Velocity Anisotropy: Temperature Effect on Thomson’s Parameter of Ogun Shale, Ogun State, Nigeria

Abstract

When a rock’s physical property is directional or orientation-dependent, the rock is described as being anisotropic. Shale is a common sedimentary rock with inherent anisotropic nature gotten from its geological evolution. Thus in studying the elastic properties of shale, the direction of the wave propagation has to be considered and reported. Velocity anisotropy of shale with temperature effect on the Thomson’s anisotropic parameters was carried out on shales from Ogun state in the southwestern part of Nigeria. Ultrasonic velocity measurement was carried out at temperatures ranging between 50 and 300 ˚C. Elastic properties, as well as the anisotropic parameters were determined. The parameters were compared with the temperature and elastic properties of the samples. Results showed that Ogun state shales are anisotropic and that both P and S wave pulses exhibited higher values in the horizontal direction (along the bedding plane) than in the vertical orientation (normal to the bedding plane) of the samples. Elastic modulus was higher in the horizontal direction while the Poisson’s ratio and velocity ratio were higher in the vertical plane. Anisotropic parameters, gamma (ϒ) and epsilon (Ԑ) exhibited value reduction up to about 150 ˚C when they started to increase with increasing temperature. Gamma however increased more notably than the epsilon. This result indicated that fractures and microcracks within the shale samples closed up as the samples expanded due to temperature increase.