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Gravel pack equipment and tools
A myriad of gravel-pack systems are available to handle virtually any conceivable well condition. Gravel pack completion equipment is the equipment that remains in the well after the gravel placement operations are complete. The equipment discussed does not represent all the types of available equipment, but it does represent a typical gravel pack completion. Certain well conditions may require compromises in the type and design of gravel-pack equipment that can be used. Another important concept is that there may be several, yet equally effective, ways to complete a well.
Typical gravel pack
Fig. 1 illustrates typical gravel packs for cased and openhole completions. These employ crossover gravel packing equipment that is state-of-the-art in the industry today. Washdown and reverse circulation methods are other alternatives that are less expensive and are to be used when costs will not support crossover equipment.
Gravel pack base
The first step in installing a gravel-pack completion is to establish a base on which the screen will rest. In cased-hole completions, the most common type base is a sump packer. The sump packer is normally run into the well on an electric wireline before perforating and is set a specified distance (5 to 10 ft) below the lowest planned perforation. The distance below the perforations must accommodate the length of the seal assembly and production screen overlap.
Although sump packers are the preferred gravel-pack base, other options such as a bridge plug or cement plug can be used. In openhole completions, provisions for a debris sump or logging access can be achieved, but these are not routine and may not be feasible in some situations. Therefore, the gravel-pack base is normally a bull plug on the bottom of the screen. The types of common gravel-pack bases are illustrated in Fig. 2.
The seal assembly is required to establish a seal in the bore of the sump packer to prevent gravel-pack sand from filling the bottom of the well during gravel packing. In the case of multiple gravel packs, the seal also provides for zonal isolation. The seal assembly used to engage the sump packer is normally a snap latch type or other type holddown.
Gravel pack screen
The purpose of the gravel-pack screen is to create an annulus between the screen and the casing/open hole and to hold the gravel in place during production. There are several different types of screens.
Filling the annulus between the screen/casing (or open hole) with gravel-pack sand is essential to the control of formation sand production. To ensure that the annulus is filled completely around the screen, centralization of the screen is required. In cased-hole completions, weld-on, blade-type centralizers are normally used. The blades are approximately 6 in. long and are cut from a 0.25- to 0.50-in.-thick plate or steel. The edges of the centralizers are beveled to ensure easy run-in. The centralizers consist of four blades welded to the screen base pipe 90° apart to result in an outside diameter (OD) approximately 0.25 in. under the inside diameter (ID) of the well’s casing. The centralizers are spaced 15 to 20 ft apart and can be positioned at the top, bottom, and/or middle of a screen joint as required.
In openhole gravel packs, centralization is accomplished with bow-spring centralizers. These centralizers consist of a top and bottom collar connected with 4 to 6 steel spring bows. The bows can be compressed (i.e., the centralizer is elongated) for running through restricted IDs. When the centralizer enters a larger ID, the bows attempt to expand to their original position, resulting in a restoring force or centralization. Sufficient centralizers are required such that the combined restoring force is capable of lifting the weight of the screen in the given hole conditions. Computer programs are available for determining optimum centralizer spacing for a specific bow-spring centralizer, hole size, and deviation. See API Spec. 10D, Specification for Bow-Spring Casing Centralizers.
The purpose of blank pipe is to provide a reservoir of gravel-pack sand above the screen to ensure that the screen remains completely packed in the event of pack settling. During gravel-pack operations, it is possible for minor voids in the annulus pack to occur. In fact, gravel packing with viscous gel transport fluids commonly produces voids, particularly opposite the short lengths of blank pipe between screen joints. Depending on deviation angle, pack settling shortly after gravel placement may fill the voids. It is important to have a sufficient reserve of gravel-pack sand available for this process to occur without uncovering the top of the screen.
Blank pipe centralization
As with the screen, the blank pipe must be centralized to ensure even gravel distribution in the blank and casing annulus. Weld-on centralizers are normally used in both cased-hole and openhole completions because the blank pipe is almost always positioned inside the casing. Bow-spring centralizers can be used if desired or required.
Blank pipe length
Several rules of thumb exist for determining the length of blank pipe. Perhaps the most scientific method would be to recognize that voids will occur within the length of screen wherever nonscreen regions exist (i.e., at screen joint connections and above the gravel pack). A long-standing guideline for gravel reserve has been to maintain a minimum of 30 ft of packed gravel in the blank pipe above the top of the screen when packing with brine. When viscous fluids are used, blank lengths may be as much as twice the screen length for short completion intervals. This allows for additional settling with these fluids when the gel breaks.
Tell-tale screens are short screen sections that are sometimes used to assist with gravel placement and determine when the gravel pack is complete. Their benefit is questionable. There are two types of tell-tale screens: the upper and lower versions.
- Upper tell-tale screens are used primarily with brine-pack systems. They are typically located about 30 ft above the main gravel-pack screen. Their function is to indicate, by an increase in pressure, when the dehydrated gravel has reached the tell-tale location. This assures that there is the desired amount of gravel reserve.
- Lower tell-tale screens are used when gravel packing with viscous fluids. Their purpose is to assist in ensuring that the gravel slurry reaches the bottom of the gravel pack before the slurry dehydrates. The gravel-pack tools are usually in the lower circulating position when the tell-tale is used in these installations.
Shear out safety joint
A shear-out safety joint is located just above the blank pipe. It consists of a top and bottom sub connected by shear screws. This device is incorporated in most gravel pack completion assemblies to allow retrieval of the gravel-pack packer and the gravel-pack extension independently of the blank pipe and screen. The joint is parted with straight tension to shear the screws while pulling the packer with a packer-retrieving tool.
Knock out isolation valve
The knock-out isolation valve is a mechanical fluid-loss device that prevents completion fluid losses and subsequent damage to the formation after performing the gravel pack. The downward closing flapper in the valve is held open by the gravel-pack service tools (normally the washpipe) during the gravel pack. When the service tools are removed from the valve, the flapper closes, preventing fluid loss to the formation. The gravel-pack service tools can be removed from the well and the completion tubing run. When the well is producing, the flapper will open. Alternatively, the flapper is made of a breakable material and can be broken hydraulically or mechanically before producing the well.
Gravel pack extension
Gravel-pack extensions are used with the gravel-pack packer and service tools to provide a flow path from the tubing above the packer and to the screen/casing annulus below the packer. The gravel-pack extension consists of the upper extension (which contains flow ports for the gravel pack fluids), sealbore (sized to match the bore of the gravel-pack packer), and lower extension (to house the gravel-pack crossover tool throughout its range of motion). The length of the gravel-pack extension is designed to work with a particular gravel-pack packer and crossover tool. Gravel-pack extensions are available in two types:
- Sliding sleeve
Gravel pack packer
At the top of the gravel-pack assembly is a gravel-pack packer. The packer may be permanent or retrievable. However, retrievable type packers are recommended for gravel packing. A retrievable packer expedites workover activities without the potential cost and risk of milling a permanent packer. The retrievable packers used for gravel packing are usually sealbore type packers that can also be used for production; therefore, the packer must be designed for the temperature, pressure, and environmental conditions present in the well.
Gravel pack service tools
Gravel-pack service tools are the equipment necessary to perform the gravel pack. They are removed from the well after gravel packing. In most cases, the type of gravel-pack equipment used dictates the service tools required for a gravel pack. Further discussion of the service tools is discussed next.
Hydraulic setting tool
The hydraulic setting tool is a hydraulic piston that generates the force required to set the gravel-pack packer. It is attached to the top of the crossover tool and has a sleeve shouldered against the setting sleeve of the packer. A setting ball is dropped to the ball seat in the crossover tool to plug off the ID of the work string. Applied pressure to the work string acts on a piston in the hydraulic setting tool to force the sleeve down to compress the slips and packing element of the packer. Special versions of the setting tool are available, which allow for rotation and high-circulating rates while running the gravel-pack assembly.
Gravel pack crossover tool
The gravel pack crossover tool creates the various circulating paths for fluid flow during gravel packing. The crossover tool consists of a series of molded seals surrounding a gravel-pack port midway down the tool and a return port near the top of the tool. A concentric tube (washpipe) design in the crossover tool along with the gravel-pack packer and gravel-pack extension allow fluid pumped down the work string above the packer to “cross over” to the screen/casing annulus below the packer. Similarly, return fluids flowing up the washpipe and below the packer can “cross over” to the work string/casing annulus above the packer.
Gravel pack crossover tools typically have three positions, as illustrated in Fig. 3:
- Reverse circulating
The squeeze position is located by positioning to seal the return ports. The squeeze position allows all fluids pumped down the work string to be forced into the formation. It is used to perform squeeze gravel-pack treatments and/or inject acid treatments into the formation. The circulating position is located by picking the crossover tool up approximately 18 in. above the squeeze position. The circulating position works with a properly sized washpipe to provide a flow path to circulate gravel-pack sand to completely fill the screen/casing annulus. The fluids flow down the work string into the crossover tool, out the gravel-pack extension, down the screen/casing annulus into the screen, up the washpipe into the crossover tool again, and up the work string/casing annulus. Special, high-rate, erosion-resistant crossover tools are available for high-rate brine or frac-pack completions.
Washpipe is run below the gravel-pack crossover tool inside the blank pipe and screen to ensure that the return circulation point for the gravel-pack carrier fluid is at the bottom of the screen. The washpipe assists in placing gravel-pack sand at the bottom of the screen and packing from the bottom up. The end of the washpipe should be as close to the bottom of the screen as possible.
Maximizing the washpipe outer diameter (OD) increases the resistance to flow, preferentially into the washpipe/screen annulus. The greater resistance to flow forces the gravel-pack transport fluid to flow in the screen/casing annulus and carry the gravel-pack sand to the bottom of the well. That causes the gravel packing of the screen/casing annulus to be more complete. The optimum ratio of washpipe OD to screen base pipe ID should be approximately 0.8. Achieving this ratio in some screen sizes will require the use of special flush-joint washpipe connections.
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