The Dynamic Evolution of Permeability in Compacting Carbonates: Phase Transition and Critical Points
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The Dynamic Evolution of Permeability in Compacting Carbonates: Phase Transition and Critical Points Xiao Chen1 · Klaus Regenauer‑Lieb1 · Adelina Lv1 · Manman Hu1,2 · Hamid Roshan1 Received: 5 March 2020 / Accepted: 16 October 2020 © Springer Nature B.V. 2020
Abstract Mechanical damage and resultant permeability evolution during compaction of highly porous reservoir rocks have strong implications on the extraction of mineral and energy resources. Laboratory Experiments can be performed to quantify this effect; however, the effect of size on these processes and the information they provide need to be evaluated before any conclusion can be drawn. As part of this study, conventional triaxial compression tests under different confining pressures were carried out on large samples (30 mm diameter and 60 mm length). These experiments were compared to the same setup for small samples with 12.7 mm diameter and 25.4 mm length which allowed monitoring of the pore structure changes through the use of an X-ray transparent triaxial cell at constant confining pressure. Both scales showed a similar mechanical response. The large-scale experiments were used to investigate the transition from brittle to ductile deformation, and the small-scale experiments allowed detailed investigation of the microstructural changes affecting the permeability evolution. The permeabilities of the specimens were continually measured during the triaxial loading at both scales. At defined increasing axial strain levels, the small sample was imaged using X-ray computed tomography (XRCT) and internal structural changes were mapped. A series of digital rock analysis techniques and Pore Network Modelling allowed accurate analysis of the evolution of the microstructure and its effect on permeability evolution using Pore Network Models. An XRCT-based, microstructurally enriched, continuum model successfully describes the permeability evolution measured during triaxial testing. Self-organized criticality of the propagating front of compaction was also shown by R2 values > 0.95 for a double Pareto fractal scaling law. Both approaches, as well as the macroscale experiments, confirmed a phase change in permeability at ~ 5% axial strain which provided a solid basis for microstructurally enriched assessment of the dynamic permeability. Keywords Compaction bands · Permeability and porosity · Phase transition · Selforganization · Fractals and multifractals · Pore network model
* Klaus Regenauer‑Lieb [email protected] 1
School of Mineral and Energy Resources Engineering, University of New South Wales, Sydney 2052, NSW, Australia
2
Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China
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List of symbols kg Gas permeability kl Liquid permeability Q Gas flow rate μ Dynamic viscosity coefficient of the gas L Length of the specimen A Cross-sectional area of the specimen Pout Outlet pressure Pin Inlet pressure q Differential stress p Effective mean stress 𝜀a Axial strain 𝜀r
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