Experimental Investigation of the Influence of Perforating on Gravel-Pack Impairment
- R.H.J. Blok (Shell Intl. E&P B.V.) | R.W.F. Welling (Shell Offshore Inc.) | L.A. Behrmann (Schlumberger Reservoir Completions) | Venkitaraman Aadinathan (Schlumberger Reservoir Completions)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2000
- Document Type
- Journal Paper
- 203 - 209
- 2000. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 2.4.5 Gravel pack design & evaluation, 2.3 Completion Monitoring Systems/Intelligent Wells, 2.7.1 Completion Fluids, 4.2.3 Materials and Corrosion, 1.8 Formation Damage, 5.3.2 Multiphase Flow, 2 Well Completion, 1.6.9 Coring, Fishing, 1.2.3 Rock properties, 2.2.2 Perforating, 1.6 Drilling Operations, 2.2.3 Fluid Loss Control, 1.14 Casing and Cementing
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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 [unconfined compressive strength (UCS)=1,200 psi] was conducted to quantify this. Castlegate sandstone rock samples, 7-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 pre-perforation, 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) vs. "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.
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 recognized 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 Saucier1 and by Sparlin2 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.3 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.4 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 IGP completion. High pressure drops over the perforation tunnel were observed, indicating reduced gravel permeability inside the tunnels.
To investigate the impact of perforating on IGP impairment, an experimental program was defined comprising single shot perforating and gravel-packing experiments in the modified API RP-43 Sec. 4 perforating facility at Schlumberger Perforating and Testing Center (SPT). The primary objective of these experiments was to identify the perforating-related impairment mechanism in internal gravel packs. Some of the experimental variables are charge type, level of underbalance, tunnel cleaning, and shaped charge perforating vs. alternative methods (abrasive jetting and drilling). In this paper we present the experimental results of the first 15 experiments. These results provide strong evidence that a key impairment mechanism in IGP wells is intermixing and invasion of fine material from rock debris generated by the perforating process into the gravel resulting in a large reduction of gravel permeability and low productivity. A more theoretical investigation and results of post-experimental analyses will be presented in a follow-up paper.
The experiments were conducted on poorly consolidated sandstone (Castlegate). Average rock properties are listed in Table 1. It should be pointed out that the Castlegate sandstone is still more consolidated than most producing gravel-packed reservoirs. While in these experiments perforation tunnels were observed, in a much weaker rock such tunnels may not have been seen. The test matrix of the 15 experiments is shown in Table 2. In this paper we cover the results of the shaped charge perforations (tests 4 through 9, 14, and 15). The results of the abrasive jetting and drilled hole experiments are mentioned briefly to allow comparison with shaped charge experiments.
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