Experimental Study on the Improvement of Wellbore Cleaning Effect by Drill Pipe Rotational Speed and Drilling Fluid Viscosity in Deep Coal Horizontal Wells

Linfen block in Shanxi Province is the first large deep coal gas field in China with 100 billion square meters of integrated gas. The coal dust produced in the drilling process is easily dispersed and expanded by hydration. When conventional parameters are used to drill CBM horizontal wells, there will be cuttings accumulation in the horizontal section and insufficient borehole cleaning. Problems such as annular blockage and circulating pump full stop occur after casing running, which seriously affect cementing quality and wellbore integrity safety. To solve the above-mentioned problems, this paper establishes a physical simulation experiment system of the transparent tube-in-tube structure, conducts cuttings migration experiments, and explores the influence laws of rotational speed and the dynamic plastic ratio of drilling fluid on the wellbore cleaning effect. The experimental results show that: (1) Increasing the dynamic plastic ratio can enhance the rock-carrying capacity and reduce the height of the cuttings bed. The dynamic plastic ratio increased from 0.33 Pa/mPa*s to 0.6 Pa/mPa*s, and the average thickness of the cuttings bed decreased by 61%. (2) The larger the dynamic plastic ratio is, the greater the decrease in the height of the cuttings bed with the variation of rotational speed. Under the conditions of dynamic plastic ratios of 0.33 Pa/mPa*s, 0.42 Pa/mPa*s and 0.6Pa/mPa*s, when the rotational speed increased from 50 r/min to 90 r/min, the cuttings bed decreased by 22%, 35% and 64% respectively. During the field test process, the dynamic plastic ratio was increased from 0.21–0.31 Pa/mPa*s to more than 0.42 Pa/mPa*s, solving the problem of cuttings accumulation in deep coal horizontal Wells. The wellbore cleaning of the long horizontal section of deep coal horizontal Wells has been achieved, which has high theoretical and practical value.