NMR T2 Profile Reveals Connectivity-Controlled Permeability Breakthrough during Pore-Scale Carbonate Dissolution

Carbonate rock formations constitute common hydrocarbon reservoirs and are often considered as candidates for geological CO₂ storage, where acid-driven carbonate dissolution may occur in the near-wellbore region. Calcite dissolution can substantially reconfigure pore networks, altering permeability and influencing storage efficiency and long-term containment integrity. While carbonate dissolution has been extensively studied experimentally and numerically, its detection and characterization using non-invasive monitoring tools remain challenging. Nuclear magnetic resonance (NMR) is a particularly promising tool as it is sensitive to pore geometry and fluid distribution. However, a quantitative framework that links regime-dependent pore-scale dissolution patterns to NMR observables remains underdeveloped. In this work, we establish such structure–signal mapping by coupling pore scale reactive transport simulations of calcite dissolution with forward modeling of low-field NMR responses, generating synthetic observables from dynamically evolving pore geometries due to calcite dissolution. By varying the relative timescales governing advection, diffusion, and surface reaction rates, we analyze the evolution of three representative dissolution patterns: uniform face dissolution, conical channeling dissolution, and wormholing dissolution. To capture the spatial heterogeneity of these features, we segment the pore geometry along the flow axis and derive an NMR T₂ relaxation time distribution for each section, constructing flow-direction T₂ profiles. In contrast, bulk T₂ distributions derived from the entire pore volume tend to average out the spatial heterogeneity of dissolution patterns. Furthermore, to capture the propagation of reaction fronts and characterize the permeability of emerging channels, we formulate specific NMR-based metrics: a pore-enlargement index E_i(t), a heterogeneity index H(t), and a connectivity index C(t). Dissolution breakthrough, defined by k/k₀ ≥ 10, occurs at PV₁₀ ≈ 314 for face dissolution, 138 for channeling, and 144 for wormholing. While H(t) consistently evolves non-monotonically, breakthrough is governed by the emergence and strengthening of an inlet-to-outlet pathway. Accordingly, C(t) closely tracks breakthrough during channeling, whereas in wormholing it indicates early connectivity without an immediate permeability increase. Our weighted pore network connectivity by cumulative enlargement yields a single metric that correlates with permeability growth across regimes. This structure–signal framework provides a workflow for using spatially distributed NMR signals to identify pathway formation and provide an early indication of permeability surges. The framework for mapping structures to signals enhances the interpretation of NMR signals in dissolution reactive settings and provides a quantitative foundation for interpreting NMR monitoring signals and informing risk assessment for geological CO₂ storage in settings where carbonate dissolution may alter flow pathways.