A successful gravel pack requires two things: perforations packed with gravel-pack sand and a completely packed annulus. Failure to obtain either one can result in low productivity and/or a sand-producing gravel pack. Alternate-path gravel packing ensures that the annulus will be completely packed with gravel-pack sand. Alternate-path gravel packing has provided long and highly productive completions without any sand production.
This paper presents completion data and production results form 39 wells completed with the alternate-path gravel-pack technique. The completion data include completion length, net perforations, carrier fluid, sand placement outside casing, workover fluid, and return rate during packing. Production results include qualitative results, including permeability-thickness product and skin on select completions.
By the end of 1992, ALLPACKSM (alternate-path gravel packing) completion technology1,2 had been used 43 completions. Included are an oil well reportedly tested at more than 40,000 BOPD by Statoil in offshore Norway, and a gas well tested at more than 70 MMcf/D by Mobil in the Gulf of Mexico. Experience with the new technology is related along with a description of insights and modifications growing out of these applications.
Alternate-path gravel packing uses bypass tubes, or shunt, attached to the screen. The shunts are perforated with small holes at various locations along the screen (Fig. 1). Although slurry can be injected directly into the bypass tubes, the preferred approach has been to leave the tubes open at the top of the annulus to act only as shunts. In a normal gravel-packing operation, the shunts have little impact until a sand bridge forms in the annulus. Then the shunts activate, carrying the slurry through that bridge and any others that subsequently form, ultimately completing the pack (Fig. 1). High well productivity and longevity result from the efficient placement of pack sand in the perforation tunnels accompanied by complete annulus packs.
In laboratory gravel-pack simulator experiments, the shunts have been shown to improve packs with all currently used carrier fluids, including water. Logic suggests that the best results should be obtained with high-quality gels. Because no special pumping procedures and carrier fluids have been used in field applications, which is described later.
The ability to pack extremely long pay intervals, highly deviated wellbores, and/or separate pay zones of highly variable permeability provides new engineering opportunities and challenges. Producing at higher rates from fewer wells can be considered. Simultaneous production from multiple zones is feasible, with possible wireline water shutoff in selected zones. Separate prepacking operations are probably no longer required, and the need for perforations washing can be re-examined.
The packing operation itself can be changed to use the advantages offered by the shunts better. Squeeze packing or severely restricting returns no longer risks incomplete packs resulting form bridging and appears to be preferred procedure. High-pressure pumping with the shunts in place appears to enhance the probabilities for frac-packing as long the screen/washpipe annulus is small enough to limit flow through that path.