1. Field of the Invention
The invention relates generally to the design of bridge plug slips.
2. Description of the Related Art
Bridge plugs are used to form closures in a flowbore. Typically, bridge plugs have a plug body with slip elements that can be selectively moved radially outwardly to bitingly engage a surrounding tubular member. One type of bridge plug is described in U.S. Pat. No. 6,167,963 issued to McMahan et al. That patent is owned by the assignee of the present application and is incorporated herein by reference.
Often, a bridge plug will need to be removed after it has been set, and this is usually done by milling through the plug. Unfortunately, milling through most conventional bridge plug designs leaves large pieces which may be difficult to circulate out of the flowbore.
The present invention provides a design for a bridge plug wherein the slip elements of the bridge plug include an inner body portion that is substantially formed of a material that is degradable by dissolution in response to a dissolving fluid and a hardened, resilient, radially outer contact portion. In described embodiments, the outer contact portion is substantially formed of a hardened material, such as cast iron, that is shaped to provide for biting into a surrounding tubular member. In described embodiments, the outer contact portion extends from the upper end of the slip element to the lower end of the slip element. Also in described embodiments, the outer contact portion includes a plurality of openings that function as stress risers.
In described embodiments, the inner body portion is substantially formed of a material that is dissolvable in response to a dissolving agent. In one current embodiment, the dissolvable material forming the inner body portion comprises magnesium powder. When the dissolvable material is magnesium powder, the dissolving agent may be potassium chloride (kcl).
As described, the slip inserts are cast within a surrounding molding to create a slip ring which can then be disposed onto the setting cone of the bridge plug. In described embodiments, the molding is a phenolic material which provides a laminate covering for the slip elements that protects the dissolvable material against premature dissolution.
In operation, the bridge plug is disposed into a flowbore and then set. When it is desired to remove the bridge plug from the flowbore, a milling device is used. During removal of the plug by milling, the molding of the slip ring is ruptured by the mill, which exposes the dissolvable material forming the inner body portions to wellbore fluid which contains the dissolving agent. The dissolving agent dissolves away the inner body portions, leaving the outer contact portions of the slip elements. The presence of openings disposed through the outer contact portions assists in disintegration of the outer contact portions into smaller component parts via operation of the milling device. The outer contact portions, or portions thereof, and other components of the bridge plug may be circulated out of the wellbore via fluid returns.
For a thorough understanding of the present invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, wherein like reference numerals designate like or similar elements throughout the several figures of the drawings and wherein:
In preferred embodiments, the slip elements 20 are cast within a surrounding molding 21, which is best seen in
The slip elements 20 are moveable upon the ramps 16 of the setting cone 12 between the retracted, unset position shown in
The structure of the slip elements 20 is better appreciated with reference to
In the instance wherein the dissolvable material is magnesium-based composite-powder compact, the dissolving agent may comprise various brines or acids often used in an oil or gas well. The brines include, but are no limited to, potassium chloride (kcl), sodium chloride (NaCl) and calcium chloride/calcium bromine (Ca2Cl/CaBr2). The acids include, but are not limited to, hydrogen chloride, acetic acid and formic acid. In particular embodiments, the dissolving agent is a solution that includes from about 2% to about 5% potassium chloride. In a particularly preferred embodiment, the dissolving agent is a solution that includes about 3% potassium chloride.
Also in present embodiments, the inner body portions 22 are entirely covered by the phenolic material forming the molding 21. As
In addition, openings 30 are preferably formed through the contact portion 24. The openings 30 introduce points of weakness in the structure of the portion 24. Thus, they serve as stress risers which assist the outer contact portion 24 in disintegration during removal of the bridge plug 10 by drilling.
The contact portion 24 (or 24′) preferably extends from the upper end 32 to the lower end 34 of the slip element 20. The outer contact portion 24 (or 24′) is preferably affixed to the body portion 22 using a suitable adhesive.
In operation, the bridge plug device 10 is run into a flowbore and then moved from its unset position to a set position, in a manner known in the art. The outer contact portions 24 (or 24′) of the slip elements 20 engagingly contact the surrounding tubular member.
When it is desired to remove the bridge plug device 10 from the flowbore, a drilling or milling device, of a type known in the art, contacts the bridge plug 10 and begins to destroy it by grinding action.
Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.