Summary To predict the effects of stress on rock permeability, the authors propose an integrated approach based on an extended rock characterization, an experimental investigation of pressure dependency of directional rock permeabilities and finally a pore-scale simulation of this dependency using equivalent pore network extracted from microtomography analysis. This study has been conducted on two analog reservoir rock types: the high-permeability Bentheimer Sandstone and a dual-porosity bioclastic carbonate, the Estaillades Limestone, having an intermediate permeability. Compression tests have been conducted using a new triaxial cell specially designed to measure directional permeabilities along and transverse to direction of maximum stress application. We measured the pressure dependency of porosity, directional permeabilities, compressibilities, and elastic moduli of the tested samples. We also performed computed microtomography (CMT) imaging of the rock samples, from which we extracted the poral skeletons and the associated characteristics lengths. Then, we calculated the macroscopic transport properties using Pore Network Modeling (PNM) based on the real pore geometry. We included a model of pressure dependence of pore and throat sizes based on pressurized cavity models derived from elasticity theory to simulate the evolution of porosity and permeability with pressure. First, we show that the experimental determination of the evolution of directional permeabilities under hydrostatic and deviatoric loading is feasible. Finally, we show that the PNM coupled with µ tomography can be a promising tool to forecast the evolution of transport properties under stresses representative of reservoir conditions, at the condition of integrating more advanced pore-scale compaction models.
This study is focused on two structures in the Baltic offshore region (E6 and E7 structures in Latvia) prospective for the geological storage of carbon dioxide (CO 2 ). Their CO 2 storage capacities were estimated recently with different levels of reliability. Petrophysical, geophysical, mineralogical and geochemical parameters of reservoir rocks represented by quartz sandstones of the Deimena Formation of Middle Cambrian in two wells and properties of Silurian and Ordovician cap rocks were additionally studied and interpreted in the present contribution. Extended methodology on rock measurements and estimation of conservative and optimistic storage capacity are presented. Uncertainties and risks of CO 2 storage in the offshore structure E6 estimated as the most prospective for CO 2 geological storage in the Baltic Region, and the largest among all onshore and offshore structures studied in Latvia, were discussed. We re-estimated the previous optimistic capacity of the E6 structure (265-630 Mt) to 251-602 Mt. Considering fault system within the E6 structure we estimated capacity of two compartments of the reservoir separately (E6-A and E6-B). Estimated by the optimistic approach CO 2 storage capacity of the E6-A part was 243-582 Mt (mean 365 Mt) and E6-B part 8-20 Mt (mean 12 Mt). Conservative capacity was 97-233 Mt (mean 146 Mt) in the E6-A, and 4-10 Mt (mean 6 Mt) in the E6-B. The conservative average capacity of the E6-B part was in the same range as this capacity in the E7 structure (6 and 7 Mt respectively). The total capacity of the two structures E6 and E7, estimated using the optimistic approach was on average 411 Mt, and using the conservative approach, 159 Mt.
A Pore Network Model is an efficient tool to account for phenomena occurring at the pore scale. Its explicit three-dimensional network of pores interconnected by throats enables to easily consider the topology and geometry effects on upscaled and homogenized petrophysical parameters. In particular, this modeling approach is appropriate to study the rock/fluid interactions. It can provide quantitative information both on the effective transport property modifications due to the reactions and on the structure evolution resulting from dissolution/precipitation mechanisms. The developed model is based on the resolution of the macroscopic reactive transport equation between the nodes of the network. By upscaling the results, we have then determined the effective transport properties at the core-scale. A sensitivity study on reactive and flow regimes has been conducted in the case of single-phase flow in the limit of long times. It has been observed that the mean reactive solute velocity and dispersion can vary up to one order of magnitude compared to the tracer values because of the concentration profile heterogeneity at the pore scale resulting from the surface reactions. As for the reactive apparent coefficient, when the kinetics is limited by the mass transfer, it can decrease by several orders of magnitude with regard to the one calculated by the usual perfect mixing assumption. That is why scale factors should be added to the classical macroscopic transport equation implemented in reservoir simulator to accurately predict the reactive flow impacts. For each study case we have also obtained the permeability variation versus the porosity evolution in a physical way which accounts for reactive transport conditions. It appears that the wall deformation pattern and its impact on petrophysical properties must be explained by considering both microscopic and macroscopic scales of the reactive transport, each one governed by a dimensionless number comparing reaction and transport characteristic times. This work contributes to improve the understanding of surface reactions impacts on reactive flow in one hand, and on permeability and porosity modifications in the other hand. Using the PNM approach, scale factors parameters and permeability versus porosity relations can be determined for various rock-types and reactive flow regimes. Once integrated as inputs in a reservoir simulator, these relations are a powerful and convenient means to enhance the modelling accuracy of the petrophysical properties evolution during a reactive fluid injection such as CO2-rich brines. Introduction The CO2 geological storage is considered as a solution to reduce the greenhouse gas emission due to human activities. However, the carbon dioxide is not inert. Its dissolution in water perturbs the thermodynamical and chemical equilibrium, which may cause rock dissolution or precipitation. An induced modification of the pore-size distribution impacts the petrophysical properties of the porous medium, in particular the permeability and the porosity. But, as the pore-size distribution and the wettability of the new rock/fluid interface evolves, other properties, such as capillary pressure or relative permeabilities, can also vary. In the aim of predicting the CO2 migration and the risks of leakage or well-injectivity damage, operators need numerical simulators able to account for the reactive transport specificities. On this base the site selection will be made and the optimal flow rate will be estimated. To improve the simulation reliability, the reactive flow understanding and modeling are key-points. To simulate the reactive transport at Darcy-scale, modifying the advection-dispersion equation by only adding a sink/source term, usually determined by a 0D geochemical module, is not sufficient. Firstly, such a chemical model, based on the "perfect mixing" concept, calculates the chemical equilibrium by using mean concentrations whereas dissolution and precipitation depend on the concentration at the rock/fluid interface. Secondly, the parameters of the macroscopic transport equation are modified when surface reactions occur: the velocity and the dispersion of a reactive solute are different from the values of an inert tracer, since the pore-scale concentration profile is not uniform any more. Thus, scale factors must be added to the macroscopic transport equation initially developed for non-reactive species (Shapiro and Brenner, 1988; Bekri et al., 1995).
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