Abandonment Operations Research Articles

A Liquid Well Barrier Element for Temporary Plug and Abandonment Operations: A Breakthrough Approach

Plug and abandonment (P&A) operations demand valuable time and resources for operational procedures and materials to establish the well barrier element. This study aims to investigate the application of a water-based fluid as a liquid well barrier element for temporary abandonment, based on estimates of its lifespan and the survival probabilities of downhole temperatures acquired through accelerated life tests. To achieve this, the water-based formulation was tested and exposed to 95, 110, 140, and 150 °C temperatures for time intervals ranging from 1 to 10 days. After the temperature exposure, the fluid properties were verified, and failure was detected by accounting for any deterioration in rheological parameters and/or a substantial increase in filtrate volume. A statistical analysis of the failure data was performed in RStudio 4.1.3 software using the Weibull Model, and the fluid average lifespans and survival probabilities were estimated for the P&A temperatures. The results obtained demonstrate that the degradation of the fluid was only observed for 140 and 150 °C temperatures. According to the results, the fluid is a promising alternative for temporary abandonment until 80 °C, with no need for monitoring once its lifetime expectation exceeds three years at this temperature. For downhole temperatures above 80 °C, the fluid is a possible alternative, however, the operation’s maximum time and monitoring requirements should consider reliability metrics for each temperature.

Revealing Microstructure and Enduring Properties of Settled Barite Extracted from an Offshore Well Two Decades Later: Well Abandonment and Slot Recovery

Summary Barite settling in the annulus behind the casing is an undesired yet common occurrence during the life of a well. Over time, the drilling fluid left in the annulus settles, leaving behind solidified barite that can hinder slot recovery and plug and abandonment (P&A) operations by impeding the cut and pull process. During a P&A operation, Equinor acquired settled barite samples from a North Sea well where the casing was held back by these weighting agents, introducing overpull and prolonging the cut and pull operation. A laboratory analysis program that included determination of the particle-size distribution (PSD), electrokinetic potential of particles (zeta potential), crystallography [X-ray diffraction (XRD)], chemical composition [X-ray fluorescence (XRF)], thermogravimetric analysis (TGA), and microstructure [scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and quantitative evaluation of materials by scanning electron microscopy (QEMSCAN)] was carried out on these settled barite samples to understand their properties. The results of this study provided valuable insights into the composition and characteristics of the settled material in the annulus. The solidified barite within the annular space exhibited no signs of chemical reactions. XRD analysis confirmed the barite to be the sole predominant solid in the solidified material, aligning with expectations. However, additional analytical techniques, including XRF, QEMSCAN, and EDS, detected interconnections between barite particles, primarily facilitated by iron or quartz particles. Additionally, trace amounts of calcite, iron oxide, and mixed compositions involving Si, S, Fe, Ba, O, and Cl were identified within the solidified material. SEM results indicated that the particles exhibited strong compaction characteristics but lacked cementation, retaining some porosity. Notably, the absence of bentonite or other clays was consistently observed in all analyses. This study highlights the process of solidification observed in settled barite, suggesting that factors other than chemical reactions may be responsible for this phenomenon. The potential mechanisms contributing to solidification include physical aggregation, compaction, and alterations in surface charge under downhole conditions. This enhanced understanding of the solidification process will contribute to the development of solutions for efficient casing removal and even the usage of settled barite as a barrier material.

Dispersing Settled Barite: An Approach to Improve Performance and Safety of Cut and Pull in Plug and Abandonment Operations

Summary Settled barite is a known challenge in well construction and well control activities. This phenomenon creates operational and safety challenges during cut and pull operations in well abandonment activities. In this paper, we introduce a novel approach to mitigate these challenges by dispersing settled barite particles using chelating agents, specifically ethylenediaminetetraacetic acid (EDTA) and diethylenetriamine pentaacetic acid (DTPA), commonly utilized in the oil industry to dissolve barite scales. Laboratory tests were conducted to assess the impact of these two chelating agents across varying concentrations, weight-to-volume ratios, pH levels, and durations. Settled barite samples retrieved from a plug and abandonment (P&A) operation were used in this test. Initially, visual inspection suggested some action of chelating agents in fragmenting settled barite materials into smaller particles. Afterward, a sequence of sieves was used to assess the particle-size distribution (PSD) and quantify the dispersion, revealing an increase in particle dispersion correlating with higher concentrations of chelating agents and weight-to-volume ratios. Contrary to scale-dissolving experiments, the dispersion of settled barite manifests across a broad pH spectrum. Furthermore, an initial increase in dispersion was observed over time, while the introduction of an activator such as potassium chloride (KCl) displayed no discernible effect on the overall dispersion process. This work shows the potential resolution of settled barite issues through the application of conventional chelating agents, such as EDTA and DTPA, commonly used within the oil field. It suggests methodologies for optimizing their performance in addressing barite settlement concerns. In addition, the study proposes the broader applicability of chelators within the EDTA and DTPA family for dispersing settled barite, thereby enhancing performance and augmenting oilfield safety in chelator use.

Mitigating Gas Migration with Eutectic Bismuth Alloy Plugs

Gas migration is an important concern to address in oil wells, especially in those that are to be abandoned. Stopping a gas leakage is not a simple task, and this can be particularly detrimental when setting a cement plug, as the migrated gas can undermine the integrity of said plug. In this context, and considering the recent attention given to metal plugs, we investigate the capability of bismuth plugs in shutting off gas leakages. The bismuth alloy employed to form the plug is to be melted downhole, and thus its proper solidification may be compromised if a leakage is underway. We test the sealing capability of two bismuth alloys – the eutectic bismuth-tin and the eutectic bismuth-tin-indium – along with two pipes – acrylic and steel. Results indicate that the bismuth plug can seal off the inner space of the pipe as long as the alloy used is eutectic; otherwise, a channel is created if the alloy is only near eutectic, effectively permitting that the leakage still continues. This sealing capability has been verified for both eutectic alloys tested, although it was also observed that voids are still present within the plugs themselves, potentially undermining their mechanical properties. Furthermore, we also observed that a slow and controlled cooling approach reduces the volume of inner voids, thus mitigating the plug degradation caused by the migrating gas. Further works should verify how these voids impact the hydraulic shear bond strength of the plug. From a microstructural perspective, it seemed that the solidification cooling rate plays a key role in the grain size of the material, which significantly impacts the microstructure of the alloy and, consequently, its mechanical properties. Considering the findings discussed in this work, we recommend that a two-plug approach is employed in leaking wells: a first, eutectic plug would shut down the leakage at the cost of its strength, and then a second – eutectic or non-eutectic – plug would be placed under no leakage ensuring the desired strength and sealability requirements. This study further highlights the potential of bismuth alloys to enhance safety and efficiency in plug and abandonment (P&A) operations by mitigating gas migration issues.

The effect of well temperature on the microstructure and the mechanical performance of bismuth-based plugs in well plugging and abandonment operations

Bismuth-tin (BiSn) alloys are considered promising candidates as potential alternatives to cement for well plugging and abandonment (P&A) applications to prevent any uncontrolled flow of deep reservoir fluids to the surface. As the temperature in the well varies with depth, the final microstructure of the BiSn alloy after its solidification might be influenced, and consequently the mechanical performance of the alloy will also be affected. The aim of this study is to evaluate the microstructural evolution of the BiSn alloy after solidification, at temperatures between 20 °C and 90 °C, to reflect in situ conditions from approximately 800m to 3 km depth. Microstructural characterization was carried out by using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The microstructural analysis revealed that the plug subjected to the lower temperature resulted in finer microstructure, whereas the grain size of the material was increased at higher temperatures. Additionally, the theoretical lever rule was employed to validate the microstructural findings. The observed microstructures were also correlated with tensile tests, which showed that the plug at 20 oC had a yield strength of 67,25 MPa, whereas the yield strength of the plug at 90 oC was only 41,64 MPa. In addition, hydraulic push-out tests further confirmed that lower testing temperatures correlate with higher pressure tolerance of the plug. The plug at 20 oC could withstand 113,5 bar, compared to the plugs at 60 oC and 90 oC which could withstand 90,4 and 50,9 bar, respectively. From all the experimental tests conducted, the outcome was that the lower temperature of 20 oC resulted in a material with a finer microstructure, which plays an important role in enhancing the mechanical properties of the bismuth-based plug. This study aims to advance the understanding of BiSn alloy performance in plug and abandonment (P&A) operations, thereby supporting the industry's efforts to qualify BiSn alloys as effective well-barrier materials.

Mechanical Performance of Bentonite Plugs in Abandonment Operations of Petroleum Wells

This study aims to evaluate how the operational procedure adopted for pellet placement and the exposure to subsurface conditions influence the mechanical integrity of bentonite plugs used as barrier elements in the abandonment of petroleum wells. To this end, the plugs were formed by hydrating the pellets directly in water, simulating the onshore procedure, while the offshore plugs were obtained from pellets hydrated in deionized water after immersion in diesel or olefin, which are suggested as displacement fluids. The plugs obtained were tested by compression and adhesion tests. These mechanical tests were also carried out for specimens obtained from plugs exposed to four formulations of synthetic formation waters. The results obtained demonstrated that, in the offshore procedure, the previous contact with olefin may adversely affects the mechanical stability of bentonite plugs, while plugs formed from pellets immersed in diesel presented satisfactory mechanical properties. However, the contact with formation water evidenced that the onshore plug presents superior resistance than the offshore plug previously immersed in diesel. The highly successful performance of the onshore plug was attested by the maintenance of the compressive strength, which exhibited a maximum reduction of 13%, even after exposure to the most saline formation waters.

Drainage flows in oil and gas well plugging: Experiments and modeling

This work studies the drainage flow of a heavy fluid from an inner pipe into an outer closed-end inclined pipe filled with an in-situ light fluid. This configuration represents the bailer drainage in the dump bailing method, which is a common technique for cement plug placement in plug and abandonment (P&A) operations of oil and gas wells, especially in Western Canada. Cement plugs are set as part of well decommissioning to prevent oil and gas leakages from hydrocarbon zones to different formations, freshwater underground resources, and the surface. The heavy fluid can be a Newtonian or viscoplastic fluid, while the light fluid is always a Newtonian fluid. The two fluids are miscible, and they have a density difference. Using experiments and modeling, the effects of the heavy fluid properties and flow parameters are examined on the drainage flow dynamics. In the experiments, high-speed imaging and non-intrusive measurement techniques are used to provide ample drainage flow characterizations. In particular, the experimental results show two distinct flow regimes inside the inner pipe, namely the slump-type and the center-type flow regimes, which are classified in a plane of the governing dimensionless numbers. The study further examines the onset of the heavy fluid drainage from the pipe based on the fluid’s yield stress and applied longitudinal buoyant stress. Once this onset is identified, a one-dimensional drainage model is developed, based on the energy balance to predict the drainage rate of the heavy fluid. The comparison between the experimental results and modeling predictions shows reasonable agreement, demonstrating that the proposed model can well present the heavy fluid drainage rate. The model also suggests that the drainage rate can be enhanced by reducing the viscosity and yield stress of the heavy fluid as well as increasing the density difference, pipes’ diameters, and inclination angle. The outcomes of this study can be helpful for improving the cementing processes in P&A operations of oil and gas wells.

Evaluation of Saline Solutions and Organic Compounds as Displacement Fluids of Bentonite Pellets for Application in Abandonment of Offshore Wells

One of the operational challenges regarding the use of bentonite pellets as sealing materials in the abandonment of offshore fields consists of their placement inside the well. This study aimed to analyze the interaction of fluid media, consisting of saline solutions (NaCl, CaCl2 and KCl) and organic compounds (diesel, glycerin and olefin), with bentonite pellets, for their applications as displacement fluids in offshore oil well abandonment operations. The physical integrity of the bentonite pellets in contact with the fluids was verified through visual inspections and dispersibility tests. Linear swelling tests were also performed to evaluate the swelling potentials of the pellets in deionized water after their contact with the fluid media. The results indicated that the NaCl, CaCl2 and KCl solutions completely compromised the physical integrity of the pellets, while diesel and olefin showed the best responses regarding the structural preservation. Furthermore, the linear swelling tests showed that, even after the contact with diesel and olefin for 1 h, the bentonite pellets reached a total swelling of 78% in water after 24 h. In this way, diesel and olefin proved to be highly promising alternatives to be used as displacement fluids for bentonite pellets in wells that will be abandoned in a submarine environment.

A novel tool to break the cementation force of a 9-5/8″ casing-cement sheath system for offshore well abandonment operations: design, tests, and feasibility analysis

Casing recovery in offshore well abandonment is challenging due to the cementation force between the casing and the cement sheath. To improve the well abandonment operation efficiency and success rate, in this paper, a newly developed metal casing recovery method for the well abandonment process and a revolutionary tool were first proposed. The proposed design of a high-pressure fluid-controlled device is discussed in detail regarding its working mechanism and structural composition. After describing the configuration of the eccentric roller mechanism and operational principle of the hydraulic rubblizer, static modeling for a mechanical part is analyzed by considering the nonlinear constitutive model of cement materials behavior, and elastic deformation of the casing. Moreover, both the linear and nonlinear behaviors of the extrusion process acting on casing and cement are analyzed and incorporated. Experimental results showed that the pulling-out force of casing is decreased by up to 61.1% under 35 MPa working pressure compared with the absence of the tool. As a result, the total well abandonment time is reduced by at least 50% compared with conventional operations. The results of this study are expected to serve as a theoretical basis for optimizing the structural and hydraulic parameters of hydraulic rubblizer.

Estimation of thermal input in thermite reaction for innovative wellbore plugging & abandonment techniques

In this work, a methodology is presented to simulate heat transfer in wellbores for plugging & abandonment operations, where the thermal input is provided by a thermite reaction. The operation of burning a thermite column inside the well's production tubing to form a metal plug is studied. The objective is to eliminate the need to simulate the thermite domain and chemical reaction during the process, simplifying the physical model and reducing computational cost. In this model, it is assumed that the thermal input from the thermite reaction is provided to the model through multiple heat flux boundary conditions along the inner wall of the production tubing. The unknown heat flux from the thermite to the inner wall of the tube is estimated by solving an inverse heat conduction problem (IHCP). The Adaptive Function Specification Method is used to estimate multiple heat flux functions at the boundary through information from multiple temperature sensors located on the outer surface along the tube's height. A characteristic behavior of the heat flux curve was verified in all segments of the tube, and the average heat flux curve was used to simulate heat transfer during a 15 m thermite burning process inside a well.

Investigation of the hydraulic integrity of cement plug: Oilwell cementitious materials

The loss of hydraulic integrity in oil and gas wells due to the loss of cement sheath sealability can lead to environmental contamination, annular pressure build-up, and safety threats. In this study, we examine the hydraulic integrity of geopolymers an alternative to cement to be used in well cementing. The hydraulic integrity of geopolymer was compared to conventional API class G and Industrial expansive cement. Down-scaled test specimens representing cement-plug in casing were prepared and tested using an in-house experimental set-up that allows continuous curing and testing of the cementitious materials under undisturbed pressure and temperature conditions. The samples were cured at 90 °C and 172 bar for 7 days after which the hydraulic sealability of the specimens was examined by applying a pressure differential to one end of the specimen and observing the resulting fluid leakage rates on the other end. The leakage rates were then expressed in terms of permeability and microannuli aperture. By injecting nitrogen and water, it was possible to compare the effects of fluid type on the hydraulic sealability of cementitious materials. Lastly, we examined the hypothesis of a linear relationship between plug length and its hydraulic sealability. The results indicate that geopolymer and Industrial expansive cement have higher hydraulic sealability compared to API class G. Geopolymers also have sufficient hydraulic bond strength to perform as much as Industrial expansive cement. The fluid type used in testing does not play a critical role in the loss of hydraulic sealability of cementitious materials. The influence of cement plug length showed varying trends on the hydraulic sealability of the cementitious materials. The results presented in this work help us understand the sealing potential of cementitious materials and the need for standards for performing laboratory-scale hydraulic sealability tests. This can benefit the improvement of cement integrity tests and well abandonment operations.

Numerical investigation of the impacts on the primary cement in a new thermal approach for plug and abandonment operations

The advancement of Plug and Abandonments (P&A) procedures is pivotal for reducing the costs associated with current operations. A novel technology concept proposes a heat emitter that will produce enough energy to melt the casing steel without critically affecting the cement layer. However, recent studies concerning this proposal have not given enough attention to the potential impact on the primary cement, which is a crucial material to guarantee the plug's integrity. This study models the heat emitter as a thermite mixture with constant volumetric heat generation, and the oil well structure was approached as a 2-D axisymmetric domain. The finite volume method with a static melting/solidification model is employed to solve the governing equations numerically. A C++ code was developed and compared with the commercial software Ansys® Fluent was performed to verify the present code. The thermal parameters of the heat emitter, including density (1983.6 and 2192.4 kg m−3), specific heat (919.6 and 1016.4 J kg−1 K−1), conductivity (5 and 15 W m−1 K−1), latent heat (1267.79 and 1147.05 kJ kg−1), volumetric heat generation (104.59 and 115.6 MW m−3), and reaction time (71.25 and 78.75 s), are evaluated through a 26 factorial design. The responses analyzed are the maximum melted volume of steel and the volume of cement critically affected. The high variability associated with thermal conductivity indicated a strong dependence on this parameter. Most importantly, this study highlights that melting the casing steel could unintentionally degrade the cement layer, increasing potential leakages paths and integrity problems.

Machine learning-based cement integrity evaluation with a through-tubing logging experimental setup

Assessing the integrity of the cement layer and the quality of its bond to the casing and formation is paramount to ensure that the wellbore is hydraulically isolated from the surrounding environment before permanently sealing the well. Such inspection is part of Plugging & Abandonment (P&A) operations, and it is usually achieved through well logging tools, which provide a vast amount of data that a skilled specialist interprets. The process is human-dependent, error-prone, and time-consuming. There has been an increasing interest in solutions that allow an automatic interpretation of the logging data since the recent panorama of the oil and gas industry indicates a growing P&A demand. Such solutions aim at reducing the dependence on human knowledge and consequently increasing the reliability and accuracy of the cement integrity evaluation. Therefore, this work presents an experimental setup capable of emulating defective cement layer configurations of real-world oil wells in single or multi-string arrangements. Such flexibility enables acquiring logging data for multiple well conditions and building a rich logging database to develop a supervised learning framework and define the most suitable model for performing the cement integrity evaluation. Several logging runs were performed, producing a database with 130 samples, including varied tubing eccentricity levels and cement layer conditions. A complete analysis of the data both in the time domain as well as in the frequency–wavenumber domain was performed, highlighting the complexity of the interpretation task. A resampling-based workflow was employed to evaluate machine learning models of different families. The models were tested under three scenarios, and accuracy and computational complexity metrics were computed to compare their performance. The results showed that shallow learning models can perform satisfactorily well even with less data available for training. The support vector machine stood out, achieving a mean accuracy score higher than 0.99 while being able to predict the cement sheath’s condition in less than 1 ms. This paper contributes to the research on the cement integrity evaluation by presenting a study that combines an experimental setup mimicking several oil well conditions and the employment of machine learning as a diagnostic tool, which has no precedents in recent literature regarding the acoustic logging knowledge field.

Expandable Geopolymers Improve Zonal Isolation and Plugging

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 212493, “Expandable Geopolymers for Improved Zonal Isolation and Plugging,” by Foster D. Gomado, SPE, Mahmoud Khalifeh, SPE, and Jan A. Aasen, SPE, University of Stavanger. The paper has not been peer reviewed. _ The sealability performance of an expanding geopolymer (E-GP) is compared with that of an expansive commercial cement (EC) with regard to shear bond strength (SBS) and hydraulic bond strength (HBS) at curing conditions of 25°C and 34.5 bar. A neat Class G (NG) cement and a neat geopolymer (N-GP) were characterized alongside corresponding expansive versions. The effect of these expansive agents on cement and geopolymers was evaluated in terms of linear expansion using the annular ring test. The complete paper reveals that geopolymers can be deployed as an alternative to Portland cement upon optimization. Geopolymer Characteristics and Use In recent years, geopolymers have been receiving attention as a viable and environmentally friendly alternative for zonal isolation and plugging and abandonment (P&A) operations because of their robust features such as higher strength, lower permeability, and durability in high-pressure/high-temperature conditions. Geopolymers can be described as binders formed by a chemical reaction between aluminosilicate materials and alkali hydroxide or soluble silicates. In this study, cement/casing bonding properties are the primary focus. Greener options for cementing, zonal isolation, and P&A must meet acceptable criteria before mass adoption. Geopolymers, like Portland cement, experience shrinkage to some extent. To improve cement bonding, expansive agents are regarded as relevant additives, correlating volume changes to bonding properties, mostly SBS. In the case of geopolymers, few studies have been directed toward the study of volumetric changes with and without expansive additives. The current literature for geopolymers lacks correlation between the performance of expansive agents on geopolymers in terms of HBS, SBS, and load-to-displacement profile. Thus, the authors explored the effect of expansive agents on SBS and accompanying interfacial friction and hydraulic sealability for both NG and EC, as well as neat geopolymers and expansive versions at equal conditions. Experimental Procedures NG cement was used as a reference because it is the most widely used cement in the Norwegian North Sea. The EC was made by the addition of a commercially available expansive agent [made of a blend of calcium oxide (CaO) and quartz] to NG at a concentration of 5% by weight of cement (BWOC). The geopolymer deployed in this study was synthesized in house. The E-GP was synthesized by the addition of a calcium salt in a concentration of 1.1% of the solid precursors. The complete paper details slurry preparation and the methodology for linear-expansion (shrinkage) tests, SBS/bond slip tests, and HBS tests.

Safety Analysis of Offshore Wells Plugging and Abandonment Process with Riserless Well Intervention System Using a DBN based Comprehensive Method

The Riserless Well Intervention (RLWI) system that performs complex offshore oil well Plugging and Abandonment (P&A) operations is a typical Multi-Mission Phased-Mission System (MM-PMS), which requires multiple missions to be completed within a phase. P&A processes involve complex operations and equipment that can contaminate local marine ecosystems if they fail. Therefore, it is necessary to evaluate the reliability of the RLWI system. This paper proposes a dynamic reliability evaluation model for analyzing the RLWI MM-PMS. The GO model of the phase operation process and the Fault Tree (FT) model used to analyze the failure of each mission were established, and a Dynamic Bayesian Network (DBN) model based on the GO model and the FT model was developed for reliability evaluation. The established model can analyze the changes in the reliability of the RLWI MM-PMS more comprehensively, and can also clarify the importance of different missions and different system components. In addition, considering the impact of the marine environment on operators, the Standardized Plant Analysis Risk-Human (SPAR-H) reliability analysis is used for quantification. These findings can guide the improvement of the reliability of the RLWI system and the success rate of P&A operations.

Introduction of emergency measures and full plug and abandonment operations for oil and gas leakage from a legacy high H<sub>2</sub>S natural dump-flood well

Introduction of emergency measures and full plug and abandonment operations for oil and gas leakage from a legacy high H<sub>2</sub>S natural dump-flood well

E&P Notes (October 2022)

E&P Notes (October 2022)

Credibility assessment in placement and verification of cement plugs in through tubing plugging and abandonment operations

Credibility assessment in placement and verification of cement plugs in through tubing plugging and abandonment operations

Structural design and mathematical cutting model of hydraulic section milling for high-strength casings in small wellbores

The section milling of casings is widely applied in the side-tracking and abandonment operations of old wells. In particular, the cutting of casings is a crucial task during section milling; however, this process is associated with problems such as low cutting speeds, especially for the high-strength casings used in small wellbores. Furthermore, theoretical support and experimental investigations related to this operation remain scarce. Accordingly, based on the working principle and structural characteristics of a hydraulic cutter in this work, a geometric model of the interactions between the piston rod and blade was established. A high-performance hydraulic section milling tool suitable for 177.8 mm casings was also developed. According to the motion characteristics of the blade, a dynamic contact force model was established for the cutting process of casings, considering changes in the cutting depth and a mathematical model of the cutting depth, considering the effect of time, was also derived according to the grinding mechanism. The critical parameters of this cutting depth model for the P110 casing were determined through ground field tests and finite-element simulations, and the changes in the trajectory were inverted. The corresponding inversion results for the cutting depth exhibited a rapid increase in the significantly short primary stage, followed by a gradual increase. The theoretical torque and cutting time were also highly consistent with the field-measured values; this further verifies the rationality and correctness of the models used in this study. The results of this study are expected to serve as a theoretical basis for optimising the structural and hydraulic parameters of hydraulic cutters and making further improvements in the cutting efficiency for the side-tracking and abandonment operations of old wells.

An evaluation of viscous deformation of chalk on wellbore stability

Wellbore instability has become an increasing concern for horizontal wells, especially with the move towards using drilling innovations that the entire lateral sections are remained without casing during the lifetime of the reservoir and after plug and abandonment operations. We aim to understand the long-term effect of viscous deformation on wellbore stability for horizontal wells through advanced laboratory experiments and viscoplastic simulations. Two-stage single lateral hole tests were conducted on chalk samples, where a horizontal wellbore was drilled laterally at the center of a cylindrical specimen and loaded under triaxial condition with a constant stress ratio. The chalk samples were imaged at two stages in the creep phase using X-ray computed tomography CT(scanning) to precisely observe the breakout development of the damaged zones formed around the borehole. The results from the experiments and CT analysis confirmed model predictions. Results presented in this work clearly demonstrate that viscous deformation can trigger failure modes and induce breakout development around the wellbore in the creep phase.