Collapse Strength Of Casing Research Articles

Prediction of Casing Collapse Resistance by 3D Finite Element Modeling

Summary 3D outside diameter (OD) and wall thickness mapping of a casing joint consists of synchronized OD laser scanning and ultrasonic testing (UT) wall thickness measurement at a resolution of every 1 in. along the length and every 1° or 2° around the circumference. This full-length mapping data can then be used to construct a 3D finite element analysis (FEA) model that captures the geometric attributes, including OD, wall thickness, OD ovality, and wall eccentricity, of a casing joint. Together with the actual nonlinear material plasticity model and the measured as-manufactured residual stress, the 3D FEA model can predict the casing collapse strength with good accuracy. Also, the Klever-Tamano ultimate limit-state (KT ULS) collapse equation per API TR 5C3 (2018) together with a moving-average (MA) scheme can calculate the joint’s collapse strength. Results predicted by 3D FEA and KT ULS collapse models are compared to the actual full-scale collapse test results. A 3D FEA model generally produces more accurate results than the KT collapse equation. In addition, both 3D FEA and KT collapse models predict a collapse pressure slightly lower than the actual collapse pressure, which is therefore on the conservative side. The prediction of casing collapse strength can be beneficial to the end user in sorting and ranking the casing joints by collapse resistance, and therefore provides valuable information that can be used in the end user’s risk assessment. Furthermore, the end user may potentially select the joints with the highest predicted collapse strength for the portion of the casing string where the highest collapse resistance is desired. In addition, the OD and wall mapping data together with the actual material tensile properties (yield strength, ultimate tensile strength, and strain hardening parameters) can be used in estimating the internal yield, ductile rupture pressures, and other performance properties. This is outside the scope of this paper and can be pursued in future work. These predicted casing performance properties may then be used in sorting and ranking the joints based on a specific performance property of interest or a combination of performance properties.

A Collapse Strength Model for a 7” Crescent-Worn Casing Connection Considering Sealing Integrity

Collapse failure under external pressure is one of the common failure forms of casing. Much research has been performed on the casing body, but few on the threaded connection, in view of the general belief that the threaded connection has a thicker wall and larger collapse strength than the casing body. However, under external pressure, the sealing capacity of a worn casing connection will decrease due to deformation of the sealing structure, so the influence of sealing ability should be considered to determine the collapse strength of casing. In this paper, we established a three-dimensional finite element model of a 7” crescent-worn casing connection and calculated the collapse strength of the connection under different wear depths. Meanwhile, the stress distribution characteristics on the sealing surface were obtained and the influence of wear on the sealing performance of the casing connection under external pressure was analyzed. The results showed that when the wear rate exceeds a certain value, the collapse strength of the connection based on sealing integrity was lower than that of the casing body. Based on these, a collapse strength model for a 7” crescent-worn casing connection considering sealing integrity was developed and a safety evaluation method of the collapse strength of the worn casing string was proposed.

Prediction of Casing Collapse Strength Based on Bayesian Neural Network

With the application of complex fracturing and other complex technologies, external extrusion has become the main cause of casing damage, which makes non-API high-extrusion-resistant casing continuously used in unconventional oil and gas resources exploitation. Due to the strong sensitivity of string ovality, uneven wall thickness, residual stress, and other factors to high anti-collapse casing, the API formula has a big error in predicting the anti-collapse strength of high anti-collapse casing. Therefore, Bayesian regularization artificial neural network (BRANN) is used to predict the external collapse strength of high anti-collapse casing. By collecting full-scale physical data, including initial defect data, geometric size, mechanical parameters, etc., after data preprocessing, the casing collapse strength data set is established for model training and blind measurement. Under the classical three-layer neural network, the Bayesian regularization algorithm is used for training. Through empirical formula and trial and error method, it is determined that when the number of hidden neurons is 12, the model is the best prediction model for high collapse resistance casing. The prediction results of the blind test data imported by the model show that the coincidence rate of BRANN casing collapse strength prediction can reach 96.67%. Through error analysis with API formula prediction results and KT formula prediction results improved by least square fitting, the BRANN-based casing collapse strength prediction has higher accuracy and stability. Compared with the traditional prediction method, this model can be used to predict casing strength under more complicated working conditions, and it has a certain guiding significance.

Calculation model and experimental study of the collapse strength of titanium alloy tubing and casing

Titanium alloy has become a promising candidate material for oil country tubular goods (OCTGs) in harsh service environments owing to its high specific strength, low density, low elastic modulus, excellent toughness, excellent anti-fatigue and corrosion resistance. However, because the high-quality natural gas resources in China are mainly concentrated deep underground, titanium alloy tubing and casing will bear great external pressure loads underground, so the collapse strength of titanium alloy tubing and casing is very important for the safety of the string in the well. In this paper, a new collapse strength calculation model, the strength collapse criterion model (SCM), was proposed for titanium alloy tubing and casing. 35 different specifications of titanium alloy tubing and casing were selected for the full-scale collapse tests to verify the reliability of the established SCM model. Furthermore, the effect of different key parameters (such as strength, ovality, eccentricity and residual stress) on collapse strength of titanium alloy pipes were investigated systematically and compared with the same specifications of steel pipes. The strength collapse criterion model and analysis results can provide a technical reference for the design and use of titanium alloy OCTGs in the petroleum and natural gas industries.

Casing Collapse Strength Analysis under Nonuniform Loading Using Experimental and Numerical Approach

Multistage fracturing is the main means of shale gas development, and casing deformation frequently occurs during fracturing of shale gas horizontal wells. Fracturing fluid entering the formation will change in situ stress nearby the wellbore. The changes of in situ stress are mainly reflected in the following two aspects: one is the increase of in situ stress and the other is the nonuniformity of in situ stress along the wellbore. And it is for this reason that the production casing is more likely to collapse under the nonuniform in situ stress load. According to the service conditions of production casing in shale gas reservoir, this paper studied the casing deformation and the collapsing strength subjected to the nonuniform loading by the experimental and numerical simulation method. The results show that under the condition of nonuniform loading, (1) the diameter variation rate of the casing reduces with the increase in the ratio of sample to tooling length. When the ratio is less than 3, the casing collapse strength will be significantly reduced. And when the ratio is greater than 6, the impact of sample length on casing collapse strength can be ignored. (2) The increase in the applied loading angle will decrease the diameter variation rate. When the loading angle increases from 0° to 90°, the critical load value increases from 1600 kN to 4000 kN. (3) The increase in load unevenness coefficient will rapidly decrease the casing collapse strength. When the load unevenness coefficient n is 0.8, the casing collapse strength reduces to 60%, and when the load unevenness coefficient n is 0, the casing collapse strength reduces to 28%. The findings of this study can help for better understanding of casing damage mechanism in volume fracturing of shale gas horizontal well and guide the selection of multistage fracturing casing type and fracturing interval design.

Experimental study of hoop stress of crescent shaped and eccentric worn casing

The decrease of collapse and burst strength of wear casing increases the risk of wellbore failure, and limits the selection of testing and completion operation parameter. For determining the residual strength of wear casing. 95/8“x 11.99mm-P110 casing is processed into the crescent and eccentric wear casing specimens with wear depth as 4 mm and 6 mm respectively. The experiments of wear casing loaded with radial centralized forces are carried out, and the finite element method is used to verify the experimental results. The circular stresses of outer surface at worn and unworn position of casing increase with the load exerted by the compression tester, and reach the yield strength of material of casing finally. The circular stress of the external surface of the worn position of casing is greater than the ones of unworn casing. The differences between the results which come from the analysis of the finite element method and the experimental data are less than 10%. So the results which are obtained by the experiment of the crescent-shaped and eccentric wear casing loaded with radial centralized forces are reliable. The circular stress of the external surface of the worn position of crescent-shaped wear casing is greater than the one of eccentric wear casing. The differences between them are less than 10%.

Calculation and prediction of casing collapse strength based on a new yield strength acquisition method

Collapse is an inevitable phenomenon. A successful and economical casing design must make use of a casing collapse equation to predict the collapse pressure of the selected tubulars to ensure that they will survive the expected downhole pressures. As the quest for hydrocarbons drilling goes deeper, the applied external pressure to well casing is becoming greater. Consequently, a well casing must have acceptable collapse strength to withstand and meet the criteria to withstand subjected external pressures without deformation of High-Pressure High-Temperature (HPHT) reservoirs and difficult conditions downhole. To account for these conditions, a modern and accurate collapse resistance equations and true yield strengths should be used. The accuracy of the collapse pressure prediction depends upon the equation selected and the value of yield strength used in that equation which normally validated through full scale experiments. These experiments are time intensive and costly.The research study proposes a novel method for the acquisition of yield strength in the hoop direction. An experimental study is carried out by through determination of tubular yield strength in the hoop direction. Applying the new testing method to the best performing collapse pressure formula to create an improved system for predicting tubular collapse resistance.The validated new method of yield strength acquisition simplifies the testing procedure in anticipations of encouraging an increase in testing frequency. The new acquisition method was compared against the tested yield strengths given by the pipe manufacturer. Testing was performed on a variety of outer diameters, wall thicknesses, and pipe grades. The new method proved to be very accurate, yielding just a 0.5% variation from the mean yield strength values.

Influence of Volume Fracturing on Casing Stress in Horizontal Wells

In horizontal wells, the casing string is affected by the gravity effect, temperature effect, swelling effect, bending effect, friction effect and other mechanical effects. In view of this situation, the mathematical models of casing swelling effect and temperature effect caused by volume fracturing are established. The case analysis shows that the length of the unsealed section in the vertical section has a great influence on the axial shortening of the casing during fracturing. With the increase of the unsealed section length, the axial shortening of the casing increases gradually under the same wellhead pressure. In the process of fracturing, repeated squeezing and pressurization lead to periodic changes of the wellhead pressure, casing deformation and load, which leads to fatigue damage and even fracture of casing. At the same time, a large amount of fracturing fluid is continuously injected through the casing during the fracturing process, which makes the wellbore temperature change greatly. The additional stress caused by the temperature change reduces the casing strength, which has an important impact on the wellbore integrity. The mathematical model of temperature stress and its effect on the casing strength during volume fracturing is established. With the increase of the temperature stress acting on the casing, the casing collapse strength decreases gradually. When the temperature stress reaches 200 MPa, the casing collapse strength decreases to 84% of the original. The research results can provide a reference for the casing integrity design and control in the horizontal well fracturing process.

Influence of expansion rate on circumferential residual stress distribution of expandable casing

To study the distribution law of circumferential residual stress after casing expansion, using the finite element explicit dynamic analysis method analyzed the expansion process of expandable casings under different expansion rates. The analysis obtained key technical parameters of circumferential residual stress, average circumferential residual stress and elastic recovery along the wall thickness direction after casing expansion. It is recognized that the maximum residual tensile stress after casing expansion locates in the middle part of the casing thickness direction. The maximum residual compressive stress locates in the outer wall of the casing. When the expansion rate exceeds 18%, the increase in expansion rate will not lead to an increase in circumferential residual stress after casing expansion. The elastic recovery after casing expansion will reduce the circumferential stress during the expansion process. Considering collapse strength and the influence of elastic recovery on casing patch sealing performance after casing expansion, 23% is the most suitable expansion rate, which can effectively reduce the circumferential residual stress and improve the casing collapse strength. The analysis in this paper can provide bases for the calculation of casing collapse strength after expansion.

Casing deformation mechanisms of horizontal wells in Weirong shale gas field during multistage hydraulic fracturing

Casing deformation during hydraulic fracturing has become a serious issue hindering the development of shale gas in Weirong field. Better understanding of the casing deformation in this field is needed. Field data shows that casing deformations herein are strongly correlated with lithologic interfaces, whilst most deformations occur in the half of wellbore near heel. The lithologic interfaces and stress accumulation by fracturing in stages are considered as main potential inducers of casing deformations in the field of interest. Numerical simulations in two scenarios were conducted to investigate casing deformation mechanisms. The results show that a large stress concentration occurs at the lithologic interface, which can cause casing deformations. The shale swelling induced by interactions between fracturing fluids and clays can intensify the influence of lithologic interfaces. Higher injection pressure and formation pressure, larger difference in formation mechanical properties can increase stress concentration at the lithologic interface. Moreover, simulation results also show that the accumulation of induced stresses from hydraulic fractures increase the maximum principal stress near the wellbore. As more frac stages are pumped, stress accumulation increases, resulting in a continuous increase in maximum principal stress, which can cause casing deformations when it exceeds the casing collapse strength. This can explain nonuniform distributions of casing deformations along the wellbore, which is a general feature of casing deformations in shale horizontal wells. The stress accumulation effect increases with an increase in the injection rate and formation elastic modulus. Finally, some countermeasures were recommended to avoid casing deformations. This study provides new understanding of casing deformations of shale horizontal wells, which can be beneficial to the solutions of casing deformations in the field of interest and other similar shale fields.

Effect of variable drill pipe sizes on casing wear collapse strength

Casing wear, on the process of drilling complex structure well, has resulted in the casing collapse strength being weaker, which could be one of the important causes of casing damage accidents. For the complex structure wells with variable drill pipe size, a bigger error would occur if the casing wear is calculated by the single size crescent wear model.In this paper, based on the hypothesis of the single size crescent wear and the size of the drill pipe joints being involved in wear, we have established two geometric models, named compound sharp wear and compound blunt wear and deduced the casing collapse strength formula considering the compound wear. Meanwhile, how the compound wear rained influence on casing collapse strength was also analyzed with two methods—analytical and finite element analysis.At the same wear depth, for the collapse strength, there are not many differences between the compound crescent wear casing and the single crescent wear casing with minimum wear radius and the maximum error, less than 5%, would decrease as the wear depth increased. That was why we regarded the latter as an approximation of the former to calculate the collapse strength of compound wear casing in our study, which will provide references for the deformation mechanism and strength analysis of casing section in complex cases.

Deformation Mechanism of Thin and Medium-Thickness Cylinder and Its Collapse Strength under Bending

Considering the influence of flattening of casing cross-section on collapse, the stress equivalence principle was applied to conclude the triaxial stress and collapse strength. The method to determine casing collapse strength considering flattening of casing cross-section was established. The diameter-to-thickness ratio had a major influence on the flattening of the casing cross-section compared with curvature. For the 5 1/2”×7.72mm P110 casing, the flattening coursed by the 0.01/m curvature decreased the collapse strength concluded by API 5C3 by approximately 7.5%.

Mechanism of collapse failure and analysis of yield collapse resistance of casing under combined load

In studies of casing strength in oil and gas wells, the focus has largely been on the influence of the non-uniformity of the external load on the casing strength. However, most equations for calculating casing strengths in the analysis of non-uniformities focus on the radial non-uniform load while neglecting the effect of shear load. To address this, the possible effects of shear load on the outer casing is discussed. A study is carried out on the hoop stress, which is the primary factor affecting casing yield failure, and this stress can be obtained by the energy method. As a result, the yield failure mechanism for the inner wall of the casing is determined under combined load. Based on these results, a new equation for calculating medium-thickness casing collapse strength under combined load is proposed. In addition, the results indicate that the casing collapse strength reaches a maximum when the shear load constant is close to the corresponding critical shear load constant of the casing, which can effectively eliminate the negative effects on casing strength caused by the radial non-uniform load. Compared with the traditional methods of increasing the casing grade and wall thickness, the results presented here provide a new concept for casing design and casing rehabilitation in complicated strata of deep and ultra-deep wells (e.g., salt and gypsum rock).

Effect of Casing Imperfection on the Casing Strength in Steam Injection Wells

In this study, effect of casing imperfection on the casing strength in thermal environment of steam injection wells was investigated. The imperfections were modelled in the form of casing pits which may represent local cavitation, wear or corrosion attacks. Several pits with various depths (C) relative to the casing thickness (t) and different casing diameter to thickness (D/t) ratios were considered in this study. For the purpose of investigation, 3D FE models consisting of defective casing-cement-formation system were developed. N80 casing with the casing length of 3.048 m is analyzed in the numerical study. In the analysis, the combined effects of cyclic thermal stress (from 25 °C to 360 °C), collapse pressure (from formation) and defects on casing were taken into account. From the numerical study, it is found that the casing imperfection greatly affected the casing strength in steam injection wells. The presence of pits markedly reduces the casing collapse strength in comparison to that of non-defective casing. For instance, the collapse strength of casing having 2 pits along its circumferential direction with depth C = 0.5t reduced about 49% and 43% for D/t = 22.09 and D/t = 17.26, respectively. In addition, the combination of high temperature, imperfection factor and formation pressure induces severe deformation on casing, even at lower formation pressure.

Equations to Calculate Casing Collapse Strength Under Nonuniform Load Based on New ISO Model

Many studies focused on casing collapse resistance under uniform load have been done, and API 5C3 and ISO standards have been formed. However, the collapse models presented by API 5C3 and ISO standards are not suitable for calculating and predicting the casing collapse resistance under nonuniform load (NUL), despite it is well known that the NUL has a significant impact on the casing collapse resistance. Hence, based on the elastic theory and new ISO collapse model, the conception of advance coefficient of plastic collapse of casing under NUL is put forward, and the new equations to compute casing collapse resistance under NUL are established, which takes into full account the effect of manufacturing defects (such as ovality and eccentricity) and residual stress on the casing collapse resistance. The influence rule of nonuniform coefficient of load (NCL) on casing collapse resistance has been analyzed. Numerical and experimental comparisons show that the calculation results (CR) of new equations are much closer to the real failure data than the current equations proposed by Han and EI-Sayed (1992, “Resistance of Cemented Concentric Casing Strings Under Nonuniform Loading,” SPE Drill. Eng., 7(1), pp. 59–64). Research results can provide an important theoretical reference for casing design in complicated stratum (such as plastic creep and dipping stratum).

Experimental investigation of the failure mechanism of P110SS casing under opposed line load

Many studies focused on casing collapse strength under uniform load have been done, and the API 5C3 and ISO standards have been formed. However, only a limited number of researches on collapse failure mechanism of casing have been done under non-uniform load, especially experimental study, although the non-uniform load has a great impact on casing collapse strength. Hence, the collapse experiment is conducted for P110SS casing under opposed line load by using electro hydraulic servo pressure testing machine. The displacement variation rules of P110SS casing have been obtained under opposed line load. The strain of casing is measured in the process of collapse testing by the method of gluing strain-gauge on the outside surface of P110SS casing. The initial yield load of casing, instability load of casing and the yield load, stress-hardening rate, strain-hardening rate of casing after hardening are obtained. The stress-strain rules of casing after hardening are analyzed. The hardening characteristic and collapse failure mechanism of P110SS casing have been clarified under non-uniform load (under opposed line load). Experimental results can provide important references for the theoretical study on failure mechanism and collapse strength of casing under non-uniform load.

A New Crescent-Shaped Wear Equation for Calculating Collapse Strength of Worn Casing Under Uniform Loading

During the practice of oil and gas drilling and exploration, the casing wear became more and more serious and well known. A crescent-shaped wear pattern is the major wear type and leads to decrease of casing collapse strength. It is obvious that the casing design and safety assessment must take casing wear into account. However, existing empirical equations generally neglect the effect of manufacturing defects on collapse strength, which leads to a large error between calculated results and experimental data. To address the need, the wear mechanism is studied in this paper, and the effects of both wear and manufacturing defects on the collapse strength are considered. Based on this study, a new equation for calculating the collapse strength of a casing with crescent-shaped wear is presented. Comparison with experiment and alternate assessment methods shows that the new equation is much better than other methods and its calculated results are much closer to the experimental data. The new equation was used for the casing design of ultra-deep wells, especially in the north–west part of China.

The Experimental Research of P110 Casing Wear-Resisting Property in Oil Base Mud

The contact and slide under high pressure between casing and tool joint leads to wear of the casing inner surface in cemented well. The reduced collapse and burst strength of worn casing threats to the safety of oil and gas well. In order to calculate the wear depth and residual strength of the casing wear, the wear efficiency and friction coefficient is measured in the different wear time, the contact force and the rotating speed. The wear efficiency is in proportion to the contact force and the rotating speed. The wear efficiency of P110 casing change from 2 to 4×10-131/Pa. Friction coefficient reaches maximum value under certain contact force and rotating speed. The rotating speed has less influence on the wear mechanism of casing, while the contact force play a greater influence on the wear mechanism.

Theoretical and experimental analyses of casing collapsing strength under non-uniform loading

Failure data from oilfield showed that casings which were designed according to API standards were deformed and collapsed in salt formations. The main reason for decrease in strength may be caused by non-uniform loading (NUL) that was not considered in traditional casing collapsing strength design or that the designing method should be improved and developed. Obviously, the calculation of casing collapse strength is one of the key factors in casing design. However, the effect of NUL on casing collapse strength was generally neglected in the present computational methods. Therefore, a mechanical model which can calculate casing collapse strength under NUL was established based on the curved beam theory of the elasticity and was solved using displacement method. Simultaneously, three anti-collapse experiments were performed on C110 casing under NUL, and the strain and deformation laws of three casings in the process of collapse were obtained by the electrical method. Yield limit of every casing was obtained by analyzing those data. Experimental results are consistent with the results of calculation of new model. It indicates that the model can be used to calculate yield limit loading of casings under NUL.

Analysis on collapse strength of casing wear

Carrying capacity of the casing will reduce after the casing is worn, which seriously affects the subsequent well drilling, well completion, oil extraction and well repair. A lot of researches on calculation of casing wear collapse strength have been done, but few of them focus on collapsing failure mechanism, and influencing factors and law of collapse strength. So, significant difference between estimated value and actual value of collapse strength comes into being. By theoretical analysis, numerical simulation and actual test, the collapsing failure mechanism of casing wear as well as the influencing factors and laws of collapse strength are investigated, and the investigation results show that collapse of crescent casing wear belongs to “three hinged” instability. The severely-worn position on the casing is yielded into the plastic zone first then deformed greatly, which causes the plastic instability of the whole structure. The casing wear collapse strength presents changes of exponent, power function and linear trend with the residual casing wall thickness, wear radius and axial load, respectively. When the flexibility is less than 10°/30 m, the borehole bending has less impact on casing collapse strength. Thus, the computation model for the casing wear collapsing strength is established by introducing wear radius coefficient and casing equivalent yield strength, at the same time, the model is tested. The test results show that the relative error for the computation model is less than 5%. The research results provide a basis for design of the casing string strength and evaluation of down-hole safety.