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We studied the heterogeneity of natural rocks with respect to their pore-size distribution, obtained from mercury-intrusion capillary pressure (MICP) tests, at a scale about one-fifth of the standard plug size (2.5 cm). We investigated two Fontainebleau sandstone and two limestone samples. We found that at the scale of the MICP tests, heterogeneities are practically nonexistent. Still, there are large differences in the capillary curves from one rock type to another. Also, carbonate rocks, unlike Fontainebleau sandstone, show heterogeneities at a scale smaller than the scale used in MICP tests, as seen by the complexity in the mercury saturation versus pressure curves. We used this diversity between the capillary curves and this complexity within a single capillary curve to obtain information about the movement and chemical reactivity of CO2 in carbonates.The method consists of three steps: first, subdividing the carbonate pore system into microstructural facies, each of them having a specific range of pore throat size (e.g., tight micrite, microporous rounded micrite, small vugs, …); second, getting a characteristic value of their petrophysical properties (namely porosity, effective surface area, and permeability) from the collected MICP data; and third, computing, for experimental conditions corresponding to a transport-controlled system, the dimensionless Péclet and Damköhler numbers, expressed as a function of the aforementioned permeability and effective surface area. These numbers allowed us to infer the dominant process (i.e., diffusion, advection, or kinetics) controlling the dissolution/precipitation reaction induced by the carbonic acid. Because of heterogeneities in the pore microstructure, we found that either diffusion or advection is locally the dominant mechanism, which renders some zones (e.g., vugs or, to a lesser extent, microporous rounded micrite) chemically more reactive than others (e.g., tight micrite or spar cement).

This study is about heterogeneities of carbonate rocks with respect to their pore-size distribution obtained by Mercury Intrusion Capillary Pressure (MICP) tests, and the implication of these heterogeneities for the geochemical evolution of a reactive fluid/rock system (e.g., CO2-rich fluid/carbonate). We found that the studied rocks show heterogeneities at a scale much smaller than the typical volume of a MICP test (1/5 of a 2.5 cm core plug). We thus subdivided the rock pore system into microstructural facies based on their pore throat sizes (e.g. tight micrite, small vugs, microporous rounded micrite...), got a characteristic value of their petrophysical properties (porosity, effective surface area and permeability) from the MICP data and rock physics relationships, and provide a new form of the Péclet and Damkhöler numbers, expressed as a function of the aforementioned permeability and effective surface area. These numbers allowed us to infer the dominant process (i.e., diffusion, advection or kinetics) controlling the dissolution/precipitation reactions induced by the CO2-rich fluid. Because of heterogeneities in the pore microstructure, we found that different processes were locally dominant, which rendered some zones (e.g., microporous rounded micrite) chemically more reactive than others (e.g., tight micrite or spar cement).

Understanding the effective stress coefficient for seismic velocity is important for geophysical applications such as overpressure prediction from seismic data as well as for hydrocarbon production and monitoring using time-lapse seismic measurements. This quantity is still not completely understood. Laboratory measurements show that the seismic velocities as a function of effective stress yield effective stress coefficients less than one and usually vary between 0.5 and 1. At the same time, theoretical analysis shows that for an idealized monomineral rock, the effective stress coefficient for elastic moduli (and therefore also for seismic velocities) will always equal one. We explore whether this deviation of the effective stress coefficient from unity can be caused by the spatial microheterogeneity of the rock. The results show that only a small amount (less than 1%) of a very soft component is sufficient to cause this effect. Such soft material may be present in grain contact areas of many rocks and may explain the variation observed experimentally.

The traditional method of fluid substitution in porous rock requires the total porosity and the elastic modulus of the mineral phase as input and assumes that the fluid reaches instantaneous hydraulic equilibrium throughout the pore space. This assumption may not be appropriate for shaley sediment because of the low permeability of shale and the resulting immobility of the water in it. To address this problem, we propose an alternative method that uses effective porosity instead of total porosity. Effective porosity is lower than total porosity if porous shale is present in the system. A new, composite mineral phase is introduced, which includes the porous water-saturated shale together with the nonporous minerals and whose elastic modulus is an average of those of its components, including the porous shale. This alternative method increases the sensitivity of the elastic properties of sediment-to-pore-fluid changes and therefore may be used as a physics-based theoretical tool to better explain and interpret seismic data during exploration as well as variations in seismic response as hydrocarbon production progresses.