Casing Sleeves with Annular Fracs
The Multistage Unlimited casing sleeve/annular frac system combines resettable frac isolation with our exclusive GripShift casing sleeves to deliver fast, economical, and effective multistage completions.
GripShift sliding sleeves
Multistage Unlimited GripShift sleeves are run and cemented as part of the completion casing string, inserted at planned frac points. GripShift 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 into the casing string.
Resettable isolation bridge plug
The unique Multistage Unlimited resettable bridge 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 bridge plug has been used for as many as 93 sleeve-shift/frac cycles without coming out of the hole.
The resettable frac plug has been used to isolate more than 42,000 stages.
Sand-jet perforating sub
Sand-jet perforating is used to add stages in blank casing without tripping out of the hole. It can also be used in lieu of sliding sleeves when completion economics dictate or when the well is already cased.
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 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 first casing 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 bridge 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).
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 7 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 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 Multistage Unlimited sand-jet perforating.