Reaction and transport in wellbore interfaces under CO2 storage conditions: Experiments simulating debonded cement-casing interfaces

Debonding-defects at the interfaces between wellbore casing and cement are widely recognized as providing potential pathways for CO2 escape from geological storage systems. This study addresses how chemical reaction between CO2, cement and steel may affect the transport properties of such defects under near-static conditions, representative for early stages in leakage pathway development, prior to formation of a fully connected defect network. Debonded cement–steel interfaces were simulated by constructing composite samples, containing a spacer-imposed gap. These were subjected to batch reaction with CO2 and a variety of aqueous solutions, in multiple consecutive runs (6 × 5 days plus 1 × 42 days). The experiments were conducted at 80 °C and 14 MPa applied CO2-pressure. Permeability was measured after each run. Two ranges in gap-width were investigated, namely 50–120 μm (SA-samples) and 270–350 μm (LA-samples). Reaction-induced permeability changes were less than 1 order in all samples, and occurred in the early stage of testing. SEM study showed Fe-carbonate scale films developed on both cement and steel. Calcium carbonates precipitated on the cement beneath the gap-spacers, where corrosion scale did not form. We infer that corrosion scale formation on the cement wall inhibited both CaCO3 precipitation and healing effects previously observed in fractured cement