The construction of a well begins during the ‘concept and select’ stage where the well objectives and deliverables are defined and ends when the well is either abandoned or handed over to production. Due consideration to well integrity during well construction offers long term effects in the well’s production life or post-abandonment.
Key areas of well integrity application during well construction are:
- Setting conductors at a verified point of refusal to prevent punch throughs while drilling subsequent hole sections.
- Tripping practices – Defined for each section.
- Hole cleaning practices – Defined for each section.
- Enhanced well control in the form of a Predict-Monitor-Prevent model.
- Monitoring and mitigating casing wear while drilling to ensure the structural integrity of casings are retained.
- Casing design and material selection considerations for potential corrosion problems during the well’s production life.
- Cement designs for life cycle zonal isolation and integrity.
- Cement handling and placement practices for cement-casing and cement-formation bond integrity.
- Wellhead designs to allow annulus pressure monitoring and maintenance for wells on production.
Casing design when carried out with life cycle well integrity at its foundation involves the application of a probabilistic casing design approach wherein the casing strings for a well, especially in deeper sections are designed by identifying all practical parameters and variables that will affect the casing over the well’s life and evaluating a probability of failure for the casing.
Such an approach utilizes reliability engineering models wherein the uncertainty in each design variable is explicitly considered. The failure criteria in such an approach is more representative of the true structural limits of a casing in its operating environment as opposed to a conventional deterministic design using the elastic limit of the steel.
Casing wear and Corrosion are not entirely preventable due to practical reasons – Casing wear is primarily caused due to drill string rotation (among others) and Corrosion mainly occurs due to the bottom hole environment. Casing wear is caused while drilling whereas corrosion may begin and progress at any stage in a well’s life.
Well Integrity assurance against casing wear and corrosion involves the application of prevention and mitigation techniques for new wells being constructed and remedial methods for wells that encounter wear-induced problems and/or corrosion-related problems later in their life.
The loss of cement integrity, especially in hole sections drilled across pressurized hydrocarbon bearing sands can lead to a myriad of problems ranging from minor cross-communication issues between formations to blowouts followed by a hydrocarbon spill.
Surface casings run in hole to shallow depths are cemented primarily for axial support to the casing and to prevent formation collapse. This offers stability as the well is drilled further. As the well is drilled deeper into pressurized fluid-bearing sands, zonal isolation across formation becomes increasingly prominent.
The cement sheath in the annulus forms two bonds – the casing-cement bond and the cement-formation bond. High quality of both bonds in the annulus is essential for life cycle cement integrity for each casing string.
Long-term cement integrity is achieved through:
- Robust cementing design – Specific to the hole section.
- Cementing practices – Defined based on the hole section being cemented and the prevailing hole conditions.
- Post-cementing quality checks and monitoring to carry out remedial jobs, if required.
Annular pressure build-up (APB) is one of the major impacts of well integrity failure experienced around the world. The causes of APB vary based on several factors such as downhole temperatures, produced fluid temperatures, mechanical leaks, gas migration, etc but the implications can be severe – production shut down for long periods of time, underground blowouts and/or uncontrolled release of well fluids on the surface.
Since the time, nature and cause of APB problems in a well’s life cycle cannot be exactly known, it is imperative to anticipate and prepare to manage APB issues in the well post-well construction.
Common APB causes such as gas migration and/or mechanical leaks can be prevented through efficient drilling and cementing practices during well construction. But other potential causes such as thermal expansion of production strings, produced fluid temperatures and bottom hole temperatures cannot be avoided and hence need to be managed effectively.
IWICF presents robust APB prediction, prevention and mitigation techniques for wells to be drilled in addition to investigation with recommendations for wells experiencing APB problems through the Reductio ad Absurdum principle.
