Fluid Migration Modelling and Treatment

Wells for production of hydrocarbons, geothermal energy and for subsurface storage of CO2 are constructed in sections, where one section is first drilled to a planned depth and then secured by running and cementing a casing string to the formation or previous casing. The casing string and well cement are critical structural barrier elements in wells. Failure to isolate the annulus hydraulically can result in leakage of formation fluids past the cement and into other permeable formations or to the surface, resulting in sustained casing pressure. Annular fluid migration represents a particularly challenging well integrity problem as it is extremely difficult and expensive to remedy. Traditional treatment of fluid migration involves perforating or cutting the casing and squeezing cement into the leaking annulus. This is a costly operation with low success rates that can even make the situation worse by fracturing the formation or generating new leakage paths. The purpose of this project is to move the industry forward by better understanding the roots of the problem, based on studying and characterizing the geometry of typical, realistic leakage paths, and then studying how fluids flow and displace each other along these channels. The results from these studies were integrated into numerical models that can be used to diagnose fluid migration paths that result in sustained casing pressure, and in a standardized full-scale test procedure for the verification of new treatment methods. Achievements in the project include full-scale testing of the mechanical expansion tool Local Casing Expander for sealing migration paths in annular cement, showing a potential for reducing permeability by up to several orders of magnitude. Unique large-scale experiments conducted on 30-year-old cemented annulus test cells from a North Sea well, using a pressure-pulse decay method proved extremely valuable insight into the actual long-term integrity of cemented well barriers, in addition to being an effective strategy for characterizing permeability and for relating log-response to seepage potential, important to perform predictive assessments of well barrier integrity. Experimental measurements were conducted to determine and compare steady-state and unsteady-state relative permeabilities in two-phase flow in micro-annuli. Alterations in cement permeability and relative permeability were investigated over time, and compared with relative permeability models. An efficient simulator for dynamic multiphase flow in rough fractures was developed, which enhanced computational efficiency and simplified input requirements for such calculations. A baseline predictive sustained casing pressure (SCP) model was tested and implemented in the P&A Leakage Calculator, using field data and model calibration to improve and strengthen interpretations of log response and predictions of actual cement permeability for forecasting barrier leakage potential. The effect of cement slurry contamination on cement sheath integrity and casing corrosion was demonstrated through spectroscopic analyses. A simplified model was developed for predicting the downhole performance of cement based on mechanical behavior and deviatoric strength. For treatment technologies, a testing approach was developed for the design and repeatable fabrication of full-scale test cells, demonstrated through the injection of various treatment solutions in three test cells. An experimental methodology for testing the treatment effectiveness of polymer sealants was developed, and tests of the sealants proved efficient in reducing leakage for both water and gas. 8 publications to international scientific journals were produced throughout the project, in addition to conference publications and presentations. A Ph.D. student has been working with factors impacting integrity of cement and alternative barrier materials at the University of Stavanger, demonstrating e.g., the applicability of expansive and neat G cement for low- and elevated-temperature applications. The project has encompassed academic and industrial cooperation nationally and internationally. NORCE has led the project with support and contributions from its Ullrigg Test Centre, UiS, University of New Mexico (UNM), University of Alberta (UoA), and the industry-research project consortium P&A Innovation Program, comprising the Norwegian Ocean Industry Authority (Havtil) and five major operators on the Norwegian Continental Shelf as well as international.

The project has through the experiments and research conducted, developed test protocols for large-scale experiments of cemented annulus sections and for the testing of the efficiency of treatment solutions, and also developed a procedure for the construction of test cells with controlled and predictable leakage properties. The combination of utilizing unique field data with measurements and predictive simulations represents factors that are crucial towards improving both the integrity of existing and future well barriers and for optimizing remedial techniques directly based on predicted efficiency. The immediate results of the project represent improved predictive capabilities for fluid flow through micro-annulus and fractures, SCP- and log response-based leakage potential, and tools and procedures for the qualification of treatment placement and efficiency. The research performed herein, and related follow-up activities, also holds an enormous long-term cost-savings potential when applied in the design and operations in industry, especially considering the volume of wells to be permanently plugged and abandoned in the coming years. This benefits both operators, the service industry, regulators and the environment.

The annulus cement is a critical well barrier in wells for oil and gas production, for geothermal energy recovery and for underground CO2 storage. Failure to isolate the annulus hydraulically can result in leakage of formation fluids past the cement and into other permeable formations or to the surface resulting in sustained casing pressure. Annular fluid migration represents a particularly challenging well integrity problem as it is extremely difficult and expensive to remedy. Traditional treatment of fluid migration involves perforating or cutting the casing and squeezing cement into the leaking annulus. This is a costly operation with low success rates that can even make the situation worse by fracturing the formation or generating new leakage paths. The purpose of this project is to move the industry forward by better understanding the roots of the problem: - What is the characteristic geometry and shape of the fluid migration paths we are trying to repair? - How do brines, hydrocarbons and repair materials flow and displace each other along these channels? These questions will be addressed using a combination of laboratory and full-scale test cells for controlled leakage and remediation experiments, combined with modelling efforts. Flow and displacement experiments in realistic microannulus geometries will be used as input for an improved leakage and sustained casing pressure simulator, and as basis for designing full-scale test cells for verifying new treatment materials. The project results will improve fluid migration diagnostics and support the choice of preferred remediation technology. The qualification methodology developed in the project will provide standardized verification of treatment technologies. This will reduce the time to market for new solutions and improve international competitiveness of Norwegian service providers, to great benefit both for the service providers and oil companies operating on the NCS.