Experimental study on the influence of hydrostatic stress on the Lode angle effect of porous rock

To investigate the deformation mechanisms of rock under hydrostatic stress, destructive experiments were conducted on sandstone under different levels of hydrostatic stress and stress Lode angles. The results reveal that the shape of the strength envelope on the π plane gradually changes from the shape of the Lade criterion to the shape of the Drucker-Prage criterion with an increase in hydrostatic stress.Normally, there exists a deviation between the strain and stress paths for porous rocks on the π plane,and the deviation decreases with an increase in stress Lode angle and hydrostatic stress. A rock failure hypothesis based on the rock porous structure was proposed to investigate the reasons for the abovementioned phenomena. It was found that the shear expansion in the minimum principal stress direction is the dominant factor affecting the Lode angle effect(LAE);the magnitude of the hydrostatic stress induces the variation of the porous structure and influences the shear expansion. Therefore, the hydrostatic stress state affects the LAE. The failure hypothesis proposed in this paper can clarify the hydrostatic stress effect, LAE, and the variation of the rock strength envelope shape.

Pore size distribution of high volatile bituminous coal of the southern Junggar Basin: a full-scale characterization applying multiple methods

Studying on the pore size distribution of coal is vital for determining reasonable coalbed methane development strategies.The coalbed methane project is in progress in the southern Junggar Basin of northwestern China,where high volatile bituminous coal is reserved.In this study,with the purpose of accurately characterizing the full-scale pore size distribution of the high volatile bituminous coal of the southern Junggar Basin,two grouped coal samples were applied for mercury intrusion porosimetry,low-temperature nitrogen adsorption,low-field nuclear magnetic resonance,rate-controlled mercury penetration,scanning electron microscopy,and nano-CT measurements.A comprehensive pore size distribution was proposed by combining the corrected mercury intrusion porosimetry data and low-temperature nitrogen adsorption data.The relationship between transverse relaxation time(T2,ms)and the pore diameter was determined by comparing the T2 spectrum with the comprehensive pore size distribution.The macro-pore and throat size distributions derived from nano-CT and rate-controlled mercury penetration were distinguishingly analyzed.The results showed that:1)comprehensive pore size distribution analysis can be regarded as an accurate method to characterize the pore size distribution of high volatile bituminous coal;2)for the high volatile bituminous coal of the southern Junggar Basin,the meso-pore volume was the greatest,followed by the transition pore volume or macro-pore volume,and the micro-pore volume was the lowest;3)the relationship between T2 and the pore diameter varied for different samples,even for samples with close maturities;4)the throat size distribution derived from nano-CT was close to that derived from rate-controlled mercury penetration,while the macro-pore size distributions derived from those two methods were very different.This work can deepen the knowledge of the pore size distribution characterization techniques of coal and provide new insight for accurate pore size distribution characterization of high volatile bituminous coal.