Improving displacement efficiency by optimizing pad fluid injection sequence during primary cementing of eccentric annulus in shale gas horizontal wells

Displacement efficiency of horizontal wells is mainly affected by casing eccentricity and slurry column structure in the wellbore, and it is an essential precondition for ensuring primary cementing quality. Most current studies use annulus two-phase flow to study the effect of eccentricity on displacement efficiency. In actual cementing conditions, there is multi-phase fluid displacement in the wellbore, including mud, spacer, flusher, and cement. In this study, the computational fluid dynamics (CFD) method is used to analyze three-phase displacement in different eccentric annuli of horizontal wells. An annulus with an eccentricity of 0.4 was selected to simulate different pad fluid injection sequences in the displacement process. The results show that the eccentricity of horizontal wells results in uneven flow velocity on the wide and narrow sides, and higher casing eccentricity results correspondingly in more mud retention volume. While the coupling effect of eccentricity and buoyancy reaches the critical equilibrium state, the displacement interface tends to be short and stable. With an eccentricity of 0.4, the efficiency of three-phase displacement is 85.88%, while the efficiency of four-phase displacement (mud-spacer-flusher-cement) reaches 89.67%. After increasing the volume of cement usage to one and a half annular volumes, the displacement efficiency increases to 95.16%. The results of this study allow better understanding of the casing eccentricity effect, the complex flow physics involved in the combination of spacer and flusher displacement, and optimization of the pad fluid injection sequence to obtain maximum cement displacement efficiency during primary cementing in horizontal wells.