Experimental and analytical study of the side cutting ability of drill bits
Christian Tellez, Kriti Singh, Stefan Miska, Evren Ozbayoglu, Małgorzata Ziaja
Vol. 35, no. 1 (2018), pp. 139-154, [1]
Side cutting ability of a drill bit is an important factor to ensure successful directional/horizontal drilling. In this study, a mathematical model of a drill bit has been developed to investigate the side cutting ability of a drill bit. In addition to the theoretical model, the study is also supported by experimental work. Experiments have been conducted for both PDC and tri-cone rotary roller drill bits. Equilibrium equations for BHA in a curved wellbore were established. The expression of internal forces and moments along the drill-string, which are the state variables, are derived. Because of the simplicity and fast computation capability, transfer matrix method (TMM) was adopted to determine these variables. Experiments were conducted on an indoor Tulsa University Drilling Research Projects (TUDRP) full-scale drilling rig. Tests have been conducted using 6 inch and 8.5 inch PDC drill bits. In addition, 8.5 inch tri-cone bit were tested as well. Bent sub was installed above the drill bit to enhance the side force at the bit. During the experiments, weight on bit (WOB) and flow rate were kept constant. The torque was recorded by torque gage. It was observed that RPM has little influence on torque. As expected, the bit side cutting abilities, also referred to as bit steer-ability, were different for different bits. For quantitative comparisons, a dimensionless quantity has been introduced that measures the relative difference in torque with bent sub and without with respect to torque without bent sub. In all tests, the torque with bent subs was found to be greater than those without. Hence, the relative difference is always positive. The higher the relative difference, the poorer the side cutting ability. For drill bits with side cutting ability equal to that of face cutting, the dimensionless quantity becomes zero. In conclusion, the main outcome of this paper is a new method of measuring face and side cutting ability for PDC and tri-cone bits. A similar approach has been applied to the analysis of data from offset wells using a computer program to predict side forces and the corresponding torques. This in turn permitted us to calculate the relative difference in torque and assess the side cutting ability of the drill bits. A comprehensive study of the side cutting ability of a drill bits missing in the current literature. Considering the important role side cutting ability plays in directional drilling, a very innovative mathematical model has been proposed to study the same. Experimental tests have been conducted to verify the model. The model can be used to analyze directional data and obtain useful information from offset wells to enhance future drilling projects.