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A new type of waterborne epoxy-cement (WEPC) system was developed by adding waterborne epoxy resin (WEP) as an external admixture to cement slurry to solve the problem of cement sheath integrity da...

Summary Annulus pressure buildup (APB) problems in shale gas wells seriously affected on the safety and efficient exploitation of shale gas all around the world. The sealing failure of the cement sheath on interfaces caused by periodically changed fluid pressure in casing during hydraulic fracturing is treated as the main reason for APB in shale gas wells. Many methods are put forward to solve the APB problem in the field, and fortunately, the preapplied annulus backpressure (PABP) method shows an excellent utility. In this paper, an analytical model is established to explain the mechanism of the PABP method increasing the sealing ability of the cement sheath. The residual strain of the cement sheath and radial stress on interfaces are considered to analyze the factors that affect the effectiveness of the PABP method. In addition, based on the field data, an experimental device is established to test the validity of the PABP method and to certify the accuracy of the analytical model established in this paper. The analytical results show that the thickness of the casing has little effect on radial stress on interfaces. The outer diameter of the casing and the thickness of the cement sheath can temperately affect the radial stress. The elastic modulus of the cement sheath and the formation rock can significantly affect the radial stress. The higher elastic modulus of the cement sheath can dramatically increase the radial stress on interfaces. On the contrary, the higher elastic modulus of formation rock will induce smaller radial stress on the interfaces. In the field, the number of newly added shale gas wells with APB problems has dramatically decreased by using the PABP method. The work in this paper can be significantly useful for researchers and engineers to reduce the APB in shale gas wells.

SummaryIn conventional wellbore-integrity analysis, the cement sheath’s initial state of stress and transient thermoporoelastic effects are often neglected. However, the initial state of stress is prerequisite information for accurately predicting the safe operating conditions that prevent a cemented well from being damaged. In addition, transient thermoporoelastic effects can have a profound effect on when damage will occur. In this paper, we propose a model that includes these effects to predict the safe operating pressures and temperatures that will prevent cement-sheath failure. For the initial state of stress, we proposed an empirical model using measurements. Subsequent stress changes are evaluated by a fully coupled transient thermoporoelastic model to analyze the mechanical behavior of the cement sheath. We predict the safe operating envelope (SOE) for shear, tensile, and debonding cement-sheath failures caused by pressure and temperature perturbations after the cement sets. Our model predicts that pore pressure is a key factor for cement failure, especially for rapid temperature changes. If the formation is low permeability, the transient pore pressures are amplified, causing the risk of damage to increase. Compared with conventional thermoelastic models, the thermoporoelastic model predicts a smaller SOE when heating the internal casing fluid and a larger envelope when cooling the internal casing fluid. Finally, the heating rate was considered with respect to field applications. The heating rate was also considered, and slower heating/cooling rates can prevent damage to the cement sheath. Finally, the thermoporoelastic model was applied to explain several laboratory and field experiments and achieved good matches.

Compared with cement stone, epoxy resin has a higher compressive strength, lower elastic modulus, and better seepage capacity in repairing cement sheath or cementing in special formations. The visc...

The cement sheath between casing and formation is required to have long-term sustainability for the longer life of the well. Different formulations based on different additives are prepared and pum...

At present, most of the studies of the stability of the formation-cement sheath-casing combination have been mainly based on the plane, and the three-dimensional model established is only one example. There is no systematic study of the influence of physical parameters and process parameters on the stability of the combined body under the three-dimensional model and the action of triaxial crustal stress. Through the establishment of three-dimensional formation-cement sheath-casing linear elastic combination model, we can study the influence of operational parameters (cement sheath pressure, casing cross section pressure, inner casing pressure, ellipticity of borehole, centrality of casing, thickness of cement sheath) by the two interfaces’ Von Mises stress and the total displacement of the combination body. It is pointed out that the pressure of cement sheath, and casing cross section pressure have no effect on the stability of formation, cement sheath and casing; The higher the ellipticity of the borehole, the eccentricity of the casing (position 1, 2) and the thickness of the cement sheath, the higher the stability of the second interface of the cementing; The higher the inner casing pressure and the eccentricity of the casing(position 3), the lower the stability of the second interface of the cementing; The higher the eccentricity of the casing (position 2,3) and the thickness of the cement sheath, the higher the stability of the first interface of the cementing; The higher the inner casing pressure, the eccentricity of the casing (position 1) and the ellipticity of the borehole, the lower the stability of the first interface of the cementing; The higher the eccentricity of the casing (position 2,3) and thickness of the cement sheath, the higher the stability of the casing; The higher the inner casing pressure, the ellipticity of the borehole and the eccentricity of the casing(position 1), the lower the stability of the casing. Through this study, according to the formation stress, the formation physical parameters (elastic modulus, Poisson's ratio, density), optimize the operational parameters, ensure the long-term integrity of the combination.

Summary We propose a novel cement additive made of graphite nanoplatelets (GNPs) for improved hydraulic isolation and durability of oil and gas wells. The primary role of the cement sheath, which is zonal isolation, can be significantly affected by the permeability of set cement (hardened cement slurry). On one hand, it is the inherent microstructural defects of cement, including pores and microcracks, that results in the intrinsic permeability of cement, and on the other hand, cracking, micro-annuli, or other flow paths developed through the disturbed cement by connecting the pre-existing microstructural defects determine the equivalent permeability of set cement. The purpose of this research is containing or at least minimizing the intrinsic and developed flow paths through the cementitious matrix with the help of surface-modified GNPs. GNPs possess high surface area to volume ratios. In this study, we focus on the effect of surface-modified GNPs on the overall mechanical properties of both cement slurry and hardened cement slurry affecting the permeability of cement. We present two dispersion methods on the basis of physical and chemical treatments of the surface properties of GNPs. The efficiency of proposed methods on the overall properties of the cement is examined before and after its setting. To mimic downhole conditions, cement slurries are cured at 3,000 psi and 190°F for 24 hours. Also, some experiments were repeated under the pressure and temperature conditions up to 5,160 psi and 126°F, respectively, to examine pumpability and behavior of cement slurry at bottomhole conditions. To examine the role of spatial distribution of GNPs on the hardened cement nanocomposite, samples with different concentrations of GNPs were tested. We investigated the effect of modified GNPs on the unconfined compressive strength (UCS), shear bond strength, thickening time, rheological characteristics, and the free fluid content. We measured zero free fluid at room temperature for different concentrations of GNPs, demonstrating uniform dispersion of nanoparticles within the cement matrix. On the other hand, the squeeze of water out of the lower parts of the cement slurry and its upward migration can develop preferential paths for oil and gas migration. Therefore, eliminating the above-mentioned water separation can enhance cement sealing properties. We found that an optimum 0.2 vol% concentration of acid-functionalized GNPs improves the compressive and the shear bond strength of the prepared cement by approximately 42 and 175% as compared to the plain cement, respectively.

The hydraulic fracturing process involves high pump pressure and large displacement, which increase the risk of debonding on the interface of the cement sheath and rock formation. Therefore...

Electrical explosion of wire (EEW) has been used in the petroleum industry to improve oil recovery because it can generate a controllable and stable shock wave. The pipe-cement-rock combination model has been established to avoid the destruction of integrity between the casing pipe and cement sheath while it is treating the formation to increase oil and gas recovery. The propagation law of shock wave in the combination is simulated by nonlinear dynamics analysis code AUTODYN. What is more, the damage of combination caused by a single explosion and multiple explosions is also studied. The results show that the shock wave attenuates greatly through the casing pipe and cement sheath, but the shock wave propagating in a perforation hole can keep higher pressure, and the multiple explosions will cause cumulative damage to the combination.

Cement sheath provides zonal isolation and structural support during oil/gas well exploitation. Mechanical behaviours of cement sheath are of increasing concern with the exploitation of shale gas. ...

The cement sheath may crack during oil and gas production. Due to the microcrack areas are difficult to locate and reach, the repair of microcrack can be achieved by self-healing materials. Potassi...

This paper presents an analytical method of predicting cement sheath integrity with incorporation of wire-line acoustic measurements on an example of a deep, high-temperature geothermal well, located in central part of the Los Humeros Geothermal Field in the northern part of the Mexican Volcanic Belt. The developed method, accounting for mechanical properties of a coupled casing-cement-rock system with the influence of temperature changes, applied internal pressures, and isotropic far-field stresses, can be easily, without additional costs and in a non-destructive way, used to evaluate cement sheath integrity during various stages of a geothermal well life cycle and help to optimize and design drilling, production, and maintenance operations.

In oil and gas fields, the repairing force (bit weight) is generally determined by experience when the deformed casing is repaired using the spinning casing swage. However, unreasonable repairing force easily damages the cement sheath around the deformed casing and causes pipe sticking, which results in failure of the wellbore integrity. Hence, based on the Hertz contact theory, the present study established a mechanical model to calculate the repairing force required to repair the deformed casing without a cement sheath by spinning casing swage, and the reshaping force was determined by combining the structural features of the spinning casing swage with the method of mechanics and kinetics analysis regarding axial loading and circumferential deformation of the deformed casing. Finally, a mechanical model was presented that could calculate the repairing force of the deformation casing with cement sheath using the inversion method. Repairing experiments involving three types of deformed casings (casing without cement sheath, casing with undamaged cement sheath and casing with damaged cement sheath) were performed, from which the accuracy and reliability of the mechanical model was validated. The damage behaviour of the cement sheath after casing repair was investigated based on the experimental results and the damage mechanism was analysed based on Saint-Venant's deformation compatibility principle. Analysis results showed that three types of damage, including micro-annulus, transverse fracture and longitudinal fracture, were caused by high contact pressure between the steel ball on the spinning casing swage and the internal wall of the deformed casing and pressure fluctuation during repairing. The research results provide important guidance and decision making for practical repairing measures.

The sustained casing pressure (SCP) problems in shale gas wells have not been effectively controlled by now. In field, the expandable cement sheath used in wells has decreased the SCP problems and in this paper, an analytical model is established to study how the expandable cement increases the sealing ability of cement sheath and give suggestions on optimized choosing of expandable cement. Meanwhile, the stress equilibrium state in well before cementing work which has not been considered in previous and it is considered in the analytical model here. During the curing process, the radial stress on the outer interface of cement sheath changes with time and increases much faster than it on the inner interface. In wells with single layer of casing, the radial stress on interfaces does not increase linearly with the cement elastic modulus (Et) and it will be affected by the elastic modulus of formation rock. In wells with double layers of casing, the expandable cement can significantly increase radial stress on interfaces during the production process and alters circumferential tensile stress into compressive stress which is useful for increasing the sealing ability of cement sheath. How the expandable cement increase the sealing ability of cement sheath in field has been analyzed and an optimized expand ratio of cement is proposed to control the SCP problems in shale gas wells.

Damage to the cement sheath during well completion, stimulation, and production leads to sealing failure in the well and poses a significant risk to safe operation and the environment. In this paper, based on a predictive model for sustained casing pressure, we propose a method for quantitative evaluation of the extent of cement sheath sealing failure. The composite permeability is selected to characterize the extent of sealing failure. Based on the calculation results for sustained casing pressure, we obtain pressure dependences of the permeability. The composite permeability is calculated based on substituting field data into the proposed model. We analyze and compare the results of the predictive model for the casing pressure in constant annular volume and annular volume expansion scenarios. The results show that the time dependences of the permeability have the shape of an inversely proportional function. The composite permeability of the cement sheath for constant annular volume is 1.90% higher than for the annular volume expansion scenario. With an increase in the time required to establish the sustained pressure (the stabilization time), the permeability decreases and consequently the extent of cement sheath sealing failure decreases. The study results are of practical importance for a quantitative evaluation of the extent of sheath sealing failure for the well and a predictive evaluation of the casing pressure.

During CO2 injection, a microannulus will be generated, leading to the transmission of gas through the cement behind the casing or before the formation when thermal stress exceeds bond strength. In this work, leakage channel prediction model was developed to predict the leakage channel of CO2 injection wells. The prediction results indicate that the thermal stresses first decreased and then increased with increasing well depth; decreased with increasing radial distance; and increased with increasing injection time. The cement sheath of a production casing is more likely to form a leakage channel than other cement sheaths during long-duration CO2 injection.

Carbon Capture and Storage (CCS) as a method for climate mitigation relies on CO2 being locked up in subsurface reservoirs with a long-term perspective. Active and abandoned wellbores are among the major potential leakage paths for CO2, and their success as "gate keepers" relies on the quality of well cement. The length requirements and recommended practices for cement sheaths in active wells vary from country to country. In this study, we investigate how stochastic properties of the microannulus may affect the recommended "safe length " of cement sheath, should assigning such length be attempted for CO2 wells. We demonstrate, by means of a simple numerical model, that variation in the width of the microannulus along the well makes it a challenging task to assign a figure to the "safe cement-sheath length", even though longer cement sheaths do indeed reduce the risk of leakage. Variation in the mean value or standard deviation of the microannulus width by only 10% changes the recommended length of the cement sheath by up to an order of magnitude. This finding sheds new light on the use and added value of more accurate leakage prediction models: due to the uncertainties in and the lack of information about the properties of microannuli, such models should focus on investigating the effects of different operational and in-situ factors on the leakage potential rather than on attempting to accurately predict the leakage flow rate, however tempting such prediction exercise might seem at first glance.

The cement sheath are damaged around the hole after perforation, and the micro-cracks in the cement sheath will cause fluid channeling during hydraulic fracturing. Focused on this problem, a numerical model was set up to calculate the crack propagation length in cement sheath during hydraulic fracturing by using the Cohesive Zone Method (CZM). Then the influence of different parameters on the unsealing length of cement sheath during hydraulic fracturing was analyzed. The results show that higher casing pressure can help to reduce the unsealing length of cement sheath. It is significant to perforate with lower density and phase to ensure the sealing integrity of the cement sheath. This research can evaluate the sealing performance of the cement sheath during hydraulic fracturing and give some guiding for the design of fracturing schemes.

The combined technology of perforation and hydraulic fracturing has been the main development method for unconventional oil and gas to date. However, perforation can cause crack damage to cement sheath or de-bonding of the cement interfaces, resulting in fluid channeling during hydraulic fracturing. From an operational perspective, it is thus critical to be able to characterize the degree and range of cement sheath damage after perforation. To achieve the objective, a numerical method of perforation is set up based on the ALE (Arbitrary Lagrange-Euler) algorithm in LS-DYNA software. The numerical results are verified by the ring target simulation test. The cement-interface damage zone shapes like a saddle after perforation, and there is a narrow range of damage around the perforation tunnel. The damage zone is minimized with the enhancement of cement compressive strength but conversely with shear modulus. Cements with lower shear modulus and higher compressive strength should be selected during well cementing for reducing cement sheath perforation damage. Additionally, a shaped charge with smaller liner diameter is more effective in enhancing the seal integrity during hydraulic fracturing.

In this article, the finite element model considering the casing eccentricity of high-pressure, high-temperature wellbore integrity is established, where the cohesive zone element is introduced to evaluate the mechanical behavior of the casing–cement sheath interface. In this analysis, the bilinear traction–separation law is used to estimate the interfacial failure, and a damage factor is simultaneously defined to describe the damage evolution of the cohesive element. In addition, to achieve the interfacial separation, the pressure fluctuation inside the casing is applied. Using the finite element approach, we investigate the effect of mechanical parameters including Young’s modulus, Poisson’s ratio, thermal conductivity of the cement sheath, and pressure fluctuation inside the casing on the interfacial failure of the casing–cement sheath. The simulation results show that the damage factor increases with the increase in the mechanical parameters including Young’s modulus and Poisson’s ratio of the cement sheath and the casing pressure fluctuation. We also observe that the maximum damage factor under higher thermal conductivity is higher than that under lower thermal conductivity for the casing concentricity, while it is lower than that under lower thermal conductivity for the casing eccentricity. This indicates that the casing eccentricity has an important impact on the damage factor. What’s more, the casing eccentricity can change the location where the maximum damage factor occurs.

Leakages of greenhouse gases, such as methane and carbon dioxide from wells, may have considerable environmental consequences. Although much emphasis is currently put on understanding well barrier failures, and thus, preventing well leakages, especially for an important barrier material as cement, there are still several knowledge gaps and unknowns. However, a step-change in well integrity understanding may be obtained by applying advanced characterization techniques and scientific approaches to studying well barrier materials and their failure mechanisms. This paper describes the development of an experimental methodology that uses X-ray computed tomography to obtain 3D visualizations of cracks and microannuli in annular cement sheaths. Several results are included that demonstrate the value of using such digital methods to study well cement, and it is shown that such experimental studies provide an improved understanding of cement sheath integrity. For example, it is seen that radial cracks do not form in symmetrical patterns and that microannuli do not have uniform geometries. Such experimental findings can potentially be used as benchmark to validate and improve cement integrity simulation tools.

Wellbore integrity is highly dependent on the integrity of the cement sheath which plays an essential role in preventing any communication between the formation fluids and the surrounding environment. Mechanical failure of the cement sheath within a wellbore is influenced and governed by many factors including cement mechanical properties. However, the paucity of cement class G mechanical parameters including lack of experimental data under different confining pressure, tensile properties, and the effect of curing temperatures on the long-term cement mechanical properties are impediments to the numerical simulations in wellbore integrity assessments. Therefore, this study expands the cement class G mechanical properties inventory. This paper investigates the mechanical behaviour of cement class G at two different curing temperatures ( at the age of 28 days. The effect of both the curing regime and confining pressures (15 MPa and 30 MPa) on the strength and post-peak response of the cement under co...

In oil field, the bit weight (repairing force) and torque are always determined by experience when the rolling reshaper is used to repair the deformed casing. However, if the repairing force and torque are unreasonable, it will easily give rise to the secondary damage of deformed casing and surrounding cement sheath as well as pipe sticking. Hence, based on the elastic-plastic theory, the three-dimensional finite element model which can simulate the repairing process of cemented casing repaired by rolling reshaper has been presented in this paper by adopting the finite element method. Based on this model, the whole repairing process of cemented casing with elliptic section has been simulated, by which the deformation law of cemented casing, the repairing force and torque required to repair the cemented casing have been obtained in the repairing process. Meanwhile, based on self-developed testing equipment, the full-scale repairing experiment of cemented casing has been conducted by using rolling reshapser, and the correlations between deformation laws of cemented casing and repairing force and torque have been analyzed. As the test results are consistent with the simulation results, it indicates that the finite element model is accurate and reliable so that the research results can provide important reference for design and optimization of technological parameters.

Cement sheath failure may occur under the influence of induced stresses during service life of the well. For many years, analytical or numerical models have been developed to study the probability of the cement sheath failure under different circumstances. Cement compressive strength is considered as a valuable input parameter during the assessment of the cement failure probability. Usually the compressive strength data are obtained by conducting several direct laboratory experiments on cement samples. The average value of these experiments are reported and used in different cement failure models. However, this approach may results in underestimating the cement failure probability. As cement have heterogeneous structures, therefore their strength variations have a statistical character. In this work, the compressive strength data are represented by the Weibull distribution function, which then will be used in the cement failure probability calculations. Results show that using the statistically distributed compressive strength data in calculations, leads to more accurate outputs of the failure model.

The integrity of the cement sheath is very important for reducing the band pressure of the empty cement sheath and improving the safety of the shaft (well hole). The injection of high pressure fluid may lead to the expansion of the fracture of the interface of the well cementation during the volume fracturing of shale gas of horizontal wells. The sealing property of cement sheath is affected. Therefore, the finite element calculation model of fracture expansion in the cement interface is established on the stepped finite element method, which is combined with the CZM model. This model is used to calculate the field examples. The fracture expansion of the cement interface is affected by the degree of interfacial cementation, formation and the properties of cement sheath, the influence of which is studied. The results of the study show that the initial stress of the cement sheath and the change of the formation pressure on the overlying formation do not have any significant influence on the length of the fracture expansion of the cement interface. The greater difference of the horizontal stress while, the smaller the length of the crack growth. The fracture expansion length of the cement interface decreases with the increase of the elastic modulus of the formation. The amplitude of the change is increased first and then reduced. The elastic modulus of the cement sheath and the degree of consolidation of the cement interface are beneficial to reduce the length of crack propagation of the interface. All results have certain implications for the evaluation and prediction of the integrity of the cement sheath during the volume fracturing of shale gas wells.

Gas migration through fresh and hardened cement slurry is an ongoing problem in the oil industry. In order to eliminate this unfavourable phenomenon, research is being conducted on new compositions of slurries for gas wells. The article presents the results of research for slurries with low and high resistance to gas migration. The proper selection of the quantity and quality of components makes it possible to design slurry with the required static structural strength values. In addition, the cement sheath of such anti-migration slurry has low porosity and a very low proportion of large pore spaces. Additionally, the mechanical parameters do not decrease during long-term deposition in borehole-like conditions. By obtaining these results, it was possible to design slurry whose cement sheath has high corrosion resistance. The new slurry has a lower water-cement ratio. Additionally, GS anti-migration copolymer, anti-filter additive and latex are used. The presence of n-SiO2 aqueous solution and microcement allows for sealing the microstructure of the hardened cement slurry. Such modifications significantly improve the technological parameters of the cement slurry and the cement coat formed from it.

Cement losses into fractures may have detrimental effect on the height of the cement sheath built in the annulus after pumping a pre-defined volume of cement slurry. On the other hand, they might help ensure zonal isolation in CO 2 storage sites by reducing permeability of the fracture network in the cap rock, at least in the near-well area. A simple, engineering model of a lost-circulation event during cementing is used to demonstrate how wider fractures result in greater losses, with the radius of the cement bank (formed after pumping a given slurry volume) being approximately proportional to the fracture width. Thicker cement results in smaller losses, but may produce higher bottomhole pressures, thereby exacerbating cement losses.

Mechanical failure of well cement sheath can result in loss of zonal isolation, which is the cause of many problems, such as sustainable casing pressure, crossflows between formation zones, and leakage of fluid to surface. Pressure- and temperature-induced stresses in the cement sheath have been numerically analyzed by a number of previous investigators with sophisticated computer models. However, these investigations do not provide a practical method for predicting the maximum permissible pressure limited by the mechanical failure of cement sheath. An analytical model was derived in this work to predict the Maximum Permissible Pressure (MaxPP) in fluid injection and well stimulation. Results given by the model were verified with data from a CO 2 sequestration operation. Case studies with the model suggest that if the cement placement efficiency in the annulus is 100% as detected by CBL, formation in-situ stress should be utilized to make a conservative prediction of the MaxPP. If the cement placement efficiency in the annulus is less than 100% detected by CBL, the outer radius of the cement sheath should be considered and formation pore pressure in the cap rock should be employed to make a conservative prediction of the MaxPP. Improving cement strength is not an effective means of increasing MaxPP. The MaxPP should be enhanced by maximizing the outer radius of the cement sheath through improving the cement placement efficiency. The Maximum Permissible Net Pressure is lower than the burst-pressure rating of casing, suggesting that cement sheath failure will occur before casing failure occurs in the burst condition. Therefore burst design should be carried out for cement sheath, not production casing. This paper provides engineers a practical guideline to design and apply fluid injection pressure in well stimulation and operations.

Summary The bulk shrinkage of cement sheaths in oil wells can result in loss of long-term zonal isolation. Expansive additives are used to mitigate bulk shrinkage. To compensate effectively for bulk shrinkage during the late plastic phase and the hardening phase of the cement system, the performance of the expansive additive needs to be regulated considering the actual cement system and placement conditions. This paper presents an introductory investigation on the potential engineering of nanosized magnesium oxide (MgO) (NM) through heat treatment for use as an expansive agent in oilwell-cement systems. In this study, the bulk shrinkage of a cement system was mitigated by introducing NM with designed reactivity to the fresh cement slurry. The reactivity of NM was controlled by heat treatment. A dilatometer with corrugated molds was used to measure the linear strain of samples cured at 40°C and atmospheric pressure. The effect of NMs differing in reactivity on tensile properties of cement systems cured for 3 days at 40°C was examined by use of the flattened Brazilian test. The reactivity of the NM played a key role in controlling the bulk shrinkage of the cement system. Addition of only 2% NM by weight of cement (BWOC) with appropriate reactivity was sufficient to maintain expansion of the cement system. Adding NM to the cement system also resulted in improved mechanical flexibility. The NM with highest reactivity caused the largest reduction in Young's modulus at 3 days and, in general, the ratio of tensile strength to Young's modulus improved through the addition of NM to the cement system. Our work demonstrates that controlling the reactivity of the additive is a promising method to mitigate bulk shrinkage of cement systems and thereby to sustain the mechanical properties of the cement sheath in the oil well at an acceptable level.

Under non-uniform in-situ stress, the casing collapse failure often happens easily, and especially in soft rock the problem is more serious. In addition, only the few scholars do some studies about failure mechanism of cemented casing under non-uniform in-situ stress which has a strong effect on collapse properties of cemented casing, especially testing investigation. Hence, the collapsing test was performed for cemented casing under non-uniform load (NFL) by adopting self-developed testing equipment, by which the radial deformation of cemented casing and damage rules of cement sheath have been measured and the stress-strain laws of cemented casing are obtained during the testing process by the electrical method. The initial yield load and plastic limit load of cemented casing as well as the subsequent yield load have been obtained. By analyzing testing data, the stress-hardening rate and strain-hardening rate after hardening have been determined. The effects of cement sheath on collapse properties of P110SS casing and strain and deformation laws of P110SS casing after hardening have been obtained. The hardening character and failure mechanism of cemented casing have been figured out under NFL.

A model for predicting the strength of a cement sheath adjacent to a production wellbore without consideration of internal and temperature stresses is proposed based on solutions of the Lame problems for one- and two-layer tubes and the Huber–Mises yield criterion using the model of a perfectly plastic isotropic body.

Cement sheaths are among the most important well barrier elements and cement sheath integrity is thus important to maintain zonal isolation. Repeated pressure cycling in a well might lead to radial cracks and/or formation of microannuli in the cement sheaths. However, most experimental tests and model simulations of cement sheath integrity do not include mudfilms at the cement-rock interface, and the effect of such mudfilms on cement degradation during pressure cycling is thus not well understood. In this paper, we have used our custom-made laboratory set-up to study how mudfilms influence cement sheath integrity during pressure cycling, where X-ray computer tomography (CT) is used to visualize microannuli and cracks formed in the cement. Our results show that the bonding towards the formation is of importance for the cement sheath integrity during pressure cycling, both in the case of a soft and a stiff surrounding formation. Additionally, we see that with mudfilm present at the cement-rock interface the cement sheath is able to withstand less casing pressure before failure compared to a cement sheath without mud. For samples with a mudfilm present, radial cracks were not observed to propagate from the cement sheath and into the surrounding rock in the area covered by mudfilm.

Cement sheath is a critical barrier for maintaining well integrity. Formation of micro-annulus due to volume shrinkage and/or pressure/temperature changes is the major challenge in achieving good hydraulic seal. Expansion of cement after the placement is a promising solution to this problem. Expanding cement can potentially close micro-annulus and further achieve pre-stress condition because of the confinement. Primary aim of this paper is to investigate mechanical integrity of different pre-stressed cement system under loading condition. To achieve the objectives, finite element modelling approach was employed. Three dimensional computer models consisting of liner, cement sheath, and casing were developed. Pre-stress condition was generated by modelling contact interference at the cement-casing interface. Three cement (ductile, moderately ductile, and brittle) were considered for simulation cases. Wellbore and annulus pressure were applied. Resultant, radial, hoop, and maximum shear stresses were investigated at the cement-pipe interface to assess mechanical integrity. For comparison purpose, similar simulations were conducted using cement sheath without pre-stress and cement system representing uniform volume shrinkage and presence micro-annulus. For constant wellbore pressure, the radial stresses observed in all three types of cement system were practically similar and decreased as pre-stress was increased. Hoop stress also reduced with increase in compressive pre-load. However, their absolute values were distinct for different cement types. These results indicate that cement system with compressive pre-load can notably reduce the risk of radial crack failure by providing compensatory compressive stress. However, on the contrary, the maximum shear stress developed at cement-pipe interface, increased because of pre-load. This can compromise the mechanical integrity by reducing the safety margin on shear failure. Thus, the selection of expansive cement should be made after carefully weighing reduced risk of radial failure/debonding against the increased risks of shear failure. This paper provides novel information on expanding cement from the perspective of mechanical stresses and integrity. Modelling approach discussed in this work, can be used to estimate amount of pre-stress required for a selected cement system under anticipated wellbore loads.

Cement sheaths are among the most important barrier elements in petroleum wells. However, the cement may lose its integrity due to repeated pressure variations in the wellbore, such as during pressure tests and fluid injections. Typical cement sheaths failure mechanisms are formation of radial cracks and microannuli, and such potential leak paths may lead to loss of zonal isolation and pressure build-up in the annulus. To prevent such barrier failures, it is important to study and understand cement sheath failure mechanisms. This paper describes a series of experiments where we have used a tailor-built laboratory set-up to study cement sheath integrity during pressure cycling, where the set-up consists of down-scaled samples of rock, cement and casing. Cement integrity before and during casing pressurization is characterized by X-ray computed tomography (CT), which provides 3D visualization of radial cracks formed inside the cement and rock. We have studied how contextual well conditions, such as rock stiffness, casing stand-off and presence of mudfilm, influence cement sheath integrity. The results confirm that the rock stiffness and casing stand-off determine how much casing pressure the cement can withstand before radial cracks are formed in the cement sheath, where the rock stiffness is significantly more important than casing stand-off. Furthermore, it is seen that the radial cracks in the cement sheath continue into the rock as well. However, when a thin mudfilm is present at the rock surface, the cracks stop at the cement-rock interface, and the cement sheath withstands less pressure before failure. The bonding towards the rock is thus of importance.

Quality of cement is an integral part of well integrity. To ensure and improve cement quality, innovative polymers, based upon a family of polymers known as polyamides, are synthesized to improve the mechanical properties of Portland cement. This results in cement systems that have high resistance to impact breakage in set cement. With the addition of polyamides into cement, mechanical properties such as unconfined compression strength (UCS), confined compression strength (CCS), Young's modulus, and the effect of temperature on cement performance are used to show significant improvements to set cement elasticity and compression strength. Under dynamic force loading conditions, up to 30 MPa (4,350 psi) and at temperatures ranging from 20 to 180°C (68-356°F), a tri-axial load cell offers a more comprehensive method of measuring mechanical properties, of designing competent cement systems, and predicting cement integrity. These measurements are further compared to analyses from more traditional methods, such as confined acoustic and unconfined load cells, for information such as compressive strength. The polymer additive is a solid comprising of a polyamide that enhances the mechanical properties of set cement by rendering it high-strength and more elastic to reduce plastic deformation. Findings show that the new polyamide-cements exhibit unconfined compressive strength improvements greater than 25% when compared to latex-treated cement. Furthermore, tri-axial load cell measurements are able to quantitatively analyze the reliability of cement by measuring cumulative fatigue damage which predicts cement failure (Reddy, 2007). Strain-controlled cyclic tests to measure mechanical properties at 20°C show that polyamide cements resist deformation better because of higher durability than latex cements. Polyamide cements had only an 11% permanent strain when compared to latex cement, which had a 27% permanent strain. It is important to assess the mechanical performance of oil well cement, which is subject to cyclic loading due to dramatic changes in pressure and temperature during production (Ravi, 2004). Lastly, the effects of temperature on cement performance is well-documented to cause phase changes at different scales and a decrease in cement strength (Reddy, 2016). With polyamide-cements, the rate of cement strength retrogression is less at temperatures up to 180°C allowing the cement to have increased strength in comparison to latex cements, as well as to cement with no additives. Collectively, the data on elasticity and high compressive strength shows the value of this new polymer additive for cement systems on long-term zonal isolation in gas and oil wells. These polyamides maintained all favorable characteristics, proving to be the best performing additive that imparts the desired mechanical properties essential to extending the endurance of wellbore cement sheaths.

The significant decreased wellbore temperature and increased casing pressure during fracturing fluid injection present a big challenge for the mechanical integrity of cement sheath in fracturing wells. Based on the theories of elastic mechanics, thermodynamics, and a multi-layer composed thick-wall cylinder, this paper proposed a new mechanical model of cement sheath for fracturing wells, coupling pressure, and thermal loads, which consider the failure modes of de-bonding, radial cracking, disking, and shear failure. The radial nonuniform temperature change and the continuous radial stress and radial displacement at two interfaces have been considered. With the proposed model, the radial distributions of failure stress and the corresponding safety factor for cement sheath during fracturing fluid injection have been analyzed and compared under four failure modes. Results show that the decreased wellbore temperature will produce significant tri-axial tensile stress and induce cement failure of de-bonding, radial cracking, and disking. The increased casing pressure will significantly lower the risk of de-bonding but also aggravate radial cracking and shear failure. For integrity protection of cement sheath, increasing the injected fluid temperature, maintaining higher circulation pumping pressures, and adopting cement sheath with a low elasticity modulus have been suggested for fracturing wells.

During drilling operations, effective displacement of fluids can provide high-quality cementing jobs, ensuring zonal isolation and strong bonding of cement with casing and formation. Poor cement placements due to incomplete mud removal can potentially lead to multiple critical operational problems and serious environmental hazards. Therefore, efficient mud removal and displacement of one fluid by another one is a crucial task that should be designed and optimized properly to guarantee the zonal isolation and integrity of the cement sheath. The present work provides an overview of the research performed on mud removal and cement placement to help the industry achieve better cementing jobs. An extensive number of investigations have been conducted in order to find some key techniques for minimizing the cement contamination and obtaining maximum displacement efficiency. Yet, even after implementing those techniques, the industry happens to encounter poor cementing jobs. The present review aims to assist with evaluating the current theories, methodologies, and practical techniques, in order to possibly identify the research gaps and facilitate the way for further improvements.

During the well-cementing setting phase, a change in temperature or pressure could fail the cement sheath or the occurrence of the microannulus at the interface between the cement and the casing, or between the cement and formation. Microannuli is a big issue in many oilfield's well-cementing operations, thus often results in production reduction and an increase in remedial cost. However, the key to successful long term well integrity of casing-cement microannulus is to understand and evaluate the hydraulic sealing of cementing annulus by detecting existing or potential fluid movements within the well that may exist between the casing and formation after the cement job. This article aims to demonstrate a new way of evaluating and understanding the behavior of microannulus sealing function of the cement sheath-casing interface using the high-resolution optical device as a function of cement curing age and pipe lengths. The technical benefit of this novel experiment is to quantify the primary functional purpose of the cement which is supporting the casing and to seal the annulus to the flow of fluid. The experiment was carried out using API Class H cement.

Failure of cement sheath due to casing expansion or formations pressure during completion or production stages of HPHT or deep vertical wells is a very common phenomenon. There have been many studies providing approaches to predict cement sheath failure, where theory of elasticity or thermo-elasticity together with the plane strain concept were taken into consideration to obtain representative results. However, sedimentary formations in subsurface layers are exhibiting a poroelastic behavior and theory of elasticity may not be able to fully describe their behaviors when changes in pore pressure and in-situ stresses are taking place. In this paper, an analytical approach based on the theory of thermo-poroelasticity was presented to predict the possibility of cement sheath failure in deep structures. A separate numerical molding was also performed to evaluate the application of the approach developed. The results obtained indicated that a thicker cement can withstand a higher load applied by the formations and protect the casing against a significant collapse pressure. The temperature was also found as a significant contributor in increasing the pressure applied by the formation and casing on the cement due to pore fluid and steel expansions. Although some discrepancies observed between the results of the numerical simulation and the analytical model, it seems that the approach presented is able to provide reliable results considering the fact that interactions of material interfaces could not be included in the analytical modeling.

Summary The annular cement sheath is one of the most-important well-barrier elements, both during production and after well abandonment. It is, however, well-known that repeated pressure and temperature variations in the wellbore during production and injection can have a detrimental effect on the integrity of the cement sheath. A unique laboratory setup with downscaled samples of rock, cement, and pipe has been designed to study cement-sheath-failure mechanisms during thermal cycling, such as debonding and crack formation. With this setup, it is possible to set the cement under pressure and subsequently expose the cement to temperature cycling under pressure as well. Cement integrity before and after thermal cycling is visualized in three-dimensional by X-ray computed tomography (CT), which enables quantification of and differentiation between debonding toward the casing, debonding toward the formation, and cracks formed inside the cement sheath itself. This paper describes in detail the development and functionality of this laboratory setup along with the experimental procedure. Several examples to demonstrate the applicability of the setup, such as tests with different types of casing surfaces and different rocks, are also shown.

Summary Lightweight cement can present unique challenges to efficient and accurate evaluation of cement-sheath integrity. Standard bond-log tools require high compressional bonding to casing to observe an attenuation of the sonic signal that is set up inside the casing. Lightweight cement exhibits high amplitude and little alteration in the log waveform on conventional bond logs because of lower fluid density when pumped and lower compressive strength after being set up. These properties can make the log appear as if no cement is present. One must still determine the definition of the cement sheath with a method that is both valid and repeatable. The industry has demonstrated the ability to accomplish this task with ultrasonic logging tools. These devices allow very precise definition of the cement sheath with a 360 ° examination of the data, but still require innovative interpretation techniques to derive useful solutions regarding the quality of the cement. Additional complication is introduced when settable efficiency-fluid material is the material used instead of cement. This fluid is not designed to be an isolating agent, but is formulated to be used as a lead-in fluid to facilitate the placement of lightweight or conventional cement. This fluid is lighter weight and has lower compressive strength when set up than even the lightest cements. The cement design for this well was to lead in with the settable efficiency fluid and then to follow up with lightweight cement. The job did not proceed as planned, and the only fluid introduced to the well was settable efficiency fluid. One must still determine segregation of the productive reservoirs from surface water and the ability to fracture treat wells with restrained fluid mobility to complete and produce the well. This paper demonstrates the application of ultrasonic cement-evaluation tools under these difficult conditions. Examples of conventional cement, lightweight cement, and settable efficiency fluids are considered. The processed logs highlight the confidence in the reservoir segregation indicated with these tools and evaluation techniques.

The physical properties of the oil-well cement, especially its strength and permeability, are changing significantly after reacting with CO2-saturated brine which will alter the cement hydration products. In this study, the effect of incorporating the olive waste into Saudi Class G oil-well cement formulation subjected to CO2 sequestration conditions at 130 °C and 10 MPa on the cement permeability, carbonation depth, compressive and tensile strength, and microstructural changes during 20 days of carbonation was evaluated. The results obtained showed that the addition of 0.1% by weight of cement (BWOC) of the olive waste to the cement enhanced the cement resistance to the carbonation process and decrease the carbonation depth from 1469 μm for the base cement to 1269 μm after 20 days of carbonation. Incorporating 0.1% BWOC of the olive waste into the cement slurry also maintained the original cement permeability of 14.3% less than that for the base cement, while the permeability of the cement samples with 0.1% BWOC olive waste after 20 days of carbonation is 33.9% lower than the base cement permeability. The permeability reduction is the main mechanism responsible for the enhancement of carbonation resistance for the olive waste-based cement. The ability of the olive waste to delay calcium leaching is attributed to the decrease in the permeability of the sample with 0.1% BWOC of olive waste.

The fracture debonding failure of cement plug/casing interface caused by the migration of underground fluid with pressure accumulation imposed serious challenges to wellbore integrity of abandoned wells. This paper presents an investigation with a view of quantifying the relationships between the fracture debonding height and the migration time of underground fluid. A new 3D numerical model of cement plug/casing/cement sheath/formation system based on the cohesive zone method, which was validated by microannulus experiment, was developed to simulate the fracture debonding process. And then the relationships between fracture debonding height and propagation pressure was studied. Additionally, the influence of horizontal in-situ stress, cement plug mechanical parameters, and interface properties on the fracture debonding height and development geometry were also analyzed. The results of research indicated that the minimum horizontal in-situ stress played a decisive role in controlling the fracture propagation of cement/casing interface. Debonding fracture propagated along the whole circumference of the interface with the same height for the case that the horizontal in-situ stress was uniform, while propagated along a certain circumference angle of the interface with the maximum height in the direction of maximum horizontal principal stress for the case that the horizontal in-situ stress was non-uniform. The results of numerical analysis showed that lower elastic modulus and Poisson's ratio of cement plug, higher cement plug permeability, and larger critical normal strength and critical shear strength were beneficial to decrease the fracture debonding height and reduce the risk of debonding failure of the cement plug/casing interface. Finally, some effective measures to maintain the sealing integrity of abandoned wells were proposed.

Robust dual barrier systems are crucial for well integrity. Regulators and industry have consistently raised the concerns in such systems regarding the leak pathways and its failure modes prediction, testing, and qualification. Liner hanger is an example of dual barrier system in which seal assembly and cement sheath act as two barrier elements. The liner hanger system is usually employed in complex operating environments such as high-pressure and high-temperature (HPHT), deep-water, and ultra-deep-water. In these harsh environments, the gas migration through such system has been attributed to several well control incidents. To investigate this problem, a thorough review of underlying complexities, leakage mechanisms, liner hanger seal assembly materials, dominant failure mechanisms in seal and cement, and the gap in current standards and regulations is conducted. Overall, the objective of this paper is to understand the nuances of the liner hanger dual barrier system and to identify the risks of failures that could compromise the wellbore integrity.

Perforation will damage the cement sheath around the hole in petroleum engineering, leading to the failure of cement sealing integrity during hydraulic fracturing. This work presents a perforating experiment in real size to investigate the damage mechanism of cement sheath. The crack characteristics inside the cement after perforation were statistically analyzed by computerized tomography (CT). Then, a numerical model was introduced to describe the effective stress state in cement sheath during perforation based on ANSYS/LS-DYNA software. The damage zone around the hole in cement sheath was partitioned by the peak stress of cement during perforation. The results show that damage zone was formed by the surface pressure around the hole after the shaped jet passed through the cement. The toothed micro-cracks around the tapered hole are close to the cement/formation interface. The cement with lower shear modulus and higher strength can reduce the damage range in cement sheath, and the scheme with lower perforation density and larger perforated phase is more effective to the maintenance of cement sealing integrity during later fracturing process.

Admixtures of mineral or waste filling materials are used to reduce slurry density. However, the sheath made of lightweight cement slurry has low mechanical performance at the initial bonding time. The required strength is achieved later. This is the main problem when evaluating the cement bond logging. The waiting time for geophysical measurements after injecting and bonding of cement is nowadays increasingly shortened. This is forced by economic factors. Too early geophysical measurements may result in obtaining a false indication of the cement bond logging. The lack of cement or partial bonding, despite the presence of slurry in the annular space is then found. The slurry developed by the author achieves high compressive strength after a short bonding time. Reducing the amount of water in the slurry resulted in a lowered filtration value. This is important in preventing gas migration after the cementing. The designed slurry also reaches the value of 3.5 MPa in a short time. This allows for an earlier commencement of a well drilling. The use of said slurry improves the effectiveness of the well sealing and makes it possible to obtain a reliable knowledge of the bond logging.

Sustained Casing Pressure (SCP) has become a serious problem in the development of shale gas fields, which is believed to relate to the multistage fracturing. However, few researches were on this, especially the study of micro-annulus at the interface between the cement sheath and casing, which was one of the mechanisms of the cement sheath integrity failure, and always caused by accumulated plastic deformation. This paper presented experimental and numerical investigations of the emergence and development of plastic deformation occurred at the interface between the cement sheath and casing during multistage fracturing. Mechanical tests and physical simulation experiments were carried out so as to analyze the plastic deformation variation characteristic of the cement sheath under cyclical loading and unloading. 3-D numerical models were established to analyze the emergence and development of the accumulated plastic deformation based on the test results, considering the actual engineering and geological conditions during multistage fracturing in shale gas wells, which presented a new method to calculate the width of the micro-annulus between the cement sheath and casing. The research showed that the numerical results were in accordance with the experimental results. With the increase of cycle times, the accumulated plastic deformation increased, which was the greatest for the first time, and then increased linearly. Decreasing elastic modulus and increasing Poisson's ratio, cohesive force and internal friction angle could facilitate the protection of the cement sheath integrity. Finally, a new cement slurry system was proposed based on the results of study and applied in 5 wells with good effects acquired, none of them showed SCP after fracturing.

Sustained casing pressure (SCP) in shale gas wells caused by cement sheath failure can have serious impacts on safe and efficient gas production. Considering the fact that horizontal wells are widely used for production from shale, the cementing quality and casing centricity is barely ensured in these wells. Among other indications, the casing eccentricity is identified very often in wells with SCP problems in the Sichuan field in China. Hence, the objective of this study is to analyze the effect of the casing eccentricity on the integrity of the cement sheath. To better understand stress distribution in eccentric cement sheaths, an analytical model is proposed in this paper. By comparing the results of this model with the one’s with centric casing, the impacts of the casing eccentricity on the integrity of the cement sheath is analyzed. During fracturing treatments, the casing eccentricity has a little effect on stress distribution in the cement sheath if the well is well cemented and bonded to the formation rock. However, on the contrary, the casing eccentricity may have serious effects on stress distribution if the cementing is done poorly. The debonding of casing–cement–formation interfaces can significantly increase the circumferential stress in the cement sheath. At the thin side of the cement sheath, the circumferential stress could be 2.5 times higher than the thick side. The offset magnitude of the casing eccentricity has little effect on the radial stress in the cement sheath but it can significantly increase the shear stress. We found that the risk of cement failure may be reduced by making the casing string more centralized, or increasing the thickness of the casing. The results provide insights for design practices which may lead to better integrity in shale gas wells.

The sustained casing pressure (SCP) in shale gas wells caused by cement sheath failure can have serious impacts on safe and efficient gas production. Although horizontal wells are widely used for production from Shales, the cementing quality and casing centericity is barely ensured. Among other indications, the casing off-center is iedtified very often in the wells with SCP problem in Sichuan field. Hence, the objective of this study is to analyze the effect of the casing off-center on the integrity of the cement sheath. To better understand stress distribution in eccentric cement sheaths, an analytical model is proposed in this paper. By comparing the results of this model with the centeric casing, the impacts of casing off-center on integrity of the cement sheath is analyzed. During the fracturing treatment, the casing off-center has little effect on stress in the cement sheath if the well is well cemented and bonded to the formation rock. But on the contrary, the casing off-center has serious effects on stress distribution if the cementing is done poorly. The debonding of casing-cement-formation interfaces can significantly increase the circumferential stress at the cement sheath. At the narrow side of the cement sheath, the circumferential stress could be 2.5 times higher than the thick side. The offset magnitude of the casing eccentricity has little effect on the radial stress in the cement sheath but it can significantly increase the shear stress. We found that the risk of cement failure may reduce by making casing string more centralized, increasing the thickness of casing. The results provide insights for design practices led to better integrity in shale gas wells.