Accurate Prediction Model of Annular Equivalent Circulating Density in “Wavy” Horizontal Wells of Shale Gas

ABSTRACT The thin thickness of shale reservoirs and the presence of faults often require frequent adjustments to the borehole trajectory during drilling, leading to the formation of “wavy” wellbores. These irregular trajectories exacerbate cuttings accumulation in horizontal sections, increasing annular pressure losses and complicating the accurate prediction of borehole pressure. To address this challenge, based on the solid–liquid two‐phase flow model, combined with the cuttings settling velocity model and the mass exchange coefficient model, this paper establishes an accurate prediction model of annular equivalent circulating density for shale gas horizontal wellbore. The effects of key parameters—including borehole trajectory, drilling fluid density, circulation rate, rate of penetration, and drill string rotational speed—on annular pressure distribution and cuttings transport were systematically analyzed. Results indicate that greater fluctuations in the horizontal section significantly increase annular pressure and debris bed formation. Although drilling fluid density has a primary influence on pressure, mechanical drilling speed and rotational speed have a limited impact on annulus pressure, about 0.3 MPa, but strongly affect hole cleaning efficiency. The effect of circulation rate on pressure shows a non‐monotonic trend: increasing the flow from 15 to 25 L/s reduces the debris bed height by up to 0.07, whereas at 35 L/s, complete borehole cleaning is achieved. This study elucidates the complex interplay between well trajectory, operational parameters, and pressure distribution in shale gas horizontal wells. The findings provide theoretical support for enhancing borehole cleaning and accurately predicting annular pressure in complex horizontal drilling environments.