Experimental Study on Sand-Washing in Horizontal Wells

In horizontal wells, sand readily settles at the bottom of the annulus under gravity, forming sand beds that may lead to string sticking in sand-washing operations. To clarify the dynamic evolution of annular sand beds, a horizontal wellbore sand-washing experimental system was developed. Clean water and a 0.3 wt% guar gum solution were used as washing fluids to investigate sand-bed front morphology, self-excited oscillation, stratified transport, and deposition height under different inlet Reynolds numbers. The results show that, during clean water sand-washing, the sand-bed front exhibited inflection-point instability, stagnation-point formation, and secondary bed branching with increasing inlet Reynolds number, accompanied by periodic self-excited oscillations. In contrast, no oscillation occurred in the guar gum solution, and the sand-bed front became more streamlined. For both fluids, stratified transport consisting of a convective layer and a shear-slip layer was observed above the sand bed, while the slip-layer thickness in the guar gum solution was approximately 45.0% lower than that in water. Based on dimensional analysis and experimental data, correlations for the dimensionless migration velocity of the sand front and the local Reynolds number were established. An implicit method for predicting the maximum sand-bed height was proposed, with an error below 1.15%. The results indicate that the sand-bed deposition height decreases nearly linearly with increasing inlet Reynolds number, whereas the absolute migration velocity of the sand front remains low, resulting in poor sand-washing efficiency. The findings provide guidance for optimizing sand-washing flow rates and assessing sand-sticking risks in horizontal wells.