DRAG BIT WITH WEAR-RESISTANT CYLINDRICAL CUTTING STRUCTURE

Description

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to cutting and chipping drag drill bits used in drilling a rock formation to produce oil, natural gas or in other applications.

A drill bit disclosed in U.S. Pat. No. 7,757,785 comprises a body with multiple blades, a conical part, a shoulder part and a gage part provided with wedge-shaped leached cutting elements mounted at a negative rake angle.

The drill bit of U.S. Pat. No. 7,757,785 may not have a sufficient durability of cutting structure.

A VTL test has shown that, when using a leached Polycrystalline Diamond Compact (PDC) cutter with a negative rake angle, a wear rate contingent on a drilled volume has an exponential relationship with three distinctive sections as shown in FIG. 1. The first section is influenced by operation of a leached diamond layer of the PDC cutter, while wear of the diamond layer in this section is minimal. In the second section, a non-leached diamond layer of the PDC cutter is engaged after the leached diamond layer is worn out, and the non-leached diamond layer is worn out faster. In the third section, a hard alloy substrate of the PDC cutter diamond layer is engaged. A wear rate in the third section is higher compared to the first and second sections due to the fact that abrasion resistant of the hard alloy substrate is significantly less than that one of the PDC cutter diamond layer. As seen in FIG. 2, a worn surface contact area of the PDC cutter diamond layer (P1, P2, P3, respectively) corresponds to each stage of the wear curve. A rapid increase in non-leached areas P2 and P3 actively generates heat leading to a rapid wear which causes an exponential increase in sections 2 and 3 of the wear curve. Such drill bit is worn quickly and loses its mechanical drilling speed.

Also, known is a drag bit of U.S. Pat. No. 6,408,958, where primary cutting elements are mounted at a negative rake angle, and secondary cutting elements are mounted behind the primary elements and arranged on a blade in such manner that their axes form an angle of 3 to 5 degrees with respect to a cutting face.

The secondary elements cut away a negligible rock volume or are engaged as a primary structure wears out. The main part of a rock is cut away by a first row of cutting elements mounted at a regular negative rake angle; therefore, energy consumption of the drag bit corresponds to that of a regular drag bit.

The closest prior art for the present invention is a drag drill bit disclosed in U.S. Pat. No. 6,332,503, the drag drill bit comprises a plurality of PDC cutting elements, each having a cylindrical shape and arranged in such manner that an axis of the cutting element each forms an angle from 5 to 40 degrees with respect to an axis perpendicular to the cutting face in a contact point between the PDC cutting elements and a rock face. The cylindrical PDC cutting elements have conical cutting ends and are spaced from the drill bit center at a distance not exceeding half of the nominal drill bit diameter.

Disadvantageously, the PDC cutter elements of U.S. Pat. No. 6,332,503 are arranged only in the central part within half diameter of the drag drill; therefore, the PDC cutter elements cannot significantly change the overall required torque on the drag drill bit. This is due to the fact that cutters in the central part of the drag drill bit require application of a smaller rotational torque thereto due to a small diameter of the apparatus. Therefore, the prior art drill bit has the same disadvantages as bits with a negative rake angle in their central part, i.e. a rotational torque required for rotation depends highly on applied axial load. In order to increase a mechanical drilling speed for such drill bit, a high specific energy for rock breakdown is required.

Another disadvantage is that the prior art drill bit is prone to intensive wear due to exponential dependence wear of cutting elements arranged at a negative rake angle outside half of a nominal diameter.

An object of the invention is to increase drilling speed while improving wear resistance of cutting elements.

The present invention provides a decrease in a cutting force acting on bit cutters, thus increasing wear resistance and drilling speed thereof.

This is achieved by a drag bit with a wear-resistant cylindrical cutting structure, the drag bit comprising a body with a connecting thread, a central channel and output openings, blades protected by a hard alloy coating and comprising a gage part, a main cutting profile with a plurality of PDC cutters, wherein some of the PDC cutters have an axis angled with respect to a cutting direction. The PDC cutters are mounted covering a surface between a central bit axis to a nominal diameter of the drag bit, wherein each PDC cutter has a diamond layer, a leached layer, a chamfer, an end face and a cylindrical face, wherein axes if the PDC cutters are arranged at an angle from 40 to 90 degrees to the cutting direction, and a cutting process is performed by the chamfer, the end face and the cylindrical face of the PDC cutter diamond layer.

The PDC cutter generates a smaller cutting force and provides increased wear resistance due to the fact that the PDC cutter is arranged such that its axis forms an angle from 40 to 90 degrees with respect to a cutting direction at a rock face contact point.

The diamond layer of each PDC cutter can have a thickness from 0.5 mm to 3 mm.

The leached layer of each PDC cutter can have a thickness from 0.1 mm to 1 mm.

The chamfer of each PDC cutter can have a size from 0.05 mm to 1.5 mm.

It is important to note that in the drag bit disclosed in U.S. Pat. No. 6,408,958, the secondary elements have a different purpose comparing to that of the present invention. In particular, the secondary elements act as an auxiliary tool, limiting penetration of a primary structure and stabilizing the drag bit during drilling. As mentioned above, the secondary cutting elements cut away a negligible rock volume or enter operation as the primary structure wears away, and the main part of the rock is cut away by the first row of cutting elements.

BRIEF DESCRIPTION OF DRAWINGS

The present invention is further described with reference to the accompanying drawings, wherein:

FIG. 1 shows an exponential wear curve obtained by VTL testing;

FIG. 2 illustrates areas of PDC cutter wear;

FIG. 3 is a perspective view of a drag bit according to the present invention;

FIG. 4 is a sectional view of a PDC cutter according to the present invention performed along the A-A line;

FIG. 5 is a sectional view of a PDC cutter according to the present invention performed along the A-A line during rock drilling;

FIGS. 6, 7 illustrate “torque/axial load” and “torque/penetration rate per revolution” power characteristics for drag bits according to the present invention and for a conventional drag bit;

FIG. 8 shows comparison between VTL test results of PDC cutters with the apparatus according to the present invention and a conventional apparatus;

FIG. 9 shows comparative wear of PDC cutter with the apparatus according to the present invention and a conventional apparatus.

As suggested by the present invention, a drag bit with a wear-resistant cylindrical cutting structure comprises a body 1 with a connecting thread 2 for a drill stem, a central channel 3 with output openings 4 for supplying washing fluid, and blades 5 protected by a hard alloy coating and comprising a gage part 6 and a main cutting profile. The main cutting profile comprises PDC cutters 9 mounted in the range from the central bit axis 7 to the nominal bit diameter 8, each PDC cutter 9 has a diamond layer with thickness D from 0.5 mm to 3 mm, having a leached layer L from 0.05 mm to 1.5 mm, a flat end face T, a cylindrical face C and a chamfer F from 0.05 mm to 1.5 mm, as shown in section A-A in FIG. 4. As seen in FIG. 5, the PDC cutter 9 is mounted such that a cutter axis O forms an angle A° from 40 to 90 degrees with respect to a cutting direction. Due to this arrangement, cutting process is performed by a combined working area 10 consisting of the chamfer F, the flat end face T, and the cylindrical face C of the PDC cutter 9.

The present invention is operated as follows.

The drag bit is attached to the end of a drilling string (not shown) and lowered into a borehole bottom where drilling occurs. As shown in FIG. 5, the PDC cutter 9 cuts away the rock with the combined working area 10 during rotation of the drag bit. An axial force appearing on the PDC cutters for supporting preset penetration rate per bit revolution and a cutting force will be lower compared to the conventional cutting with a negative rake angle. Therefore, as shown in FIG. 6, a cutting force on the PDC cutters 9 mounted as described above creates a lower required rotational torque on the drag bit according to the present invention compared to a grad bit having conventionally mounted PDC cutters with a negative rake angle at equal axial loads on the drag bits. Further, as shown in FIG. 7, a lower required rotational torque is formed on the drag bit according to the present invention compared to a drag bit with conventionally mounted PDC cutters at equal penetration rates per bit revolution. As a result, the disclosed drag bit requires less specific energy for rock breakdown compared to a drag bit with negative rake angles, thus providing high drilling speeds. The advantages of the lower rotational torque of the disclosed drag bit can be implemented by increasing bit rotation speed, thus increasing mechanical drilling speed.

The disclosed arrangement of the PDC cutters 9 is more efficient in terms of impact resistance of the structure. During rock cutting, when unstable operation modes occur, the entire impact is taken, as illustrated in FIG. 5, by the combined working area 10 of the diamond layer D, the combined working area 10 consisting of the flat end face T, the chamfer F, and the cylindrical face C. In the disclosed arrangement of the PDC cutters 9, the combined working area 10 is significantly larger than that in the conventional arrangement with a negative rake angle, therefore breaking stress on the cutter will be significantly lower, thus preventing its breakdown.

Abrasion resistance according to the VTL test in the first wear area (that is the operating area of the leached layer) of the leached PDC cutter 9 mounted according to the above method is more effective compared to a conventional arrangement with a negative rake angle (FIG. 8). As shown in FIG. 9, at equal wear height W of leached layers, reserve of the diamond layer of the PDC cutter 9 is up to 10 times higher compared to a conventional arrangement of a PDC cutter with a negative rake angle, thus increasing operating life.

Therefore, the above advantages increase abrasion resistance of the PDC cutter.

The present invention provides decrease in cutting force on drag bit cutters decreasing torque capacity and increasing structural resistance of the drag bit during rock drilling.