Salt transport experiments in fractured media
Creators: name: Daher, Ibrahim
Publisher: name: Imperial College London
Year: 2016

description: During the sequestration of CO2 into down-hole rock formations, salt precipitation may occur due to the drying of the formation brine if the injected CO2 is dry. This can negatively affect the performance of injection wells and can even lead to well clogging, which is a serious risk for such operations. Further, the salt deposition can alter the flow of the CO2 in the formation altering the storage capacity. Therefore, it is very important to explore the effect on CCS (Carbon Capture and Storage) process of drying out and salt precipitation during CO2 injection. This study is focused on CCS in fractured aquifers, which has received less attention than their un-fractured counterparts and particularly, the flow impairment associated with salt precipitation during the injection of dry CO2. When CO2 is injected into a conductive fracture network, the brine will rapidly be displaced from the fractures near the point of injection and the subsequent mass transfer between the matrix and the fracture; orthogonal to the flow direction in the fracture, is the major target of the project. The dry-out that occurs due to the evaporation of water from the brine filled region of the matrix into the under-saturated CO2 filling the fracture can cause deposition of salt in the matrix or the fracture, locally reducing permeability. This thesis reports on an investigation of the evaporative drying kinetics and salt precipitation using a combination of gravimetric and X-ray µ-CT techniques to measure the water and brine saturation, salt precipitation and distribution of salt deposition in two rocks; a sandstone, Bentheimer and a carbonate, Ketton. Based on the experimental results for de-ionised water, two main regimes occur during the dry-out process: a capillarity driven regime which seems to be dominant for most of the dry-out process in the experiments, during which evaporation happens only at the surface of the fracture, followed by a diffusion limited regime after the liquid bridge to the surface breaks and pores near the surface become dry for the first time. In pure water, this results in an almost constant evaporation flux in the first regime followed by a mass loss that is linear when plotted against the square root of time. The experiments with brine were initially similar with an evaporative flux almost constant with time. However, a short time into the process the evaporative flux started to decrease approximately linearly with the square root of time, following the deposition of salt at the surface of the fracture. At the end of gravimetric dry-out tests, µ-CT images were obtained showing that salt was mainly precipitated at the surface of the sample; however, relatively small amount of salt was observed precipitated in the interior of the sample. The pore structure of the precipitated salt at the end of the dry-out tests maintained connectivity between the surface of the deposit and the rock matrix. Dynamic µ-CT imaging of Bentheimer during brine drying showed that during the early stage of evaporation, salt was continuously deposited at the surface of the matrix. During this stage in the evaporation of brine, advection dominates the transport of dissolved salt, indicated by a large Peclet number, and this resulted in an increased salt concentration very local to the site of evaporation. The ongoing formation of an efflorescence therefore, is evidence for the continuity of the liquid connections to the outside of the sample, despite the evaporation becoming linear against the square root of time. Unfortunately, the liquid bridges to the surface were too small to be seen directly in the µ-CT imaging. The volume of precipitated salt increased with time and this resulted in a change in the pore structure at the surface of the sample structure, consequently reducing the brine-drying rate. However, as the salt deposition and therefore the location of the evaporation continued to be at the exposed surface, vapour diffusion cannot account for the mass lost by evaporation becoming linear in the square root time as is usually stated. Some other mechanism must account for the observed behaviour and we speculate that the surface area for evaporation was reduced by the appearance of dry patched on the surface. At a very late stage of evaporation, it was observed that no further salt precipitated at the surface of the sample; and subsequently, salt precipitation progressed with time towards the interior of the sample core with small amount of salt. At this stage the liquid connection to the surface must finally have broken and a true diffusion controlled process occurred. In the limited sample size used in this study, this mechanism accounted for only a small fraction of the total salt deposited. From permeability measurements before and after the complete drying of the samples, it was demonstrated that the permeability of Bentheimer was reduced by 81 % from 2.2 D to 0.41 D by the salt deposition. However, Lattice Boltzmann simulations of single phase permeability in the segmented µ-CT images, showed a reduction by 54% from 2.27 D to 1.28 D at 6 µm scanned voxel resolution and 54% from 2.7D to 1.48D at 15um scanned voxel resolution. From these results, it can be concluded that salt precipitation during the injection of CO2 into a fractured porous medial result in a significant reduction in formation permeability, but connectivity between the matrix and the fracture is maintained.