Downhole-milling-tool method

Abstract

A downhole-milling-tool method for milling through hard substances, such as barite, found in underground wells, providing a stepped increase of diameters and positioning of carbide cutters and appropriate positioning of fluid ports and channels, to provide removal of cuttings and cooling and lubricating of the cutting head, in turn providing more efficiency and a better rate of penetration (ROP).

Claims

1. A downhole-milling-tool method for downhole drilling operations in a well through hard material with a coiled-tubing workstring, having a fluid motor using drilling fluid, generating cuttings of hard material to be flushed away by drilling fluid, and having, in use, a downhole direction and a wellhead direction, and a direction of fluid-motor spin, the downhole-milling-tool method comprising: (i) providing a downhole milling tool comprising: (a) a tool body adapted to being mounted on the downhole end of a coiled-tubing workstring, said tool body having a cylindrical tubular form with a perimeter and an internal axial conduit for passage of drilling fluid, having a maximum external-surface diameter portion towards the wellhead end, and at least one stepped-down external-surface portion towards the downhole end, and having a shoulder at each step-change of external-surface diameter; (b) a plurality of fluid ports adapted to allow passage of drilling fluid from the internal axial conduit of said tool body out through the external surfaces of said tool body, at least one said fluid port being located on the downhole end of said tool body, and at least one said fluid port on each shoulder of said tool body; (c) a forward-bits group comprising carbide bits affixed to the downhole end of said tool body; (d) at least two leading-bits rows, each comprising carbide bits affixed to the external surface of said tool body, and having a first average profile radially perpendicular to said tool body, and being affixed in a rotationally balanced relationship with maximal spacing from each other around the perimeter; and (e) at least two following-bits rows, each comprising carbide bits affixed to the external surface of said tool body, and having a second average profile, lower than the first, radially perpendicular to said tool body, being affixed in a rotationally balanced relationship with maximal spacing from each other around the perimeter; where each said following-bits row is further affixed to said tool body adjacent to a corresponding said leading-bits row, such that, in use, each said leading-bits row precedes the corresponding said following-bits row along the direction of spin; where each adjacent pair of a said leading-bits row and said following-bits row are affixed in a rotationally balanced relationship with maximal spacing from each other around the perimeter, and defining an axially oriented continuous no-bit area on the external surface of the tool body between each said adjacent pair; and where each adjacent pair of a said leading-bits row and a said following-bits row provides a gap defining a no-bit area on the external surface of said tool body along each said adjacent pair, and each said no-bit area gap provides communication across said adjacent pair between said axially oriented no-bit areas; (ii) mounting said downhole milling tool on the end of the coiled-tubing workstring; (iii) entering the well; and (iv) pumping drilling fluid under pressure through the workstring and fluid motor, to said downhole milling tool; where, in use, said forward-bits group makes initial contact with a smaller central cross-sectional area of the hard material and begins breaking it up, the drilling operation being cooled and lubricated, and the cuttings being flushed away by drilling fluid expelled from said at least one fluid port located at the downhole end; where, as said downhole milling tool advances, a slightly-larger-circumference area of material is chipped away by said leading-bits rows, and each said leading-bits row is followed immediately by a said following-bits row, which further chips or crushes the cuttings, and where additional drilling fluid is expelled from said fluid ports at the shoulders and flows upwards through a channel formed by the arrangement of said no-bit areas, flushing the cuttings upwards; and where, as said downhole milling tool advances further, a larger-circumference area of material is removed by the next-larger portion of said downhole milling tool, the process repeating for each step up in diameter.

2. The downhole-milling-tool method of claim 1, where said tool body is made of steel.

3. The downhole-milling-tool method of claim 1, where said tool body has a largest external-surface diameter of between 2 and 2.5 inches, inclusive.

4. The downhole-milling-tool method of claim 1, where said at least one stepped-down external-surface portion further comprises at least two stepped-down external-surface portions.

5. The downhole-milling-tool method of claim 1, where said no-bit area gaps are further arranged to provide a helical path of gaps.

6. The downhole-milling-tool method of claim 1, where said fluid ports further comprise two said fluid ports at each shoulder, arranged in a 180-degree relationship each to the other.

7. The downhole-milling-tool method of claim 1, where said hard material is barium sulfate.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Reference will now be made to the drawings, wherein like parts are designated by like numerals, and wherein:

(2) FIG. 1 is a schematic view illustrating the downhole milling tool of the invention in use;

(3) FIG. 2 is a nominal top view of the downhole milling tool of the invention;

(4) FIG. 3 is a nominal top view of the downhole milling tool of the invention schematically showing fluid flow in use;

(5) FIG. 4 is a nominal front view of the downhole milling tool of the invention;

(6) FIG. 5 is a perspective view of the downhole milling tool of the invention;

(7) FIG. 6 is a perspective view of the downhole milling tool of the invention; and

(8) FIG. 7 is a perspective view of the downhole milling tool of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(9) Referring to FIG. 1, the downhole-milling-tool method of the invention uses a downhole milling tool 10 in the bottom hole assembly (BHA) on a workstring in coiled-tubing drilling and workover operations. It is particularly effective in drilling through barite (barium sulfate, BaSO.sub.4) that has either leached into the hole or has been placed deliberately in order to seal the hole. Typically, the bottom hole assembly is run on 1.25 in. coiled tubing inside 2.875 in. 8.7 lb./ft. production tubing.

(10) Referring to FIG. 2 & FIG. 3, the downhole milling tool 10 provides a tool body 2 which mounts on a bottom hole assembly at an up-hole end. The tool body 2 is essentially tubular or cylindrical, with an axial channel for the flow of drilling fluid or mud under pressure. The tool body 2 also provides at least one step down of the diameter of the outer surface. A preferred embodiment has two steps down, with a largest diameter of the tool body between 2 and 2.5 inches, inclusive, stepping down twice in increments of one-half inch. Each step down creates a shoulder. The tool body 2 can be made of steel.

(11) Fluid ports 3 are provided at each shoulder and at the downhole or leading end. Pressurized drilling fluid or mud from the axial channel of the tool body 2 is expelled through the fluid ports 3 to provide cooling and lubrication, and to flush cuttings or debris up the annulus.

(12) Tungsten carbide inserts or bits are attached by welding directly to the tool body 2 in order to provide cutting faces. The bits are attached so that the farthest-out edge of a given bit is at one of two heights, a higher one and a lower one. This difference in heights can be achieved either by using two different sizes of bits, or by mounting the same bits in two different orientations. The bits are attached to the external surface of the tool body 2 in double rows 4, 5, and also as a forward-bits group 6 at the downhole end of the tool body 2. Each double row of bits is arranged as a leading-bits row 4 and a following-bits row 5, with the leading-bits row 4 containing higher-reaching bits, and the following-bits row 5 containing lower-reaching bits. The alignment of each row does not have to be as precise as illustrated, but can be somewhat varied. The double rows 4, 5 are distributed around the circumference of the tool body 2 in a balanced orientation, such as the 90 degrees for four double rows illustrated, or 120 degrees for three double rows. Between each double row 4, 5 and any adjacent double row a no-bit area 7 is left between the double rows, where no bits are attached. These no-bit areas 7 therefore form rows parallel to the double rows. These are axially oriented no-bit areas, which form channels for spoil-laden drilling fluid to travel upward. Additionally, each double row 4, 5 contains a gap along the rows where no bits are attached, forming additional no-bit areas 7. Each no-bit area gap is located between two axially oriented no-bit areas, and merges those no-bit areas, forming lateral channels. In a preferred embodiment, as illustrated, the gaps are located at different places along each double row so that a continuous helical channel is formed. Where the downhole milling tool 10 is spinning in the standard right-hand or clockwise direction, the helical channel is arranged to conduct spoil-laden drilling fluid up the hole.

(13) Referring additionally to FIG. 4, in a preferred embodiment, the fluid ports 3 on the shoulders of the tool body are placed in the axially oriented no-bit areas.

(14) In use, spinning in a standard right-hand or clockwise direction, the forward-bits group 6 makes initial contact with a smaller central cross-sectional area of the hard material and begins breaking it up. The operation is cooled and lubricated, and the cuttings are being flushed away by, drilling fluid or mud expelled from the fluid port 3 at the downhole end. As the downhole milling tool 10 advances, a slightly-larger-circumference area of material is chipped away by the leading-bits rows 4. Each leading-bits row 4 is followed immediately by a following-bits row 5, which further chips or crushes the cuttings to an optimal size for being flushed away by the drilling fluid, but without reducing the cuttings to a powder, which would become cementitious and would resist flushing. Additional drilling fluid is expelled from fluid ports 3 at the shoulders. The arrangement of no-bit areas 7 forming a helical channel allows the flow of drilling fluid to flush away the cuttings or spoil upwards. As the downhole milling tool 10 advances further, a larger-circumference area of material is removed by the next-larger portion of the downhole milling tool 10. The process repeats for each step up in diameter.

(15) In use, the downhole milling tool 10 provides a clean and cool cutting surface, which equals more efficiency and therefore a better rate of penetration (ROP). The internal flow path or channel allows for better cutting face cooling as well as better flushing of debris.

(16) Many changes and modifications can be made in the present invention without departing from the spirit thereof. I therefore pray that my rights to the present invention be limited only by the scope of the appended claims.