Experimental simulation of chemomechanical processes during deep burial diagenesis of carbonate rocks

Abstract

Chemomechanical processes involved in the deep burial diagenesis of carbonate petroleum reservoirs are still poorly understood. To better understand these processes and explain how porosity and permeability can be preserved at the great depth of DBRs (deeply buried reservoirs), we developed an experimental device allowing both the simulation of high-pressure/stresses/temperature conditions (80°C, 60MPa of confining pressure, and differential stress up to 40MPa) of DBR and the circulation of different fluids in rock samples. We tested (triaxial multistep creep tests) four core samples of a cemented limestone and analyzed creep deformations, fluids chemistry, and petrographical and petrophysical properties of samples. Different flow conditions (no flow and flow through) and chemical compositions (natural meteoric water with and without phosphate ions) were considered. Our study showed that the precipitation of calcite on free pore walls of micrites blocks the microporosity between micrite crystals, thus rendering the microporosity inaccessible to fluids. Hence, the connected porosity decreased strongly after experimentation. This is due to the PSC (pressure solution creep) which is themain process implied in the porosity reduction of a carbonate rock during deep burial. The preservation of macropores during PSC allows the preservation of permeability. In addition, calcite solubility is positively dependent onmechanical parameters (axial compaction and axial stress), thus suggesting that calcite can precipitate during decompression of deep basinal fluids, resulting in changes in porosity. A comparison of experimental results with theoretical calculations showed that the integration of the PSC process into calculation databases would greatly improve the modeling of DBR. © 2014. American Geophysical Union.