiment of casing is completed, and the strength change law of casing under shear load is obtained. On this basis, the numerical model of wellbore deformation under the action of fault interface slip is established, and the deformation of casing under the action of fault slip and related influencing factors are analyzed. The casing shear test and numerical simulation results are consistent. The results show that there is a linear positive correlation between formation slip and casing deformation. The fault angle, that is, the angle between the fault plane and the wellbore, has a great influence on the deformation of the casing. When the angle is 90°, the casing deformation is the largest. Reducing the elastic modulus of cementing cement stone is beneficial to reduce the casing deformation. The research results can lay a theoretical foundation for the prevention and control of shale gas well casing deformation. [ABSTRACT FROM AUTHOR]