History of gravel packs

History

The earliest gravel packs were performed in shallow, vertical wells, typically by simply pouring gravel into the tubing/casing annulus and allowing the gravel to settle around a screen. Some screens were even washed into place after the gravel was placed. The technique is still employed in water wells but now is seldom used in oil/gas wells. As equipment and technology improved, gravel packing of oil/gas wells was accomplished by mixing sand in brine and pumping the mixture into the hole. Brine represents the simplest of the transport fluids. Before the early 1960s, brine was the most commonly used gravel-pack fluid because other fluid systems had not been developed at that time.

The early equipment used to mix brine and gravel was inefficient and resulted in the “slugging” of gravel into the hole, as opposed to a consistent brine-to-gravel mix ratio. The brine was seldom filtered, and no specifications were in place to ensure the quality of gravel-pack sand. Overall rig housekeeping was poor, and the perforating techniques available were limited to low-shot density, small-diameter guns that produced entrance-hole diameters that were less than 0.5 in. in diameter. The combination of all these factors resulted in unsatisfactory gravel-pack completions that were commonly damaged.

In the late 1960s, research efforts by several companies focused on improving gravel packing. The research efforts culminated in the introduction of viscosified gravel transport fluids, HEC being the fluid of choice. One of the most attractive features of viscous fluids is that it permits the transport of high gravel concentrations (up to 15 lbm/gal). HEC gel provided a reasonably clean medium for transporting the gravel-pack sand, the gel allowed consistent batch mixing, and it protected the gravel from crushing and contamination during pumping. Because of its apparent advantages, HEC fluids rapidly replaced brine as the gravel packing fluid of choice. HEC gels remained the “state-of-the-art” gravel transport fluid for many companies until the early 1990s.

Despite the advances in gravel quality, wellbore cleanliness, fluid filtration, and perforation quality, gravel-packed wells were not, in general, producing as efficiently as theoretically possible. Gravel-pack skins from 20 to 100 were common when gels were used. Also, it became common knowledge that gravel packs performed with gelled fluids commonly produced voids in the packs. HEC was evidently not as nondamaging as originally assumed, and as a consequence, improved shear mixing procedures were developed. Despite better mixing, damage because of residual gel remained likely. Research also indicated that HEC did not pack perforations efficiently in deviated wells with a large interval zone length. Alternatives to HEC, such as crosslinked (XC) polymers and other special gels, were proposed as the ideal gravel-pack fluid but were never completely accepted.

Research and operating data presented in the early 1990s showed that water was a general-purpose gravel transport fluid that produced low-porosity packs that did not contain voids and was capable of efficiently prepacking perforations, provided that fluid loss was acceptable. Improved mixing equipment was developed for handling brine-sand mixtures in water-pack systems. The equipment allowed consistent mixing of gravel in brine and redirected attention to brine as the gravel transport fluid of choice. Coupled with research data and positive field results, these developments initiated the trend for most of the industry to accept brine as a gravel-pack carrier fluid. Although gel represented an improvement in technology at the time and is still applicable for certain well situations, brine is the most widely used gravel-pack fluid in the industry today. However, gelled fluids are used extensively for frac packing.

Continued evolution of procedures saw the introduction of DE filtration systems (circa 1980) that were able to filter large quantities of brine quickly at a reasonable cost. Coupled with the increasing use of clear brine, DE filtration systems resulted in substantially cleaner wellbores than previously had been possible.

In 1986, the API introduced specifications for gravel-pack sand (API RP 58, Testing Sand Used in Gravel-Packing Techniques) that established rigorous requirements. The API specifications called for gravel, sieved to strict tolerances with low crush resistance and acid solubility, that was capable of passing through pumping equipment with little or no degradation. Finally, in the early 1980s, underbalanced-tubing-conveyed perforating became a common and well-established technique for achieving the high-shot density, large-hole diameter, clean perforations required for maximum gravel-packed well productivity. All of these improvements, developments, and changes significantly improved the gravel-packing systems that are now offered on a routine service.

Noteworthy papers in OnePetro

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External links

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See also

Gravel pack design

Gravel pack equipment and tools

Gravel placement techniques

Sand control