Experimental Investigation of the Influence of Perforating on Gravel-Pack Impairment
Authors: Blok R.H.J. , Welling R.W.F. , Behrmann L.A. , Venkitaraman A. 
Published: December 2007

Abstract
Analysis of the performance of perforated, gravel-packed wells compared with openhole gravel-packed wells has indicated that the perforation could be responsible for the low productivity of internally gravel-packed wells. A series of laboratory experiments on weak rock (UCS = 1200 psi) was conducted to quantify this. Castlegate sandstone rock samples, 7 in. in diameter and 18 in. long, were perforated with shaped charges under downhole conditions. The perforations were gravel packed by circulating a gravel slurry past the tunnel entrance while applying a differential pressure. The preperforation, post-perforation and post-gravel-pack productivities of the core samples were monitored at different flow rates (5 to 20 b/d/perf). Sectional photographs of the core samples were taken at post-gravel-pack conditions for qualitative analysis. Performance evaluation of gravel-packed "realistic perforations" (debris and loose sand in the tunnel removed by drawdown-induced flow) versus "ideal perforations" (debris and loose sand removed by external means) showed the post-gravel-pack productivities of realistic perforations to be much lower than ideal perforations in oil-phase as well as water-phase experiments. Injection of fluid after gravel packing only temporarily unplugged the perforation tunnel. The study shows the importance of fines- and debris-free perforation tunnels in maintaining the productivities of gravel-packed completions.

Introduction
Gravel packs have been applied extensively over past decades for sand control. Although efficient for sand exclusion, gravel packs are often associated with relatively high values of skin and low completion efficiencies. In particular, internal (cased hole) gravel packs often represent a significant flow restriction in a well completion. The cause of impairment of gravel packs has been studied extensively in the past. The impact of poor completion operation practices such as dirty completion fluids and poorly selected fluid-loss control pills has been addressed in many studies. Although these factors are now generally recognised as contributing factors to impairment, field implementation of the results of these studies has not resulted in the anticipated improvement in gravel-pack productivity. Other studies have focused on the cause and consequences of low gravel permeability. Permeability reduction resulting from intermixing with formation material has long been suspected as a key factor in gravel-pack impairment. Work done by Saucier and Sparlin shows that gravel permeability reduces significantly when intermixing of gravel and formation sand occurs.
A common observation in gravel-packed wells is a lower average completion efficiency of internally gravel-packed (IGP) wells compared to externally (openhole) gravel-packed (EGP) wells. This suggests that a significant part of IGP impairment may be directly related to the perforations. This is supported by a study carried out by Welling et al. on a large number of gravel-packed completion intervals. This study shows that besides well-known impairing factors, such as contaminated completion fluids and fluid-loss control pills, the perforation tunnel is the most critical area in an IOP completion. High pressure drops over the perforation tunnel were observed, indicating reduced gravel permeability inside the tunnels.
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