Experimental Study on the Impact of Wellbore Collapse in Sand–Mudstone Reservoirs on the Productivity of Horizontal Wells

During the drilling and well test operation cycles of horizontal wells, wellbore instability and collapse frequently occur in sand–mudstone reservoirs. The fallen wellbore debris and rock fragments accumulate inside the wellbore, triggering local wellbore blockage. At present, research regarding the influence of collapse-induced blockage on test productivity under various blockage conditions is lacking. A self-developed visualized experimental system for horizontal well productivity testing was adopted in this study to investigate the fluid flow characteristics and pressure loss in the wellbore under variable blockage degrees and fluid flow parameters. The influence mechanism of collapse blockage on flow pressure loss was analyzed. The results show that at low production rates (less than 100 m3/d) under blockage degrees of 1/3 and 2/3, the fluid flow velocity in the wellbore is low, and the collapsed solid fragments exhibit no migration behavior. Severe blockage can hardly form in the wellbore, and the proportion of hydraulic pressure loss along the wellbore to the total production pressure difference of the reservoir is relatively low. Under high production rates (greater than 150 m3/d), fine debris fills the voids between large collapsed fragments and forms blockages. Furthermore, the pipe flow friction factor of horizontal wells under different collapse blockage degrees was revised based on measured experimental data, which fills the theoretical gap for the selection of pipe flow friction factors under blockage conditions.