Establishing, verifying and remediating well barriers are all extremely critical operations that affect the safety, productivity and the abandonment costs of wells for hydrocarbon or geothermal power production. The state of original well barriers also affect the suitability of existing wells to be used for e.g. carbon storage. On the Norwegian Continental Shelf (NCS), thousands of wells are approaching the end of their productive lifetime during the coming decades. Permanent well abandonment requires establishing cross-sectional barriers that prevent cross-flow of fluids and do so indefinitely. As the condition of the original annulus cement can be uncertain or difficult to establish, limitations in current technology implies that expensive installations are often required for a considerable part of the P&A operation. Finding more efficient, cost-effective and safer methods for well abandonment is of great importance for the industry and the society.
Plug and abandonment is a prioritized, strategic research direction for NORCE. Initially through DrillWell (www.drillwell.no) and now through the P&A Innovation Program, we support operators and the service industry through projects that target operational challenges, field-relevant conditions and full-scale testing. P&A-related research within DrillWell include the following recent projects:
- Tubing left in hole: Leaving the production tubing in the well during abandonment can save considerable time and costs, especially for subsea wells. Full-scale through-tubing cementing experiments have been performed to investigate tubing and annulus cement quality.
- Technologies for barrier verification: Establishing the condition of annulus barriers outside casing is critical for P&A operations. Full-scale reference test assemblies for barrier logging has been established to facilitate testing and development of new verification technologies.
- Cementing irregular wellbore geometries: An important goal of primary cementing is to achieve zonal isolation between casings and between casing and formation. Full-scale cementing experiments and computational fluid dynamics simulations have been performed to investigate effects of wellbore geometry on mud displacement and cement quality.
- Leakage risk assessment for plugged and abandoned oil & gas wells: Current prescriptive requirements means that the P&A operation of a low temperature, low pressure well is equally complicated and costly as for a high temperature, high pressure well. Results from the above projects are integrated into a leakage calculator to develop a methodology for quantitatively compare different P&A designs and approach a risk-based approach to P&A.
The P&A Innovation Program launched in 2018 and is supported by the Norwegian government regulator and major operators. Through close collaboration with program participants, Ullrigg Test Centre and a major service company, the first activity to kick off in the program is cement logging and leakage testing of well sections that were cut and retrieved from a thirty year-old production well in the North Sea. The activity has generated a unique database of results that correlate logs to actual leakage potential. Other activities running within the program focus on full-scale testing of barrier placement and barrier performance, methods for barrier verification, fluid migration and treatment of failed annulus barriers. Supported by the Research Council of Norway and now in collaboration with a Brazilian university and operator, we also extend the risk-based approach to P&A operations.
We utilize an offshore-standard batch mixer for mixing of cement slurries and other fluids required for full-scale barrier testing and a wide range of annulus test cells with different well-defined features are available for logging purposes. Ullrigg wells provide relevant well environments for demanding test programs.
We also perform applied R&D related to well completions. In particular, a unique test methodology has been developed for evaluating the sand retention behaviour of sand screens in the laboratory. Our clients are primarily oil and gas companies and sand screen manufacturers. The slurry test methodology was developed during 2009 – 2010, and we have since performed a considerable amount of sand retention tests involving both formation and model sands.