The influence of hydrating mineral components on wellbore sealing capacity of salt-plugged wells

Abstract

Rock salt forms an important hydrocarbon caprock and a source for salt (solution) mining. With many associated wells approaching the end of their lifetime, effective Plugging & Abandonment strategies are required. At the same time, as the energy transition progresses, many new wells will likely be drilled for specific use during geological CO2 storage or temporary hydrogen storage. These wells, too, will eventually need to be plugged and abandoned safely. We investigated the sealing effectiveness of a potential alternative for Portland cement as plugging material, consisting of a mixture of metal oxides (CaO, MgO) and salt (NaCl), wherein hydration leads to significant volumetric expansions. In 11 out of 17 flow-through experiments, the apparent plug permeability fell to 10−17–10−18 m2 upon hydration, under differential pressures of 0.2–1.8 MPa. One CaO:NaCl sample could withstand up to 2 MPa differential pressure across its 5-cm-length, attaining a minimum apparent permeability of 10−21 m2. Apparent plug permeability correlated closely with the expected final solid volume fraction, i.e., the amount of void space remaining. Upscaling to realistic wellbore dimensions (10–100 m) suggests that expanding metal oxide-salt plugs set in steel casing could withstand differential pressures of 3.6–40 MPa/m. For evaporitic caprocks, this implies that metal oxide-salt plugs can be a potential alternative to conventional Portland cement, ensuring plug closure and potentially sealing within several hours. However, sealing highly depends on the amount of metal oxide available and the volume of void space requiring closure.