CROSS-REFERENCE TO RELATED APPLICATIONS
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
REFERENCE TO MICROFICHE APPENDIX
1. Technical Field
This invention relates to improved methods and apparatus for completing wells, and more particularly, to improved methods and apparatus for gravel packing or frac packing by providing multiple flow paths for slurry flow via blank (nonperforated) tube segments in the well annulus.
2. Background of the Invention
The production of hydrocarbons from unconsolidated or poorly consolidated formations may result in the production of sand along with the hydrocarbons. The presence of formation fines and sand is disadvantageous and undesirable in that the particles abrade pumping and other producing equipment and reduce the fluid production capabilities of the producing zones in the wells.
Particulate material (e.g., sand) may be present due to the nature of a subterranean formation and/or as a result of well stimulation treatments wherein proppant is introduced into a subterranean formation. Unconsolidated subterranean zones may be stimulated by creating fractures in the zones and depositing particulate proppant material in the fractures to maintain them in open positions.
Gravel packs with sand screens and the like have commonly been installed in wellbores penetrating unconsolidated zones to control sand production from a well. The gravel packs serve as filters and help to assure that fines and sand do not migrate with produced fluids into the wellbore.
Cased-hole gravel packing requires that the perforations or fractures extending past any near-wellbore damage as well as the annular area between the outside diameter (OD) of the screen and the inside diameter (ID) of the casing be tightly packed with gravel. See Brochure: “Sand Control Applications,” by Halliburton Energy Services Inc., which is incorporated herein by reference for all purposes. The open-hole gravel-pack completion process requires only that the gravel be tightly packed in the annulus between the OD of the screen and the openhole.
Several techniques to improve external gravel-pack placement, either with or without fracture stimulation, have been devised. These improved techniques can be performed either with the gravel-pack screen and other downhole equipment in place or before the screen is placed across the perforations. The preferred packing methods are either 1) prepacking or 2) placing the external pack with screens in place, combined with some sort of stimulation (acid-prepack), or with fracturing or acidizing. The “acid-prepack” method is a combination stimulation and sand control procedure for external gravel-pack placement (packing the perforations with gravel). Alternating stages of acid and gravel slurry are pumped during the treatment. The perforations are cleaned and then “prepacked” with pack sand.
Combination methods combine technologies of both chemical consolidation and mechanical sand-control. Sand control by chemical consolidation involves the process of injecting chemicals into the naturally unconsolidated formation to provide grain-to-grain cementation. Sand control by resin-coated gravel involves placing a resin-coated gravel in the perforation tunnels. Resin-coated gravel is typically pumped as a gel/gravel slurry. Once the resin-coated gravel is in place, the resin sets up to form a consolidated gravel filter, thereby removing the need for a screen to hold the gravel in place. The proppant pumped in a frac treatment may be consolidated into a solid (but permeable) mass to prevent proppant-flow back without a mechanical screen and to prevent formation sand production. U.S. Pat. No. 5,775,425, which is incorporated herein by reference for all purposes, discloses an improved method for controlling fine particulates produced during a stimulation treatment, including the steps of providing a fluid suspension including a mixture of a particulate coated with a tackifying compound and pumping the suspension into a formation and depositing the mixture within the formation.
A combined fracturing and gravel-packing operation involves pumping gravel or proppant into the perforations at rates and pressures that exceed the parting pressure of the formation. The fracture provides stimulation and enhances the effectiveness of the gravel-pack operation in eliminating sand production. The fracturing operation produces some “restressing” of the formation, which tends to reduce sanding tendencies. See Brochure: “STIMPAC Service Brochure,” by Schlumberger Limited, which is incorporated herein by reference for all purposes. The high pressures used during fracturing ensure leakoff into all perforations, including those not connected to the fracture, packing them thoroughly. Fracturing and gravel packing can be combined as a single operation while a screen is in the well.
“Fracpacking” (also referred to as “HPF,” for high-permeability fracturing) uses the tip-screenout (TSO) design, which creates a wide, very high sand concentration propped fracture at the wellbore. See M. Economides, L. Watters & S. Dunn-Norman, Petroleum Well Construction, at 537-42 (1998), which is incorporated herein by reference for all purposes. The TSO occurs when sufficient proppant has concentrated at the leading edge of the fracture to prevent further fracture extension. Once fracture growth has been arrested (assuming the pump rate is larger than the rate of leakoff to the formation), continued pumping will inflate the fracture (increase fracture width). The result is short but exceptionally wide fractures. The fracpack can be performed either with a screen and gravel-pack packer in place or in open casing using a squeeze packer. Synthetic proppants are frequently used for fracpacks since they are more resistant to crushing and have higher permeability under high confining stress.
In a typical gravel pack completion, a screen consisting of screen units, which are connected together, is placed in the wellbore within the zone to be completed. The screen is typically connected to a tool having a packer and a crossover. The tool is in turn connected to a work or production string. A particulate material, usually graded sand (often referred to in the art as gravel) is pumped in a slurry down the work or production string and through the crossover whereby it flows into the annulus between the screen and the wellbore. The liquid forming the slurry leaks off into the subterranean zone and/or through a screen sized to prevent the sand in the slurry from flowing there through. As a result, the sand is deposited in the annulus around the screen whereby it forms a gravel pack. The size of the sand in the gravel pack is selected such that it prevents formation fines and sand from flowing into the wellbore with produced fluids.
Circulation packing (sometimes called “conventional” gravel-packing) begins at the bottom of the screen and packs upward along the length of the screen. Gravel is transported into the annulus between the screen and casing (or the screen and the open hole) where it is packed into position from the bottom of the completion interval upward. The transport fluid then returns to the annulus through the washpipe inside the screen that is connected to the workstring.
Horizontal gravel packs can be placed in open or cased hole completions of varying lengths. The alpha/beta wave approach has been used extensively for gravel packing horizontal wells. This method is a two-step procedure, which includes an alpha wave sand deposition in one direction and a beta wave sand deposition in the opposite direction. See M. Economides, L. Watters & S. Dunn-Norman, Petroleum Well Construction, at 533-34 (1998), which is incorporated herein by reference for all purposes. Water-based sand slurry is pumped down the vertical work string out the horizontal portion of the screen-casing annulus. A sand dune builds up in the borehole both in the forward direction (away from the vertical borehole) and in the reverse direction (back toward the vertical borehole). The sand dune fills the horizontal borehold annulus to about 50% to over 80% fill (the alpha sand wave deposition). The leading edge of the sand dune progresses toward the toe of the wellbore until it reaches the end of the screen. Then the beta wave deposition of sand in the horizontal borehole begins. The sand movement in the beta deposition occurs in successive waves. However, this approach depends on maintaining a very limited fluid loss. If fluid loss is too great, it will stall wave development, causing a bridge to form that prevents the annular pack from being completed.
A problem often encountered in forming gravel packs, particularly gravel packs in long and/or deviated unconsolidated producing intervals, is the formation of sand bridges in the annulus between the sand retainer screen and the casing wall (for in-casing gravel packs) or the formation (for open-hole gravel packs). Non-uniform sand packing often occurs as a result of the loss of carrier liquid from the sand slurry into high permeability portions of the subterranean zone. This in turn causes the formation of sand bridges before all the sand has been placed. Sand bridges in the interval to be packed prevent placement of sufficient sand below that bridge for top-down gravel packing, or above that bridge for bottom-up gravel packing. When the well is placed on production, the flow of produced fluids is concentrated through the voids in the gravel pack, which soon causes the screen to be eroded, and the migration of fines and sand with the produced fluids to result.
The key to successful frac packs and gravel packs is the quantity of gravel placed in the fracture, perforations and casing/screen annulus. The development of bridges in long perforated intervals or highly deviated wells can end the treatment prematurely, resulting in reduced production from unpacked perforations, voids in the annular gravel pack, and/or reduced fracture width and conductivity.
To prevent the formation of sand bridges and create uniform distribution during gravel packing, “alternate-path” (or “multiple-path”) well screens using perforated “shunt tubes” extending along the screen have been proposed. See, e.g., Jones, L. G., et al.: “Alternate Path Gravel Packing,” SPE 22796, 1991, which is incorporated by reference herein for all purposes. In these well screens, the alternate-paths (e.g., perforated shunts or by-pass conduits) extend along the length of the screen and are in fluid communication with the gravel slurry as the slurry enters the well annulus around the screen. If a sand bridge forms in the annulus, the slurry is still free to flow through the conduits and out into the annulus through the perforations in the shunt tubes to complete the filling of the annulus above and/or below the sand bridge.
Shunt technology is suited for frac-packing. See L. Jones: “Spectacular Wells Result From Alternate Path Technology,” article reprint from Petroleum Engineer International, which is incorporated by reference herein for all purposes. When a shunt system is placed in the squeeze position, the slurry dehydrates as with any fracturing operation until the pack in the fracture builds back into the annulus at some high leak-off location. Then, instead of a sand out, the slurry bypasses the bridge through the shunts and finishes packing the downstream interval. The shunts can be used in multiple intervals isolated by packers. See Brochure: “Alternate Path Service Brochure,” by Schlumberger Limited, which is incorporated herein for all purposes. The shunts are compatible with cup-type annular packers. Different sized tubes can be used for treating and packing different intervals. Shunts in different sizes can result in different flow rates.
In many alternate-path well screens, the individual shunt tubes are carried externally on the outer surface of the screen. U.S. Pat. No. 4,945,991, which is incorporated herein by reference for all purposes, proposes a well screen with perforated shunt tubes attached to the outside of a screen. This patent proposed attaching long, perforated shunt tubes to the exterior of the screen to form a continuous shunt path extending along the entire length of the screen, even when the screen was comprised of multiple sections. The shunt tubes were connected together between all sectional lengths of the screen, to provide a continuous flow path along the exterior of the screen sections for the gravel-laden fluid. (The patents and/or other references mentioned in the Background Section are not admitted to be “prior art” with respect to the present invention merely because they are mentioned herein).
As explained at col. 1, ln. 66-col. 2, ln. 24 in U.S. Pat. No. 6,220,345 external shunt tubes suffer from numerous disadvantages and problems. Problems with the device of U.S. Pat. No. 4,945,991 are that it is troublesome to hang down the device in the wellbore and it is difficult to lift up the device from the wellbore due to the danger of the screen sticking to the wellbore. Besides, it is extremely difficult to connect respective shunt tubes attached to the outside of the screen to shunt tubes attached to the outside of a following screen in the course of assembling the screen and lowering it into the wellbore.
Another disadvantage in mounting the shunts externally is that the shunts are exposed to damage during assembly and installation of the screen. Due to the relative small size of the alternate-path shunt tubes, it is vitally important that they are not crimped or otherwise damaged during the installation of the screen. One proposal for protecting these shunts is to place them inside the outer surface of the screen; see, e.g., U.S. Pat. Nos. 5,476,143 and 5,515,915, which are incorporated herein by reference for all purposes. However, it may be more desirable from an economic standpoint to merely position and secure the by-pass conduits or shunt tubes onto the external surface of a commercially-available well screen.
Another technique proposed for protecting externally positioned shunt tubes involves concentrically mounting a perforated, protective shroud over the screen and the associated shunt tubes. U.S. Pat. No. 5,934,376, which is incorporated herein by reference for all purposes, discloses a new method, called CAPS™, for concentric annular pack screen system, basically comprising the steps of placing a perforated shroud with an internal sand screen disposed therein, in the zone to be completed, isolating the perforated shroud and the wellbore in the zone and injecting particulate material into the annuli between the sand screen and the perforated shroud and the wellbore to thereby form packs of particulate material therein. The system enables the fluid and sand to bypass any bridges that may form by providing multiple flow paths via the perforated shroud/screen annulus. The shroud protects the screen during placement and prevents subsidence problems during production.
U.S. Pat. No. 5,890,533, which is incorporated herein by reference for all purposes, proposes a gravel-pack, well screen having a shunt tube positioned inside the base pipe of the screen. The shunt tube extends substantially throughout the length of the base pipe. A threaded connector or the like is provided on either end of the length of the internal shunt tube to connect the adjacent lengths of shunt tube together.
As mentioned, it is also difficult and time consuming to make all the fluid connections between the respective shunt tubes which are required in making-up a typical alternate-path well screen. The use of thread joints to interconnect adjacent lengths or joints of screen often makes it difficult to circumferentially align each pair of shunt tubes that must be interconnected to maintain axial continuity in the overall shunt flow path. Additionally, a supplemental connection fitting must be used to interconnect and operatively communicate the interiors of each pair of shunt tubes to be connected.
For example, the length of a typical alternate-path well screen is normally substantial (e.g., 1000 feet or more) and is made up of a plurality of 20 foot or longer joints. Each joint is basically similar to the others in that they all are comprised of a permeable section (i.e., length of screen material) having a plurality of axially-extending, individual shunt tubes positioned thereon. In making-up or assembling most of these alternate-path well screens, the desired number of joints are secured together by first coupling the “base pipes” of the screen joints together and then individually, fluidly connecting each of the shunt tubes on a joint to its respective shunt tube on the adjacent joint. Since a typical joint normally has a plurality of parallel, axially-extending shunt tubes thereon with individual connections for each shunt tube is required in making up the necessary fluid connections between the shunt tubes of any two adjacent joints thereby requiring eight different physical manipulations for each joint (i.e., one at each end of each individual connector).
One proposed technique for reducing both the number of connectors and the time required in assembling an alternate-path screen is contained in U.S. Pat. No. 5,390,966, which is incorporated herein by reference for all purposes, and wherein a single connector is used to make a fluid connection between four sets of respective shunt tubes. This device still requires that each shunt tube be substantially aligned with its respective shunt tube on an adjacent joint before the single connector will function.
U.S. Pat. No. 5,868,200, which is incorporated herein by reference for all purposes, proposes an alterate-path, well screen made-up of joints and having a sleeve positioned between the ends of adjacent joints which acts as a manifold for fluidly-connecting the alternate-paths on one joint with the alternate-paths on an adjacent joint. These sleeves surround the respective lower and upper ends of the shunt tubes on adjacent screen joints, which are to be fluidly connected. The sleeves are affixed to the joints, by threads, welding, etc. In some instances, a sealing means (e.g., O-rings or the like) may be required between the sleeves and the joints, to prevent excessive leakage at the manifolding of the joints together. The alternate flowpaths are comprised of perforated conduits, which extend longitudinally along the external surface of the screen. To protect the shunt tubes, a protective shroud is concentrically positioned about each screen joint. The shroud has a plurality of openings through the wall thereof to provide an exit for fluid (e.g., gravel slurry) to pass out of the shroud as it flows out the openings in the shunt tubes and an entrance for fluids into the shroud and through the permeable section of the screen during production.
Another problem that may arise in typical alternate-path well screens is in maintaining adequate and consistent flow of fluid through the relatively small perforations (or “exit ports”) at each of the delivery points along the lengths of the shunt tubes. For example, the flow of the gravel-laden slurry in a gravel pack operation is substantially parallel to the axis of the delivery or shunt tubes until the slurry reaches the respective exit ports along the length of a shunt tube. The flow must then make a “right-angle” turn before it can flow through a respective exit port. This results in a tendency for at least some of the particulates (i.e., sand) to by-pass the ports. This, in turn, causes the sand concentration of the carrier fluid to build-up inside the shunt tube thereby adversely affecting the distribution of the gravel pack. In fracturing operations, at least a portion of any particles (e.g., sand) in the fracturing fluid will have the same tendency to by-pass the exit ports and build-up within the delivery conduit of the tool. This results in a diluted fracturing fluid (i.e., lower concentration of sand) being delivered through the exit ports. Further, in order to maintain the proper pressures at each level along the tool and to prevent premature dehydration of the slurry, each of the exit ports must be relatively small. Unfortunately, the small size (e.g., diameter) of the exit ports severely restricts the volume of fracturing fluid, which can be delivered to each fracturing level thereby further adversely affecting the fracturing operation. Too many holes will provide too much leak-off from the shunts and reduce shunt fluid velocities. Plugging of smaller shunt holes is also a problem.
Thus, there are needs for improved methods and apparatus for completing wells, including providing a simpler, more cost-effective system that uses the alternate path or “bridging bypass” phenomenon to enhance gravel packing and fracturing operations.
The present invention provides improved methods and apparatus for completing wells, including gravel packing and frac packing operations, which meet the needs described above and overcome the deficiencies of the prior art. More specifically, the present invention provides an alternate-path, well screen without requiring that the alternate paths (i.e., delivery or shunt tubes) on adjacent joints of screen be axially aligned or individually connected. This allows the joints to be made-up quickly which speeds up the assembly and installation of the alternate-path, well screen. The delivery tubes (as opposed to the relatively smaller shunt-tube perforations, in typical alternate-path well screens) provide alternate flow paths for the sand-laden fluid to reach the well annulus, so larger volumes of fluid can be delivered and premature dehydration of the slurry and/or sand build-up within the tubes is inhibited. No structure projecting outside the screen such as shunt tubes is provided and therefore, there is no danger of the screen sticking to the wellbore when the screen is lowered or lifted through the wellbore.
The improved methods basically comprise the steps of placing a sand screen (e.g., screens, screened pipes, slotted liners, prepacked screens, etc.) in the wellbore adjacent the zone to be completed, positioning an alternate flowpath comprised of a plurality of blank (non-perforated) tube segments which are open at both ends in the annulus between the sand screen and the wellbore, isolating the annulus between the screen and the wellbore in the zone, and injecting particulate material into the annulus between the sand screen and the wellbore and into the zone, whereby the particulate material is uniformly packed into the annulus between the sand screen and the zone. As used herein, the terms “blank tubes” or “conduits” are used to define a structure forming an elongated closed fluid passageway effectively having only two spaced opening points for flow into and out of the passageway. The permeable pack of particulate material formed prevents the migration of formation fines and sand with fluids produced into the wellbore from the unconsolidated zone.
The present alternate-path, well screen is comprised of one or more basically identical screen joints (or “screen units”). A threaded coupling or the like is provided on either end of the base pipe of the screen joints to connect adjacent joints together. The improved well screen has a cross-over sub or the like attached at its upper end which, in turn, is connected to and suspended in the wellbore by a work string or tubing string.
The alternate-paths (e.g., blank tube segments) are secured or otherwise provided on the external surface of the screen for delivering a gravel slurry to the interval of the wellbore being completed. The blank tubes extend in the axial direction of the screen disposed cylindrically at a predetermined interval in the circumferential direction of the screen. Each of the blank tubes extends only a portion of the length of the screen. The blank tubes are open at both ends and adapted to receive the gravel slurry as it reaches the apparatus and direct it to the interval. The upper and lower ends of the blank tubes preferably have an arcuate or beveled shape. If a sand bridge forms in the annulus between the sand screen and the wellbore, the slurry is still free to flow through the blank tubes and out into the annulus through the lower, open lower ends of the blank tubes to complete the filling of the annulus above and/or below the sand bridge. The use of the larger (in area) open, lower ends of the delivery tubes to deliver the slurry to the well annulus alleviates the problem of the relatively smaller exit ports along the length of a typical shunt tube often becoming blocked with sand prior to the completion of the operation.
In some instances, it may be desirable to also include a protective shroud mounted over the sand screen and the associated multiple flowpaths (e.g., blank tubes) to protect the alternate flowpaths during installation of the well screen. In such instances, the protective shroud has a plurality of openings in the wall thereof to allow fluid from the lower, open ends of the blank tubes to flow through the shroud and into the well annulus during a gravel pack operation and for fluids to flow into the shroud and through the sand screen during production.
The present methods can be combined with other techniques, such as prepacking, fracturing, chemical consolidation, etc. The methods may be applied at the time of completion or later in the well's life. The unconsolidated formation can be fractured prior to or during the injection of the particulate material into the unconsolidated producing zone, and the particulate material can be deposited in the fractures as well as in the annulus between the sand screen and the wellbore.
The improved spacing methods and apparatus of this invention provide a simpler, more cost-effective system with multiple paths, so that a slurry can bypass any premature annulus bridges that form during gravel packing or frac packing and halt the packing process. The system may be used in long intervals and variable formations.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows when taken in conjunction with the accompanying drawings, in which: