Mechanical testing and modelling of oil & gas well cement sheath

Description of proposed research
The oil industry primarily uses cement as a primary barrier (sheath) to isolate and protect steel casings from the surrounding rock and soil formations and the environment, to protect against fluid and gas leaks, and to enhance the stability of the bored structure. However, there is an overall concern that the current cement sheaths do not function flawlessly as is apparent due to pressure buildups in the annular space between casings and the well heads. Recent information obtained from wells in the Norwegian North Sea on methane leaks indicates that the quality and performance of a cement sheath as the barrier element is in need for critical revision and improvement.
The functions of cement sheaths for wells include anchoring of the casing, protecting the casing from the corrosive environment, support of the borehole walls, and to provide a zonal isolation of segments along the borehole. Complete and durable isolation is the primary goal of the cementing installation, both for zonal isolation purposes, maintaining well integrity, and abandonment of exhausted sections or wells. During the life of an oil / gas production or water injection well, the quality of the cementing installation and cement workover(s) directly impact the economic longevity of the well. From the time the well is first taken into use until the well is fully abandoned, appropriate cement-slurry design and placement techniques will affect the physical and economic efficiency of the well.
The cement sheath is expected to provide a durable and stable barrier, capable of withstanding mechanical, chemical and thermal loads during its lifetime. However, there is no specific definition of the expected service-life of the system, and most of the recommendations presume a quasi-infinite durability of an uncracked cementing material. The cement sheath, however, is known to be brittle and to suffer from damage due to several cracking mechanisms as illustrated in Figure 1.
The faults can be divided into two categories:
•Malfunctions that form in the fresh cement during placement around the casing, and
•Mechanically-induced failures usually in the form of radial or annular cracks that initiate due to casing or formation deformations.
These faults are described in further detail in the following section “Pre-study on Cement Problems in Oil & Gas Wells” which provides an overview on the sprint projects that led to the development of this project proposal. 
While cement has been long used as the only barrier material in oil and gas wells, newly researched and developed classes of cementitious materials have the potential to significantly improve the design and performance (i.e., reducing the occurrences of faults) compared to conventional approaches with traditional materials. To fully appreciate the potential of these new materials, the currently used design and material evaluation methods must be examined against the actual structural conditions faced by the installed cement in the well to better understand whether appropriate material properties are being considered. This approach will highlight fully the potential benefit of these new materials. An improvement in the understanding of the faults in the cement sheath and the actual load (mechanical and otherwise) faced by the cement in the underground conditions will lead to refinements in the design of materials for cement sheaths. New materials will provide opportunities to reduce the lifetime cost of wells by reducing the workover costs and/or increasing the lifetime of the well.

Project Objectives and Potentials for Intra-Center Collaborations
The main objectives of the proposed project are to develop an enhanced mechanical evaluation model for prediction of behavior of cement sheath in Oil & Gas wells. The evaluation model will consist of two parts – experimental methods to assess material properties and numerical modeling of realistic load/environmental conditions. Experimental test methods and data derived in the proposed study would serve as a basis for developing a numerical model to characterize and predict mechanical damage of cement sheaths.

Progress on this field will directly assist and gain from other developments in interrelated fields such as material development, service-life design and degradation modelling. The most obvious collaboration opportunities include:
•The ongoing self-healing material development project which could utilize test methods developed through the proposed project for evaluation of their solutions, and
•Involvement of geophysics and structural geology experts in estimations of possible external loads/boundary conditions from the formation

Pre-study on Cement Problems in Oil & Gas Wells
The formation of cracks and interfacial damage at the primary cement sheath, as shown in Figure 1, compromises the overall structural stability, imperviousness and durability of the system. These faults also provide a flow path for fluids through the cement sheath. There is still a limited understanding regarding the crack formation and propagation processes in well cements due to the unique geometric and boundary conditions. A sprint project was performed in DTU BYG in the autumn of 2016 to improve the understanding of the actual quality of the cement sheath in underground conditions and to develop possible experimental methods to investigate the occurring damage mechanisms. A brief synopsis of the findings and potential future work from the sprint project is given below.

Figure 1 Experienced and observed damage mechanisms in cement sheaths

A basic test setup was developed to replicate a pressure testing of casing and to monitor the formation of cracks in the cement sheath using digital image correlation measurements (Figure 2). The outcome of the sprint project gave good indications that this type of test setup could be further developed and used to monitor the behavior of the cement sheath during external loading, and was considered to be of interest to technical experts from Haliburton (Gunnar Lende) and Maersk Oil (Sofia Lassen). The development of additional experimental assessment methods will be necessary to consider other situations and conditions (e.g., loads from the formation, other pressure and temperature conditions present in the underground conditions, etc.). There is a large potential for further research on representative experimental setups, investigating the impact of additional variables on the crack initiation and propagation processes, such as:
•Tri-axial loading,
•Fatigue or creep,
•Geometry of the cement sheath, e.g. eccentricity between casing and formation, consideration of typical casting defects, etc.,
•Combinations of pressure and temperature loads on the plastic, hardening, and hardened cement from both the casing and the formation simultaneously, and
•Casting and curing regime, comprising simulated downhole conditions.

Figure 2 Ring test, loaded test sample showing cracks due to casing pressure testing (DIC picture of a test result captured during sprint project)

In addition to various other innovative cementitious materials, the ongoing research in the Center on self-healing materials will be aided by the work proposed here. Additional mechanical evaluation will be necessary to prove the benefit of the self-healing process.
The experimental methods developed in this proposed project will be ideal for this evaluation and to answers the following questions:
•What are the residual/recovered strength and deformability parameters of the self-healed cracks?
•How successful is the self-healing process in cracks subject to flow of liquid/gas during healing?
•What limitations exist in the ability of the self-healing process (e.g., maximum ‘healed’ crack width, maximum load transfer across the healed crack, permeability of the healed crack, potential to control crack formation and crack width to enable self-healing, etc.)
The initial study for this project is currently starting in DTU BYG. The current project is focusing on a literature study on research related to mechanical evaluation of the cementitious materials used for the oil and gas industry and the further development of the test methods to evaluate the performance of self-healed cementitious materials. The currently ongoing project is a short term project and will be used as a starting point for the proposed project.