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 = Ds r /Ds a 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.Citation: Dautriat, J., N. Gland, A. Dimanov, and J. Raphanel (2011), Hydromechanical behavior of heterogeneous carbonate rock under proportional triaxial loadings,

Deformation bands in porous carbonate grainstones: Field and laboratory observations

“…In the laboratory, the mechanical behavior of carbonate rocks of a wide range of porosities has been documented by many studies (Fruth et al, 1966;Teufel et al, 1991;Renner and Rummel, 1996;Fabre and Gustkiewicz, 1997;Baud et al, 2000Baud et al, , 2009Vajdova et al, 2004;Risnes et al, 2005;Zhang and Spiers, 2005;Baxevanis et al, 2006;Croizé et al, 2010a, b;Zhu et al, 2010;Dautriat et al, 2011;Vajdova et al, in press). These studies investigated the distinct mechanical responses of carbonates to different stress conditions and various level of plastic strain.…”

Microstructures Induced in Porous Limestone by Dynamic Loading, and Fracture Healing: An Experimental Approach

“…Two types of porosity were observed: inter-granular macro-pores which are well connected (size approx. 500 lm), and intra-granular micro-pores found in some red algae fossil grains (HAN et al 2007;DAUTRIAT et al 2011) of size approximately 10-50 lm.…”

Porosity and Permeability Change under Stress and Correlation to Rock Texture

“…While such data are relatively abundant for sandstones (Bernabé , 1991;Davies and Davies, 1999;Bernabé et al 2003), not much data are available for carbonates (Jones, 1975;İşcan et al, 2006;Dautriat et al, 2011) and dual-porosity fractured rocks. X-ray computed microtomography (micro-CT) showed strong porescale heterogeneity in a carbonate sample with high power-law exponent.…”