Slant Fracture Research Articles

Improvement Plan Using Fatigue Analysis Minimizes Jar Twistoff

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 210723, “Service Quality Improvement Plan Using FEA Fatigue Analysis Minimizes Jar Twistoff in Challenging Applications,” by Chaouki Boufama, SPE, Baozhong Yang, and Sultan Alfaraedhi, Schlumberger, et al. The paper has not been peer reviewed. _ Jar twistoff is a well-known service quality issue for some applications, especially in relatively large borehole sizes such as 22 and 16 in. The jar is one of the weakest points in the drillstring because of moving parts, which include complex shapes, resulting in stress concentration. The complete paper presents results of a study focusing on placing the jar in the bottomhole assembly (BHA) to reduce bending moments and bending stresses on the jar connections. As a result, the risk of twistoffs during drilling is minimized, while the effectiveness of the jar is maintained if a stuck event occurs. Introduction Jars traditionally are placed between drill collars in the applications described in this study. These applications include 22- and 16-in. vertical holes with simple BHA designs, either rotary or with positive displacement motors. In cases of lateral vibration that creates high bending stress, this design is likely to cause the jar to experience extreme stress levels. The design concept of placing the jar between other components with lower weight and stiffness appears to be the most logical approach. This design would have to be validated with finite element analysis (FEA) dynamic modeling simulations to confirm lower stress at the jar and that a new weak point has not been introduced in the string. Impact and impulse also must be evaluated to confirm the effectiveness of the jar in the case of a stuck-pipe condition. The study is based on the work flow shown in Fig. 1 of the complete paper. Statement of Theory and Definitions A more in-depth analysis was performed on a 22-in. vertical drilling application with nonhomogeneous formations. A downhole real-time recording tool was added to the drillstring above the BHA to evaluate and verify the magnitude of vibration and bending moments acting on the BHA during drilling operations. The 22-in. vertical section was selected because it was determined to be the most challenging section. The following key observations were made during the drilling run when the circulation was lost to the formation: - Significant lateral vibration levels - Chaotic whirling motions - High bending moment Mechanical specific energy (MSE) and drilling parameters were controlled during the run to alleviate vibration. Vibration severity could have been influenced by the absence of the mud column as well as the borehole quality because of formation washout and an oversized hole. However, this was never verified using a caliper log. The string entered a state of chaotic whirl as the loss of circulation fluid began. The parameters, and thus the MSE, were reduced, but the severe vibrations persisted. Even though the run did not cause any jar-component failure or crack, multiple previous incidents showed similar failure modes in the same downhole conditions. These failures, which included the fractured face of each of the parted components, were analyzed and evaluated (Fig. 1). The slanted fracture indicated a high combined stress-related fracture. In addition, beach marks present near the fracture area also indicated fatigue-induced damage.

Experimental study on bamboo-shaped buckling-retrained energy dissipater with different grades of steel

Buckling-restrained energy dissipaters are miniature energy dissipaters that constrain an inner core through an outer tube and have been widely utilized to improve the seismic performance of fabricated structures. Low yield point steel is often used to produce energy dissipaters, but the mechanical behavior of low yield point steel energy dissipater is rarely investigated. In this paper, a quasi-static test of bamboo-shaped energy dissipaters with different grades of steel was carried out. The influence of the material and configuration of the core on the fatigue performance and failure mode was evaluated. Test results showed that all specimens demonstrated stable hysteretic performance. The equivalent viscous damping ratio of low yield point steel energy dissipaters was relatively large at small strain amplitudes. At large strain amplitudes, both had a similar energy dissipation capacity. The compressive strength adjustment factors and strain hardening factors of low yield point steel energy dissipaters were higher because of their lower post-yield stiffness and significant isotropic hardening. The fracture mode on the low yield point steel energy dissipaters was similar to flat rupture, and significant necking was observed here. However, the normal strength steel energy dissipaters exhibited slant fracture, and no evident necking occurred. Two deformation modes were proposed. A calculation method for the buckling wavelength of the energy dissipater was proposed. When the effective length of the yielding region was smaller than the buckling wavelength, the fatigue performance of low yield point steel energy dissipater was better than that of normal strength steel energy dissipater, and the low yield point steel energy dissipater had a better reserve performance.

Vibration-induced crack tip flipping: A closer look at an unfamiliar phenomenon in pipeline failure analysis

Fracture propagation control is an essential strategy to avoid a catastrophic event involving both economic losses and environmental damage. The Dynamic Tensile Tear Test (DT3) was introduced as an alternative tool to characterise dynamic fracture behaviour of high-strength pipeline steels. Mimicking the in-service loading conditions by imposing a dynamic tensile load, the obtained fracture surfaces closely resemble those observed in full-scale pipeline burst tests. A phenomenon called Crack Tip Flipping (CTF) could also be observed in combination with the formation of Arrowhead Markings (AHMs). The mechanisms provoking these phenomena and their effect on the fracture resistance of the pipeline material have not yet been investigated with respect to pipeline failure. Therefore, a high-speed stereo DIC setup was used to study the fracture behaviour of ×70 grade pipeline steel. The DIC setup identified a mechanical out-of-plane vibration that resulted in an additional torsional loading component. Numerical models were constructed with implementation of the Modified Bai-Wierzbicki (MBW) damage model to validate the experimental observations. Imposing a vibration-induced out-of-plane component resulted in a nearly identical fracture behaviour as observed during experiments. Both the crack path and the final fracture surface could be reproduced. Based on the obtained data, it is suggested that this oscillation causes a rotation of the stress tensor controlled by the through-thickness shear component. As a result, the stress state at the crack front is oriented towards the plane of maximum shear, forming a slanted fracture surface. Notably, the additional torsion load did not affect the load-displacement curve, as the value of the effective stress remains unaffected. Consequently, the fracture resistance, computed from energy values extracted from the load-displacement curve, are not influenced by the occurrence of CTF or the formation of AHMs. Conversely, based on the discussed mechanism of CTF, its presence could indicate the conditions under which pipeline failure occurred.

A gradient-based non-local GTN model: Explicit finite element simulation of ductile damage and fracture

Non-local models have over the years been established as an effective approach to solve the pathological mesh dependency problem observed in finite element simulation of strain-softening materials. This paper presents the formulation, implementation, and application of a gradient-based non-local Gurson–Tvergaard–Needleman (GTN) model for explicit finite element analysis. The porosity is taken as the non-local variable where the increment in porosity is averaged over the volume using an implicit gradient model. The gradient model is implemented in Abaqus/Explicit by utilising the coupled thermal–mechanical solver, which proves to be both a simple and computationally efficient approach. Due to the use of an explicit integration scheme, a transient term is introduced to the partial differential equation of the gradient formulation. The non-local GTN model is compared to the local counterpart for increasing mesh refinements using a plane strain shear band specimen, a plane strain tension specimen, and a plane strain compact tension specimen. The proposed approach can remedy the pathological mesh dependency problem. For the plane strain tension specimen, it is shown that the non-local GTN model will preserve the fracture mode (slant versus cup–cup fracture mode) when the mesh is refined. The non-local GTN model is also able to predict the same fracture mode as observed in ductile tearing experiments. However, non-local averaging can over-smooth the fields and exclude the slant fracture mode from occurring if the material length Lc is too large.

Simulations of complex crack paths using a robust and cost-efficient continuous–discontinuous approach

Prediction of complex crack propagation is necessary to guarantee performance in service of critical parts of transport vehicles despite the presence of a crack. In cases where linear fracture mechanics assumptions are not valid, e.g. for ductile rupture, this prediction may require continuous–discontinuous strategies to insert a strong discontinuity (i.e. a crack) into a continuous model based on a scalar variable related to material degradation. These strategies still face difficult challenges regarding robustness and cost-efficiency, in particular for 3D problems. To overcome these challenges, it is useful to consider the different ingredients that constitute a continuous–discontinuous strategy and try to optimize each of them. With this perspective, this work addresses the simulation of crack initiation and propagation in ductile metals and proposes new contributions to some rarely addressed points: (i) a simple geometrical approach to initiate realistic crack shapes in 3D meshes, (ii) a new 3D insertion criterion to minimize the number of transfer operations, (iii) a pragmatic remeshing procedure to refine only Active Process Zones, and (iv) an equilibrium recovery method meant to improve the convergence rate after the insertion of a discontinuity.The effectiveness of these new ingredients is demonstrated in 2D axisymmetric, 2D plane strain, and 3D cases in the case of test specimens used to characterize ductile failure in metals. This includes axisymmetric specimens exhibiting cup-cone fracture, plane strain specimens with slant fracture, and pre-cracked CT-like specimens with crack blunting and large crack propagation.

Study of the slant fracture in solid and hollow cylinders: Experimental analysis and numerical prediction

This paper is devoted to the numerical and experimental study of ductile fracture in bulk metal forming of the 2017A-T4 aluminum alloy. From an experimental standpoint, the ductile fracture of the 2017A-T4 aluminum alloy is investigated under compressive load. Two cross-sections of solid and hollow specimens are considered. The mechanical behavior and the microstructure of the 2017A-T4 aluminum alloy were characterized. It is found that the well-known barrel shape is obtained when a compressive load is applied. Analyses of fracture topographies show a ductile fracture with dimples under tension and coexistence of ductile fracture with dimples and slant under compression. The classical physically-based Gurson-Tvergaard-Needleman (GTN) model and its extension to incorporate shear mechanisms to predict failure at low-stress triaxiality are considered. These two models have been extended to take into account the thermal heating effect induced by the mechanical dissipation within the material during the metal forming process. The two models have been implemented into the finite element code Abaqus/Explicit using a Vectorized User MATerial (VUMAT) subroutine. Numerical simulations of the forging process made for hollow and solid cylindrical specimens show good agreement with experimental results. In contrast with the GTN model, the modified GTN model incorporating shear mechanisms can capture the final material failure.

A Gradient-Enhanced Plasticity Based Phase-Field Model for Ductile Fracture Simulations

Ductile fracture is modeled by using a novel phase-field method of geometric type to avoid the use of the complicated discretization approaches for crack discontinuities. The plasticity model is defined by an over-nonlocal implicit gradient-enhanced framework, which is equivalent to the integral-type plasticity models and therefore strongly nonlocal. A modified phenomenological barrier function is used as the crack phase-field driving force by mainly considering the effects of the nonlocal plastic deformation under shear-dominated stress states. The ductile damage is assumed to solely affect the plastic energy stored capacity from the micro-mechanical perspective such that the proposed approach can be easily extended to more general loading conditions. The implementation of the proposed phase-field method is shown to be easily integrated into the commercial codes (e.g., ABAQUS) through the coupling use of several user interfaces. We present simulations of the shear band formation under axial compression and the ductile crack propagations in a single-edged notched plate, a slanted fracture specimen and a pure shear test specimen to elucidate the viability of the current nonlocal method. The numerical results adequately demonstrate that mesh dependency can be apparently alleviated if material softening occurs.

Microstructural and Mechanical Characterization of the Aging Response of Wrought 6156 (Al-Mg-Si) Aluminum Alloy

The impact of the artificial aging response on the microstructure and tensile mechanical properties of aluminum alloy 6156 was investigated. Specimens were artificially aged at three different artificial aging temperatures and for various holding times to investigate all possible aging conditions, including the under-aged (UA), peak-aged (PA) and over-aged (OA) tempers. Microstructural investigation as well as tensile tests were performed immediately after the isothermal artificial aging heat treatment. An almost 50% increase in yield stress (around 340 MPa) was noticed in the PA temper and this was attributed to the precipitation of β′ and Q′ phases, consistent with the modelling predictions. This high yield stress value is accompanied by high values of elongation at fracture (>10%) that is essential for damage tolerance applications. The lack of large or interconnected grain boundary precipitates contributes to this high elongation. Slanted fracture was noticed for both UA and PA tempers, exhibiting a typical ductile and shear fracture mechanism. At the OA temper, coarsening of the precipitates along with broadening of the precipitate free zones resulted in a reduction in the strengthening effectiveness of the precipitates, and a small increase in the tensile ductility of approximately 12% was noticed.

Precipitation, mechanical properties and early slant ductile fracture in cyclic and naturally aged Al-Zn-Mg(-Cu) alloys

Natural and artificial ageing treatments are compared with ageing by cyclic deformation at room temperature. Atom probe tomography and transmission electron microscopy revealed that a fine distribution of GP-zones, was responsible for the strength after cyclic ageing. Ten days further natural ageing allowed these early-stage clusters to grow into more stable ones, thereby improving the mechanical response. This resulted in similar tensile strength and uniform elongation as for the peak aged condition by artificial ageing. It was found that fracture of cyclic aged samples occurred earlier, in the form of a ductile slant shear fracture without necking in advance. In all cases, the slant fracture started at about the same point as reaching maximum strength, i.e., at the uniform limit. However, by an artificial ageing treatment after the cyclic hardening treatment, an ordinary necking behavior was seen, similar as for the peak aged samples. Natural aged samples had similar GP-zones as the cyclic hardened ones, and the same slant fracture mode, indicating that the precipitation structure controls the fracture type. Similar behavior was observed for two different Al-Zn-Mg alloys, with and without copper.

A two characteristic length nonlocal GTN model: Application to cup–cone and slant fracture

Ductile failure prediction is essential to avoid loss of structural integrity or to control crack propagation during forming processes. One of the main difficulties is the prediction of complex crack paths. This study proposes a nonlocal extension of a local Gurson–Tvergaard–Needleman (GTN) ductile damage model at finite strains able to capture cup–cone or slant fracture. The proposed model is based on an implicit gradient formulation which enables to solve the problem of spurious strain and damage localization. The model integrates two different material characteristic lengths, which are used to separately regularize damage by void growth and damage by damage nucleation. After parameter fitting using data for a line pipe steel, conditions to obtain converged solutions are studied, which can be used to select the mesh size in the localization band. With the appropriate mesh design, it was possible to study the effect of the characteristic lengths on the formation of cup–cone fracture and slant fracture. It is observed that, for a given specimen size, larger characteristic lengths favor flat crack advance. Similarly, for given material lengths, size effects can be predicted with small specimens being more prone to flat fracture. This paves the way to a more direct determination of material lengths by using homothetic specimens so as to obtain different crack paths.

Investigation on Strain Hardening and Failure in Notched Tension Specimens of Cold Rolled Ti6Al4V Titanium Alloy.

Uniaxial and notched tension samples are utilized to investigate the damage and failure of titanium alloy Ti6Al4V. The strain fields on the samples are obtained by the digital image correlation (DIC) method. Strain localization occurs before fracturing in all samples, and the width and size of the localized zone are characterized. Slant fractures are observed in uniaxial and notched tension specimen, which indicate that the initiation and propagation of cracks in thin sheet specimens are highly affected by the shear stress. Numerical simulations were performed for identification of hybrid hardening laws, and the results were compared with the experiments. The influence of the stress triaxiality on damage mechanism of Ti6Al4V was analyzed by observation of the specimen fracture surfaces using SEM. The results show that a higher stress triaxiality facilitates the formation and growth of micro-voids, which leads to a decrement of strain at failure.

Effects of hydrogen pressure and prior austenite grain size on the hydrogen embrittlement characteristics of a press-hardened martensitic steel

The effects of hydrogen gas pressure and prior austenite grain size (PAGS) on the susceptibility of a 22MnB5 press-hardened martensitic steel (PHS) to hydrogen embrittlement were studied. The hydrogen test apparatus at NIST-Boulder was modified for tensile testing of plate-type and sheet-type specimens in gaseous hydrogen. This modification made it possible to evaluate the slow strain rate tensile (SSRT) properties of the PHS with three different PAGS at various hydrogen pressures (0.21 MPa–5.5 MPa). SSRT testing in gaseous hydrogen resulted in significant reductions of both the tensile strength and ductility, as compared to those measured in air. In addition, the presence of gaseous hydrogen resulted in a transition in fracture morphology from the near-45° slant fracture to a more brittle fracture along a plane perpendicular to the tensile axis. The hydrogen-affected fracture zones were connected to the sheet specimen free surfaces, signifying the effect of external hydrogen. The fracture surfaces of the hydrogen-embrittled specimens contained relatively flat, “cleavage-like” facets, the size of which depended on the PAGS or packet size. The PHS having the largest PAGS represented generally larger secondary cracks and straighter crack paths in addition to a greater area fraction of the “cleavage-like” facets, likely indicative of a lower frequency of crack deflections. Compared to the largest PAGS condition, the two PHS with smaller PAGS were more resistant to the hydrogen-induced fracture especially at relatively low hydrogen gas pressures (<0.52 MPa). In contrast, with an increase in hydrogen pressure, all PHS specimens exhibited significant decreases in tensile strength and ductility. The positive effect of refining martensitic microstructure, at the low hydrogen pressures, is likely associated with improved toughness of the smaller grain-sized specimens.

An analysis of deformation and failure in rectangular tensile bars accounting for void shape changes

A three-dimensional finite deformation study of necking and failure in rectangular tensile bars is carried out using a constitutive relation for porous material plasticity. The fully dynamic formulation accounts for void nucleation and growth along with thermal and rate effects, but here focus is on quasi-static response with a specified initial void volume fraction. The constitutive relation takes into account void shape changes and associated void rotations for three-dimensional voids. The constitutive update is carried out using a generalized rate tangent scheme for an elastic-viscoplastic solid. The sensitivity of necking and failure patterns to the aspect ratio of the rectangular bar is investigated with focus on the plane strain limit and a square tensile bar. The calculations predict the well-known slant fracture in plane strain tension and the emergence of a cup-cone like failure region for a square cross-section. Details are provided for the development of porosity in the bar with a square cross-section, including void shape changes and void rotations. The numerical examples show the capability of a constitutive relation for porous plasticity that can model details of void evolution, thus paving the way for advanced analyses of ductile failure under arbitrary loadings.

Ductile fracture of high strength steels with morphological anisotropy, Part II: Nonlocal micromechanics-based modeling

The ductile fracture behavior of a high strength steel is addressed in this two-part study using a micromechanics-based approach. The objective of Part II is to propose, identify, and validate a numerical model of ductile fracture based on the Gurson–Tvergaard–Needleman model. This model is enhanced by the Nahshon–Hutchinson shear modification in combination with a generalization of the Thomason coalescence criterion within a fully nonlocal form and relying on a damage-to-crack transition technique. The material model involves parameters of different natures either related to the micro-mechanics of porous materials or to semi-empirical formalisms. The void nucleation model and elastoplastic behavior have been developed and identified in Part I. The other parameters are identified in this part using inverse modeling based on both the numerical results of void cell simulations and the experimental measurements. The model is shown to adequately predict the effect of stress triaxiality and Lode parameter on the fracture strain as well as the fracture anisotropy. While the cup-cone and slant fracture paths in the round bars and in the plane strain specimens, respectively, cannot be captured using the pure continuum approach, the damage-to-crack transition framework reproduces these experimental observations.

Effect of Lüders and Portevin–Le Chatelier localization bands on plasticity and fracture of notched steel specimens studied by DIC and FE simulations

The impact of the Portevin–Le Chatelier (PLC) effect on plasticity and fracture ahead of a severe notch is investigated by DIC field measurements for C–Mn steel. The PLC effect causes an intermittent activity of highly localized strain rate bands. This first high temperature investigation of the effect of a notch on the PLC localization bands is successfully reproduced numerically in terms of strain localization in 3D FE simulations. This thereby validates quantitatively the McCormick-type model under these complex conditions. It is found that, when the PLC effect is at play at elevated temperature (175°C), a flat to slant crack transition is observed. In contrast, at room temperature, when only Lüders bands are active, the crack remains mostly flat, i.e. normal to the loading direction of the SENT samples. This behaviour may be related to the early loss of symmetry and intermittency of the plastic zone that is found for PLC even affecting the initial Lüders bands at elevated temperature. During the slant fracture at high temperature, only half the fracture energy is absorbed compared to flat fracture at room temperature. The McCormick-type model simulations predict slant strain rate bands through the sample thickness, that are consistent with the slant fracture found at elevated temperature. Accordingly, no slant bands are found for simulations outside the PLC domain. Both experiments and simulations show PLC strain localization bands that are flip-flopping up and down during crack propagation. By combining the McCormick-type model with a Rousselier porous plasticity model, the flat fracture of the sample at room temperature and the associated macroscopic curve are reproduced successfully, but the toughness at elevated temperature is overestimated.

A Fractographic Analysis of Additively Manufactured Ti6Al4V by Electron Beam Melting: Effects of Powder Reuse

Metal additive manufacturing (AM) is being rapidly adopted in the aerospace and biomedical industries. Powder bed fusion AM processes are leading this trend. To maximize process economy, excess “unmelted” powder retrieved from the build chamber is used in subsequent build cycles. The metal properties and component reliability could undergo degradation with powder reuse. This study investigates the effects of powder reuse on fracture surface characteristics of Ti6Al4V specimens fabricated by electron beam melting AM over 30 sequential build cycles. Optical microscopy and scanning electron microscopy were used to evaluate the changes in fracture surface features of tensile failures with powder reuse. Macroscopically, slant fractures were most common in early builds, which transitioned to orthogonal fracture surfaces with poorly defined shear lips with increasing reuse. Regardless of the build number, the fracture origins were consistently from the as-built surfaces. Microscopically, ductile features such as micro-void coalescence were evident throughout the 30 build cycles. However, increasing flute content with reuse suggests that rising oxygen levels causes solution strengthening and limits the participation of active slip systems. These results highlight the importance of surface roughness and powder oxidation to metal performance in AM, and the evolution of fractographic features with powder reuse.

Application of the modified Bai-Wierzbicki model for the prediction of ductile fracture in pipelines

The complex mechanical and corrosive loads of modern pipeline systems transporting oil, natural gas and CO2 impose steadily increasing requirements on material properties. The majority of current design standards still limit the application of modern high toughness linepipe steels due to the simple specification of material requirements in terms of energy levels from Charpy impact or Battelle Drop-Weight-Tear (BDWT) tests. In consequence, research activities have been conducted recently aiming at developing modified or novel experimental methods for the characterization of the ductile fracture behavior. To quantify the effects of various parameters on fracture behavior and derive suitable correlations, it is necessary to accompany these activities by numerical simulations with appropriate ductile damage models. In this paper, the MBW model is applied to study the structural behavior of pipelines in ductile fracture regime. Due to its precise incorporation of the underlying load conditions, the damage model is successfully used to simulate the slant fracture behavior in Battelle Drop weight tear test specimens and pipe sections. In comparison to ductile damage models applied in former studies, namely the Gurson-Tvergaard-Needleman and Cohesive Zone model, the presented numerical methodology allows for a more detailed investigation of loading, material and geometry effects on fracture and crack arrest behavior of pipelines.

Quantification and microstructural origin of the anisotropic nature of the sensitivity to brittle cleavage fracture propagation for hot-rolled pipeline steels

This work proposes a quantitative relationship between the resistance of hot-rolled steels to brittle cleavage fracture and typical microstructural features, such as microtexture. More specifically, two hot-rolled ferritic pipeline steels were studied using impact toughness and specific quasistatic tensile tests. In drop weight tear tests, both steels exhibited brittle out-of-plane fracture by delamination and by so-called “abnormal” slant fracture, here denoted as “brittle tilted fracture” (BTF). Their sensitivity to cleavage cracking was thoroughly determined in the fully brittle temperature range using round notched bars, according to the local approach to fracture, taking anisotropic plastic flow into account. Despite limited anisotropy in global texture and grain morphology, a strong anisotropy in critical cleavage fracture stress was evidenced for the two steels, and related through a Griffith-inspired approach to the size distribution of clusters of unfavorably oriented ferrite grains (so-called “potential cleavage facets”). It was quantitatively demonstrated that the occurrence of BTF, as well as the sensitivity to delamination by cleavage fracture, is primarily related to an intrinsically high sensitivity of the corresponding planes to cleavage crack propagation across potential cleavage facets.

Failure in lithium-ion batteries under transverse indentation loading

Deformation and failure of constrained cells and modules in the battery pack under transverse loading is one of the most common conditions in batteries subjected to mechanical impacts. A combined experimental, numerical and analytical approach was undertaken to reveal the underlying mechanism and develop a new cell failure model. When large format pouch cells were subjected to local indentation all the way to failure, the post-mortem examination of the failure zones beneath the punches indicates a consistent slant fracture surface angle to the battery plane. This type of behavior can be described by the critical fracture plane theory in which fracture is caused by the shear stress modified by the normal stress. The Mohr-Coulomb fracture criterion is then postulated and it is shown how the two material constants can be determined from just one indentation test. The orientation of the fracture plane is invariant with respect to the type of loading and can be considered as a property of the cell stack. In addition, closed-form solutions are derived for the load-displacement relation for both plane-strain and axisymmetric cases. The results are in good agreement with the numerical simulation of the homogenized model and experimentally measured responses.

Feature of fractures induced by hydrofracturing treatment using water and L-CO2 as fracturing fluids in laboratory experiments

Recently, the use of CO2 as a treatment fluid in the petroleum and gas industry as well as enhanced geothermal system projects has attracted the attention of many scholars. In this paper, hydrofracturing experiments were performed on shale and coal specimens employing CO2 and water as fracturing fluids, respectively. The differences of hydraulic and L-CO2 fracturing in the breakdown pressure and fracture features were investigated under different geological and engineering conditions. The experimental results indicate that preexisting flaws in reservoirs have a significant effect on the stimulation mechanism of the hydrofracturing treatment. The non-reactivation of preexisting flaws leads to tensile stimulation. In contrast, the reactivation of preexisting flaws might lead to shear stimulation under a high horizontal stress difference. The shear stimulation would impact fracture propagation, and can even induce the slip of slant fractures, that generates splay cracks at the fracture tips. Because shear slip requires a high horizontal stress difference, which reduces the conductivity of reservoirs. Comparing with hydraulic fracturing, the low viscosity of CO2 is beneficial for shear stimulation.