Experimental investigation on the anisotropy of rock brittleness for reservoir evaluation based on digital drilling mechanical properties

Rock brittleness is a key indicator for reservoir excavation in energy engineering, especially in oil and gas field exploitation. In this study, we developed an energy-balance model of the rock energy characteristics during rock drilling. To evaluate the anisotropic effect of rock brittleness, a drilling-based brittleness index (BI) is derived. Digital borehole tests were conducted on substrates comprising limestone, mudstone, sandstone, and shale, probing into the energy characteristics and anisotropic nature of brittleness across distinct drilling directions (0°, 90°, 270°, and 360°). A meticulous analysis was undertaken to unravel the energy traits and the progressive evolution of the BI throughout diverse drilling directions. Results suggested that the BI of limestone and sandstone exhibited an initial descent (in the drilling direction in 0-90°) followed by an ascent (in the drilling direction in 90-180°), culminating at a critical direction of 90°. The order of anisotropy of rock brittleness of the tested specimens was as follows: limestone (0.95) > sandstone (0.94) > mudstone (0.83) > shale (0.82). To ascertain the reliability of proposed methodology, a comparative analysis was conducted against various methods for determining the rock brittleness, revealing a predicted error margin below 5%. The digital drilling method has potential practical applications. [Received: July 18, 2024; Accepted: August 05, 2024]