Brief description of the Data: This folder contains data for a study on reactive sealing of microannuli along wellbore casing-cement interfaces. The data set consists of four Microsoft Excel Worksheets, containing volume, flow rate, and pressure data with time for reference and reactive flow measurements on two samples, denoted BC-1 and SIL-1 in the aforementioned study. This document provides basic descriptions of the samples, experimental procedures and data processing. For more detailed information, the reader is referred to the methods section of the main manuscript. Summary of the Study: Achieving zonal isolation along wellbores is essential for upholding the containment integrity of subsurface reservoirs and preventing fluid seepage to the environment. The sealing integrity of the Portland cements conventionally used to create hydraulic barriers can be severely compromised by the presence of defects, such as fractures or micro-annuli along casing-cement and cement-rock interfaces. A possible remediation method would be to circulate reactive fluids through the compromised cement sections and induce defect clogging via mineral precipitation. Here, we assess the sealing potential of two prospective fluids, namely sodium bicarbonate and sodium silicate-based solutions. Reactive flow-through permeametry experiments were performed on 6 m-long cemented steel tubes, bearing debonding micro-annuli of ~20 μm wide. The experiments were conducted at 50 °C and fluid pressures of up to 6 MPa. For sodium bicarbonate solution (90 g/kg-H2O concentration), reactive flow yielded only a minor reduction in the equivalent permeability of the wellbore sample, with values remaining within one order. Injection of sodium silicate solution (37.1 wt.% concentration, SiO2:Na2O molar ratio M= 2.57) resulted in a large decrease in flow rate, effectively reaching the lower measurement limit of the experimental setup within 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.