Developing NMR/MRI Techniques for Characterisation of Reactive Transport in Porous Media
Reactive flow in porous media is a complex transport phenomenon which involves the simultaneous flow and reaction of a fluid in a porous medium and plays a critical role in improving our understanding of carbon sequestration. When CO2 is introduced into a storage site, which might be a hosting rock formation, high temperatures and pressures result in the production of a supercritical phase, which can then react with the existing brine to generate carbonic acid. This process is defined as reactive flow and may lead to both dissolution and/or precipitation. Major effort has been invested in researching the former since it may induce significant structural changes in the formation and has previously been examined for acid stimulation treatments in the oil and gas industry. Various dissolving regimes may be encountered depending on the reservoir characteristics and the physical conditions of the hosted fluid, ranging from uniform to *wormholing*, which is characterised by the formation of dissolution channels where the majority of the fluid transfers to. X-ray microtomography is widely adopted to study this kind of systems, since it can provide information about the rock structure resolved at the micron-scale. This is usually coupled with numerical simulations in order to predict the behaviour of the reactive fluid. Nuclear Magnetic Resonance (NMR) methods are a powerful tool for characterising reactive flow because, in addition to imaging, they can give a range of information such as transport characteristics and pore-size distributions. This work attempts to characterise dissolution in a reactive flow system using NMR techniques, which is accomplished in a variety of ways. First an innovative experimental protocol is defined to dynamically study the dissolution of carbonate rocks of various heterogeneities at the core-scale, allowing for the detection of major structural changes as well as temporally-resolved direct measurements of the system transport properties via the use of spatially resolved propagators. This was accomplished through optimisation of Compressed Sensing methods, a set of image reconstruction algorithms that allow for faster acquisition times using under-sampling strategies. Second, this work presents the development of novel experimental techniques based on PFG-NMR to directly measure reactive flow system’s flow field, especially in dominant wormhole regimes which are challenging to study due to extremely heterogeneous range of velocities experienced. These approaches were used to demonstrate flow mechanisms that were not previously experimentally verified and to provide experimental validation on reactive flow modelling. Moreover, it has been possible to study transport properties in the form of velocity maps and propagators at different dissolution regimes as well as at different scales with the acquisition, for the first time, of high-resolution co-registered MRI/µ/CT flow maps. Structure-flow correlations were used in particular to quantify the primary changes in the flow field during the dissolution process, and both Ketton and Estaillades experienced a focusing of velocities into high-flowing channels with subsequent stagnation of the other regions. Finally, the developed NMR/MRI techniques are used to study reactive flow in realistic systems. This is achieved by applying chemical selectivity to assess transport properties of multiphase reactive flow systems and by working towards the development of a rig to study the behaviour of CO2 flowing through rock core plugs.
MRI, Reactive Flow, Wormhole dissolution
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