2.1. Description of Samples
 The samples used for the injection experiments are all carbonate rocks spanning different microstructures and/or mineralogical compositions. Samples range from white, chalky mudstones, to brownish calcite limestones stained with residual oil, to calcite/dolomite limestones. Before and after injection, both the microstructure and transport properties are fully characterized. Sample characterization also includes Helium porosity and Klinkenberg-corrected nitrogen permeability measurements, with an uncertainty of ±1% and ±2%, respectively. Before injection, the porosity and permeability of the samples range from 15.4% to 29.8% and from 5 to 255 mD, respectively.
2.2. Experimental Device and Methodology
 The experimental device consists of a hydrostatic pressure vessel, a core holder, and a fluid-injection system. Core samples of about 1 inch in length and diameter are jacketed with rubber tubing and loaded into the pressure vessel under a constant confining pressure of 1.1 MPa or 8MPa.
 The core holder is equipped with three linear potentiometers measuring the change in length of the sample as a function of stress (with a maximum uncertainty of ±1%). This enables us to correct porosity from the resulting volume change of the sample due to pressure. This change in porosity, due to mechanical compaction alone, is given by:
where is the initial sample porosity, V0, sample is the initial volume of the sample, ΔV is the volume change due to pressure and V0, matrix is the initial volume of the matrix.
 The core holder is also equipped with two stainless steel end-cups having a pore-fluid inlet/outlet system, which allows the injection of the pore fluid, an acidic solution of aqueous CO2 with a pH of about 3.2, which is monitored over the experiment. The downstream flow is maintained at 4 to 8 mL/min. During the experiment, the injected volume (Vnorm) is normalized with respect to the initial pore volume of the sample. The fluid is regularly sampled at the outlet to measure pH and calcium content using the complexometric titration method. We use a digital titrator (HACH LANGE 16900) and a commercial 0.08M EDTA (Ethylenediaminetetraacetic acid) solution as titrant. The uncertainty of pH measurements is ±0.1 pH unit, and the uncertainty of calcium concentration is less than ±3%.
 The calcium concentration in the injected fluid being equal to zero, we used the calcium concentrations measured in the output fluid to monitor the total mass of carbonate material (grains, matrix and/or cement) dissolved and transported out of the sample while injecting the carbonated water. We then translated the dissolved mass into the corresponding change in porosity. Since the initial volume is used in this calculation, this represents the change in porosity due to the effect of chemical dissolution alone. The change in porosity over the period [ti-1, ti] between two fluid samplings is thus given by:
where Δmi is the change in mass over the period [ti-1, ti], i is the mean calcium concentration over the whole period, ΔVf,i is the volume of the fluid injected over that period, V0,sample is the initial volume of the sample, and ρgrain is the initial density of the minerals composing the rock. This value is then compared with that measured by He-porosimetry at the end of the experiment. The total change in porosity over the experiment is thus the sum of the two contributions of porosity change (equations (1) and (2)).
 The stainless steel end-cups also incorporate a stack of lead-zirconate-titanate (PZT) piezoelectric crystals of frequency 1MHz and 0.7MH for the measurement of P- and S-wave velocities, respectively. Ultrasonic velocities are measured by using a pulse-transmission technique [Birch, 1960] with a high-viscosity bonding medium that ensures good coupling between the sample and the end-cups. The uncertainty in measuring VP and VS is estimated to be about ±1%. P- and S-wave velocities are first measured on the dry sample before injection and then monitored as the injection proceeds under fully saturated conditions. To monitor velocities, we stopped the flow and took the ultrasonic measurements under a pore-fluid pressure of 1.0MPa. We also monitored the variation in the elastic properties induced on the rock frame alone after drying the samples and before proceeding with the next injection. We dried the sample within the vessel for about 10 hours by alternating the injection of warm, dry air with that of dry helium. After the last drying process, the sample is removed from the pressure vessel, weighed for the calculation of the residual saturation, and dried in an oven for 48 hours at 70°C. Velocities in “oven dry” samples were only 3 to 5% lower than in the “in situ dry” ones.
 SEM images of both the tops and bottoms of the samples are taken before and after the injection experiments. Images are acquired at different magnifications using the Variable Pressure (VP-SEM) mode of the Hitachi 3400N Scanning Electron Microscope. It allows charge-up-free observation with good resolution without coating the samples surfaces, which would have affected the samples surface reactivity. We use a beam intensity of 15 kV and a vacuum pressure of 40 Pa. A metal wire is glued onto the surface for the purpose of sample registration and easy localization of the imaged spots. This procedure proved necessary because of dramatic changes in the microstructure.