ALBERT

Description: The electrical rock conductivity is a sensitive indicator for carbon dioxide (CO 2 ) injection and migration processes. For a reliable balancing of the free CO 2 in pore space with petrophysical models such as Archie's law or for the detection of migrating CO 2 , detailed knowledge of the pore water conductivity during interaction with CO 2 is essential but not available yet. Contrary to common assumptions, pore water conductivity cannot be assumed constant since CO 2 is a reactive gas that dissolves into the pore water in large amounts and provides additional charge carriers due to the dissociation of carbonic acid. We consequently carried out systematic laboratory experiments to quantify and analyse the changes in saline pore water conductivity caused by CO 2 at thermodynamic equilibrium. Electrical conductivity is measured on pore water samples for pressures up to 30 MPa and temperatures up to 80 °C. The parameter range covers the gaseous, liquid and supercritical state of the CO 2 involved. Pore water salinities from 0.006 up to 57.27 g L –1 sodium chloride were investigated as well as selective other ion species. At the same time, the CO 2 concentration in the salt solution was determined by a wet-chemical procedure. A two-regime behaviour appears: for small salinities, we observe an increase of up to more than factor 3 in the electrical pore water conductivity, which strongly depends on the solution salinity (low-salinity regime). This is an expected behaviour, since the additional ions originating from the dissociation of carbonic acid positively contribute to the solution conductivity. However, when increasing salinities are considered this effect is completely diminished. For highly saline solutions, the increased mutual impeding causes the mobility of all ions to decrease, which may result in a significant reduction of conductivity by up to 15 per cent despite the added CO 2 (high-salinity regime). We present the data set covering the pressure, temperature, salinity and ion species dependence of the CO 2 effect. Furthermore, the observations are analysed and predicted with a semi-analytical formulation for the electrical pore water conductivity taking into account the species’ interactions. For the applicability of our results in practice of exploration and monitoring, we additionally provide a purely empirical formulation to compute the impact of CO 2 on pore water conductivity at equilibrium which only requires the input of pressure, temperature and salinity information.

Description: The spectral complex conductivity of a water-bearing sand during interaction with carbon dioxide (CO 2 ) is influenced by multiple, simultaneous processes. These processes include partial saturation due to the replacement of conductive pore water with CO 2 and chemical interaction of the reactive CO 2 with the bulk fluid and the grain-water interface. We present a laboratory study on the spectral induced polarization of water-bearing sands during exposure to and flow-through by CO 2 . Conductivity spectra were measured successfully at pressures up to 30 MPa and 80 °C during active flow and at steady-state conditions concentrating on the frequency range between 0.0014 and 100 Hz. The frequency range between 0.1 and 100 Hz turned out to be most indicative for potential monitoring applications. The presented data show that the impact of CO 2 on the electrolytic conductivity may be covered by a model for pore-water conductivity, which depends on salinity, pressure and temperature and has been derived from earlier investigations of the pore-water phase. The new data covering the three-phase system CO 2 -brine-sand further show that chemical interaction causes a reduction of surface conductivity by almost 20 per cent, which could be related to the low pH-value in the acidic environment due to CO 2 dissolution and the dissociation of carbonic acid. The quantification of the total CO 2 effect may be used as a correction during monitoring of a sequestration in terms of saturation. We show that this leads to a correct reconstruction of fluid saturation from electrical measurements. In addition, an indicator for changes of the inner surface area, which is related to mineral dissolution or precipitation processes, can be computed from the imaginary part of conductivity. The low frequency range between 0.0014 and 0.1 Hz shows additional characteristics, which deviate from the behaviour at higher frequencies. A Debye decomposition approach is applied to isolate the feature dominating the data at low frequencies. We conclude from our study that electrical conductivity is not only a highly sensitive indicator for CO 2 saturation in pore space. When it is measured in its full spectral and complex form it contains additional information on the chemical state of the system, which holds the potential of getting access to both saturation and interface properties with one monitoring method.

Description: The electrical rock conductivity is a sensitive indicator for CO2 migration processes. CO2 dissolves into the pore water in large amounts and provides additional charge carriers due to the dissociation of carbonic acid. We present laboratory measurements of the spectral complex electrical conductivity of water-bearing sand samples during exposure to and flow-through by carbon dioxide. Pressures up to 300 bar and temperatures up to 80°C were applied. Steady-state experiments serve for investigating the physicochemical equilibrium of the fluid phases. Dynamic experiments aim at analyzing the impact of partial saturation and chemical interaction on complex conductivity. The steady-state dissolution experiments show that besides the conductivity-increasing dissociation a second opposing process may be observed which results in a significant reduction of conductivity at high salinities despite the added CO2. We explain our observations with a semi-analytical formulation for the electrical conductivity taking into account the interactions of ion and neutral species. A significant reduction of saturation is observed during CO2 flow and drainage. The spectral complex conductivity maps both changes in saturation and chemical interaction. recovery usually take place on active or abandoned oil respectively gas fields. Considering the amount of steel infrastructure in the subsurface of these investigation sites it is crucial to take account of the effect of steel infrastructre on electromagnetic fields. Therefore we present three-dimensional finite element (FE) simulations of transient electromagnetic fields in the present of steel infrastructure. As a first approach we consider two common scenarios. The first scenario covers the case of surface transient electromagnetic measurements with a crossing pipeline beneath a receiver profile. The second scenario covers borehole transient electromagnetic measurements in a partially steel cased borehole. We demonstrate that steel infrastructure has a significant effect on the electromagnetic response.

Description: The electrical properties of rocks are sensitive indicators for CO2 migration processes. Due to their sensitivity to the pore fluids, electric and electromagnetic geophysical methods bear a great potential for monitoring underground CO2 storage reservoirs and for early-warning leakage detection. However, CO2 is a reactive gas which massively interacts with other pore fluids as well as the solid rock matrix. Therefore, classic petrophysical relationships do not necessarily apply. With laboratory studies we investigate the impact of CO2 on the complex electrical conductivity of water-bearing rocks with and without a reactive mineral matrix under geologically relevant pressure and temperature conditions. By combining the rock physics findings with site-specific 3D time-lapse simulation studies, we can develop optimized electromagnetic monitoring setups with respect to coverage and resolution. The presented results provide insight into the reactive processes in rocks subject to CO2 and can be used to correct for chemical interactions during the interpretation of electrical measurements in terms of saturation. Our experimental and numerical results have implications for leakage detection methods and monitoring the evolution of mineral dissolution and/or precipitation processes at the grain-water interface during underground CO2 storage. Additionally, the results also advance our understanding of geogenic CO2-rich geosystems, such as volcanic terrains with gas emissions or low-temperature CO2 discharges.