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Experiments were conducted to assess the potential impact of fractured well-cement degradation on leakage rate. Permeability was monitored while CO2-enriched reservoir-equilibrated brine was flowed at constant rate through a single fracture in a class G cement core under conditions mimicking geologic sequestration environments (temperature 60 °C, pressure 10 MPa). The results demonstrate that, at least for the conditions used in the experiment, an initial leakage in a 42 μm aperture fracture (permeability = 1.5 × 10(-10) m(2)) can be self-mitigated due to the decrease of the fracture hydraulic aperture after about 15 h. This decrease results from the development of continuous highly hydrated amorphous Si-rich alteration products at the edge of the fracture and the dense carbonation of the bulk cement that mitigate the penetration of the alteration front.

Numerical programs for simulating flow and reactive transport in porous media are essential tools for predicting reservoir properties changes triggered by CO(2) underground injection. At reservoir scale, meshed models in which equations are solved assuming that constant macroscopic properties can be defined in each cells, are widely used. However, the parameterization of the dissolution-precipitation problem and of the feedback effects of these processes on the flow field is still challenging. The problem arises from the mismatch between the scales at which averaged parameters are defined in the meshed model and the scale at which chemical reactions occur and modify the pore network geometry. In this paper we investigate the links between the dissolution mechanisms that control the porosity changes and the related changes of the reactive surface area and of the permeability. First, the reactive surface area is computed from X-ray microtomography data obtained before and after a set of dissolution experiments of pure calcite rock samples using distinctly different brine-CO(2) mixtures characterizing homogeneous to heterogeneous dissolution regimes. The results are used to validate the power law empirical model relating the reactive surface area to porosity proposed by Luquot and Gouze (2009). Second, we investigate the spatial distribution of the effective hydraulic radius and of the tortuosity, two structural parameters that control permeability, in order to explain the different porosity-permeability relationships observed for heterogeneous and homogeneous dissolution regimes. It is shown that the increase of permeability is due to the decrease of the tortuosity for homogeneous dissolution, whereas it is due to the combination of tortuosity decrease and hydraulic radius increase for heterogeneous dissolution. For the intermediate dissolution regime, identified to be the optimal regime for increasing permeability with small changes in porosity, the increase of permeability results from a large increase in the mean effective hydraulic radius of the sample., (Copyright © 2010 Elsevier B.V. All rights reserved.)

Carbonation of ultramafic rocks in geological reservoirs is, in theory, the most efficient way to trap CO2 irreversibly; however, possible feedback effects between carbonation reactions and changes in the reservoir permeability must be considered to realistically assess the efficiency and sustainability of this process. We investigated changes in the hydrodynamic properties of sintered dunite samples by means of percolation experiments, under conditions analogous to that of in situ carbonation. Our results show that carbonation efficiency is controlled by the local renewal of the reactants and the heterogeneity of the pore structure. Preferential flow zones are characterized by the formation of magnetite and of a silica-rich layer at the olivine surfaces, which eventually inhibits olivine dissolution. Conversely, sustainable olivine dissolution together with coprecipitation of magnesite, siderite, and minor Mg-TOT-phyllosilicates, occur in reduced-flow zones. Thus carbonate precipitation only decreases porosity in zones where diffusion-controlled transport is dominant. Consequently, while high flow rates will decrease the carbonation efficiency of the reservoir and low flow rates may reduce the permeability irreversibly close to the injection point, moderate injection rates will ensure a partial carbonation of the rock and maintain the reservoir permeability.

Abstract: A set of four reactive flow-through experiments at temperature T =100 °C and total pressure P =12 MPa was performed in limestone reservoir samples. By using various ranging from 0.7 to 10 MPa, these experiments mimic mass transfers occurring (1) near the injection well, where the brine is almost saturated with CO2 (i.e. P CO2 ≈ P), and (2) at increasing distances from the injection well, where the fluid displays lower values and higher divalent cation concentrations due rock dissolution along the fluid pathway. Results for P CO2 =10 MPa show non-uniform dissolution features associated with transport-controlled mass transfer, while reaction-controlled uniform dissolution is observed for P CO2 =2.5 MPa. The experiment with P CO2 =6 MPa allows investigating in detail the transition from transport- to reaction-controlled dissolution. Conversely, the experiment reproducing conditions far from the injection well (P CO2 =0.7 MPa), shows a decrease of porosity triggered by the precipitation of Mg-rich calcite. For all the dissolution experiments, the time-resolved porosity ϕ(t) can be modeled by a simple non-linear equation including parameters that characterise the dissolution regime triggered by the reactivity of the inlet fluid (measured by the Damköhler number, Da). Furthermore, all dissolution experiments display power scaling between permeability (k) and porosity (ϕ) with distinctly different scaling exponents characterising the reactivity of the fluid percolating the sample, independently from the decrease with time of the reactive surface area. It is shown also that dissolution at moderate positive values of Da seems the most efficient to increase permeability and promote a rapid spreading of the reaction front, while inducing minimal modification of the porosity in the vicinity of the CO2 injection well. These results can be used to parameterize the k–ϕ function for modeling the earliest dissolution processes occurring in the vicinity of the reaction front. [Copyright &y& Elsevier]

• Two dedicated field experiments carried to quantify the CO 2 residual trapping in-situ at 1.6 km depth. • The experimental procedures and results of these experiments are presented. • Hydraulic withdrawal test as a characterization method was robust and gave a clear signal. • Tracer experiments can give more detailed information of CO 2 residual distribution but are more complicated to carry out. Two dedicated field experiments have been carried out at the Heletz, Israel pilot CO 2 injection site. The objective has been to quantify the CO 2 residual trapping in-situ, based on two distinctly different methods. Both experiments are based on the principle of a combination of hydraulic, thermal and/or tracer tests before and after creating the residually trapped zone of CO 2 and using the difference in the responses of these tests to estimate the in-situ residual trapping. In Residual Trapping Experiment I (RTE I), carried out in autumn 2016, the main characterization test before and after the creation of the residually trapped zone were hydraulic withdrawal tests. In this experiment, the residually trapped zone was also created by fluid withdrawal, by first injecting CO 2 , then withdrawing fluids until CO 2 was at residual saturation. The second experiment, Residual Trapping Experiment II (RTE II), was carried out autumn 2017. In this experiment, the residually trapped CO 2 zone was created by CO 2 injection, followed by the injection of CO 2 -saturated water, to push away the mobile CO 2 and leave the residually trapped CO 2 behind. In this test, the main reference test carried out before and after creating the residually trapped zone was injection and recovery of gas partitioning tracer Krypton. This paper presents the experimental procedures and results of these experiments. A hydraulic withdrawal test as a characterization method was robust and gave a clear signal. Given the difficulties in injecting water optimally saturated with CO 2 , in order not to dissolve the residually trapped CO 2 or to create situations with excess mobile gas, withdrawal test may also be a generally preferable hydraulic testing method, in comparison to injection. The limitation of any hydraulic test is that it only gives an averaged value over the test section. At Heletz additional information about CO 2 distribution was obtained based on thermal measurements and by monitoring the pressure difference between the two sensors in the bolehole. The latter could be used to estimate the amount of mobile CO 2 in the well test section. Tracer experiments with gas partitioning tracers can in principle give more detailed information of CO 2 residual distribution in the reservoir than hydraulic tests can, but are also far more complicated to carry out, involving sophisticated and sensitive equipment. In the Heletz case the optimal injection of CO 2 -saturated water turned out to be difficult to achieve. Creating the zone of residual saturation by means of fluid withdrawal rather than by injecting CO 2 -saturated water seemed a more robust approach. Monitoring the gas contents in the test interval gave good guidance on the state of the system. Model interpretations of the two experiments to obtain values for CO2 residual saturation are presented in companion papers in this same Special Edition. [ABSTRACT FROM AUTHOR]

Experiments were conducted to assess the potential impact of fractured well-cement degradation on leakage rate. Permeability was monitored while CO2-enriched reservoir-equilibrated brine was flowed at constant rate through a single fracture in a class G cement core under conditions mimicking geologic sequestration environments (temperature 60 °C, pressure 10 MPa). The results demonstrate that, at least for the conditions used in the experiment, an initial leakage in a 42 μm aperture fracture (permeability = 1.5 × 10-10 m²) can be self-mitigated due to the decrease of the fracture hydraulic aperture after about 15 h. This decrease results from the development of continuous highly hydrated amorphous Si-rich alteration products at the edge of the fracture and the dense carbonation of the bulk cement that mitigate the penetration of the alteration front. [ABSTRACT FROM AUTHOR]

Carbonation of ultramafic rocks in geological reservoirs is, in theory, the most efficient way 10 trap CO2 irreversibly; however, possible feedback effects between carbonation reactions and changes in the reservoir permeability must be considered to realistically assess the efficiency and sustainability of this process. We investigated changes in the hydrodynamic properties of sintered dunite samples by means of percolation experiments, under conditions analogous to that of in situ carbonation. Our results show that carbonation efficiency is controlled by the local renewal of the reactants and the heterogeneity of the pore structure. Preferential flow zones are characterized by the formation of magnetite and of a silica-rich layer at the olivine surfaces, which eventually inhibits olivine dissolution. Conversely, sustainable olivine dissolution together with coprecipitation of magnesite, siderite, and minor Mg-TOT- phyllosilicates, occur in reduced-flow zones. Thus carbonate precipitation only decreases porosity in zones where diffusion-controlled transport is dominant. Consequently, while high flow rates will decrease the carbonation efficiency of the reservoir and low flow rates may reduce the permeability irreversibly close to the injection point, moderate injection rates will ensure a partial carbonation of the rock and maintain the reservoir permeability. [ABSTRACT FROM AUTHOR]