Estimating rock strength parameters using drilling data

Estimating rock strength parameters using operational drilling data can be a fast and reliable method. In this case, several researchers have proposed different analytical models based on force or energy equilibrium methods. Most of them propose methods to estimate uniaxial compressive strength through the investigation of interaction between the bit and rock in drilling process. Although in the proposed models, operational drilling system, rock strength parameters, bit geometry and contact friction were considered, some of the important factors such as crushed zone and its mechanical properties, contact frictions between the bit and rock and friction between the rock and crushed zone need to be explicitly considered. In this research work, a theoretical model is developed based on limit equilibrium of forces and considering contact frictions, crushed zone and bit geometry in the rotary drilling process by a T-shaped drag bit. Based on the model, a method is used to estimate rock strength parameters form operational drilling data. The operational drilling parameters such as thrust force, torque, rate of penetration and speed of rotation were obtained by a developed portable drilling machine. The portable drilling machine is able to drill the rocks with different strength range coincident with measure and record the parameters. A set of drilling experiments were conducted on three different rocks ranged from weak, medium and hard strength. Obtained results based on proposed model for uniaxial compressive strength, cohesion and internal friction angle of rock are well fitted to the results of the conventional standard tests.