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Efficient computational models are desirable for simulation of large-scale geological CO2 sequestration. Vertically integrated models, which take advantage of dimension reduction, offer one type of computationally efficient model. The dimension reduction is usually achieved by vertical integration based on the vertical equilibrium (VE) assumption, which assumes that CO2 and brine segregate rapidly in the vertical due to strong buoyancy and quickly reach pressure equilibrium. However, the validity of the VE assumption requires small time scales of fluid segregation, which may not always be fulfilled, especially for heterogeneous geological formations with low vertical permeability. Recently, Guo et al. (2014a) developed a multiscale vertically integrated model, referred to as the dynamic reconstruction (DR) model, that relaxes the VE assumption by including the vertical two-phase flow dynamics of CO2 and brine as fine-scale one-dimensional problems in the vertical direction. Although the VE assumption can be relaxed, that model was limited to homogeneous geological formations. Here we extend the dynamic reconstruction model for layered heterogeneous formations, which is of much more practical interest for saline aquifers in sedimentary basins. We develop a new coarse-scale pressure equation to couple the different coarse-scale (vertically integrated) layers, and use the fine-scale dynamic reconstruction algorithm in Guo et al. (2014a) within each individual layer. Together, these form a multiscale multilayer dynamic reconstruction algorithm. Simulation results of the CO2 plume from the new model are in excellent agreement with full three-dimensional models, with the new algorithm being much more computationally efficient than conventional full three-dimensional models. © 2016. American Geophysical Union. All Rights Reserved.

Serpentine minerals serve as a Mg donor in carbon capture and storage by mineralisation (CCSM). The acid-treatment of nine comprehensively-examined serpentine polymorphs and polytypes, and the subsequent microanalysis of their post-test residues highlighted several aspects of great importance to the choice of the optimal feed material for CCSM. Compelling evidence for the non-uniformity of serpentine mineral performance was revealed, and the following order of increasing Mg extraction efficiency after three hours of acid-leaching was established: Al-bearing polygonal serpentine (<5%) ≤ Al-bearing lizardite 1T (≈5%) < antigorite (24-29%) < well-ordered lizardite 2H1 (≈65%) ≤ Al-poor lizardite 1T (≈68%) < chrysotile (≈70%) < poorly-ordered lizardite 2H1 (≈80%) < nanotubular chrysotile (≈85%).It was recognised that the Mg extraction efficiency of the minerals depended greatly on the intrinsic properties of crystal structure, chemistry and rock microtexture. On this basis, antigorite and Al-bearing well-ordered lizardite were rejected as potential feedstock material whereas any chrysotile, non-aluminous, widely spaced lizardite and/or disordered serpentine were recommended.The formation of peripheral siliceous layers, tens of microns thick, was not universal and depended greatly upon the intrinsic microtexture of the leached particles. This study provides the first comprehensive investigation of nine, carefully-selected serpentine minerals, covering most varieties and polytypes, under the same experimental conditions. We focused on material characterization and the identification of the intrinsic properties of the minerals that affect particle's reactivity. It can therefore serve as a generic basis for any acid-based CCSM pre-treatment. © 2016 The Authors.

In this proposed CO2 injection system, brine is extracted from the target storage aquifer by means of a lateral horizontal completion located near the top of the formation. It should be noted that the brine is not lifted to the surface. An Electrical Submersible Pump (ESP) is used to extract the brine and boost its pressure, before it is mixed with CO2 that is injected down the vertical section of the well. The mixing takes place in the vertical section of the well below the upper lateral. The CO2–brine mix is then injected into the same formation through a lower lateral. A down-hole tool would be used to maximise agitation and contact area between CO2 and brine in the vertical mixing section of the well, which may be tens to hundreds of metres long, depending on the thickness of the formation. The advantages of this method are that there is little overall pressure increase, because CO2 is mixed with brine extracted from the formation, and also the extracted brine is already at high pressure when it is mixed with the CO2, greatly increasing the solubility of CO2 and reducing the volume of brine required. Energy is not expended lifting the brine to surface nor is there any concern about handling large volumes of acidic brine in the surface equipment. In this study, in addition to the concept of the down-hole mixing (DHM) method which is presented, the application of the DHM method in a hypothetical storage site (Lincolnshire—Smith et al., 2012) is also examined. The calculations are performed to identify the optimum rates of water extraction and injection of dissolved CO2 in brine. © 2016

The injection of CO2 into a depleted reservoir will alter the pore pressure, which if sufficiently perturbed could result in fault reactivation. This paper presents an experimental study of fault reactivation potential in fully saturated kaolinite and Ball Clay fault gouges. Clear differences were observed in fault reactivation pressure when water was injected, with the addition of mica/illite in Ball Clay seen to reduce the pressure necessary for reactivation. Slip occurred once pore-pressure within the gouge was sufficient to overcome the normal stress acting on the fault. During gas injection localised dilatant pathways are formed with approximately only 15% of the fault observing an elevated gas pressure. This localisation is insufficient to overcome normal stress and so reactivation is not initiated. Therefore faults are more likely to conduct gas than to reactivate. The Mohr approach of assessing fault reactivity potential gave mixed results. Hydro-mechanical coupling, saturation state, mineralogical composition and time-dependent features of the clay require inclusion in this approach otherwise experiments that are predicted to be stable result in fault reactivation. © 2016