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Keywords:

  • CO2 storage;
  • depleted gas field;
  • drying out;
  • intentional salt clogging;
  • salt precipitation;
  • well abandonment

Abstract

Well abandonment in the context of CO2 storage operations demands a mitigation strategy for CO2 leakage along the wellbore. To prevent possible CO2 transport toward the surface and to protect the wellbore material from contact with acid brine, we propose forming a salt seal around the wellbore at reservoir level, after CO2 injection into a depleted gas field. The mechanism of water evaporation into dry gas and subsequent salting-out of the dissolved halite is a well-known issue in hydrocarbon production. We propose alternating injection of brine and CO2 to facilitate intentional salt clogging of the reservoir. Salt clogging was studied with TOUGH2, simu-lating multiple cycles of brine and CO2 injection. We developed two near-well reservoir models – a homogeneous 2D model and a more complex heterogeneous 2D model – using certain characteristics of the K12-B depleted gas field. The homogeneous 2D model yields a 50-cm thick salt bank around the well and complete permeability impairment after eight cycles of brine-CO2 injection. Addition of vertical permeability heterogeneities causes variation in the lateral extent of salt precipitation and hence vertical discontinuities in the salt bank. A carefully designed injection strategy – with more injection cycles and lower injection rates – improves the vertical sealing. Predictions of the amount of required clogging for effective sealing would benefit from more accurate porosity-permeability relationships and insights in the long-term stability of the salt bank regarding re-dissolution. We conclude that alternating brine-CO2 injection could be a promising method for intentional salt-clogging of the near-wellbore area. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd