Fluid distribution and pore structure multifractal characteristics analysis of coal measure mudstone

It is of great significance to delineate the features of fluid migration and pore structure heterogeneity of coal measure mudstone and reveal its influencing factors for improving shale gas production and recover. Based on the response of Low-field nuclear magnetic resonance under fully brine water saturated, centrifugal at 12000 rpm and heating-treatment at 80 °C, the T2 of capillary bound fluid and clay bound fluid of coal measure mudstone ranged from 0.57 to 0.987 ms and 0.596–0.909 ms, respectively. Correspondingly, the movable fluid, capillary bound fluid, and clay bound fluid saturations were between 25.76% and 83.73%, 1.24%–15.31% and 13.42%–69.33%, respectively. And the calculated lower limits of the effective pores in coal measure mudstone were 1.84–185.84 nm. The pore structures of coal measure mudstones presented strong heterogeneity. The total organic carbon (TOC) was negatively correlated with Hurst Index (H) and Δα, while the pore connectivity and heterogeneity increased with maturity increase at over mature stage. The effects of clay minerals on pore connectivity and heterogeneity were the opposite of the effects of feldspar. Clay mineral was negatively correlated with H and Δα, resulting from the positive effects of the clay minerals on the clay bound fluid. Additionally, pore heterogeneity increased with the increase of quartz content, while the pore connectivity of mudstone did the reverse. In addition, carbonate minerals and pyrite were negatively correlated with heterogeneity, while increases in carbonate minerals and pyrite content tends to be improve the pore connectivity of coal measure mudstone. Furthermore, increases in porosity, permeability, reservoir quality index and flow zone indicator have been found to obviously reduce the pore connectivity and heterogeneity of coal measure mudstone. Meanwhile, the increases in T2 tend to weaken the pore connectivity. However, such increases may have certain enhancement effects on the heterogeneity. •The fluid distribution features of mudstone were revealed using saturation, centrifugal, and heat-treatment LF-NMR.•The multifractal characteristics of coal measure mudstone were further discussed using LF-NMR experiments.•The multifractal characteristics of the physical properties and pore structures of mudstones were discussed.