An analytical drilling model of drag bits for evaluation of rock strength

Abstract

To evaluate the unconfined compressive strength (UCS) of rocks from drilling data is a promising in-situ method and has been studied by many researchers. In most studies, experimental methods have been used to determine the relationship between UCS and drilling data. In this paper, an analytical model is proposed to describe rock drilling processes using drag bits and rotary drills, and to deduce the relations among rock properties, bit shapes, and drilling parameters (rotary speed, thrust, torque, and stroke). In this model, a drilling process is divided into cycles, each of which includes two motions: feeding and cutting. Feeding is treated as an indentation motion. There is a linear relation between indentation pressure (thrust) and the indentation depth (penetration rate). The cutting forces and friction forces of both the rake surface and the flank surface are examined. Also, a virtual base is set to the model to simulate the contact surface between the flank surface of the bit and the rock.According to this model, drilling torque consists of four parts respectively generated from cutting, friction, feeding, and idle running. Torque caused by friction and idle running is ineffective for drilling, whereas that caused by cutting and indentation is effective. Similar to torque, specific energy also has four parts respectively from cutting, friction, feeding, and idle running. For the purposes of this study, effective specific energy is defined as the sum of specific energy consumed by cutting and feeding. Effective specific energy is independent of the penetration rate. Since it is proportional to the UCS of the rocks, it is not influenced by the penetration rate, and is more useful in the evaluation of UCS than other parameters. Some laboratory and field tests were conducted, and the results verified the usefulness and effectiveness of the proposed model.