What Laboratory-Induced Dissolution Trends Tell us about Diagenetic Trends and Reservoir Properties of Carbonate Rocks

Vanorio, Tiziana; Ebert, Yael; Grombacher, Denys

Carbonate rocks show no one-to-one relationship between porosity and permeability nor between porosity and velocity. While carbonate deposits form only once, diagenetic processes overlap in time and space overprinting the original depositional texture of the rock. Studying how the rock microstructure of carbonates re-adjusts itself while being chemically altered under stress is key to understand the evolution of its rock properties. This chemo-mechanical coupling enormously increases the number of possible deviations from first-order relationships between porosity, velocity, and permeability, leading to significant scatter in the trends of data.

There is an objective difficulty in unraveling continuous sequences of diagenetic events, which ultimately define patterns in the generated rock physics properties. We will show a series of experiments driven by a working-forward approach. We directly monitored the evolution of porosity, permeability, compressional and shear velocities of different depositional facies (i.e., different pore stiffness) undergoing both chemical and mechanical processes. Chemical processes are induced by flooding the rock samples with a CO2 aqueous solution while monitoring the mechanical deformation under stress and the induced-changes in transport and elastic properties. Though the number of injected pore volumes is the same for each sample, the evolution of both permeability and velocity differ. However, similar evolution trends are observed for samples belonging to the same facies/formation. In micrite-rich packstones, both porosity and permeability linearly increase upon injection, while velocities decrease. Large compaction is observed. In tight limestones, porosity and velocity show negligible change upon injection, while permeability suddenly increases after reaching a critical threshold. Minimal compaction is observed. Trends are also compared to the natural diagenetic trends of untreated carbonate samples. Surprisingly, the evolution of velocity and permeability of the injected samples follows that expressed by the natural diagenetic trends. These results, along with observations from time-lapse SEM and Ct-scan images, imply that pore attributes and microstructure are modified in a way that is characteristic of each carbonate facies.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013