Abstract
The anisotropy of tight sandstone (a type of unconventional gas reservoirs) is a significant factor influencing the characteristics of cracks network under hydrofracturing; thus, it also has a large influence on the final production capacity of the gas reservoirs. To improve the understanding of anisotropy degree and mechanical properties of the tight sandstone of Xujiahe Formation and thus to provide reliable reference for the establishment of hydrofracture model and parameter designing in fracture field, a series of experiments including ultrasonic wave velocity and uniaxial and triaxial compression tests of the tight sandstone samples obtained from Xujiahe Formation with different inclination angles (the angle between sample drilling direction and bedding plane) have been conducted. With the increase of inclination angle, the velocity of the longitudinal wave and elastic modulus both show the tendency of decreasing, whereas the compressive strength shows a “U” shape varying pattern, which is high on sides and low in the middle region. The values of uniaxial compression strength (UCS) are the lowest of sandstone with the inclination angles of 30° and 45°. The fracture patterns are dominant by splitting fracture under uniaxial compression tests. However, shear fracture and dilatancy morphology is the main pattern under triaxial compression test. But the local morphology of the failure surfaces behaves different if the inclination angle is changed. Combining the mechanic theory of transversely isotropic material, the anisotropy parameters of the tight sandstone are analyzed, as well as the influence on the hydrofracturing technology for tight sandstone in the field.
Highlights
In China, under the situation of increasing natural gas supply, tight sandstone gas has become one focus of developing nonconventional/unconventional natural gas due to its wide distribution and rich reserves [1,2,3,4,5]
We find that the elastic modulus changes with the dip angle. at is, when the axial direction of the sample has a different dip angle, its elastic modulus changes
Because cohesion strength c and internal friction angle φ of the samples with different dip angles change a little, in the numerical simulation of hydraulic fracturing model of the tight sandstone reservoir of the Xujiahe Formation, we suggest that the cohesion strength and internal friction angle can be replaced by the average values of cohesion strength and internal friction angle
Summary
In China, under the situation of increasing natural gas supply, tight sandstone gas has become one focus of developing nonconventional/unconventional natural gas due to its wide distribution and rich reserves [1,2,3,4,5]. Chen et al [16] studied the anisotropic properties of low Cambrian black shale at Niutitang; the effects of confining pressure and bedding angle on the black shale are demonstrated by triaxial compression tests. Anisotropy of the rock reservoirs is important for unconventional gas exploitation, as it is a significant parameter for production in the field. As a type of sedimentary rock, tight sandstone has obvious anisotropy with bedding angle, and this is the prerequisite for the designing of hydrofracturing in reservoirs. E samples are prepared with different bedding angles, and uniaxial and triaxial compression tests are carried out to investigate the anisotropy characteristics of the tight sandstone from the viewpoint of strength, mechanical parameters, and failure characters. The tight sandstone samples are collected from Xujiahe reservoir, which is a tight sandstone reservoir in Hubei Province. e samples are prepared with different bedding angles, and uniaxial and triaxial compression tests are carried out to investigate the anisotropy characteristics of the tight sandstone from the viewpoint of strength, mechanical parameters, and failure characters. e anisotropic index of the Xujiahe tight sandstone is analyzed and discussed by using the transversely isotropic theory of rock mass. is study can provide effective technical support for designing and optimizing hydraulic fracturing in the tight sandstone fields
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
