Predicting Fluid Flow Regime, Permeability, and Diffusivity in Mudrocks from Multiscale Pore Characterisation

Amirsaman Rezaeyan,Timo Seemann,Leon Leu,Vitaliy Pipich,Andreas Busch,Lester C Barnsley,Jingsheng Ma,Gernot Rother

doi:10.1007/s11242-021-01717-9

Abstract

In geoenergy applications, mudrocks prevent fluids to leak from temporary (H2, CH4) or permanent (CO2, radioactive waste) storage/disposal sites and serve as a source and reservoir for unconventional oil and gas. Understanding transport properties integrated with dominant fluid flow mechanisms in mudrocks is essential to better predict the performance of mudrocks within these applications. In this study, small-angle neutron scattering (SANS) experiments were conducted on 71 samples from 13 different sets of mudrocks across the globe to capture the pore structure of nearly the full pore size spectrum (2 nm–5 μm). We develop fractal models to predict transport properties (permeability and diffusivity) based on the SANS-derived pore size distributions. The results indicate that transport phenomena in mudrocks are intrinsically pore size-dependent. Depending on hydrostatic pore pressures, transition flow develops in micropores, slip flow in meso- and macropores, and continuum flow in larger macropores. Fluid flow regimes progress towards larger pore sizes during reservoir depletion or smaller pore sizes during fluid storage, so when pressure is decreased or increased, respectively. Capturing the heterogeneity of mudrocks by considering fractal dimension and tortuosity fractal dimension for defined pore size ranges, fractal models integrate apparent permeability with slip flow, Darcy permeability with continuum flow, and gas diffusivity with diffusion flow in the matrix. This new model of pore size-dependent transport and integrated transport properties using fractal models yields a systematic approach that can also inform multiscale multi-physics models to better understand fluid flow and transport phenomena in mudrocks on the reservoir and basin scale.

Highlights

Technologies utilising the subsurface are impacted by the presence and properties of mudrocks
Given that transport phenomena are pore size-dependent, we show, for the first time, how different fluid flow regimes are controlled by different pore size ranges at different reservoir depths and how they are related to total porosity
Based on the pore structure information obtained from small-angle neutron scattering (SANS), we developed three fractal models to predict: (i) Darcy permeability for continuum flow, (ii) apparent permeability for slip flow regimes and (iii) effective diffusion coefficient for diffusional flow regimes in mudrocks

Summary

Introduction

Technologies utilising the subsurface are impacted by the presence and properties of mudrocks. This includes the energy industry evaluating top seals for hydrocarbons or the properties of shale gas reservoirs, and applications relating to the energy transition like permanent storage of C O2, or intermittent storage of H 2 or C H4 (Amann-Hildenbrand et al 2013; Beckingham and Winningham 2020; Busch and Kampman 2018; Ilgen et al 2017). Our understanding of how fluid flow regimes and transport properties (e.g. permeability and diffusivity) are controlled by the pore structure in mudrocks across different scales is limited. Pore sizes generally range over several orders of magnitude, including macropores > 50 nm, mesopores 2–50 nm, and micropores < 2 nm according to the International Union of Pure and Applied Chemistry (IUPAC) pore size classification (Sing et al 1985)

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Discussion

Conclusion