INFLUENCE MECHANISM OF IRREGULAR BOREHOLE ON CEMENTATION DISPLACEMENT DURING COALBED METHANE DRILLING

摘要

Due to some characteristics of the coal seam such as the low pressure, easy to collapse and leak-age, resulting in the irregular borehole in coal seam segments and serious hole enlargement, the dis-placement efficiency in the process of cementing is reduced, which affects the sealing effects of slurry in cementing. Aiming at the case of casing misa-lignment and borehole irregularity in coalbed me-thane wells,a simplified model of coalbed methane well casing misalignment and borehole irregularity was established in this study. A description method of the coordination of displacement contact inter-face was defined, and the VOF model was selected by computational fluid dynamics method, therefore, the Heba model cement displacement process was controlled and solved. The simulation results show that the eccentric flow of cement slurry advances at the upper end and lags at the lower end, and the gravity effect has a certain inhibitory effect on it. The sudden change of the borehole produces vortex flow and displacement dead angle, and the dis-placement effect accelerates and decreases with the increase of borehole irregularity. Therefore,im-proving casing centering and ensuring borehole continuity are important measures to improve the displacement effect.