[1] Pressure solution is widely regarded as a mechanism of ductile deformation in the upper crust. It is driven by stress differences and its rate is affected by temperature, grain size, and fluid chemistry. Pressure solution involves dissolution at grain contacts under high stress and precipitation at grain contacts on pore surfaces under low stress, leading to porosity reduction by precipitation in the pore space or by grain indentation. For a system closed at a grain scale, pressure solution is traditionally described by a mechanism involving three steps: (1) dissolution at intergranular interfaces, (2) diffusion of solutes inside the contact between two grains, and (3) precipitation on the surface of the grains in contact with the pore fluid. In this paper we propose a model where we have added a fourth step to this process, diffusive transport to other open pores, to account for the macroscopic diffusion of solutes in pore fluids, such that the deformation is not closed at the grain scale. In this model, differences in mineral solubility due to variations in stress and grain size produce concentration gradients which drive diffusive mass transport. The interaction between pressure solution at a grain scale and transport over distances of several grains can lead to the amplification of initial porosity heterogeneities and subsequent localization of deformation. Regions of intense dissolution compact and form ''bands'' in close proximity to regions where the porosity reduction is mainly due to cementation. Pressure solution augmented by large-scale diffusional transport will cause mass transport from fine-grained to coarse-grained rock volumes. We show that such processes are important during both diagenesis of sediments and compaction of fault gouge.
Résumé -Simulation numérique de l'effet du stockage souterrain de dioxyde de carbone sur la déformation des calcaires par dissolution sous contrainte : résultats préliminaires -Lors de l'injection de dioxyde de carbone dans un réservoir déplété ou dans un aquifère, la dissolution du CO 2 dans l'eau de formation produit une acidification. Ce phénomène accélère les réactions de dissolutionprécipitation avec la matrice rocheuse, et par conséquent, peut modifier notablement les propriétés mécaniques et hydrauliques des roches. De tels effets sont particulièrement importants dans les calcaires pour lesquels la solubilité et la réactivité des minéraux dépendent directement du pH, qui est lié à la pression partielle de CO 2 . Le mécanisme de déformation par dissolution-précipitation sous contrainte est contrôlé par un couplage entre des processus de dissolution et de précipitation des minéraux et une déformation macroscopique de la matrice. Ce mécanisme implique une dissolution aux joints de grains où la contrainte normale est élevée, une diffusion de la matière dissoute dans le fluide intergranulaire, et une précipitation de matière dans les pores où la pression est plus faible. Cela induit une compaction de la roche et une diminution de porosité contrôlées à la fois par l'indentation des grains et par la précipitation dans les pores. La percolation de fluides riches en CO 2 tend à accélérer la compaction et peut ainsi modifier les propriétés mécaniques du réservoir à long terme. Dans cet article, nous avons cherché à quantifier ce processus à l'aide d'un modèle numérique 2D qui couple les processus de dissolution et de précipitation à l'échelle des grains avec des transferts de matière à une échelle plus importante (quelques décimètres). Nous montrons que des pressions élevées de CO 2 (jusqu'à 30 MPa) accélèrent la vitesse de compaction des roches calcaires d'un facteur ∼50 à ∼75 et diminuent aussi leur viscosité. Abstract
Pressure solution is an efficient mechanism for ductile deformation and local mass transport in the upper crust. In this paper we model pressure solution as a mechanism involving four steps: (1) dissolution at the grain contacts; (2) diffusion of solutes through fluid films at the contact between two grains; (3) transport of solutes by diffusion through the pore fluid into other adjacent open pores; and (4) precipitation on the surface of grains at their contact with the pore fluid. In this study we constrain under which conditions pressure solution is limited by one of the four processes: dissolution; contact diffusion; precipitation; and global diffusion. From our model of pressure solution, based on thermodynamic relationships we derive three dimensionless numbers which represent the competetion between the four mentioned processes. With these numbers we can define the crossover from a situation where one process acts as the limiting process to a new situation controlled by another process. We also see how the different rate-limiting processes influence the amount of mass transported during the compaction process. In addition we study the effect of clays, as it has been suggested that these minerals speed up the rate of pressure solution. We propose two models, a chemical related and a mechanical model for how the clay particles may affect the dissolution process of quartz.
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