Re-evaluating the accurate multiphase water distribution in coals: Unifying experiment and theory

Abstract

An in-depth understanding of multiphase water (adsorbed and free phase) distribution in coals is critical to the coalbed methane engineering application. However, the multiphase water quantitative characterization method, together with its microscopic mechanism still remains unavailable. This article proposed an innovative and more accurate methodology to characterize the multiphase water in coals by combining theory and experiment, which is applicable not only in water-saturated coals but also in the sub-saturated state. Firstly, traditional nuclear magnetic resonance (NMR)-centrifugal experiments were performed for six coals with strikingly different ranks to evaluate the experimental multiphase water distributions. Results show that the free water amounts significantly increase with increasing centrifugal pressure before stabilizing to a limiting value, similar to a Langmuir-like equation. An innovative modified multiphase water analysis model was established by integrating the centrifugation and typical pore structure parameters with the adsorption ratio equation. Results indicate that the adsorbed water density and mean adsorption thickness value as 1.59 g/cm3 and 0.63 nm, respectively. The estimated optimal surface relaxivities of coals range from 2.2 nm/ms to 5.2 nm/ms. The experimental estimated multiphase water NMR spectra are inconsistent with those from a theoretical model, leading to inaccurate analysis in the laboratory and field logging based only on experimental results. The multiphase water theoretical calculation method proposed in this article can be applied for quantitatively characterizing both the adsorbed/free water contents and its distributions in (un) saturated coals.