What laboratory-induced dissolution trends tell us about natural diagenetic trends of carbonate rocks

Abstract

The pivotal idea of this study is to unravel the processes that control heterogeneity in the attributes of the pore space in carbonate rocks (i.e. stiffness, connectivity and tortuosity), and, in turn, in the transport and elastic properties. We use starting rocks of variable fabric (i.e. a depositional-dependent microstructure) to induce a specific process (e.g. chemical dissolution under stress) and then observe the development of the microstructure, permeability, porosity and velocity due to the induced chemomechanical processes.We find that the changes in the two end members of the analysed rock types (mudstones and packstones) can lead to two different evolutionary trends of permeability and velocity, depending on the effectiveness of dissolution with respect to compaction. The balance between the two depends on: (a) the fraction of the carbonate phases characterized by large surface area; and (b) the pore stiffness of the rock. Packstones are characterized by low pore stiffness and compact significantly upon dissolution. This behaviour leads to a decrease in velocity because of a reduction in the stiffness at the grain contacts and a slight increase in permeability. The latter is curbed by the ongoing compaction. Mudstones are characterized by higher pore stiffness, experiencing minimal or negligible compaction. This behaviour leads to a slight change in porosity and velocity. However, large permeability changes are observed, related to enhanced connectivity or decreased tortuosity of the pathways.

Fundamental controls on fluid flow in carbonates: current workflows to emerging technologies

“…These insights support the prediction of reservoir quality from geophysical data. Experiments by Vanorio et al (2014) examine the coupling between chemical dissolution, mechanical compaction, and the original rock compositions and textures. Their simulations of competing chemico-mechanical processes during diagenesis highlight the ways by which the original rock texture (linked to pore stiffness and reactive surface area) may define distinct evolutionary paths for velocity and permeability for different carbonate rock types.…”

“…Expanded efforts are needed to validate seismic interpretations and bulk-volume attributes and to link them to data obtained at well and interwell scales. Overall, time-lapse objectives would benefit from improved velocity models, rooted in robust knowledge of physical property evolution of carbonate rocks, which include more sophisticated representations of physico-chemical changes during burial/uplift and diagenesis (Vanorio et al 2014). Further advances require a full understanding of the physics, as well as the limitations and complementary nature of tools and techniques, used to acquire information on different scales.…”

Chemomechanical evolution of pore space in carbonate microstructures upon dissolution: Linking pore geometry to bulk elasticity

“…Mechanical sediment consolidation and chemical microstructure changes are often treated separately, despite the coupling that has been observed between the two [ Scholz et al , ]. For instance, under a constant stress, dissolution lowers rock strength and leads to grain sliding and compaction [ Vanorio et al , ]. Chemical compaction is accomplished by solution transfer [ Durney , ]: dissolution at points of greatest stress (i.e., grain contacts) and reprecipitation in lower stress regimes in the adjacent pores.…”

“…In a porous medium, pore space compressibility is defined as the ratio of the fractional change in pore volume, v p , to an increment of applied mean stress σ [ Mavko and Mukerji , ]: in which Φ is porosity and K s is the bulk modulus of the solid grains. Vanorio et al [] found that the pore space compressibility 1/ K Φ of the original rock plays a key role in defining different styles of modifications of the pore network.…”

“…Gassmann's model [ Gassmann , ] is used as a general basis for interpreting the effect of fluids on both log and seismic velocity data. Vanorio et al [] emphasized that the model scheme performs a simple fluid substitution, predicting the change in moduli and density of the rock by replacing one pore fluid with another, and then converting the predicted moduli and density back to velocities. In such a scheme, the rock‐fluid interaction is treated as a purely mechanical problem: that is, the change in seismic velocity depends only on the compressibility and density of the fluid ( ρ fl ; K fl ), the physical parameters controlling them [ Batzle and Wang , ], and the density and elastic moduli of the rock frame ( ρ dry ; K dry ; G dry ).…”