Compaction and Failure in High Porosity Carbonates: Mechanical Data and Microstructural Observations

Abstract

We investigated systematically the micromechanics of compaction in two carbonates of porosity above 30%, Majella grainstone and Saint Maximin limestone. The composition, grain size and pore surface area of these rocks were determined. Hydrostatic compression experiments were performed under dry and wet conditions beyond the onset of grain crushing. A significant water weakening effect was observed in both rocks. A set of conventional triaxial experiments was also performed on both rocks under dry conditions at confining pressures ranging from 3 to 31 MPa. Microstructural observations were carried out on the deformed samples. The mechanical behavior of these high porosity carbonates is dominated by shear-enhanced compaction associated in most cases with strain hardening. Stress-induced cracking and grain crushing are the dominant micromechanisms of deformation in both rocks. In Majella grainstone, compactive shear bands appeared at low confinement, in qualitative agreement with the deformation bands observed in the field. At higher confining pressures, compaction localization was inhibited and homogeneous cataclastic flow developed. In Saint Maximin limestone, compaction localization was observed at all confining pressures. An increasing number of compactive shear bands at various orientations appeared with increasing strain. These new data suggest that compaction localization is important in the mechanical compaction of high porosity carbonates.

Deformation bands in porous carbonate grainstones: Field and laboratory observations

“…After a certain amount of strain hardening (this amount typically increases with confining pressure), a switch from compaction to dilatancy is observed Vajdova et al, 2004). The high-porosity carbonates (4 > 30%) studied in Baud et al (2009) show similar behavior to the less porous ones, but no dilatancy occurs at low confining pressures. The results of microstructural analyses performed, on the laboratory deformed samples, by previous authors highlighted microcracking and plastic/cataclastic pore collapse as the main deformation micromechanisms.…”

“…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.…”

Detection of moving capillary front in porous rocks using X-ray and ultrasonic methods

“…area, smaller particle size and larger throat size (derived from mercury injection) in the former [16]. For both rocks the velocity and wave amplitude have been normalized to the value of the dry rock, before imbibition starts: for Majella grainstone, the initial P wave velocity is 2750 m/s and the first peak amplitude is −16.6 mV, while the initial P wave velocity is 2810 m/s and the first peak amplitude is −3.1 mV for Saint-Maximin limestone.…”

“…Information on the mineralogical and petrophysical properties of both carbonate rocks can be found in David et al [15] and Baud et al [16]. As explained in Section Material and Methods (Section Standard Method for Capillary Imbibition Studies), the rock sample is hanging under an electronic balance (Figure 4).…”

“…The Saint-Maximin used for the test was different from the one presented in Section Ultrasonic Method for the Detection of Moving Capillary Fronts, however the tested samples had a similar porosity (37%, Baud et al [16]) and permeability (2.1 D). The Savonnières limestone is a relatively homogeneous clean biogenic limestone (98% of calcite) with a porosity of 31.6% and a moderate permeability of 51.2 mD.…”