Experimental investigation of the influence of perforating on gravel-pack impairment

Analysis of the performance of perforated, gravel-packed wells compared wit h openhole gravel-packed wells has indicated that the perforation could be responsible for the low productivity of internally gravel-packed wells. A s eries of laboratory experiments on weak rock [unconfined compressive streng th (UCS)=1,200psi] was conducted to quantify this. Castlegate sandstone roc k samples, 7-in. diameter and Is-in. long, were perforated with shaped char ges under downhole conditions. The perforations were gravel packed by circu lating a gravel slurry past the tunnel entrance while applying a differenti al pressure. The pre-perforation, post-perforation and post-gravel-pack pro ductivities of the core samples were monitored at different flow rates (5 t o 20 B/D/perf). Sectional photographs of the core samples were taken at pos t-gravel-pack conditions for qualitative analysis. Performance evaluation o f gravel-packed "reaIistic perforations" (debris and loose sand in the tunn el removed by drawdown-induced flow) vs. "ideal perforations" (debris and l oose sand removed by external means) showed the post-gravel-pack productivi ties of realistic perforations to be much lower than ideal perforations in oil-phase as well as water-phase experiments. Injection of fluid after grav el packing only temporarily unplugged the perforation tunnel. The study sho ws the importance of fines- and debris-free perforation tunnels in maintain ing the productivities of gravel-packed completions.