ABSTRACT: Horizontal wells have been frequently used in many oil reservoirs, which has significant advantages in improving oil development efficiency. During horizontal well drilling, the cuttings can easily accumulate in the wellbore to form a cuttings bed at the bottom. The formed cuttings bed would directly increase the contact area so that wellbore friction during drilling is also increased. The increase in friction can cause serious underhole accidents. Hence it is necessary to study the parameters affecting the friction between the drillpipe and cuttings bed. The viscosity of drilling fluid and the rotation speed of drillpipe are important factors affecting the cuttings transport. In this paper, a laboratory experiment was designed to study the effects of fluid viscosity and drillpipe rotation speed on the sliding friction coefficient, then a general correlation model is built according to experimental results. Genetic programming is used here to analyze the effects accurately and optimize the structure and parameters of the model. This new way could greatly improve the model's accuracy and provide guidance for the sliding friction coefficient prediction. 1. INTRODUCTION Horizontal wells have unique advantages in the development of these reservoirs, which is an important technology to improve the development efficiency of shale oil and gas reservoirs. The long horizontal well may greatly increase the exposed area of the reservoir in the wellbore and effectively improve the oil and gas recovery. However, horizontal wells also face some problems, such as cuttings accumulation during drilling, which will lead to out-of-control wellbore trajectory, excessive friction and torque, stuck drilling and wellbore instability. Borehole cleaning is one of the key problems affecting the safety and efficiency of drilling in horizontal wells. The distribution of cuttings bed in horizontal well has great influence on drillpipe friction, drillpipe position and drillpipe buckling. Cuttings bed accumulation is common in horizontal well, especially in inclined and horizontal well section. The cuttings bed accumulation increases the roughness of the contact surface between drillpipe and wellbore, and the friction coefficient between drillpipe and cuttings bed may increase significantly with the extension of the inclined section and horizontal section length. The excessive friction of drillpipe is easy to affect the drilling speed and wellbore trajectory, aggravate the fatigue damage of drill tool, affect the service life of drill tool, increase the probability of complex downhole conditions, and restrict the extension of horizontal section. Therefore, the frictional characteristics between drillpipe and cuttings bed during horizontal well drilling becomes to a key problem.

Experimental Study of Fluid Rheological Properties on the Friction of Cuttings Bed in the Horizontal Well