Lithologic Reservoirs ›› 2018, Vol. 30 ›› Issue (4): 149-160.doi: 10.12108/yxyqc.20180418

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Influences of bedding planes on mechanical properties and prediction method of brittleness index in shale

WANG Yuepeng, LIU Xiangjun, LIANG Lixi   

  1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
  • Received:2017-09-13 Revised:2018-01-14 Online:2018-07-21 Published:2018-07-21

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Abstract: In order to study the mechanism of the effects of bedding planes on the mechanical properties of shale, we carried out the following related experimental researches. After observing the shale samples taken from Long-maxi Formation in Jiaoshiba area in Sichuan Basin by polarizing microscope and scanning electron microscopy, we got that the shales are rich in bedding planes, micro-fractures and micro-pores. The clay minerals are arranged in a directional arrangement, and the cementations of the layers are weak. RFPA numerical simulation basic parameters were acquired by the results of uniaxial physical compression experiments at different bedding plane angles. These are the basis on which the influence of bedding plane angles, the densities of bedding panes and the mechanical properties of bedding planes on uniaxial compressive strength, elastic modulus, Poisson's ratio and brittleness can continue to be studied by using digital simulation method. A new prediction method for shale brittleness index was presented by statistical methods when we only know the densities of bedding planes, the angles of bedding planes, uniaxial compressive strength and the mechanical properties of bedding planes. Numerical simulation experiments results indicated that the characteristics of uniaxial compressive strength and brittleness index at different angles of bedding plane roughly showed the U-shaped changing trend. The maximum value occurred at the bedding plane angles of 0° and 90°, whereas the minimum value occurred at the bedding plane angle of 45°. The elastic modulus of shale decreased with the increase of bedding angles, then increased and then decreased, finally increased. With the increase of bedding plane angles, the value of Poisson's ratio firstly decreased, then increased and finally decreased. And Poisson's ratio got the minimum value at bedding plane angle of 30°, whereas the maximum value at bedding plane angle of 70°. Elastic modulus, uniaxial compressive strength, and brittleness index overall decreased with the increase of the density of bedding planes. Poisson's ratio showed different states at the different bedding plane angles with the increase of the density of bedding planes. The types of rock fracture of uniaxial physical compression experiments are different at different bedding plane angles. The strength parameters, elastic parameters, brittleness and fracture modes all showed very strong anisotropy. The study about the influence of bedding planes on the mechanical properties and the prediction of brittleness in shale can provide a necessary technical foundation for the stability of shale gas wells and rational development of shale gas.

CLC Number: 

  • P313.1
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