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The dissolution of carbonate rocks usually leads to both porosity $$(\phi )$$ and permeability $$(k)$$ increase. We present experimental evidences and physical-based models of positive and anti-correlated dynamics of $$k$$ and $$\phi $$ observed during dissolution experiments of carbonate rocks. We study the way the rate of change of $$\phi $$ and $$k$$ is controlled by the degree of undersaturation of the percolating solution for two different types of carbonate rocks. We document the occurrence of an anti-correlated $$k-\phi $$ trend when the flowing fluid (deionized water) has a weak capacity of dissolution. A positive correlation is found when $$\hbox {CO}_{2}$$ is added to the deionized water to increase the potential dissolution rate. Detailed analyses of the microstructures of the rock performed by X-ray microtomography reveal that low dissolution rate favors detachment of solid particles and their subsequent accumulation at the pore-throat inlet. Particles are detached from the rock matrix due to the differential dissolution rate of the indurated grains and the microporous cement. We then propose a simple phenomenological model to interpret the effect of the pore-throat clogging by the accumulation of partially dissolved carbonate particles. We conjecture that permeability is controlled by the decrease of the effective hydraulic radius and the increase of the tortuosity due to partial and localized obstruction of the pore network. Conversely, increasing the level of undersaturation of the flowing solution leads to an augmented potential of dissolving most of the transported particles before they reach the throats. In this case, both $$k$$ and $$\phi $$ increase and display power-law correlations. [ABSTRACT FROM AUTHOR]

Summary In this paper we solve the Navier Stokes’ equation using finite difference method, on a simulated porous rock structure in 2-D, to study the velocity distribution of fluid flowing through it under a constant pressure gradient. A reactive solute carried through the fluid is allowed to interact with the minerals in the rock. Depending on the rock composition, both dissolution and precipitation reactions may occur. However precipitation occurs only through the cations that are released in the solution due to dissolution. These combined dissolution–precipitation reactions change the porosity, permeability and pore geometry of the sedimentary rock. We study the temporal changes of these properties as functions of Peclet number, concentration of the reactive solute and ratio of Damkholer numbers of dissolution to precipitation. The final flow property is decided by a combination of these parameters. [ABSTRACT FROM AUTHOR]

Limestone dissolution by $$\hbox {CO}_2$$ -rich brine induces critical changes of the pore network geometrical parameters such as the pore size distribution, the connectivity, and the tortuosity which govern the macroscopic transport properties (permeability and dispersivity) that are required to parameterize the models, simulating the injection and the fate of $$\hbox {CO}_2$$ . A set of four reactive core-flood experiments reproducing underground conditions ( $$T = 100\,^{\circ }\hbox {C}$$ and $$P = 12$$ MPa) has been conducted for different $$\hbox {CO}_2$$ partial pressures $$(0.034 < P_{\mathrm{CO}_2}< 3.4\; \hbox {MPa})$$ in order to study the different dissolution regimes. X-ray microtomographic images have been used to characterize the changes in the structural properties from pore scale to Darcy scale, while time-resolved pressure loss and chemical fluxes enabled the determination of the sample-scale change in porosity and permeability. The results show the growth of localized dissolution features associated with high permeability increase for the highest $$P_{\mathrm{CO}_2}$$ , whereas dissolution tends to be more homogeneously distributed for lower values of $$P_{\mathrm{CO}_2}$$ . For the latter, the higher the $$P_{\mathrm{CO}_2}$$ , the more the dissolution patterns display ramified structures and permeability increase. For the lowest value of $$P_{\mathrm{CO}_2}$$ , the preferential dissolution of the calcite cement associated with the low dissolution kinetics triggers the transport that may locally accumulate and form a microporous material that alters permeability and produces an anti-correlated porosity-permeability relationship. The combined analysis of the pore network geometry and the macroscopic measurements shows that $$P_{\mathrm{CO}_2}$$ regulates the tortuosity change during dissolution. Conversely, the increase of the exponent value of the observed power law permeability-porosity trend while $$P_{\mathrm{CO}_2}$$ increases, which appears to be strongly linked to the increase of the effective hydraulic diameter, depends on the initial rock structure. [ABSTRACT FROM AUTHOR]

Abstract: A set of four reactive flow-through experiments at temperature T =100 °C and total pressure P =12 MPa was performed in limestone reservoir samples. By using various ranging from 0.7 to 10 MPa, these experiments mimic mass transfers occurring (1) near the injection well, where the brine is almost saturated with CO2 (i.e. P CO2 ≈ P), and (2) at increasing distances from the injection well, where the fluid displays lower values and higher divalent cation concentrations due rock dissolution along the fluid pathway. Results for P CO2 =10 MPa show non-uniform dissolution features associated with transport-controlled mass transfer, while reaction-controlled uniform dissolution is observed for P CO2 =2.5 MPa. The experiment with P CO2 =6 MPa allows investigating in detail the transition from transport- to reaction-controlled dissolution. Conversely, the experiment reproducing conditions far from the injection well (P CO2 =0.7 MPa), shows a decrease of porosity triggered by the precipitation of Mg-rich calcite. For all the dissolution experiments, the time-resolved porosity ϕ(t) can be modeled by a simple non-linear equation including parameters that characterise the dissolution regime triggered by the reactivity of the inlet fluid (measured by the Damköhler number, Da). Furthermore, all dissolution experiments display power scaling between permeability (k) and porosity (ϕ) with distinctly different scaling exponents characterising the reactivity of the fluid percolating the sample, independently from the decrease with time of the reactive surface area. It is shown also that dissolution at moderate positive values of Da seems the most efficient to increase permeability and promote a rapid spreading of the reaction front, while inducing minimal modification of the porosity in the vicinity of the CO2 injection well. These results can be used to parameterize the k–ϕ function for modeling the earliest dissolution processes occurring in the vicinity of the reaction front. [Copyright &y& Elsevier]

Abstract: Several studies, both theoretical and experimental, show that sedimentary rocks have fractal pore–grain interface. The real rocks are 3-d structures with highly tortuous and often fractal pore spaces. Before attempting simulation of this daunting geometry, we present here, as a preliminary study, a simpler 2-d version. In this paper, a computer simulated 2-d sedimentary rock structure is generated by the relaxed bidisperse ballistic deposition model. Grains of two different sizes are dropped ballistically on a linear substrate. By changing the fraction of the two types of particles, the porosity of the rock structure can be tuned. The structure undergoes compaction through the relaxation of possible unstable overhangs. The micro structure of the pore space is investigated. The pore mass and the rock–pore interface show a fractal behaviour with the same fractal dimension indicating that the pore volume is a fractal. Our simulation results indicate that the process of compaction of grains during the deposition process seems to erase the dependency of the fractal dimension on the grain size distribution. The two point density correlation is measured for the pore space. It shows anisotropy which is an outcome of the growth rule. X-ray tomography of two-dimensional sections of real sedimentary rocks obtained from Mallorca Island is subjected to the same study and the results compared with those obtained from simulation. The simulation results agree qualitatively with the real rock sample. We also study diffusion on the pore space. Diffusion is found to be anomalous as is expected in fractal spaces. It also bears the signature of anisotropy of the structure. Diffusion studies on the real rock sample could not yield conclusive results as the system size is not large enough. [Copyright &y& Elsevier]