Reactive transport of CO2-rich fluids in simulated wellbore interfaces: Flow-through experiments on the 1-6 m length scale

- Debonded well interfaces were simulated using 1-6 m long cement-filled steel tubes.
- Flow-through of CO2-rich water at in-situ P-T decreased permeability by 2-4 orders.
- Microstructural observations revealed dissolution-dominated reaction upstream.
- Carbonate precipitation occurred downstream, sealing debonding apertures.
- Long-range reactive flow strongly enhances self-sealing potential of wellbores.

Debonding at casing-cement interfaces poses a leakage pathway risk that may compromise well integrity in CO2 storage systems. The present study addresses the effects of long-range, CO2-induced, reactive transport on the conductance of such interfacial pathways. This is done by means of reactive flow-through experiments, performed on simulated wellbore systems consisting of cement-filled steel tubes, measuring 1.2-6.0 m in length. These were prepared by casting Class G HSR Portland cement into steel tubes (inner diameter 6-8 mm), followed by curing for 6-12 months. The tubes were subsequently pressurized to permanently inflate them off the cement, creating debonded cement-steel interfaces. Four experiments were performed, at temperatures of 60-80 °C, employing flow-through of CO2-bearing fluid at mean pressures of 10-15 MPa, controlling the pressure difference at 0.12-4.8 MPa, while measuring flow-rate. The results show decreases in sample permeability of 2-4 orders, which microstructural observations reveal to be associated with downstream precipitation of calcium carbonates, possibly aided by migration of fines. This demonstrates that reactive-flow on the metre-scale significantly enhances the self-sealing potential of cement-casing interfaces relative to near-static reaction experiments. The results and method presented can be used not only to understand the long-range behaviour of annuli in wells qualitatively, but also to test reactive transport models which can then be applied at the field scale.