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.
From a hydrocarbon perspective, the Caney Shale has historically been evaluated as a sealing unit, which resulted in limited studies characterizing the rock properties of the Caney Shale and its suitability for hydraulic fracturing. The objective of our research is to help bridge the current knowledge gap through the integration of multiscale laboratory techniques and to characterize the macro- and microscale rock properties of the Caney Shale. We employed an integrated approach for the characterization of the Caney using 200 ft (61 m) of Caney core from a target well in southern Oklahoma. Core observation and petrographic analysis of thin sections were combined to characterize the general rock types and associated fabrics and textures. Mineralogical composition, pore system architecture, and rock fabric were analyzed using x-ray diffraction (XRD), scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS), and focused ion beam (FIB)-SEM. In addition, rebound hardness and indentation testing were carried out to determine rock hardness (brittleness) and elasticity, respectively. With the integrated multiscale characterization, three mixed carbonate-siliciclastic rock types were identified — mudstone, calcareous siltstone, and silty carbonate — likely representing a spectrum of deposition from low to relatively high energy environments in the distal portions of a ramp system. Silty carbonate contains mostly interparticle pores. The calcareous siltstones and silty mudstones contain a combination of organic matter pores and interparticle pores. Each of the rock types shows unique mineralogical compositions based on XRD. The mudstone lithofacies has the highest clay content and the least carbonate content. Calcareous siltstones show moderate carbonate and clay content. Silty carbonate indicates the highest carbonate content with the least clay content. In an order of mudstone, calcareous siltstone, and silty carbonate, rebound hardness and Young’s modulus show an increasing trend. As a result of rock-fluid interactions, there are potential scaling reactions during completion and production that could ultimately affect permeability and production rates. Overall, the proposed multiscale integration approach is critical for the geologic characterization of most rocks. However, in shale reservoirs dominated by microporosity and microstructure where engineered fractures are expected to provide permeability at a reservoir scale, successful integration is essential. An optimized, integrated geological characterization of the Caney Shale that is well aligned with the engineering designs in drilling, completing, and producing wellbores will ultimately lead to optimal production while providing safe and environmentally responsible operations.
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