Cement Annulus Research Articles

Quantitative assessment of well leakage, part I: Cement stress evolution

A damaged cement sheath in wells can open a leakage pathway to shallow freshwater aquifers and atmosphere. Quantitative assessment of leakage along wells has become an area of interest for both the industry and the regulatory bodies. The well leakage can be of importance in both active and legacy wells. In order to estimate leakage through cement sheaths, the size of the leakage pathway and the damage in the cement sheath must be estimated. In this work, we have developed a hydro-thermo-mechanically coupled near-well model that aims to calculate the evolution of cement’s stress as it cures. This process takes into account the cement’s gradual increase in stiffness, chemical shrinkage, and the heat of hydration. The results are verified using lab measured cement stress and pore pressure data from the literature. A case study was developed based on a low-enthalpy geothermal doublet in the Netherlands. The results show that during the cold water injection, an outer microannulus may open to 60 µm. The presence of an external source of water and formation stiffness are of significant importance in determining the damage to the cement sheath. The heat of hydration in cement increases the temperature of cement during curing. The subsequent drop in temperature due to drilling or completion reduces the cement stress and exacerbates the damage to the cement sheath. The producer well may not form a microannuli, however shear and cyclical failure may be of higher likelihood. The modelling framework presented here allows for estimation of annular cement stress in the well. The analysis provides quantitative estimates of the size of the leakage pathway along a well that can be used to estimate well leakge. Quantitative estimate of well leakage provides crucial information for quantitative risk analysis and provides a framework to optimize well operations to minimize leakage risk.

Permeability and self-healing behavior of alkali activated geopolymers for well cementing applications

Geopolymers, also identified as alkali-activated materials (AAM), are alternative well-cementing materials with a lower carbon footprint than traditional ordinary Portland cement (OPC). Geopolymers are formed by combining an aluminosilicate precursor, such as fly ash (FA), with an alkaline activator solution. Among their merits is an ability to self-heal after becoming damaged, which can be exploited to restore permeability, sealing ability, zonal isolation, and well integrity in wells that suffer from cracking, fracturing, or debonding in their annular cement sheaths and abandonment plugs.This study investigates the self-healing ability of various alkali-activated Class F fly ash-based geopolymers. Liquid sodium hydroxide (LSH), liquid sodium silicate (LSS), and solid sodium silicate (SSS) were used as activators for the preparation of geopolymer specimens, while Class H OPC was used as the control material. Permeability was characterized using pressure transmission tests (PTT). The tested samples were of different compositions, curing ages (7-day and 28-day), damage stages (pristine, damaged after freeze-thaw cycling, self-healed after post-damage curing), and damage severity (low, medium, and high). Fourier-transform infrared-spectroscopy (FTIR) and loss on ignition (LOI) tests were performed to examine the geopolymer reaction over time, in an attempt to relate the various permeability results to the geopolymerization process. The experimental results show that geopolymers possess low permeability, comparable to – or lower than – OPC, and that effective permeability restoration occurs in geopolymers, even after high degrees of damage. The implication of this work is that geopolymers are promising alternative barrier materials to OPC with a superior ability to guarantee low barrier permeability and appropriate zonal isolation, keys to wellbore integrity over the entire lifecycle of wells, even if those wells suffer from damage.

Distributed fiber optic temperature and strain sensing in cementing and water injection: Insights to well integrity monitoring and multisensing optical fiber cable design

In this study, we installed two fiber optic cables with different designs into a new well, a soft-flat cable and a stainless-steel cable, for distributed fiber optic sensing in cementing and water injection. The two fiber cables were cemented behind casing and the soft-flat cable was used for temperature sensing in cementing as well as cement curing period. The high-speed Rayleigh scanning successfully visualized the cement front tracking from the combined effects of temperature and pressure changes by cement pumping. The temperature variations in cement curing reveled the cement volume and have the potential to identify defects in annular cement and formation damage caused by cementing. The water injection test for strain sensing was carried out with increasing injection rate from 100 L/min to 200 L/min. Strain profiles from the two fiber cables suggested that the strain responses reveal alternation of sand and silt, and reservoir heterogeneity. The water injection results showed that the measured strains of the two cables matched fairly well and the stainless-steel cable showed a high strain sensitivity responding to injection pressure (1 με/5 kPa). Comparison of strain measurement results of the two cables provided valuable insights for future strain fiber cable design. Strain sensing results also indicated a high capability to capture small pressure changes in the heterogeneous layers. Thus, our results demonstrate the high potential of using strain sensing for leak detection and well integrity monitoring in fluid injections, particularly for the long-term monitoring at CO2 storage site.

Modeling thermal-induced wellhead growth through the lifecycle of a well

During production from high-yield wells, hot reservoir fluids may significantly heat the casing string which may lead to the wellhead growing out of the surface once the induced thermal stress exceeds the wellhead load. This issue involves some challenges that lie in the initial gravity-driven elongation of casing strings before cementing and then their combined expansion with annular cement as one assembly due to later temperature changes. Here, we proposed an integrated modeling method for wellhead growth by incorporating heat transfer and thermodynamics. Element-death technique is used to simulate free elongation of the casings in the absence of cement before cementing and their subsequent coupling thermomechanical behavior with cement after cementing job. Arbitrary Lagrange-Eulerian (ALE) adaptive meshing is employed to avoid mesh distortion due to uncoordinated deformation between casings and cements. Numerical results indicate that the combined effect of gravitational-induced tensile strain and thermal-induced compressive strain may potentially result in a strain-neutral point within the casing. Beyond this point, the casing experiences a tensile state, whereas, below it, the casing is subjected to a compressive state. Furthermore, casing running friction can mitigate tensile strain, while higher production can enhance the thermal strain in the casing. Both factors contribute to shifting the strain neutral point upwards and exacerbating the risk of wellhead growth. However, the application of pre-tensioned casing while cementing can effectively mitigate wellhead growth by offsetting the thermal-induced compressive strain through the increasing tensile strain.

Gas and water migration in cement slurries at different curing times

Gas and water migration in the hydrating cement slurry is a major cause of well completion failure. Gas and water migration may occur at different curing times due to varying well conditions and cement slurries. The current study has not been carried out to investigate the gas and water migration at different curing times. In this paper, theoretical calculations and migration experiments are used to investigate the gas and water migration in cement slurry at different curing times. The gas and water migration process can be divided into the formation rock to annular cement slurry (F-A) stage and the bottom to top (B-T) stage. At relatively short curing time, the cement slurry is more like a viscous fluid and there is clear interface between the gas and slurry. The pressure difference required for gas migration in F-A stage is very small. Maximum velocity of gas bubble migration in B-T stage is closely related to gas bubble size and cement viscosity. The smaller the size of gas bubble and the greater the viscosity of the cement slurry, the smaller the maximum velocity of the gas bubble will be. There is no clear interface between the water and slurry at relatively short curing time. The pressure difference of water migration is small. At relatively long curing time, the cement slurry has a certain of gel strength and there is also clear interface between the gas/water and slurry. The water migration is similar to gas migration. The pressure difference required for gas and water migration in F-A stage increases as the static gel strength increases. The size of the gas and water bubbles formed after invasion increases as the static gel strength increases. The critical radius at which gas and water bubbles can move is closely related to the static gel strength of the cement slurry. The greater the static gel strength, the larger the critical radius will be. The channeling path sizes of the gas and water migration are all small at relatively short curing times. The dimensions of the channel are gradually increasing from bottom to top. The pressure difference of gas migration and the dimension of channel increase as curing time increases. When the cement slurry cures for a long enough period, the gas cannot migrate in the cement slurry, but only along the weak cement slurry-wellbore interface.

Analytical solutions for dispersions and waveforms of acoustic logging in a cased hole

Acoustic logging is one of the most promising methods for the quantitative evaluation of cement bond conditions in cased holes. However, inefficient use of full-wave information yields unsatisfactory interpretation accuracy. Fundamentally, this is because the wavefield characteristics have not been thoroughly investigated under various cement bonding conditions. Thus, this study derives analytical solutions of wavefields for a single-cased-hole model and emphasizes the dispersion calculation algorithm. To solve the dispersion equation when solving for the poles of the propagating modes with real wavenumbers, we renormalize the Bessel function related to the borehole fluid by multiplying it with an attenuation factor. For leaky modes with complex wavenumbers, we develop a novel method to find local peaks of the matrix condition number (LPMCN) in the frequency domain to determine dispersion poles, avoiding the local optimization issues resulting from the traditional Gauss-Newton iteration method. Combining these two methods, we establish a fast and accurate workflow for evaluating the dispersion of all modes in cased holes using a relatively fast bisection method to manage the dispersion of the propagating modes and using the LPMCN method to derive the dispersion curves of leaky modes. Furthermore, all propagating modes are individually investigated in the monopole measurement by evaluating the residues of the real poles in a casing-free model. The analysis demonstrates that the first-order pseudo-Rayleigh wave (PR1) and inner Stoneley wave (ST1) are the two strongest modes. Finally, we focus on the waveforms and dispersion characteristics of the outer Stoneley wave (ST2) related to the fluid channel in the cement annulus. The results reveal that as the fluid thickness increases, the phase velocity of the ST2 mode decreases, and its amplitude increases. Therefore, the ST2 mode can potentially evaluate the thickness of the fluid channel in a cement annulus if an effective weak-signal-extraction method is used.

The Sealing Performance of Cement Sheaths under Thermal Cycles for Low-Enthalpy Geothermal Wells

The repetitive process of shut-in and production in geothermal wells promotes thermal stress on the wellbore components, including annular cement. A cement sheath at a relatively shallow depth undergoes the most significant stress change due to the high differential temperature between the geothermal gradient and the production fluid’s temperature. Understanding the impact of cyclical thermal stresses on cement is critical for assessing the barrier integrity at a shallow depth that serves as aquifer protection. A novel large-scale setup simulating a 1.5 m-long casing-cement-casing well section was built to study the changes in cement’s sealing performance of low-enthalpy geothermal wells during production. Using this setup, a cement sheath can be cured similarly to the in situ conditions, and the annular temperature can be cycled under realistic operating conditions. The change in flow rate through the cement sheath before and after cycling is quantified through leak tests. UV dye is injected at the end of the experiment to identify the location and type of damage in the cement sheath. A hydromechanically coupled finite element model was used to estimate the stress evolution in cement during the tests. The model incorporated the impact of cement hydration and strength development during curing. The numerical results were used as a guide to ensure the test design closely mimicked in situ conditions. The results show the presence of a small microannulus immediately after curing due to hydration shrinkage. Thermal cycles reduced the permeability of the microannulus. The size of the micro-annulus was observed to be sensitive to the backpressure applied to the cement sheath, indicating the need for pressure to maintain an open microannulus. Thirty-nine thermal cycles between 80 and 20 °C did not change the permeability of the cement sheath significantly. Tensile cracks in the cement sheath were not continuous and may not be a significant pathway. The new setup allows for measuring cement’s effectiveness in withstanding in situ stress conditions when exposed to thermal cycles such as geothermal and CCS wells.

Preparation and Characteristics of a pH Intelligent Response Blocking Agent for the Microcrack in the Cement Sheath.

Microcracks in the annular cement sheath of oil wells frequently cause annular water channeling. Traditional cement squeeze technology has a low success rate in controlling this issue. Based on the conventional profile control water-blocking agent for underground in situ gelling and polyacrylic acid, a pH intelligent response microcrack-blocking agent was developed to block the microcrack in the cement sheath. The study investigated the influence of pH on the viscosity characteristics of the new blocking agent, the impact of polyacrylic acid on the compression recovery ability of the new blocking agent after gelling, the change in viscosity of the blocking fluid after flowing through the microcrack in the cement sheath, and the blocking effect after gelling. The results indicated that the new blocking agent has excellent viscosity-increasing ability with pH. After flowing through the microcrack in the cement sheath, the viscosity of the blocking agent increased significantly with the extension of the contact distance between the blocking agent and the microcrack in the cement sheath, which is very conducive to the retention of the blocking agent in the microcrack of the cement sheath. Polyacrylic acid had a negligible effect on the compression recovery ability of the blocking agent after gelling. At a fracture length of 5 cm, the pressure-bearing capacity of the blocking agent could reach 6 MPa.

An Integrated Modeling Approach for Vertical Gas Migration Along Leaking Wells Using a Compressible Two-Fluid Flow Model

Gas migration behind casings can occur in wells where the annular cement barrier fails to provide adequate zonal isolation. A direct consequence of gas migration is annular pressure build-up at wellhead, referred to as sustained casing pressure (SCP). Current mathematical models for analyzing SCP normally assume gas migration along the cemented interval to be single-phase steady-state Darcy flow in the absence of gravity and use a drift-flux model for two-phase transport through the mud column above the cement. By design, such models do not account for the possible simultaneous flow of gas and liquid along the annulus cement or the impact of liquid saturation within the cemented intervals on the surface pressure build-up. We introduce a general compressible two-fluid model which is solved over the entire well using a newly developed numerical scheme. The model is first validated against field observations and used for a parametric study. Next, detailed studies are performed, and the results demonstrate that the surface pressure build-up depends on the location of cement intervals with low permeability, and the significance of two-phase co-current or counter-current flow of liquid and gas occurs along cement barriers that have an initial liquid saturation. As the magnitude of the frictional pressure gradient associated with counter-current of liquid and gas can be comparable to the relevant hydrostatic pressure gradient, two-phase flow effects can significantly impact the interpretation of the wellhead pressure build-up.

An investigation of fatigue response of well cement under cyclic loading and impact on zonal isolation performance

Cyclic bottomhole pressure fluctuation is very prevalent in modern well construction and enhanced petroleum recovery applications and poses a great challenge to annular seal integrity and successful zonal isolation, even in cases where the cement seal exhibits excellent initial quality. Thus, annular sealants must be designed to mitigate or minimize the deleterious effects of cyclic pressure fluctuations on the annular seal. Optimal cement designs for this purpose require a proper understanding of damage modes and mechanisms associated with a cyclic pressure regime, as well as the impact of the cement mechanical integrity evolution on the leakage of the cemented annulus. Using four cement formulations with distinct mechanical and microstructural properties, we investigated the mechanical integrity evolution and leakage characteristics of cemented pipe specimens subjected to confinement pressure fluctuations. Via microscopic inspection of the cement/pipe interface after pressure cycling and permeability measurements during pressure cycling, we attempted to correlate cement properties with leakage and suggest a recommendation for optimal cement design and successful zonal isolation in wellbores subjected to cyclic pressure. Test data indicate that under high enough pressure variation, interfacial cracking at the cement pipe interface is unavoidable and the cement seal's ability to maintain zonal isolation will be dependent on the microstructural morphology and microscale mechanical resilience of the cement formulation. Stiffer conventional cements also appear to have a higher chance of leakage through non-interfacial fractures than their more flexible counterparts. Latex cements show great promise for effective zonal isolation under cyclic bottomhole pressure conditions and should be studied in more detail.

Imaging and Characterization of Cement Annulus and Bonding Interfaces in Cased-wells with Fully Connected Neural Network

Well integrity evaluation is of great significance to the production life and productivity of oil and gas wells, as well as the protection of reservoir layers. However, traditional sonic logging technology has been unable to meet the current requirement for evaluating unconventional oil and gas resources. For instance, it falls short when it comes to detecting cementing quality between the cement and formation, as well as achieving a quantitative assessment of annulus. In addition, factors such as mud invasion and wellbore damage have the potential to compromise the integrity of the cement sheath and lead to radial alteration in the medium in the wellbore, which can affect cementing quality. To address these issues, a novel approach based on deep learning is proposed to reconstruct slowness models from wellbore data using a fully connected neural network (FCNN) to evaluate the cementing quality. This interpretation workflow requires forward modeling with the three-dimensional dyadic Green’s function to explore borehole acoustic signals and thus generate training and test datasets. Afterward, the spatial feature maps are learned to reconstruct the slowness models by utilizing all the wellbore data. Additionally, the Real-ESRGAN method is applied to borehole imaging to further improve the above inversion. The inversion results regarding the slowness value, wellbore structure, and interface are more consistent with the true model. Finally, the image pixels are optimized using the thresholding method, resulting in an image of superior quality in terms of sharpness and detail compared to previous results. This research reveals that the FCNN-based method can effectively invert the radial profile of slowness around the wellbore from monopole sonic logging data, and the Real-ESRGAN method can further enhance the imaging results to better approximate the true models. This method has also been applied to realistic borehole scenario models, demonstrating its feasibility.

The effect of PVA microfiber reinforcement on the mechanical and rheological behavior of class G oil well cement pastes

During the life cycle of an oil well, the annular cement sheath will be exposed to load combinations that can result in failure. To enhance the structural integrity and ductility of the cement paste, and to improve the resistance to tensile crack propagation, fiber reinforcement can be used. In the present work, the effects of different polyvinyl alcohol (PVA) fiber concentrations on the rheological and mechanical properties of a class G oil well cement paste was investigated. The study focuses on impacts of fibers on the cement paste viscosity, its confined and unconfined stress–strain responses, and resistance to shear and bending loads. An increase in the effective viscosity of the cement paste with increasing fiber content was observed, although the impact was minor for the smallest (semi-dilute) fiber concentration. All cement paste specimens showed a modest frictional strengthening with increasing confining pressure, with the fiber reinforced samples exhibiting improved post-peak load-bearing capacity compared to the base formulation with no fiber additive. Fibers were found to significantly improve cement paste resistance to shear and bending loads, to enhance the toughness and to arrest tensile crack growth. Despite all the benefits already known about adding fibers, for application in the oil and gas industry, it is essential to evaluate the rheology of the mixture that will directly impact pumpability and placement. In this work, flow curves were measured only for concentrations ranging from 0.1% to 0.5% since the other cases approached the concentrated suspension regime with the fluid behaving like a plug. Still, the results elucidate the potential benefits of adding even a relatively small concentration of high-aspect-ratio PVA fibers to oil well cement pastes, particularly for enhancing early-age cement’s shear and bending strength.

Prior assessment of CO2 leak rate through cracks sealed by nanoparticle gels

The leakage of hydrocarbon fluids through cracks in the annular cement and CO2 storage is a major concern to the Petroleum Industry. A significant risk is posed when repairing leakage in a micro annuli channel with smaller apertures. A low-viscosity sealant that can generate a long-lasting resilient seal is desired. The solution to sealing these channels might lie in a novel application using nano-silica Gel. In this study, laboratory tests were carried out to examine the capabilities of nano-silica gels to seal the cracks. Analyzing its rheological property, the gel strengths of nano-silica gels were found to increase with an increase in nano-silica concentration. Additionally, it was discovered that as the concentration of nano-silica increases, the sealing and leakage pressures, defined as the pressures before and after water breakthrough, respectively, increase as well. With a typical 15% concentration of nano silica in gel, a sealing pressure gradient of 80.2 psi/in and a leakage pressure gradient of 30 psi/in at a leaking rate of 1 cc/min were noted. To validate the validity of the experimental results, a mathematical model was developed to predict the leakage rate of sealed fractures. The model suggests that the young’s modulus of sealant is a key property of nano-sealants and further investigations are needed to validate the mathematical model for quantitative use. This study suggests a novel strategy for enhancing cement zonal isolation and reducing cement failure in oil and gas sector.

Reverse Circulation Displacement of Miscible Fluids for Primary Cementing

AbstractPrimary cementing is the well construction operation where drilling fluid is displaced from the annular space behind the casing string, and replaced by a cement slurry. The annular cement sheath is a critical barrier element that should provide zonal isolation along the well and prevent uncontrolled flow of formation fluids to the environment. We present a combined experimental and computational study of reverse circulation displacement of the annulus, corresponding to operations where cementing fluids are pumped down the annulus from the surface. We focus on iso-viscous displacements in a vertical and concentric annulus, and vary the density hierarchy among the fluids to study both stable and density-unstable displacement conditions. While the interface between the two fluids is advected according to the laminar annular velocity profile for density-stable and iso-dense displacements, considerable secondary flows and fluid mixing is observed for density-unstable cases. Increasing the imposed velocity from the top is seen to provide a certain stabilizing effect by suppressing backflow of the lighter fluid and reduce the magnitude of azimuthal fluctuations. Computational results are in qualitative agreement with the experiments, and support the categorization of the displacement flows as either inertial or diffusive, in accordance with previous work on buoyant pipe displacements.

Wellbore cement fracture permeability as a function of confining stress and pore pressure

Cemented annulus fractures are a major leakage path in a wellbore system, and their permeability plays an important role in the behavior of fluid flow through a leaky wellbore. The permeability of these fractures is affected by changing conditions including the external stresses acting on the fracture and the fluid pressure within the fracture. Laboratory gas flow experiments were conducted in a triaxial cell to evaluate the permeability of a wellbore cement fracture under a wide range of confining stress and pore pressure conditions. For the first time, an effective stress law that considers the simultaneous effect of confining stress and pore pressure was defined for the wellbore cement fracture permeability. The results showed that the effective stress coefficient (λ) for permeability increased linearly with the Terzaghi effective stress (σ-p) with an average of λ=1 in the range of applied pressures. The relationship between the effective stress and fracture permeability was examined using two physical-based models widely used for rock fractures. The results from the experimental work were incorporated into numerical simulations to estimate the impact of effective stress on the interpreted hydraulic aperture and leakage behavior through a fractured annular cement. Accounting for effective stress-dependent permeability through the wellbore length significantly increased the leakage rate at the wellhead compared with the assumption of a constant cemented annulus permeability.

A novel apparatus and method for lab-scale study of wellbore integrity using CT imaging and analysis

Well drilling, completions, stimulation, and enhanced oil recovery operations induce downhole conditions that may negatively impact the integrity of the annular seal and consequently hinder zonal isolation. Thus, the ability to accurately quantify the evolution of the annular seal in response to the prevailing downhole environment is critical for the optimal design of the annular barrier for the life of a well. Thanks to increased accessibility and recent advancements in computing power and techniques, X-ray computed tomography has gained popularity as a non-destructive analysis method in materials science and geomechanics due to its ability to reveal details about the interior volume of objects in real-time without physical disassembly.Therefore, in this study, a novel apparatus is presented for the construction of a lab-scale wellbore, with the purpose of simulating downhole processes while simultaneously monitoring wellbore elements of interest in real-time via x-ray computed tomography. The benefits of this novel setup for wellbore integrity are demonstrated via applications to two test cases: the mechanical evolution of annular cement under stresses induced by cyclic water injection as a function of the mechanical properties of the cased and cemented wellbore system; the evaluation of nano magnesium oxide performance as an additive for autogenous shrinkage mitigation in annular cement.The results of the studies presented illustrate the benefits of combining x-ray computed tomography with lab-scale wellbore process simulations. The results of the cyclic water injection study suggest that residual strain in the cement is the major factor in annular seal degradation under cyclic downhole pressure fluctuations. Nano magnesium oxide is also shown to be very effective in preventing autogenous shrinkage of Class H cement. However, more study is required to characterize its effectiveness in a wider range of cement formulations. Finally, suggestions are offered on how to improve the experimental procedure presented while future potential applications of the apparatus are discussed.

Measured Properties of Annular Cement Compared With Ultrasonic Cement Logs

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 204015, “A Comparison of the Measured Properties of Annular Cement With Ultrasonic Cement-Evaluation Logs,” by Dave Gardner, SPE, Hans Joakim Skadsem, and Katherine Beltrán-Jiménez, SPE, Norwegian Research Centre, et al. The paper has not been peer reviewed. In this work, different measurement methods have been used to investigate the quality of the cement sheath inside two cemented “sandwich” sections, including high-resolution ultrasonic cement-evaluation logs, sample analysis, mechanical loading, and fluid-seepage measurements. The examined sections were recovered from a North Sea well during a permanent plug-and-abandonment operation. The measurements have been analyzed with an aim of describing in detail the spatial variations in the cement properties and relating them to the logs. The data analysis has improved the understanding of the cement-sheath mechanical properties, seal quality, and response of the ultrasonic cement-evaluation logs. Introduction A well drilled at the Valhall Field was permanently abandoned in 2018 after being in production since 1985. To install the surface plug, the 9⅝-in. and 13⅜-in. casings were both cut and retrieved to surface in sections simultaneously. After the well abandonment, two of the cemented casing-in-casing sandwich sections were sent to shore for further analysis, including relogging. The deepest section, recovered from the interval between 251 and 263 m, was named Fish 11. A visual inspection of the section showed complete annular cement coverage at both ends but with signs of mud contamination, especially at the casing/cement interface and primarily on the narrow side of the annulus. The 9⅝-in. casing had an eccentricity of 70% compared with the outer casing and equivalent to the standoff created by the 9⅝-in. casing-collar thickness. The uppermost sandwich section covering the top of the cement was retrieved from 119 to 131 m and was named the transition joint. The eccentricity of the inner casing compared with the outer casing was 60% for this section. The bottom of the section had complete cement coverage; however, some mud intrusions were apparent in the bulk cement as well as mud contamination between the casing/cement interface. The 9⅝-in. casing was originally cemented in two stages because of the presence of a weak formation. Both the transition joint and Fish 11 were retrieved from the upper part of the section, which was cemented in a second stage by bullheading slurry from the surface and down the annulus. In this operation, the denser slurry was displaced from above the spacer fluid and the oil-based mud was used to drill the 12¼-in. section into the formation. The authors speculate that the mud displacement during the bullheading operation was instantaneous, with no backflow. The fact that the sections were cemented by bullheading may have influenced the properties of the cement, particularly close to the top of the cement.

Prediction of Methane Leakage Through Primary Cement Barrier in the High Island OPD, Gulf of Mexico

With the increasing concern of greenhouse gases emitted from oil and gas wells, wellbore leakage has been a topic of interest lately. Previous research has shown that the annular cement used to complete the well and is a primary barrier of fluid migration can experience debonding to the casing causing a microannulus. The microannulus can propagate up the cement sheath to the top of cement depth causing a continuous leakage pathway. Previous methods quantify the corresponding leakage using laminar Hagen-Poiseuille flow assuming an incompressible fluid. That methodology is valid assuming water or oil as the leaking fluid but is voided for gas leakage. Therefore, an analytical laminar gas flow model including the effect of gravity was derived. The objective of this work is to investigate the difference between using Hagen-Poiseuille flow for analyzing gas flow data versus the developed gas flow model. The gas flow model is verified with experimental data, and the potential leakage of a previously studied Gulf of Mexico well is compared using the two models. The results show that analyzing gas leakage with Hagen-Poiseuille flow overestimates leakage rates by an order of magnitude when compared to the gas flow model.

Modeling carbonation and chloride ingress in well cements

The long-term durability of the wellbore system has become an important topic in recent decades. Specifically in a CO2 underground storage system, both the ingress of chloride and the leaking of carbon dioxide (CO2) can lead to severe degradation of the system, including corrosion of steel casing, and carbonation of the cement annulus. In this paper, a model was developed to consider the coupling effect of the carbonation reaction and chloride diffusion in well cement. The stoichiometric model was first applied to calculate the chemical compositions change of the well cement during the hydration process and the carbonation process. The chloride diffusion coefficient of carbonated well cement was calculated based on the general self-consistent (GSC) model and pore tortuosity. After that, the concentration of free chloride ions was predicted based on Fick's law, which takes into consideration released bonded chloride in the carbonated cement. The proposed model was validated based on available experimental data, and a comparison was made between the proposed model and other previously published models. The model has great significance for the safe and reliable geological storage of CO2. As described in this paper, the model can be used to predict the durability of a CO2 wellbore system and to inform the design of future systems.

Characterization of Annular Cement Permeability of a Logged Well Section Using Pressure–Pulse Decay Measurements

Abstract The cement behind casings is an important barrier element in wells that should provide zonal isolation along the well. The hardened cement does not always isolate permeable formations, either due to placement issues or loads that over time compromise the integrity of the barrier. The modern method used to characterize the annular material is ultrasonic logging which provides essential information concerning the type of material behind casing, but no measurement of the annular permeability. This study provides permeability characterization of a casing-cement sandwich joint retrieved from a 33 years old production well that has been logged at surface using a state-of-the-art ultrasonic tool. The joint contains an interval of low-permeable cement that previously has prevented permeability measurement by gas injection. A pressure–pulse decay test method has now been performed that is based on monitoring the evolution of a pressure pulse through the joint. Long-term pressure measurements show communication through the entire joint and are in qualitative agreement with the log. A pressure diffusion model is used to estimate local permeability along the joint, enabling comparison of log response and permeability. The low-permeable region is relatively short, situated directly on top of a casing collar, and has permeability that is orders of magnitude lower than the cement above and below. In the longer term, results from this and related studies can be used as input for future sustained casing pressure evaluations or for quantifying seepage risk behind casings for abandonment designs.