The potential to transfer deposited scale into a competent barrier component in CO2 storage reservoirs: A study on in-situ consolidation of low-strength carbonate composite

The use of non-producing oil and gas reservoirs for geological Carbon Capture and Storage (CCS) is a promising strategy to reduce carbon emissions, while its success relies on creating impermeable plugs/barriers to prevent leaking of the stored CO 2 . Mineral scale deposited in the reservoir pipes poses significant challenges during the plugging phase, which needs to be removed in the current practice. However, the conventional removal via milling is costly, inefficient, and leads to significant amounts of scale waste. This study investigated a novel alternative: in-situ consolidation of porous carbonate scale into a competent barrier component using a diammonium hydrogen phosphate (DAP) treatment. A limestone-cement composite simulating downhole scale was treated by immersion in DAP solutions, systematically varying treatment duration, DAP concentration, ethanol content, and temperature (25–95 °C). The treatment markedly improved the performance: compressive strength increased by up to 43.7 % and water absorption decreased by up to 21.4 % relative to the untreated samples. Temperature was the most influential parameter, revealing a critical trade-off: 75 °C maximized compressive strength, whereas 25 °C minimized water absorption. Comprehensive characterization confirmed that the strengthening mechanism was the precipitation of hydroxyapatite (HAP) within the pore network. The divergent temperature optima were attributed to a competing mechanism of temperature-accelerated calcium dissolution from the cementitious matrix. This work provides a proof-of-concept for a cleaner production strategy that advances a circular, lower-impact CCS pathway, reducing the environmental and operational burdens of conventional milling and supporting more sustainable, cost-effective well plugging and abandonment technologies.