Hydromechanical behavior of heterogeneous carbonate rock under proportional triaxial loadings

Abstract

[1] The influence of stress paths representative of reservoir conditions on the poromechanical behavior and coupled directional permeabilities evolution of a heterogeneous carbonate has been studied. Our experimental methodology is based on performing confined compression tests keeping constant a stress path coefficient K = Δσr/Δσa ratio of the radial and axial stress magnitudes, commonly assumed to be representative of reservoir stress state evolution during production. The experiments are performed in a triaxial cell specially designed to measure the permeability in two orthogonal directions, along and transverse to the direction of maximum stress. The tested rock is a heterogeneous bioclastic carbonate, the Estaillades limestone, with a bimodal porosity, of mean value around 28% and a moderate permeability of mean value 125 mdarcy. Microstructural analyses of initial and deformed samples have been performed combining X-ray tomography and microtomography, scanning electron microscopy (SEM) observations, and mercury injection porosimetry. The microstructural heterogeneity, observable by SEM, is characterized by the arrangement of the micrograins of calcite in either dense or microporous aggregates surrounded by larger pores. The spatial distribution of the two kinds of aggregates is responsible for important density fluctuations throughout the samples, recorded by X-ray tomography, which characterizes the mesoheterogeneity. We show that this mesoheterogeneity is a source of a large directional variability of permeability for a given specimen and also from sample to sample. In addition, the fluctuation of the porosity in the tested set of samples, from 24% to 31%, is an expression of the macroheterogeneity. Macroscopic mechanical data and the stress path dependency of porosity and permeability have been measured in the elastic, brittle, and compaction regimes. No significant effect of the stress path on the evolution of directional permeabilities is observed in the elastic regime. At failure, according to the selected stress path, either a limited or a drastic permeability decrease takes place. From the postmortem observations at different scales, we clearly show the impact of the mesoheterogeneities on the localization of compaction, and we identify the precursor of the shear-enhanced compaction and pore collapse mechanisms (for K ≥ 0.25) as an intense microcracking affecting only the denser aggregates. Applying an effective medium theory adapted to our observations, we propose a porosity scaling to normalize the pressures at failure. It is then found that the normalized critical pressures evolve linearly with the stress path coefficient. Consequently, we put forward a new definition of the yield cap for this type of carbonate, which is parameterized by the stress path coefficient.