The impact of temperature and confining pressure upon mechanic characteristics and failure modes of the tight quartz sandstone in the second member of the Xuejiahe Formation(T

3

x

2

) in Xinchang gas field of Sichuan Basin

was studied through a conventional triaxial compression test by using a MTS 815 servo-controlled testing machine to simulate subsurface environment of high temperature and high confining pressure.The results show that

when the temperature remains constant

the average values of compressive strength and elastic modulus of quartz sandstone samples increase with growing confining pressure.And when under a constant confining pressure

the value changes in a two-stage way below and over the temperature of 60℃.Before the temperature reaches 60℃

the samples have a quadratic nonlinear decrease in terms of cohension and a nonlinear increase for their internal friction angle

triaxial compression strength

and elastic modulus.When the temperature is higher than 60℃

the above-mentioned parameters vary in a totally reverse direction.The temperature of 60℃ is then the critical point for the mechanical properties of the samples to reverse under joint control of temperature and confining pressure.Within the ranges of pressure from 0 to 100 MPa and temperature from 15 to 150℃

the deformation or failure modes of the samples depends on temperature and confining pressure

with the latter having stronger impact than that of the former.Based on deformation process and failure modes

we roughly categorize the stress-strain curves into five types

which in turn loosely correspond to failure modes of tensile

shear

shear dilatancy

and tensile-shear

respectively.