Leak Path Development in CO2 Wells

Abstract:

Wells have in numerous scientific works been denoted the "weak link" of safe and cost-efficient CO₂ Capture and Storage (CCS). Whether they are active or abandoned, all wells are man-made intrusions into the storage reservoir with sealing abilities depending on degradable materials like steel and cement. If dense CO₂ is allowed to expand (e.g. due to leakage) it will cool down its surroundings and cause strong thermal and mechanical loading on the wellbore. In addition, CO₂ reacts chemically with rock, cement and steel. To ensure long-term underground containment, it is therefore necessary to study how, why, where and when leakage occurs along CO₂wells.

If cement bonding to rock or casing is poor, leak paths can form already during drilling and completion of the well. In the present work, we have mapped the bonding quality of cement-rock and cement-steel interfaces – and measured their resistance towards CO₂ flow. This involved a large experimental matrix including different rocks, steels, cement types and well fluids. The bonding qualities were measured on composite cores using micro computed tomography (µ-CT), and CO₂ was flooded through the samples to determine leakage rates. These were further compared to numerical simulations of leakage through the digitalized µ-CT core data, and CO₂chemical interactions with the materials were mapped using electron microscopy. We also present a new laboratory set-up for measuring how well integrity is affected by downhole temperature variations – and we showcase some initial results.

Our work concludes that leak path development in CO₂ wells depends critically on the drilling fluids and presflushes/spacers chosen already during drilling and completion of a well. Fluid films residing on rock and casing surfaces strongly degrade the quality of cement bonding. The operation of the well is also important, as even slight thermal cycling (between 10°C and 95°C on casing) leads to significant de-bonding of the annular cement.