Shale has significant bedding structure and different mechanical properties in various directions. The formation temperature increases with well depth. The principle of shale drillability changing with drilling direction and formation temperature affects the safety and efficiency of shale gas drilling.
This paper conducted shale drillability testing experiment in different directions and at different temperatures with shale samples from Longmaxi formation, Sichuan Basin, China. Firstly, the differences of shale drillability at different formation temperature with different drilling directions are analyzed. The shale drillability anisotropy index and the change rule of drillability are evaluated. Then, a shale drillability prediction model considering well trajectory and the formation temperature is built up based on geometry and regression. Finally, the principle of shale drillability changing with the drilling direction and the temperature is analyzed with rock mechanics theory.
The results show that the drillability of the shale in Longmaxi formation has obvious anisotropy, and the drillability grade in vertical direction is about 24.6% higher than that in parallel direction. Under the same drilling direction, the shale drillability increases with the rising temperature. The anisotropy of shale drillability decreases gradually with the increase of temperature and it follows approximatively power law in the experimental range. Optimizing drilling trajectory to adjust the drilling direction and controlling suitable bottom hole temperature are the guarantee of the optimal and fast drilling.
This study proposed a novel prediction method for the drillability of the shale at different formation temperature, which has important significance for providing guidance for the design of well trajectory and temperature control.
Shale gas is an important unconventional oil and gas resource, which has great mining and economic value. As the reservoir of shale gas, shale has a significant bedding and anisotropy characteristics. At present, many international scholars have studied the anisotropy of shale (Mokhtari, M. et al., 2013; Heng Shuai et al., 2014). The failure criterion of the isotropic rock is established by Jaeger, J. C. et al based on the experimental and theoretical analysis (Jaeger, J. C., 1960; Yong, M. T. et al., 2001; Nasseri, M. H. B. et al., 2003). Simpson, N. D. J. et al studied the effect of the anisotropic shale on the Brazilian tensile test (Simpson, N. D. J. et al., 2014). Wang Hu et al analyzed the anisotropy of the elasticity modulus, Poisson's ratio and compressive strength of the shale (Wang Hu et al., 2017). Ai Chi analyzed the brittleness anisotropy of the shale and calculated the shale brittleness index of the different direction with a new method (Ai Chi et al., 2017).