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 ∼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.

This study explores the geochemical reactions that can cause permeability loss in hydraulically fractured reservoirs. The experiments involved the reaction of powdered-rock samples with produced brines in batch reactor system at temperature of 95 °C and atmospheric pressure for 7-days and 30-days respectively. Results show changes in mineralogy and chemistry of rock and fluid samples respectively, therefore confirming chemical reactions between the two during the experiments. The mineralogical changes of the rock included decreases of pyrite and feldspar content, whilst carbonate and illite content showed an initial stability and increase respectively before decreasing. Results from analyses of post-reaction fluids generally corroborate the results obtained from mineralogical analyses. Integrating the results obtained from both rocks and fluids reveal a complex trend of reactions between rock and fluid samples which is summarized as follows. Dissolution of pyrite by oxygenated fluid causes transient and localized acidity which triggers the dissolution of feldspar, carbonates, and other minerals susceptible to dissolution under acidic conditions. The dissolution of minerals releases high concentrations of ions, some of which subsequently precipitate secondary minerals. On the field scale, the formation of secondary minerals in the pores and flow paths of hydrocarbons can cause significant reduction in the permeability of the reservoir, which will culminate in rapid productivity decline. This study provides an understanding of the geochemical rock–fluid reactions that impact long term permeability of shale reservoirs.