A Review of Past, Present, and Future Technologies for Permanent Plugging and Abandonment of Wellbores and Restoration of Subsurface Geologic Barriers

摘要

Leakage of oil & gas during fossil fuel exploration, production, and transportation poses a major environmental challenge that impacts the quality of air, water, soil, and ultimately the life on Earth. The result of uncontrolled spills & leakages may cause the contamination of groundwater as well as methane emission into the atmosphere increasing global warming, while the spills in open waters in the case of offshore wellbores impact fragile marine ecosystems. The United States alone has similar to 1.7 million wellbores with an American Petroleum Institute (API) number indicating that they have not been permanently plugged and therefore require Plugging and Abandonment (P&A) in the future. However, there remain thousands of wells drilled before the 1950s that were not properly P&A. Improper P&A of boreholes are potential pathways for leakage as well as already existing leaky plugs. The objective of this literature review is to (1) assess subsurface conditions where P&A materials need to be placed, (2) assess the challenges encountered during barrier placement in wellbores under current P&A technologies, (3) appraise contaminations of barrier materials by drilling fluid and possible mitigation, and (4) discuss the future requirements of P&A materials and technology involved in restoring subsurface sealing barriers interrupted by drilling. The review indicates that to achieve permanent P&A in the future, two major challenges must be addressed: (1) innovation & improvement of the barrier materials, and (2) advancements and innovation of the placement methods. The major insight from this article is that re-establishment of a permanent seal capable of withstanding subsurface geomechanical and geochemical changes will involve engineered materials and processes providing sealing in the early stages (1-50 years) and geomaterials and geological processes take over and provide permanent seals, for thousands of years to geological times. Thus no single solution can be successful for future P&A campaigns.