Casing-cement Interface Research Articles

Guest Editorial: The Difference Between CO2 EOR and CCS Injection Well Metallurgy

_ The recent surge in development of CO2 carbon capture and storage (CCS) projects has brought focus on the requirements for corrosion resistance in the injection wells as required by US Environmental Protection Agency (EPA) permitting. In some cases, the long successful history of various metallurgies in enhanced oil recovery (EOR) wells has been cited as sufficient to allow the same completions for CCS injection wells. The lack of actual data on the long-term performance of these alloys in EOR wells in combination with the more stringent requirements for Class VI wells suggests otherwise. Introduction The recent significant influx of large amounts of government incentives for a variety of green initiatives including CCS and CCUS has created a rush to drill and complete CO2 injection wells. However, the necessary corrosion data to make informed choices for corrosion resistance in these wells is minimal at best. Some oil and gas professionals have argued that there is no difference between the more than 40 years of petroleum experience with CO2 EOR and planned CCS wells. This comparison is not a valid one and can be risky considering the need for very long-term containment of CO2 required by regulators. This article presents a comparison between CO2 EOR and CCS for injection well metallurgy and explains why this comparison is invalid. Background and Comparisons There is a 50-year successful history of CO2 EOR in petroleum production that has led to a consensus, largely without merit, that corrosion of injection-well equipment is manageable and does not result in long-term degradation of certain components such as casing. The reason there is little or no merit to applying this experience to CO2 CCS wells is the lack of data specific to the long-term corrosion resistance of casing that remains in the EOR wells. Rather, the industry generally simply plugs these wells with cement, and in the majority of cases without examining the casing before abandoning. Furthermore, industry rarely returns to inspect them after years of abandonment. In a few instances, logging and retrieval of cement cores and casing have been performed and analyzed. For example, analysis of cement and casing from the SACROC field that had been exposed to CO2 for over 30 years showed corrosion along the casing-cement interface (Carey et al. 2007). Laumb et al. (2016) reported substantial casing corrosion for a CO2 EOR well in the Weyburn field based on a series of logs over several years. Koplos et al. (2007) provided survey data from numerous CO2 EOR wells and found that 11.1% of the wells failed mechanical integrity tests and concluded that injection-well mechanical integrity is a concern for CO2 injection and storage due to the corrosivity of CO2 on downhole materials.

Study on the mechanism and controlling method for annulus pressure buildup problems in shale gas wells

The Annulus Pressure Buildup (APB) problems in shale gas wells is mainly caused by the crack at casing-cement interface. The stresses on casing-cement interface and cement-formation interface are calculated by the established model which the constrain of formation rock is considered. The elastic modulus of cement sheath has great effect on the stress on casing-cement and cement formation interface. The radial stress on interfaces changed linearly with fluid pressure. The cement rock is treated as a porous elastic material and the residual strain in cement sheath is mainly caused deformation or collapse of pore structure in the cement rock due to the large radial stress on interfaces. Ultimately, micro-annulus will be generated on the casing-cement interface due to the residual strain of cement sheath. High flexibility cement rock is proposed to decrease the radial stress on the interfaces which can reduce the residual strain of cement sheath. On the other hand, PABP is proposed to offset the decreased radial stress due to residual strain. A model for calculating the stress on interface by considering residual strain of cement sheath and PABP is established. The radial stress on the interfaces caused by PABP is quantitative calculated. The mechanism on how the method for solving APB problems in shale gas wells is clearly explained. The field application certificated the validity of method for solving the APB problems and it also proved the correctness of the calculation method.

Thermomechanical behaviour of well cement in different geological formations under the coupled effects of temperature and pressure

It is crucial to ensure that the cement sheaths remain intact in all types of deep well applications subject to high temperatures and high pressures underground. It should be noted, however, that the behaviour of annulus cement sheath is not self-controlled and is governed by the response of steel casing and the surrounding rock formation in such extreme underground conditions. In this study, we investigate the thermomechanical response of wellbore materials in a typical oil well consisting of steel casing, annulus cement sheaths and surrounding rock formations subjected to continuous steam injection. A section of an oil well was simulated by using the COMSOL Multiphysics, a finite element method, using Class G Portland cement as the annulus cement. In two formation scenarios, stiff (carbonate) and compliant (sandstone), temperature and pressure were coupled to evaluate wellbore material behaviour. It was found that wellbore materials in carbonate were more susceptible to stress than those in sandstone. Despite the fact that all materials reached their highest temperatures in both formations, the retention time of the maximum temperature in the cement sheath in sandstone was shorter than in carbonate. Regardless of the thickness of the cement sheath, the highest strains were located at the casing-cement interface in carbonates. As a result of the temperature and pressure ramp, the cement sheath in sandstone failed by generating tensile radial cracks along the lowest layer thickness of the cement sheath. In compliant formations such as sandstone, the cement sheath would likely fail due to tensile cracking along its lowest thickness.

Experimental and Numerical Investigations of Cement Bonding Properties at Elevated Temperatures-The Effect of Sample Cooling.

Well integrity is currently defined through the concept of well barriers, in which one or more barriers are used to prevent unwanted fluid flow. Many papers have highlighted that the casing–cement interfacial bonding is critical for well integrity, but many discrepancies between laboratory experiments and field data have been noticed. The use of finite element analysis is now established as an alternative to complex in situ tests, but these simulations are sensitive to the input parameters, which results in many discrepancies across published works. Currently, the cohesive zone material (CZM) method is considered to offer good results if the correct parameters are selected or experimentally determined. The novelty of this paper lies in the development of a better workflow that enables the simulation of three processes that are acting on the laboratory-scale casing–cement system: temperature changes, debonding, and post-debonding behavior. The aim of this paper is to fully understand the debonding process within laboratory-scale samples, and thus to eventually enable upscaling in the near future. The paper presents a new workflow generated using FEM that allows us to determine the contact stresses at the casing–cement interface during temperature changes at the moment of debonding and post-debonding. The results presented within this paper show that temperature samples tested according to the push-down setup will provide similar interfacial bonding shear strength values; however, post-debonding, there is a remaining frictional force slightly higher than that of the room-temperature samples. In this case, the results are within a 5% error of the average field data, which is slightly higher than in our previous experiments, where only room temperature data were considered. A major outcome of our paper is the demonstration of the existence of friction forces after debonding, which are a result of radial stresses induced during the debonding process.

Numerical investigation of the factors affecting the cement sheath integrity in hydraulically fractured wells

The current industrial practice calls for wells to be hydraulically fractured in as many as 50 stages, depending on the formation. The integrity of the cemented wellbore sections around the perforations has been a major concern in most hydraulically fractured wells. In this paper, we have conducted a comprehensive numerical modelling study to understand the possible impact of hydraulic fracturing operation on the integrity of the cemented wellbore sections, especially around the perforations where the fracture fluid is injected. A numerical model allowing full coupling of solid/liquid flow has been adopted to simulate the fracturing fluid injection and the resultant deformation of cemented casing sections around the perforations. Field data from a hydraulically fractured deep shale gas well in China were used to demonstrate the impact of the operational variables on the integrity of the cemented wellbore section. We also studied the influence of fracture fluid injection pressure, mechanical properties of the cement, casing and the reservoir, perforation azimuth, and fracture fluid viscosity on the cement sheath failure. Simulation results have shown that the cement sheath failure at the cement-casing interface may occur in the form of debonding in the radial direction. The cement debonding occurring at the casing-cement interface due to high frac fluid injection pressure, would be one of the main reasons for the poor hydraulic fracturing job performance commonly observed in shale gas wells. Following conclusions can also be drawn from simulation results: 1) Injection pressure has an important impact on the cement debonding. For the specific reservoir conditions investigated in this paper, to avoid cement debonding, the injection pressure should be no more than 75 MPa; 2) Use of a casing with a larger elasticity modulus and cement with lower elasticity modulus and lower permeability are conducive for improved cement bonding quality; 3) For a shale gas reservoir with lower permeability, these requirements for the casing and cement should be even stricter. Resultsof this study can be useful for optimum selection of the limiting pump pressure, cement and casing properties, and the perforation azimuth to be used for the design of a successful hydraulic fracturing job.

Numerical modeling to evaluate tensile mechanical and shear failure of cement in the casing-cement interface

This paper presents a numerical model of an integrated 3D casing-cement-formation system, to evaluate the mechanical tensile and shear failure of the cement at the casing-cement interface as the pressure and temperature conditions of the formation and borehole vary during production. The model which includes the formation pressure, unlike others proposed, was developed by stages under finite element discretization and compared to analytical models. Results show that increasing the formation temperature increases the probability of tensile and shear failure in the cement, while increasing the wellbore temperature decreases these probabilities. On the other hand, the decrease in the well pressure reduces the probability of shear failure and increases the tensile failure. In the case of formation pressure, the opposite occurs.

Bonding Mechanism of Zonal Isolation Materials to Clean and Rusted Casing

Summary In oil and gas and geothermal well construction, a cementitious material is pumped in the wellbore to provide zonal isolation and support the casing during the life cycle of the well. Thus, the cementitious barrier materials must be durable in terms of chemical and mechanical properties and have chemical compatibility with casing pipe. The complex region of casing-cement interface is considered a key parameter to fulfill long-term zonal isolation. This interface must be chemically stable and impermeable to block unwanted formation fluid communication. Shortcomings of conventional Portland cement under operational conditions and increasing sensitivity to its carbon footprint are motivations for a green alternative. Bond strength and sealability of cement with steel surface have been measured previously. But few research works cover surface characterization and morphological analysis of barrier materials and the connected steel surface. This study provides a full picture of selected alternative materials in terms of shear bond strength, hydraulic sealability, and interface morphology analysis of the materials. Materials include API Class G cement, an industrial expansive cement, noncement-based pozzolanic material, geopolymer, and thermosetting resin. Also, clean and rusted steels were considered as a representative for the casing pipe in the field. The samples were prepared under elevated pressure and temperature. The results proved that higher shear bond strength is not an indication of good sealability, and the ingredients used to mix slurries have a critical role in the structure of the interfacial zone between casing and barrier material.

IMPROVING MECHANICAL PROPERTIES OF OIL WELL CEMENT USING POLYPROPYLENE FIBERS AND EVALUATING A NEW LABORATORY METHOD FOR MEASURING THE CASING CEMENT BONDING STRENGTH

The main objective of this study is to enhance the poor performance of oil well cement in terms of mechanical properties by using pure polypropylene fibers. Polypropylene fibers were added in increasing concentrations from 0 to 0.1%, 0.3%, 0.5%, 0.7%, and 0.9% by weight of cement (BWOC). Rheological parameters, density, fluid loss, permeability, porosity, compressive strength, tensile strength, and flexural strength were all tested. A new method for measuring the tensile strength of cement samples in the presence of a casing is also evaluated in this research. In addition, the interfacial bonding shear strength, which represents the strength of cement adhesion to the casing, was measured using a new laboratory procedure. The influence of adding polypropylene fibers on rheology, density, and fluid loss can be ignored, according to the results of the experiments. The permeability and porosity of cement samples increased as the proportion of polypropylene fibers increased, according to the findings. Further, an increase in polypropylene fibers concentration up to 0.3% BWOC led to improving the mechanical properties at different curing times. The bonding strength of the casing cement interface improved with increasing polypropylene fibers concentration up to 0.5% BWOC.

Investigation of Stress State During Cement Hardening and Its Effect on Failure of Cement Sheath in Shale Gas Wells

Abstract Consideration of initial stress state after cement hardening provides a vital basis for the prediction of cement failure, which has been overlooked in previously published methodologies partly due to the difficulties in examining this problem rationally. In the present study, the hoop stress at casing-cement interface during cement hardening is investigated experimentally based on the full-scale casing-cement sheath-formation system (CCFS) facility, which is equipped with the real-time stress-strain measurement capability. The hoop stress at casing-cement interface during cement hardening drops sharply, rather than equating with the initial annulus pressure of cement slurry. It presents a higher drawdown under higher annulus pressure and thinner casing, and a lower drawdown under elastic cement slurry and thicker cement sheath. Furthermore, an analytical model taking the effect of cement hardening into account is developed to predict the integrity of cement sheath. Reliability of the model is verified by comparison with field observations. Excellent agreements are observed. The results illustrate that the tensile cracks are likely to occur at the inner cement (inner surface of cement sheath) by the effect of cement hardening, since the hoop stress at inner cement during cement hardening drops greatly and even becomes tensile. A detailed sensitivity analysis illustrates that an elastic cement slurry with a lower elastic modulus works more effectively, which can resolve the sustained casing pressure (SCP) problem in shale gas wells.

A Parametric Study for Radial Cracking in Cement Under Different Loading Events Based on the Stress Intensity Factor

AbstractCement is one of the primary barriers in a wellbore and critical to well integrity. Radial cracking is a pervasive failure mode in cement due to the temperature and pressure variation during drilling, completion, or production. This work presents a comprehensive analysis of radial cracking in cement under various loading events. The proposed model estimates the stress intensity factor and fracture surface displacement as indicators for crack propagation and opening, respectively, through a distributed dislocation technique. Three types of radial cracks, divided by their tips terminating at the casing–cement interface, inside cement, or at the cement–formation interface, are considered. Based on this model, we conduct a parametric study for radial cracking under typical loading events such as steam injection, CO2 injection, and high-pressure and high-temperature (HPHT) drilling. Results indicate that the crack near the casing–cement interface has an increased risk for steam injection and HPHT drilling, while all three types of radial cracks are destructive during CO2 injection. The thermal expansion coefficient of cement is a significant parameter for steam and CO2 injection wells. The fluid pressure and the cement’s thickness are crucial to radial cracking under HPHT conditions. Stiffer cement could promote crack opening for steam injection but prohibit the crack deformation for CO2 injection or HPHT wells. Thicker cement would accelerate radial cracking under the three loading events. These findings are helpful in designing cement to maintain long-term integrity.

A study on the hydraulic aperture of microannuli at the casing–cement interface using a large-scale laboratory setup

Presence of microannuli at the cement interfaces in wellbores could provide a path for fluid leakage. In this study, we combined experimental analysis and numerical modelling to assess microannuli development at casing–cement interfaces. Lab experiments were conducted using two-metre long samples consisting of a 7” casing cemented inside a rock analogue (stiffer, stronger cement) encased with a steel outer shell. Hydraulic aperture of the microannuli was calculated by measuring water flow through the casing–cement interface at various casing pressures and after axial displacements. The results show that once a microannulus forms, it remains open at internal casing pressures as high as 40 MPa. The measured hydraulic apertures were in the range of tens of microns, with residual apertures in the range of 15 to 30μm. Axial displacement of the casing did not lead to a significant change in the hydraulic apertures. A numerical model was created with a comparable geometry to the experiments, and used to calculate the mechanical apertures of the microannuli. A relationship is proposed to link the mechanical apertures from the numerical models to the hydraulic apertures measured in large-scale experiments. The findings enable the operators and regulators to improve safe well operation practices by forecasting the conditions that lead to possible loss of zonal isolation and the associated well leakage rates.

Investigation of the interface cracks on the cement sheath stress in shale gas wells during hydraulic fracturing

Wellbore integrity problem induced by insufficient seal ability of the cement sheath has become a technical challenge of shale gas wells during hydraulic fracturing. In this paper, a new model has been established to characteristic the mechanics of the casing-cement interface (CCI) and the cement-formation interface (CFI) of the shale gas wells. In this model, the radial stress and the stress intensity factor has been analyzed to predict the prolongation of the initial cracks caused by perforation through comparison of the stress intensity factor and the interfacial fracture toughness of the CCI and CFI. In addition, the influence of the casing and cement parameters on the interfacial stress and the crack width of the CCI and CFI has been discussed in detail, and the optimal design of these parameters has been proposed according to the field data. Finally, the conclusions and some realistic suggestions have been put forward based on the discussion.

Chemical Change Induced by CO <sub>2</sub> Leakage through Wellbore Cement and Casing Interfaces

Damaged wellbores pose some of the greatest risks for CO2 leakage from underground storage reservoirs. If CO2 is injected into the subsurface, leakage may occur through fractures and cracks formed at the cement-casing interface due to deficient well completion, poor mud displacement, or damage during operation or abandonment. A review of the literature has shown that a range of cement fractures can be sealed through calcium carbonate precipitation, given appropriate fluid residence time and initial fracture aperture. In the presence of steel casing, studies have observed that iron carbonate precipitation may also play a key role in fracture sealing. Here we discuss results of cement-steel, cement-cement, and steel-steel core-flood experiments simulating CO2-brine leakage in these systems. The aim of this study is to quantify the chemical changes in the cement and steel materials upon exposure to CO2-equilibrated brine. Using a combination of surface profilometry measurements, X-ray tomography (XRCT) and solution chemistry analyses, we quantified the chemical changes within both the cement and steel phases. These new experimental data are needed to extend chemical degradation models and predict the long-term behavior of the casing-cement interface in the wellbore environment.

Evaluation of azimuth cement bond quality based on the arcuate phased array acoustic receiver station

Cement bond quality is a crucial parameter in oil production, and acoustic logging is among the main methods of evaluating this property. By analyzing the characteristics of casing and formation waves, the cement bond quality of the casing–cement interface (interface I) and cement–formation interface (interface II) can be evaluated. However, the use of both monopole sources and receivers render the evaluation of the azimuth and angle range of cement channeling difficult using conventional acoustic logging tools. Herein, we developed an improved azimuthally acoustic bond tool (AABT 2.0) comprising a monopole acoustic radiator and arcuate phased array (APA) acoustic receiver stations with enhanced azimuth resolution capability. AABT 2.0 can determine the azimuth and angle range of cement channeling and improve the resolution of azimuth measurement. We studied these factors through numerical simulation and physical experiments. Numerical simulation results show that the azimuth of cement channeling can be determined by measuring the amplitude of the casing wave's head wave in multiple azimuths received by the APA acoustic receiver stations in a casing well. Compared with individual-reception mode, APA acoustic receiver stations with the scanning-reception mode can both determine the azimuth of cement channeling more accurately and confirm the angle range of cement channeling. Data processing results from experimental wells show that AABT 2.0 can evaluate the cement bond quality of interface I with a high azimuth resolution in the circumferential and axial directions, through the amplitude imaging display of the casing wave's head wave. The reliability of AABT 2.0 and the accuracy of our measurement results are verified through comparison with results from an ultrasonic imaging logging tool and information concerning the well condition.

Modified discrete element method (MDEM) as a numerical tool for cement sheath integrity in wells

Cement sheaths undergo extreme loading conditions in wells during subsurface operations. A damaged cement sheath may lead to fluid communication between different formation layers and fluid migration up to the surface, which can cause environmental, technical, and economic problems. In this study, we numerically analyze cement sheath stability using the modified discrete element method (MDEM). MDEM considers the cement sheath and rock formation as porous media and can model discontinuous fracturing of the materials. Analyses are performed based on a small-scale cement sheath integrity test and field cement pressure data measured by Cooke et al. (1983). Oil well class H cement was used, and its poroelastic properties were estimated using a micromechanics model and a multi-scale homogenization technique. First, the evolution of the stress state was approximated in the cement sheath and rock formation. Then, radial fracturing, shear failure, and interface debonding formation were studied under pressure increase/decrease operations. The effects of several parameters, such as the casing size, rock elastic parameters, and loading time, were also investigated. During the hydration of cement, the compressive and shear stresses evolved in the cement sheath, and the stresses in the surrounding formation also changed. The simulation results showed that a decrease in the casing pressure can lead to debonding of the casing-cement interface, and the calculated interface opening was within 1–50 μm. A pressure increase in the casing can lead to progressive shear and tensile failures in the cement sheath. A further pressure increase did not extend those failures into the rock formation; rather, it increased the number of radial fractures in the cement sheath. The analyses showed that a lower decrease and increase in the casing pressure is required to generate debonding and radial fracture in the cement sheath, respectively, in the early stages of hydration after the cement is set. This is also the case when the casing has a larger diameter and smaller thickness. When the cement sheath is bonded to a softer rock formation, the pressure increase required to create a fracture in the cement is lower compared with the case in which the sheath is bonded to a stiffer formation. However, in the case of softer formation, debonding was not observed.

Micro-annulus generation under downhole conditions: Insights from three-dimensional staged finite element analysis of cement hardening and wellbore operations

A micro-annulus (MA) is defined as a high permeability zone or gap initiating/occurring at the casing-cement and cement-formation interfaces during the wellbore life span. An MA can significantly compromise wellbore integrity by establishing enhanced fluid flow pathways. This study uses a staged finite element approach to simulate wellbore integrity during various loading steps of wellbore operations under downhole conditions. Particular emphasis is placed on the processes of cement poro-elastic property evolution, volume variation, and pore pressure variation as part of the cement hardening step. The resulting state of stress during the life cycle of a typical injection well (i.e. hardening, completion, and injection) is analyzed to assess the onset and evolution of micro-annuli at various interfaces of the composite wellbore system under downhole conditions. The results show that cement shear failure is observed at the casing-cement interface during pressure testing (excessive wellbore pressure); and tensile debonding failure initiates at the cement-formation interface due to cement shrinkage during hardening and injection-related cooling (thermal cycling). Sensitivity analyses considering several parameters show that: (1) the degree of poro-elastic bulk shrinkage has significant implications for both shear and tensile failure initiation – the less the cement shrinks, the less likely the failure initiation is; (2) cement integrity increases with increasing depth; (3) cement pore pressure evolution has significant implications for tensile failure – if cement pore pressure decreases more, higher temperature differences can be sustained before an MA occurs; and (4) cement temperature fluctuations during hardening promote initiation of debonding failure. In summary, the results presented indicate that establishing downhole conditions to quantitatively analyze MA generation is necessary. The results are different compared to laboratory studies without considering/simulating downhole conditions. The knowledge from this study can raise the awareness of predicting and evaluating MA under downhole conditions and can be used to supplement and improve future laboratory experiments.

Experimental Evaluation of Geopolymer, Nano-Modified, and Neat Class H Cement by Using Diametrically Compressive Tests

Abstract Cement is the prime barrier material to maintain zonal isolation during wellbore life-cycle. Different types of the cement have different performances on sealability. The primary objective of this study is evaluation of three types of cement systems, neat Class H, nano-synthetic graphite, and rock-based geopolymer cements, on interfacial bond integrity. Diametrical compressive tests are used on casing–cement–formation systems. Cement performances are tested in a conventional configuration (Berea Sandstone). A universal testing system is utilized to analyze the relationship applied load–axial strain and the digital image correlation (DIC) technique is used to generate the strain field maps. Based on the relationship and DIC results, bond strengths in different cement systems are evaluated. Additionally, microscope images provide insight explanations of the bond from micro-view. Based on the experimental results, failures of all cases initiate and propagate from the casing–cement interface. Then, radial cracks develop through the formation. This is because the mechanical interlocking bonds are formed only at the cement–sandstone interface to enhance the bond strength. The comparison of geopolymer and nano-modified cements with the conventional cement (neat Class H) shows that the unconventional cements have a lower maximum load and higher axial strain before failure of the samples. It indicates that both the two kinds of cements are more ductile than conventional cement and can support more deformation before the initiation of cracks.

Distribution laws and characteristics of mud films at casing–cement interfaces in horizontal wells

The mud film thickness on the outer surface of casing and its distribution laws are important factors affecting the cementation quality at casing–cement interfaces. So far, however, the distribution laws of mud film thickness on the outer surface of casing at different inclinations and its effect on the interfacial cementing strength have never been reported in literatures. In this paper, the mud film thickness at different parts on the outer surface of simulated casing (steel pipe) was measured at the inclination of 0°, 30°, 60° and 86°, respectively, using the independently designed experimental device. Then, the distribution law of mud film thickness on the outer surface of casing and its effect on the interfacial cementing strength were summarized. And the following research results were obtained. First, when the inclination increases from 0° to 86°, the mud film thickness changes from uniform distribution to thin-and-thick distribution, and the area proportions of thinner and thicker mud films both increase. And especially at the inclination of 86°, the mud film thickness on the upper surface is very large, and there are large exposed areas on the left and right surfaces of the steel pipe. Second, the cementing strength at the casing–cement interface of the exposed casing is much higher than that at the interface with mud film. And with the increase of inclination, the interfacial cementing strength with mud film tends to increase. Third, with the increase of inclination, the interfacial cementing strength increases successively, which is closely related to the area proportion of the exposed steel pipe to the solid mud film on the outer surface. In conclusion, the exposed area of the simulated casing on the outer surface is a key factor affecting its interfacial cementing strength.

Experimental and finite element modelling evaluation of cement integrity under diametric compression

With the increased exploration activity in unconventional and extreme operational environment, well integrity is becoming a challenging problem. Cement sheath, being an important barrier element, should be designed properly for adaptation of complicated loads surrounding the wellbores. This experimental and modelling study evaluates casing-cement-formation system under diametrically compressive load.Digital image correlation (DIC) technique was employed to examine the strain distribution and failure in experimental setup. A 3D numerical model was developed and the comparison between simulation and experimental results indicated a reasonable match. Using validated model, the effect of cement elastic properties on stresses in cement was studied. The stress distribution in the cement-formation system shows that casing-cement interface is the weakest point in the system, thus it has the highest risk of failure. The failure initiates at orientation parallel (θ = 0°) to the applied load followed by in perpendicular (θ = 90°) direction. Hoop stress is more sensitive to Young's modulus and Poisson's ratio than radial stress, so it is more critical for the failure assessment. In general, flexible cement with low Young's modulus and high Poisson's ratio has low risk of failure.

Microannulus Formation Mechanism at the Cementing Interface of a Thermal Recovery Well during Cyclic Steam Injection

During the thermal recovery of heavy oil when using cyclic steam injection technology, a microannulus tends to form at the cementing interface subjected to high temperature and pressure during steam injection, and large temperature and pressure differences after injection can lead to wellbore integrity failure. In this study, a thermomechanical coupled finite element casing‐cement‐formation model of a thermal recovery wellbore is established. The deformation of the wellbore during both the steam injection stage and the steam shutdown stage is analyzed. The microannulus formation mechanism at the cementing interface of the wellbore is studied. During steam injection, under the large thermomechanical coupling load, the wellbore generates a high stress that leads to elastic‐plastic deformation. In the steam shutdown stage, with the load on the wellbore decreasing, elastic deformation recovers mostly, while plastic deformation continues. If the plastic deformation is large enough, a microannulus will form at the cementing interface. Increasing the elastic moduli of the casing, cement, and the formation can enlarge their plastic deformation during steam injection. The increase of plastic deformation of the cement or formation can enlarge the microannulus of the casing‐cement interface or the cement‐formation interface correspondingly in the steam shutdown stage.