Extension limit analysis of drillstring with drag reduction oscillators in horizontal drilling

Abstract

Horizontal wells are currently the main mode for developing unconventional oil and gas resources, of which the extension limit prediction and the corresponding optimal design method have attracted a lot of attention. However, the extension limit laws for drillstring with drag reduction oscillators in horizontal drilling under different drag reduction parameters and drilling parameters have not been fully revealed. This paper studies the extension limit while considering the drag reduction effect. Firstly, the mechanical model of drillstring with drag reduction oscillators is established by considering ground and down-hole constraints, and solved by the finite difference method. Based on the optimal installation position model and propagation distance model of the drag reduction oscillator, the main controlling factors affecting the drag reduction efficiency are obtained while considering the constraints of drillstring fatigue and hydraulic loss. Next, the prediction models of three kinds of extension limits are built, and the main control factors that affect the extension limits are determined. The values of the main control factors are optimized with the extension limit as the objective function. At last, the above models are applied to a horizontal well in the Chinese Jimsar oilfield.The results show that the drag reduction oscillator can effectively improve the transmission of axial force and drag reduction efficiency. The closer to the drill bit, the larger the drag reduction efficiency. Compared with the model without cuttings, three kinds of extension limits of the horizontal section obtained by considering the cuttings are significantly reduced. The amplitude and the number of oscillators are the main factors affecting the drag reduction efficiency, and the drilling fluid density, the flow rate and the number of oscillators are the main factors affecting the extension limit.In the case study, it is recommended that an oscillator should be installed 204 m away from the drill bit and the optimal values of drilling parameters are set as follows: drilling fluid density of 1380 kg/m3, the flow rate of 27.3 L/s, WOB of 30 kN, and ROP of 10 m/h. Then, the extension limit can be increased to 6752 m, and the drag reduction efficiency of 4.72% is obtained.