[S-CG44] Rheology, fracture and friction in Earth and planetary sciences

[SCG44-P11] Mechanical behaviour and porosity evolution of porous sandstone and limestone during triaxial compaction

*Puspendu Saha¹, Ashmita Dasgupta¹, Abinash Bal¹, Santanu Misra¹ (1.Experimental Rock Deformation Laboratory, Department of Earth Sciences, IIT Kanpur, India)

Keywords:Porous reservoir rocks, Porosity evolution, Shear-induced dilation, Shear-enhanced compaction

Triaxial compression experiments were carried out to explore the failure behaviour of sandstone samples from the Krishna Godavari (KG) Basin, with porosities ranging between 9.5% and 12.7%, and limestone samples from Bombay High, with porosities ranging from 20.6% to 25%. A broad range of effective pressures was applied to these rocks to observe both brittle faulting and cataclastic flow in the deformed samples. The sandstone samples deformed under effective pressures varying from 5 MPa to 70 MPa are representative of the brittle faulting regime. The differential stress attained a peak, beyond which strain softening was observed, and the stress progressively dropped to a residual level. Peak stress increased with increasing effective pressure, indicating characteristics of Mohr-Coulomb brittle failure. The porosity initially decreased, but near the peak stress it reversed to an increase, indicating dilation of the pore space. The dilation decreased with increasing effective pressure. These experimental results typically indicate ‘shear-induced dilation’. However, in an experiment with 100 MPa effective pressure, the porosity decreased initially with loading and then became steady/constant after the failure of the sample. This type of failure might be related to the ‘shear-enhanced compaction’, developing cataclastic flow. Further experiments will give more insight into the transition of the stress regime from ‘shear-induced dilation’ to ‘shear-enhanced compaction’ in porous KG basin sandstones.

Experiments conducted at effective pressures ranging from 5 MPa to 30 MPa revealed a compactive cataclastic flow regime in the porous limestone samples at lower effective pressure range, contrasting with that observed in sandstones. The slopes of the differential stress-axial strain curve were mostly nonnegative, except for a small regime immediately after sample failure, with porosity decreasing consistently with deformation. Analysis of the effective mean stress and porosity change plot indicated deviation from hydrostatic behavior, suggesting additional porosity changes induced by deviatoric stress. Within the cataclastic flow regime, the curve for a given effective pressure aligned with the hydrostat until reaching a critical stress state (C_s). Beyond C_s, there was an accelerated decrease in porosity compared to the hydrostat, indicating a significant contribution from the deviatoric stress field to compactive strain and the development of ‘shear-enhanced compaction’ in limestone samples. Therefore, these triaxial experimental results are crucial to gain acute insights into the evolution of failure regimes in different porous reservoir rocks, considering variations in initial porosity, mechanical strengths, and mineralogical compositions.