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Heletz sandstone (Israel) reactivity is studied by means of CO 2 -rich brine percolation experiments and modeling. Experiments are performed at in situ temperature and pressure conditions with two distinctly different injection flow rates and for two different brine compositions: the genuine Heletz brine and a similar brine but in equilibrium with gypsum. The Heletz sandstone is composed of quartz, dolomite, ankerite, K-feldspar, clay minerals and minor amounts of gypsum and pyrite. The main reaction is the dissolution of the carbonates which is fastest for the highest flow rate injection and for the genuine Heletz brine. Permeability increases during the five percolation experiments, regardless of flow rate and brine type. This increase in permeability is associated to a decrease in porosity during the gypsum-equilibrated brine experiments due to gypsum precipitation. The precipitation of secondary minerals (kaolinite, muscovite and smectite) is mainly controlled by the dissolution of K-feldspar and largest at low flow rate injection, indicating that these precipitation processes are diffusion-controlled and localized in immobile zones where favorable chemical micro-environments exist. These processes are quantified by means of a reactive transport model. These results indicated that the CO 2 injection at the Heletz site should trigger both permeability increase in interface between brine and CO 2 , which may extend over a large extension and where mixing processes are very active, especially near the well under intermittent injection conditions. [ABSTRACT FROM AUTHOR]

• 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]