Identification and evaluation of well integrity and causes of failure of well integrity barriers (A review)

Volatile markets and harsh locations and downhole conditions pose severe challenges for ensuring safe and long-lasting intact well conditions. Well integrity is a crucial issue in the life cycle of all sub-surface boreholes. Failure of wellbore integrity leads not only to negative financial consequences, but also potentially to significant environmental impacts, such as groundwater contamination, gas leakage to the atmosphere, and fluid spills and seepage at the surface. Many studies have specifically focused on well integrity issues related to particular types of conventional and unconventional oil and gas reservoirs. Specific types of wells and well operations (e.g., high pressure high temperature, enhanced oil and gas recovery, deepwater, water and gas injection, geothermal, and plugging and abandonment) pose their specific issues. To understand the barriers to well integrity, and what is required to sustain it, a holistic study encompassing a wide range of issues is highly required. From a practical point of view, there are several factors affecting well integrity issues which can be classified based on chemical, mechanical, and operational factors. The consequence of these well integrity issues is mainly the fluid migration over time within or escaping from the wells. Past studies reveal that well integrity barriers are highly impacted by cement carbonation and casing corrosion processes, fluid migration, in-situ conditions, cement and casing mechanical properties. Cement is the main physical barrier able to seal fluid flow into unintended zones from the wellbore. The sealing efficiency of cement is highly dependent on in-situ environment conditions and cement chemical compositions, influencing the time-dependent stress geometry in the vicinity of wellbores. Casing corrosion is another challenging issue which is often unavoidable due to acidic environments imposed mainly by CO2 and H2S “sour” gasses. Modern studies have also shown the importance of cement fatigue degradation. Pressure regulation during production and temperature variation are the most common influencing variables impacting the mechanical aspects of well integrity. These variables induce extra stresses on the established barriers which can initiate and/or promote fluid migration. In addition, to chemical and mechanical aspects of well integrity, operational interventions can play crucial roles in improving well integrity. This aspect contributes to establishing zonal isolation, not limited to, but specific requirements of plugging and abandonment operations. Continuous evaluation and monitoring using different logging techniques and tests during drilling, completion, and production are required to address the issues that compromise the robustness of the well integrity. Nuances of the interpretation of multiple well logs must be understood in order to effectively respond to the potentially damaging situation, without risking the amplification of negative downhole conditions. Well integrity compensatory factors such as pressure, temperature, chemical changes, corrosion are interdependent. For instance, the variation in temperature or chemical changes will affect the extent of corrosion, hence a comprehensive design and monitoring system is essential to clearly understand the well integrity issues. This paper presents a broad review of research and field experiences related to well integrity.