Cement Annulus Research Articles

Influence of the Casing Eccentricity on the Cement Sheath Sealing Integrity in the Tie-Back Interval Under the Complex Temperature and Pressure Conditions of Deep-Water Oil and Gas Wells

The integrity of cement sheath seals in casing connection sections is an important part of preventing air channelization. It is of great significance to evaluate the failure mechanism of cement sheath seals in casing connection sections under complex temperatures, pressures, and casing eccentricities. Based on the self-developed high-temperature and high-pressure full-size casing connection period of the cement sheath seal integrity evaluation device and the casing loading/unloading casing eccentricity influence on the cement sheath seal failure under the experimental conditions, it is concluded that the casing pressure changes, annulus preset pressure, and casing eccentricity influence the law of cement sheath seal integrity. The results show that the following: ① Casing eccentricity has an important influence on the failure of the cement sheath seal when the casing is loaded, and to ensure the integrity of the cement sheath seal the casing should be greater than 75%; ② in the case that the annular pressure is not set, casing eccentricity has little influence on the integrity of the cement annular seal during casing unloading, and sealing failure occurs when pressure unloading is 15–20 MPa; ③ when there is a preset pressure in the annulus, casing eccentricity has a great influence on seal failure during unloading. The greater the preset pressure in the annulus is, the better the seal integrity of the cement annulus. The research results provide an important basis for ensuring the integrity of cement sheath seals in casing connection sections.

Experimental study on nanoparticle injection by using a lab-scale wellbore system

After long-term underground storage of CO2 gas, leakage generated from the cement annulus in the wellbore system is often detected due to unbalanced pressure, chemical attack, etc. In this paper, nanoparticle injection technology is introduced to inject different types of nanoparticles into the cementitious material for repairing the leakage paths through electrochemical means. The pressure vessel is designed and built to provide a realistic underground environment with high pressure and high temperature. The testing results are evaluated by the saturate-drying method, charge-passed method, and X-ray microscopy (XRM) test. The results consistently show that all types of nanoparticles tested in this paper can be successfully driven into the cement annulus within the wellbore system. The pressure (6.89 MPa) and temperature (80 °C) in the pressure vessel, and the surrounding rock can directly affect the injection effectiveness. The nanoparticle injection technology is proven to be beneficial to the cracked cement annulus for preventing the leakage of CO2 gas through the lab-scale wellbore system.

Restoration of annular zonal isolation using localized casing expansion (LCE) technology: Treatment of near-horizontal test sections containing a free-water channel

Fluid migration behind casings is a well integrity problem that can result in sustained casing pressure, undetected leaks to the environment, and potentially very difficult remediation attempts. Traditional methods for treating annular fluid migration are perf-and-squeeze cementing or section milling followed by recementing. The main disadvantage of the former is the limited penetrability of cement slurry into narrow cracks and microannuli, while for the latter it is the requirement of a full rig to perform the operation. Casing expansion is a recent alternative remediation technology that involves imposing a permanent radial expansion of the casing and external cement layer, with the purpose of mechanically closing pathways for migrating fluids. A necessary requirement for the treatment to be effective is that the annular cement is confined between casings or between casing and competent rock formations, such that the cement can sustain significant mechanical strains without failure. Recent laboratory experiments and field trials have shown casing expansion to be a promising alternative to traditional treatment methods. We build on these insights and perform controlled treatment experiments involving a 7-in Local Expander tool and full-scale cemented annulus test assemblies, which contained much larger-scale defects than previously tested. Prior to treatment, the test assemblies had continuous migration channels on the top side of the cement, leading to high effective permeabilities ranging from approximately 80 to hundreds of darcy. The origin of the defects was likely separation of free water from the cement slurry, which is considered a relevant failure mode for primary cementing of inclined wellbores. We study the effect of imposing single and multiple local dents on the annular seepage rates, and probe for potential alteration of the cement and casing properties using Vickers hardness testing. We find that casing expansion can be highly effective in treating relatively major defects in cemented annuli, even when the defect is several millimeters wide and located adjacent to the outer casing. The effectiveness of the treatment is found to be linked to the degree of casing expansion and the initial condition of the annular cement. The deformed cement shows a tendency toward increased Vickers hardness, which is likely linked to densification of the cement microstructure. Vickers hardness testing of casing steel indicated no significant changes in hardness at the dent locations compared to average hardness values away from these points.

Novel laboratory setup for realistic wellbore cement and formation integrity studies

Oilfield wellbore integrity relies on a good well cement formulation, placement, and resistance to solicitations when aging, either due to chemical or stress changes. Requirements are that the cement sheath and plugs remain as proper hydraulic barriers for all foreseeable future. Thus, to qualify the cement formulations with respect to various types of stress changes, it is important to carry out these tests in the field relevant conditions in the laboratory. These tests will contribute to improved understanding of the limiting factors influencing the well integrity and assist in design of remediation strategies. Hence, a new experimental setup designed for studying near wellbore systems in field conditions is introduced. The setup comprises of an advanced X-ray cell with the possibility of three independent pressure controls of casing, pore and formation pressures. The experiments are carried out under simultaneous X-ray Micro-CT imaging allowing to monitor the in-situ mechanical changes. Numerical calculations in the finite element domain are carried out for the same test conditions with first results showing the evolution of radial cracks in the cement sheath and the formation rock submitted to excessive well pressures. The test results confirm with the numerical simulation. These results may provide valuable input for near-wellbore integrity assessments. • A unique laboratory set-up with more relevant field conditions for study cement and near wellbore formation integrity. • X-ray imaging system can visualize radial fractures in the annular cement and formation in the laboratorysetup. • Fracture structures are qualitatively and quantitively different, with and without confinement pressure. • Using a finite element tool, OOFEM, to model the obtained experimental results.

Effects of confinement pressure on the mechanical behavior of an oil well cement paste

Oil well cement paste is a key structural component in wells that should provide mechanical support to the casing and prevent uncontrolled flow of formation fluids along the wellbore and to the environment. Since the annular cement paste is subjected to both hydrostatic pressure and to loads from formation or through the casing, it is important to understand the mechanical behavior and strength of oil well cement pastes under confining conditions. We study cement specimens cored from test sections that were cemented using a full-scale batch mixer. The specimens were tested mechanically without confinement and under 10, 20 and 40 MPa confining pressures using a state of the art triaxial test system. Unconfined samples are found to exhibit linear elastic behavior up to axial strains of approximately 0.2% with an average Young’s modulus of 14.9 GPa and a Poisson ratio of 0.21. At larger strains, the stress–strain response deviates from the initial slope, terminating with a brittle shear failure at axial strains of approximately 0.5%. The corresponding average uniaxial compressive strength is 58 MPa, in good agreement with previously published results. When subjected to confining pressures of 10 MPa or higher, the cement paste accommodated larger strains at a given level of deviatoric stress, and maintained its load-carrying capacity through the entire test cycle. The Young’s modulus for the initial loading phase was found to decrease with increasing confining pressures, and the ultimate deviatoric stress approached approximately 80 MPa, independent of the magnitude of the confining pressures used in this study. The results suggest that the main effect of increasing confining pressure is to increase the sample ductility and the axial strain corresponding to the ultimate deviatoric stress. It is further found that the confined stress–strain behavior of the oil well cement paste can be described by a simple nonlinear constitutive equation. The more ductile and flexible response of well cement under relevant confining pressures is considered to be a positive characteristic of cement as a barrier material for zonal isolation. This study is based on a commercial well cement slurry that is mixed and placed using field equipment. The results are therefore considered novel and unique, and can contribute toward improved knowledge and evaluation of mechanical well cement behavior under realistic, confined conditions.

Distributed fiber optic strain sensing of bending deformation of a well mockup in the laboratory

Well integrity is critical to the safety and success of subsurface energy exploration and management, as leakage of fluids from subsurface reservoirs is often induced by compromised wells. This study investigates bending deformation of a mockup of an oil/gas well that is subjected to loads expected in deviated wells under reservoir compaction and assesses the feasibility of utilizing distributed fiber optic strain sensing to monitor the deformation. A well mockup, which consists of outer and inner steel pipes with a cemented annulus, is tested under three-point bending loading and its strain and curvature development is monitored by Brillouin optical time domain reflectometry/analysis (BOTDR/A) as well as optical frequency domain reflectometry (OFDR). The primary objective of this research is to assess the strain sensing performance of newly fabricated fiber optic cables and to identify key cable characteristics which could improve the quality of distributed strain measurements with BOTDR/A. Results show that the tight-buffered cable is best suited for strain sensing as its maximum errors in the strain measurement were −36% and −24% against conventional sensors at the maximum elastic and plastic bending loads, respectively, whereas those of the non-tight-buffered cable were −45% and −71%, respectively. Similar trends were obtained in the bending curvature measurement. The detailed design of the tight-buffered cable is presented to elucidate key characteristics of such a cable, which will facilitate accurate distributed strain sensing in oil and gas wells.

Fluid Migration Characterization of Full-Scale Annulus Cement Sections Using Pressure-Pulse-Decay Measurements

Abstract Fluid migration behind casings is a well integrity problem that can result in sustained casing pressure, undetected leaks to the environment, and potentially very challenging remediation attempts. Understanding the geometric dimensions and extent of annular migration paths is important for diagnosing and effectively treating fluid migration and sustained casing pressure problems in wells. In this study, permeability and micro-annuli sizes in two full-scale cemented annulus test sections are measured using transient pressure-pulse-decay and steady-state seepage measurements. One of the studied sections is a cemented 9 5/8-in. and 13 3/8-in. casing section from a 30 years old Norwegian North Sea production well. A model for predicting the transient pressure decay in annular sections with non-uniform permeability is presented and the permeabilities of the two sections are determined by fitting the transient model to pressure measurements at either side of the test sections and at selected axial positions. For both sections, measured micro-annulus sizes are within the range of effective wellbore permeabilities based on sustained casing pressure records and previous vertical interference tests from other wells. The test sections display measurable axial permeability variations with the bottom part of these vertical sections having the lower permeability. For the retrieved casing section, the axial permeability variation occurs close to the middle of the test section and is attributed to the top-of-cement. Increasing internal casing pressure is found to slightly reduce the equivalent micro-annulus size, indicative of fracture-like response of the migration paths. Using two independent test protocols, we have measured effective permeabilities as well as local permeability variations in full-scale test sections and found consistent results. The study suggests that the transient test procedure can be used to more effectively characterize low-permeable annular cement where it is otherwise time-consuming or difficult to establish steady-state flow conditions.

Ureolysis-induced calcium carbonate precipitation (UICP) in the presence of CO2-affected brine: A field demonstration

Biomineralization is an emerging biotechnology for subsurface engineering applications like remediating leaky wellbores. The process relies on ureolysis to induce precipitation of calcium carbonate in undesired flow paths. In geologic storage of CO2, there is a potential for leakage and low pH conditions, thus, ureolysis-induced calcium carbonate precipitation (UICP) was tested at field scale to seal a channel in the wellbore cement annulus in the presence of CO2-affected brine. Conventional oil field methods were used to deliver UICP-promoting fluids downhole to the treatment zone approximately 1000 feet (305 m) below ground surface (bgs). Over 4 days, 242 L (64 gal) of heat-treated Sporosarcina pasteurii cultures (22 bailers) and 329 L (87 gal) of urea – calcium chloride solution (30 bailers) were injected. The UICP treatment resulted in a 94% reduction of injectivity and ultrasonic well logging showed a noticeable increase in the percentage of solids in the channel outside the casing, including more than 30 m (100 ft) above the injection point. Subsequent well logging 11 months after the field demonstration showed that a significant portion of the new solids remained but the seal was compromised following sustained pumping. The results of this experiment suggest that UICP can be promoted in the presence of CO2-affected brine to seal leakage pathways. Additional research is required to optimize long term seal integrity to ensure storage of CO2 in geologic carbon sequestration scenarios.

Development of a Probabilistic Framework for Risk-Based Well Decommissioning Design

Summary In the next decades, tens of thousands of well plugging and abandonment (P&A) operations are expected to be executed worldwide. Decommissioning activities in the North Sea alone are forecasted to require 2,624 wells to be plugged and abandoned during the 10-year period starting from 2019 (Oil&Gas_UK 2019). This increase in decommissioning activity level and the associated high costs of permanent P&A operations require new, fit-for-purpose, P&A design tools and operational technologies to ensure safe and cost-effective decommissioning of hydrocarbon production wells. This paper introduces a novel modeling framework to support risk-based evaluation of well P&A designs using a fluid-flow simulation methodology combined with probabilistic estimation techniques. The developed well-centric modeling framework covers the full range of North Sea P&A well designs and allows for quantification of the long-term (thousands of years) evolution of hydrocarbon movement in the plugged and abandoned well. The framework is complemented by an in-house visualization tool for identification of the dominant hydrocarbon flow-paths. Monte Carlo methods are used to account for uncertainties in the modeling inputs, allowing for robust comparison of various P&A design options, which can be ranked on the basis of hydrocarbon leakage risks. The proposed framework is able to model transient conditions within the well P&A system, allowing for the development of new key performance indicators (e.g., time until hydrocarbons reach surface and changes in hydrocarbon saturation within the P&A well). Such key performance indicators are not commonly used, because most published work in this area relies on steady-state P&A models. The developed framework was used in the assessment of several P&A design cases. The results obtained, which are presented in this paper, demonstrate its value for supporting risk-based decision-making by allowing for quantitative comparison of the expected performance of multiple P&A design options for given well/reservoir conditions. The framework can be used for identifying cost-effective, fit-for-purpose P&A designs, for example by optimizing the number, location, and length of wellbore barriers and evaluating the effectiveness of annular cement sheath remedial operations. Additionally, this framework can be used as a sensitivity analysis tool to identify the critical parameters that have the greatest impact on the modeled leakage risks, to guide data acquisition plans and model refinement steps aimed at reducing the uncertainties in key model parameters.

Pilot Test of pH Triggered Polymer Sealant for CO2 Storage

Wellbore integrity is a critical subject in oil and gas production and CO2 storage. Successful subsurface deposition of various fluids, such as CO2, depends on the integrity of the storage site. In a storage site, injection wells and pre-existing wells might leak due to over-pressurization, mechanical/chemical degradation, and/or a poor cement job, thus reducing the sealing capacity of the site. Wells that leak due to microannuli or cement fractures on the order of microns are difficult to seal with typical workover techniques. We tested a novel polymer gelant, originally developed for near borehole isolation, in a pilot experiment at Mont Terri, Switzerland to evaluate its performance in the aforementioned scenario. The polymer gel sealant was injected to seal a leaky wellbore drilled in the Opalinus Clay as a pilot test. The success of the pH-triggered polymer gel (sealant) in sealing cement fractures was previously demonstrated in laboratory coreflood experiments [1-3]. pH-sensitive microgels viscosify upon neutralization in contact with alkaline cement to become highly swollen gels with substantial yield stress that can block fluid flow. The leaky wellbore setup was prepared by heating-cooling cycles to induce leakage pathways in the cased and cemented wellbore. The leakage pathways are a combination of fractures in the cement and microannuli at the cement-formation interface. The exact nature of these leakage pathways can be determined by over-coring at the end of the experiment life. We used polyacrylic acid polymer (sealant) to seal these intervals. The process comprises of three stages: (1) injection of a chelating agent as the preflush to ensure a favorable environment for the polymer gel, (2) injection of polymer solution, and (3) shut-in for the polymer gelation. Then, we evaluated the short-/long-term performance of the sealant in withholding the injected fluids (formation brine and CO2 gas). The novel sealant was successfully deployed to seal the small aperture pathways of the borehole at the pilot test. We conducted performance tests using formation brine and CO2 gas to put differential pressure on the polymer gel seal. Pressure and flow rate at the specific interval were monitored during and after injection of brine and CO2. Results of performance tests after polymer injection were compared against those in the absence of the sealant. Several short-term (4 min) constant-pressure tests at different pressure levels were performed using formation brine, and no significant injection flow rate (rates were below 0.3 ml/min) was observed. The result shows more than a ten-fold drop in the injection rate compared to the case without the sealant. The polymer gel showed compressible behavior at the beginning of the short-term performance tests. Our long-term (1-week) test shows even less injectivity (~ 0.15 ml/min) after polymer gelation. The CO2 performance test shows only 3 bar pressure dissipation overnight after injection compared to an abrupt loss of CO2 pressure in the absence of polymer gel. Sealant shows good performance even in the presence of CO2 gas with high diffusivity and acidity. Pilot test of our novel sealant proves its competency to mitigate wellbore leakage through fractured cement or debonded microannuli, where other remedy techniques are seldom effective. The effectiveness of the sealing process was successfully tested in the high-alkaline wellbore environment of formation brine in contact with cement. The results to date are encouraging and will be further analyzed once over-coring of the wellbore containing the cemented annulus occurs. The results are useful to understand the complexities of the cement/wellbore interface and adjust the sealant/process to sustain the dynamic geochemical environment of the wellbore.

Nano Sealant for CO2 Seal Integrity and Overcoring at Mont Terri

Wellbore integrity is an important consideration for the viability of subsurface CO2 sequestration. Wellbores may leak due to micro annuli that develop between annular cement and steel casing and/or the host formation, as well as fractures and flaws in the cement itself. Leakage pathways may be problematic for the long-term containment of CO2. To address the need for a repair material that can effectively seal and repair wellbores, a new nanotechnology-based sealant has been developed at the University of New Mexico, USA. The sealant, referred to TS-Nano 20™, has superior flowability, excellent bond strength to steel and rock, is very ductile, and is thermally stable. Moreover, the sealant has been demonstrated to readily flow into microcracks below 20 µm under modest pressure gradients. The sealant showed excellent bond strength and superior fatigue resistance to debonding under stress cycles than microfine cement and other sealants. TS-Nano 20™ was used in a field test at the Mont Terri Underground Research Laboratory (MT-URL) located in NW Switzerland. This experimental work at MT-URL was performed under the auspices of the CO2 Capture Project (CCP), a three-company collaboration comprising Chevron, BP, and Petrobras. The CCP effort is directed to develop more cost-effective and efficient processes for CO2 concentration, capture and sequestration applied to the energy, refining, chemical, and power generation industries in order to reduce greenhouse gas emissions. A wellbore with a cemented annulus was located beneath the floor of the underground laboratory. The wellbore was intentionally damaged by thermal cycling inside the casing to produce leakage pathways in the wellbore system. The wellbore system included injection ports at several locations along its length to allow pressure and flow tests to be conducted to document the leakage along the length of the wellbore system. Subsequently, various sealants were injected into the injection ports to seal the leakage pathways within the wellbore system. The sealant was then successfully injected into two of the intervals on site. After injection of sealants, the entire wellbore system needed to be stabilized for eventual overcoring and forensic analyses. To this end, a large open hole test interval (uncemented) at the bottom of the wellbore was first filled with a very flowable cement slurry to chase out water and reduce the dead volume, and then the sealant was injected on top of the cement and squeezed into leakage pathways that remained near the bottom of the wellbore system and cracks in the clay formation. Additionally, the sealant was used to further stabilize portions of the wellbore by injecting into all the previously used injection ports. We report on the ability of the nano sealant to maintain its flowability and mechanical characteristics under relatively low temperature and meet all field injection requirements, including having a relatively long-time to gel while hardening after injection. Pressure and permeability measurements from the field tests are reported documenting the ability of the sealant to seal relatively very thin microcracks and create appropriate conditions for overcoring.

Characterization of Oil Well Cement Performance during Early Hydration under Simulated Borehole Conditions

Summary Experiments on oil well cement (OWC) slurries were performed using the newly developed laboratory-scale wellbore simulation chamber (WSC). The WSC can simulate hydrostatic pressure reduction in the cemented annulus and possible gas migration under representative conditions. Forensic analysis shows that pressurized fluids can result in porous cement and gas channeling during cement slurry gelation. The effects of different factors on slurry pore pressure were also studied, including formation permeability, initial overburden pressure (OBP) representing the depth of interest, wellbore temperature, water/cement (w/c) ratio, cement composition, and the use of a calcium chloride (CaCl2)-based accelerator. By analyzing the temperature history of hydrating cement using degree of hydration, the evolution of cement hydration was characterized for slurry designs cured at different hydration rates. This provides the opportunity to parameterize the slurry designs and other important factors associated with wellbore conditions.

Wood-cement-composite behaviour of beech circular hollow sections

Beech laminated veneer lumber (LVL) circular hollow sections for use as nails in temporary geotechnical soil nailing systems were developed within this research. For the soil wall stability, beside a sufficient load carrying capacity of the nail cross section, a load carrying connection between the cement annulus and the nail is necessary. As cement grout ensures the bond between the nails and soil for the mobilisation of a soil reinforcement effect, a reliable connection between cement and wood surface is required. Occurring shear forces must be transferred from the nail into the soil through the wood-cement interface similar to reinforced concrete. Pull-off tests on clear LVL specimens with different cement and wood surface modifications as well as the results of large-scale push-out tests are presented. The results reveal a sufficient short-term bonding strength between the cement annulus and the wooden member to transfer the geotechnical loads, but also highlight the necessity of further research on long-term behaviour.

Development of mathematical models to control the technological properties of cement slurries

Oil and gas producing enterprises are making increasingly high demands on well casing quality, including the actual process of injection and displacement of cement slurry, taking into account requirements for the annular cement level, eliminating possible hydraulic fracturing, with developing a hydraulic cementing program. It is necessary to prevent deep invasion of cement slurry filtrate into the formation to exclude bridging of productive layers. It is impossible to fulfill all these requirements at the same time without application of modifying additives; complex cement compositions are being developed and applied more often. Furthermore, need to adjust cement slurries recipes appears for almost every particular well.
 In order to select and justify cement slurries recipes and their prompt adjustment, taking into account requirements of well construction project, as well as geological and technical conditions for cementing casing strings, mathematical models of the main technological properties of cement slurries for cementing production casing strings in the Perm Region were developed. Analysis of the effect of polycarboxylic plasticizer (Pl) and a filtration reducer (fluid loss additive) based on hydroxyethyl cellulose (FR) on plastic viscosity (V), spreadability (S) and filtration (F) of cement slurries is conducted. Development of mathematical models is performed according to more than 90 measurements.

Digital Cement Integrity: A Methodology for 3D Visualization of Cracks and Microannuli in Well Cement

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.

Mechanism of gas migration through microstructure of cemented annulus in deep-water environment

Gas migration has long been recognized as one of the most difficult problems in the oil and gas industry. Due to the complicated interactions between temperature, pressure, and cement hydration reaction, the temperature and pressure in a deep-water environment makes gas migration in cement a complicated process, by effecting the driving forces and channels of gas migration. Accordingly, a transient prediction model for cement temperature and pressure based on cement hydration kinetics was established in this study. Using this model, cement microstructure under different temperatures and pressures was studied. In addition, a mathematical model for gas seepage through the microstructure of a cement sheath was also suggested. The proposed models were validated by comparing the results obtained from those of experimental studies from the literature, where a good agreement was obtained. Furthermore, numerical simulations were conducted by applying the proposed models to a simulation well. The simulation results demonstrated that the microstructure of the cement sheath varied with time and space under the effect of temperature and pressure. The cement pressure first decreased with the deepening of cement hydration, and then increased as a result of gas migration. In addition, the gas migration resulted in a sustained casing pressure at the wellhead as high as 14.26 MPa. This would pose a threat to safety and subsequent oil and gas production.

Numerical study of the effect of cement defects on flexural-wave logging

Cement-bond evaluation is needed for new wells and plug and abandonment activities. The ultrasonic leaky Lamb-wave (also called the flexural-wave) technique, in combination with the pulse-echo technique, has been widely used for cement-quality evaluation. Using a 2D time-domain staggered-grid stress-velocity finite-difference methodology, we have numerically investigated the attenuation and group velocity of flexural waves, and the scattering from defects, in the presence of a water-filled void in the cement annulus. The position, length, thickness, and burial depth of a defect are considered. The numerical study suggests that the combination of the attenuation and group velocity of the flexural wave allows for a discrimination between solids and liquids. The scattering from voids can be used to indicate the existence of a hidden defect, which cannot be detected by using the attenuation and group velocity if it is located larger than 5 mm away from the casing. The void signatures can even be used to characterize the geometry of the defect for neat cement. The numerical results provide improved understanding of flexural-wave logging results.

The Potential of CO2 Leakage Along De-Bonded Cement-Rock Interface

Leakage along wellbores is one of the key concerns for geological storage of CO2. De-bonding of annular cement - casing/rock interfaces during wellbore construction or operations could result in formation of microannuli as leakage pathways. A field scale reactive transport model was developed in TOUGHREACT to evaluate potential leakage of supercritical CO2 along microannuli. The model is axisymmetric, consisting of a cemented casing and surrounding rock formations: the CO2 reservoir, impermeable caprock and overlying permeable aquifer. Between the annular cement and surrounding rock formation, a microannulus with 100 micron aperture is implemented from the reservoir towards the surface. Simulated leakage rates with and without considering chemical reactions between CO2, formation water and cement minerals are compared to analytical equations which are often used to predict well leakage. The results show that the analytical equations highly overestimate leakage rates because they oversimplify the transport processes and do not consider relevant parameters such as gas saturation, relative permeabilities and even gravitational effects. If chemistry is excluded, the numerical simulations predict leakage rates which are one to two orders of magnitude lower than the analytical equations. In addition, the chemical interaction between the rock formation waters with the cement minerals result in relatively fast cement mineral dissolution and precipitation of calcite within the microannulus, with bicarbonate and calcium supplied by respectively the formation water and the dissolved cement minerals (mainly portlandite). Only if both the reservoir pressure and microannulus permeability are high and the resulting upward migration rate of the supercritical CO2 is fast enough, leakage of CO2 into the aquifer overlying the caprock is predicted to take place before calcite precipitation can seal the leak path. Future studies should focus on improving aperture-permeability relationships to be used in the numerical simulations. With reliable aperture and permeability input, numerical simulations can be used by operators to define maximum CO2 injection pressures for their storage sites and minimize leakage risks.

Experimental investigation of well integrity: Annular gas migration in cement column

Cementing operations present various difficulties attributed to shallow flows all around the world. Cement ability to seal the annular space, aids in sustaining the well integrity by mitigating and preventing fluid migration to other formations and to surface. Failure of cement to seal the annular space can result in serious problems that might jeopardize the well integrity.In this study, a wellbore model setup was designed and fabricated to mimic the gas migration in cemented annulus. A series of experiments were conducted on this setup to examine the cement sealability of neat Class H and neat Class G cements, and to also evaluate the effect of anti-gas migration additive on the cement sealability. Different additives were used such as bentonite and latex. Experiments conducted revealed that neat Class H, Class G, and any other slurry design with additives (but without anti-gas migration additives) does not have the ability to serve in mitigating gas migration. The wellbore permeability of annular cement using neat Class H and Class G cement was ranging between 10−3 to 10−1 mD. In addition, it has been found that the wellbore permeability of annular cement increases as cement age increases. Annular cement should not be considered as the primary barrier when liner-hanger is used.

A Bayesian Belief Network to Assess Risk of CO2 Leakage Through Wellbores

One of the approaches to tackle climate change is preventing emissions of carbon dioxide via CO2 storage. Leakage out of the storage complex, especially along wellbores which penetrate the caprock, is a concern and difficult to quantify. This paper presents a quantitative approach to a well integrity risk assessment, which examines the likelihood of a microannulus forming between the annular cement and casing or rock formation, and the leakage through that microannulus throughout the post-abandonment period of a CO2 storage site. The structure of our methodology is threefold, comprised of (i) a geomechanical structural integrity model of a wellbore to predict the probability of microannulus formation, (ii) a transport model to estimate CO2 leakage rates through the microannulus, if present, and (iii) a Bayesian belief network (BBN) for leakage risk assessment. The geomechanical and transport models are based on a particular CO2 storage site and well history, with variations in input parameters and operational conditions. The uncertain parameters are treated as probabilistic and sampled from statistical distributions using the Monte Carlo method. The geomechanical model is used to assess if debonding along the well cement interfaces has occurred over the life of the well (completion, production, re-injection) and to estimate the width of microannulus created by debonding. The transport model simulates migration of fluids through the microannulus and estimates leakage rates. The outputs of numerical simulations are assimilated, organized into ranges and used to define conditional probabilities, which are added to the BBN. The BBN allows the user to compare different scenarios by examining the associated probabilities of debonding and leakage. In this manner, the network serves as a decision support tool, to inform the operator of the risks of a certain scenario and to aid in deciding which mitigation or abandonment measures should be taken.