Sliding-Sleeve System with Annular Fracs
The Multistage Unlimited sliding sleeve/annular frac system combines resettable frac isolation with our exclusive Grip/Shift sliding sleeves to deliver fast, economical, and effective multistage completions.
Grip/Shift sliding sleeves
Multistage Unlimited Grip/Shift sleeves are run as part of the completion casing string, inserted at planned frac points. These mechanically shifted sliding sleeves can be cemented along with the rest of the casing string, eliminating the need to use casing packers for zone isolation. Grip/Shift sleeves are built to match the host casing and are shipped with an upper handling pup joint and conventional pin-down/box-up configuration for easy handling and make-up at the rig.
Sand-friendly, resettable frac plug
The unique Multistage Unlimited resettable frac plug has three functions: isolate the frac zone from the hole below, mechanically shift the sliding sleeves open, and anchor the isolation assembly during frac pumping.
- automatic J-slot mechanism sets, releases, and resets the tool with straight up/down coiled tubing motion.
- integral equalization valve facilitates releasing the frac plug.
- Extremely sand-friendly and compatible with frac operations. Pulling it up through clean fluid a short distance completely flushes any sand or proppant from the tool, leaving it ready to set at the next stage.
- Engineered elastomer seal contains differential pressure up to 8,500 psi, well above typical formation breakdown pressure and temperatures up to 350° F. The frac plug has been used for as many as 44 sleeve-shift/frac cycles without coming out of the hole.
The resettable frac plug has been used to isolate more than 32,000 stages and has been cycled as many as 44 times during a one-trip completion operation.
Sand-jet perforating sub
Sand-jet perforating is used to add stages in blank casing without tripping out of the hole. It is also used in lieu of sliding sleeves when completion economics dictate or when the well is already cased (see Sand Jet Perforating for details). See Sand-jet perfs with annular fracs
The sleeve locator is a unique one-piece sleeve with a special profile that matches a recess at the bottom of the grip/shift sleeves. As the frac-isolation assembly is pulled up through the sleeve, the keys snap into the recess. The instant increase in coiled tubing weight is positive indication that the assembly is properly positioned inside the sleeve. When the sleeve shifts, the recess closes, forcing the locator to retract; this provides a secondary confirmation that the sleeve has shifted.
Decompression sub (not shown)
A flow path is required at the toe of the casing to relieve the pressure when the first sleeve is shifted. The optional Multistage Unlimited decompression sub, made up below the sleeve locator, is a sealed atmospheric-pressure chamber with a burst disc exposed to the coiled tubing annulus. When the frac plug moves downward, fluid pressure ruptures the disc, allowing displaced fluid to flow into the empty chamber in a virtually instantaneous operation.
Other components (not shown)
The typical frac-isolation assembly also includes fluted centralizers and a coiled tubing disconnect sub.
Typical frac sequence
Completions begin at the toe of the well, with the frac-isolation assembly positioned below the sliding sleeve. As the isolation assembly is pulled upward, the sleeve locator keys snap into a recess at the bottom of the sliding sleeve. Then coiled tubing set-down weight is applied to set the resettable frac plug. The slips grip the inside of the sleeve, and the packer element seals against the sleeve’s inner barrel to seal off the wellbore below. An increase in wellbore pressure from above forces the assembly and the inner barrel down, opening the sleeve’s frac ports to the formation. Sleeve shifting is clearly indicated at the surface by weight and pressure changes. As the sleeve shifts, the locator recess closes, disengaging the locator keys and providing an additional verfication that the sleeve has shifted (useful in the event that the formation will not take fluid).
The frac is pumped down the coiled tubing/casing annulus, except in the case of low-rate fracs, when it can be pumped through the coiled tubing (see the Half-Straddle system). [Link]
During the frac, the coiled tubing serves as a deadleg, transmitting bottom-hole treating pressure to the surface to assist in adjusting pad size, sand concentration and ramp, and pump rate in real time.
After the frac is away, a pull on the coiled tubing opens the integral equalizing valve and unsets the frac plug. The isolation assembly is moved up to the next sliding sleeve, and the sequence is repeated. In about 5 minutes, the stage is ready to frac. With planning and coordination, stages can be completed quickly and smoothly at a rate of more than one per hour.
After the last stage, the isolation tool assembly is pulled from the well and the full-open wellbore is ready to produce.
Adding stages on the fly
If the formation at a sliding sleeve will not take fluid, the isolation assembly can be released, moved up, and set in a section of blank casing. Sand-laden fluid is pumped down the coiled tubing and through the perforating sub nozzles. The high-velocity slurry cuts through the casing and cement and into the formation in about 7 minutes, providing a fresh frac-initiation point. See page 6 for details on Multistage Unlimited sand-jet perforating.