Abstract
Achieving zonal isolation along wellbores is essential for upholding the containment integrity of subsurface reservoirs and preventing fluid seepage to the environment. The sealing performance of Portland cements conventionally used to create barriers can be severely compromised by defects like fractures or micro-annuli along casing–cement–rock interfaces. A possible remediation method would be to circulate reactive fluids through compromised cement sections and induce defect clogging via mineral precipitation. We assess the sealing potential of two prospective fluids: sodium bicarbonate and sodium silicate solutions. Reactive flow-through experiments were conducted on 6-m-long cemented steel tubes, bearing ~20-μm-wide micro-annuli, at 50 °C and 0.3–6 MPa fluid pressure. For the sodium bicarbonate solution (90 g/kg-H2O), reactive flow yielded only a minor reduction in permeability, with values remaining within one order. Injection of sodium silicate solution (37.1 wt.%, SiO2:Na2O molar ratio M= 2.57) resulted in a large decrease in flow rate, effectively reaching the setup’s lower measurement limit in hours. However, this strong sealing effect can almost certainly be attributed to gelation of the fluid through polymerisation, rather than defect clogging via mineral precipitation. For both fluids investigated, the extent of solids precipitation resulting from single-phase injection was less than anticipated. This shortfall is attributed to ineffective/insufficient liberation of Ca-ions from the alkaline phases in the cement.
Highlights
Ensuring wellbore integrity and long-term zonal isolation is of utmost importance to many of our activities in the subsurface, such as geothermal energy production [1,2], temporary underground storage of energy carriers, such as hydrogen or compressed air [3], geological sequestration of wastes like CO2 [4], and oil and gas production [5,6,7,8].While designed to have a low permeability, the Portland cements, typically used to create hydraulic barriers against unwanted fluid seepage along wellbores, may become impaired by permeable defects, such as fractures within the cement or debonding micro-annuli along its interfaces with the casing pipe or rock formations
The results obtained from our cemented steel coil samples showed that, while initial flushing with water did not affect the samples’ transport properties [87], subsequent introduction of a sodium silicate solution into sample SIL-1 caused a decrease in flow rate of several orders, effectively reaching the lower detection limit of the flow setup in a few hours (Figure 5)
The impact of sodium bicarbonate and sodium silicate solutions has been assessed in reactive flow-through experiments performed on 6-m-long cemented steel tube samples with about 20-μm-wide interfacial debonding defects
Summary
While designed to have a low permeability, the Portland cements, typically used to create hydraulic barriers against unwanted fluid seepage along wellbores, may become impaired by permeable defects, such as fractures within the cement or debonding micro-annuli along its interfaces with the casing pipe or rock formations. Such defects can have various causes, including (i) ineffective cement placement [9,10,11], (ii) autogenous shrinkage and debonding upon setting of the cement [12,13,14,15,16,17,18], or (iii) mechanical damage sustained by the set cement [19,20,21,22,23,24,25,26]. If the seepage poses a safety or environmental risk in terms of its surface pressure, volume flux or chemical composition, remedial action will generally be required
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