Challenges and Current Advances in the Rock Physics of Carbonate Rocks
Tiziana Vanorio, Yael Ebert, and Ammar El Husseiny
Stanford Rock Physics Laboratory, Stanford, CA
Carbonate rocks show no one-to-one relationship between porosity and permeability nor between porosity and velocity. On the one hand, permeability depends on the ability to conduct fluid flow, which is controlled by both the size of the pores and the tortuosity of the interconnected pore space. On the other hand, velocities depend on pore compliance and stiffness at the grain contacts, which are controlled respectively by pore shape and the amount of intergranular cement. Diagenesis during burial, cement dissolution and compaction under stress, all contribute to the evolution of the rock connectivity and stiffness over time. Although this is applicable to all porous rocks, carbonate rocks are especially susceptible to these phenomena because of their proneness to react chemically. In the long-term, this contributes to their heterogeneous texture and fabric, thus greatly increasing the chance for variability and complicating how transport and elastic properties relate to the diagenetic trends of these rocks. In the short-term, complex rock-fluid interactions violate most of the assumptions of purely mechanical models used for predictive property modelling, thus challenging any attempt of quantitative, geophysical monitoring.
So far, laboratory experiments showed that fluid-solid chemical reactions inducing mechanical deformations in carbonates are highly nonlinear, strongly coupled, and controlled by size and geometry of porosity as well as spatial distribution and surface area of the solid phases composing carbonate rocks. That is intimately connected to the rock depositional environment and facies. Because of the complexity of carbonates and the need to understand the link (and its changes) between seismic and transport response and rock microstructure so that coupled reactive fluid flow-mechanical deformation models can be developed at the pore-scale, we are also conducting laboratory experiments and time-lapse imaging on synthetic samples mimicking composition, microstructure, and pore structure (i.e., pore shape and size) of carbonates. Trends of the measured rock properties and their evolution upon injection on controlled microstructures may provide physical and well-controlled bounds which help in understanding how to model the processes controlling the rock physics of carbonates and their geophysical response to deformation upon rock-fluid interaction.
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012