Patent Application
20070017709
Roller cone bits, variously referred to as rock bits or drill bits, are used in earth drilling applications. Typically, they are used in petroleum or mining operations where the cost of drilling is significantly affected by the rate that the drill bits penetrate the various types of subterranean formations. That rate is referred to as rate of penetration (“ROP”), and is typically measured in feet per hour. There is a continual effort to optimize the design of drill bits to more rapidly drill specific formations so as to reduce these drilling costs.
Roller cone bits are characterized by having roller cones rotatably mounted on legs of a bit body. Each roller cone has an arrangement of cutting elements attached to or formed integrally with the roller cone. The most common type of roller cone drill bit is a three-cone bit, with three roller cones attached at the end of the drill bit. A prior art three-cone bit is shown in
When drilling smaller boreholes with smaller bits, the radial bearings in three-cone drill bits become too small to support the weight on the bit that is required to attain the desired rate of penetration. In those cases, a two-cone or a single cone drill bit is desirable. A single cone drill bit has a larger roller cone than the roller cones on a similarly sized three-cone bit. As a result, a single cone bit has bearings that are significantly larger that those on a three cone bit with the same drill diameter.
Single cone drill bits sometimes experience difficulty while drilling through changes in the earth formation, such as when a “stringer” is encountered. A “stringer” refers to a relatively small portion of harder earth formation, such as a section of sedimentary rock, encountered within a relatively softer formation. A problem that is sometimes encountered with hard stringers is that the single cone drill bit will pivot based on the indentation of the lowermost inserts in the bottom contact zone. Because the roller cone is a unitary structure, the inserts in the wall contact zone are unable to continue cutting. This can cause the single cone drill bit to hang up and stall when it encounters a stringer while drilling. Excessive scraping action and limited crushing of the stringer by the inserts in the bottom contact zone of roller cone are thought to be causes of the single cone drill bit getting hung up by a stringer. Although this issue is especially prevalent in single cone drill bits, multiple roller cone drill bits (e.g. two cone and three cone drill bits) can experience similar difficulties in drilling into stringers.
In light of the difficulties in drilling stringers and other hard formations with prior art roller cone drill bits, and especially single cone drill bits, what is still needed, therefore, are improved roller cones that are suited to drill stringers and other hard formations.
In one aspect, the present invention relates to a roller cone drill bit. The roller cone drill bit includes a bit body configured to be coupled to a drill string and a journal depending from the bit body. A split roller cone is rotatably attached to the journal. The split roller cone includes an upper section and a lower section. The upper section has a plurality of cutting elements disposed at selected positions thereon. The lower section has a plurality of cutting elements disposed at selected positions thereon. The lower section is able to rotate independently of the upper section.
In another aspect, the present invention relates to a method of designing a roller cone drill bit. The method includes identifying a wall contact zone and a bottom contact zone of the roller cone drill bit. The roller cone drill bit includes a bit body configured to be coupled to a drill string and a journal depending from the bit body. A split roller cone is rotatably attached to the journal. The split roller cone includes an upper section and a lower section. The upper section has a plurality of cutting elements disposed at selected positions thereon. The lower section has a plurality of cutting elements disposed at selected positions thereon. The lower section is able to rotate independently of the upper section. The method further includes locating an intersection of the upper section and the lower section such that all of a plurality of cutting elements disposed on the upper section cut in the wall contact zone of the split roller cone.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
In one or more embodiments, the present invention relates to a drill bit having a at least one roller cone divided into two or more sections. More specifically, the two or more sections of the at least one roller cone are able to rotate relative to each other while drilling an earth formation.
In this disclosure, “rotatably mounted” means that the roller cone is axially constrained on the journal, but able to freely rotate.
As discussed above with respect to
In one embodiment, the lower section 201 corresponds with the bottom contact zone 18, while the upper section 202 corresponds with the wall contact zone 17. By allowing the upper section 202 to rotate relative to the lower section 201, the upper section 202 is able to continue rotating should the lower section 201 have difficulty cutting into a stringer. As the single cone drill bit continues to rotate, the lower section 201 will be forced to start rotating because of the journal angle θ, which will allow the single cone drill bit to continue drilling the stringer.
In one embodiment, the cutting elements 20 on the lower section 201 may be arranged to cut all of the bottom contact zone 18 and a portion of the wall contact zone 17, while all of the cutting elements 20 on the upper section 202 are arranged to only cut the wall contact zone 17. In this particular embodiment, if some of the cutting elements 20 on the lower section 201 begin to scrape the hole bottom without crushing or turning, other cutting elements 20 on the lower section 201 may engage with the hole wall, causing the lower section 201 to turn rather than pivot about the cutting elements 20 contacting the hole bottom.
In one embodiment, the split roller cone may include one or more intermediate sections disposed between the upper section and the lower section. In one embodiment, the split roller cone may be divided by rows of cutting elements instead of cutting zones. Further, the sections of the split roller cone need not be equal in size. Although in some embodiments the upper section and the lower section are each about 50 percent of the split roller cone, in other embodiments the split roller cone may be about 60 percent lower section and about 40 percent upper section, or vice versa. The relative size of the sections of the split roller cone is not intended to be a limitation of the present invention.
Although the embodiment shown in
Turning to
The lower section 201 and the upper section 202 of the split roller cone 24 is formed from steel or other high strength material, and may, in some embodiments, be covered about their exterior surfaces with hardfacing or similar coating intended to reduce abrasive wear of the split roller cone 24. In some embodiments, the split roller cone 24 may include a seal 303 disposed between the lower section 201 and the upper section 202 to exclude fluid and debris from entering the junction of the lower section 201 and the upper section 202 and the space between the inside of the split roller cone 24 and the journal 5. In one embodiment, a seal 304 may be disposed in the upper section 202 to further exclude fluid and debris from entering the space between the inside of the split roller cone 24 and the journal 5. Such seals are well known in the art, and the particular seal(s) used are not intended to limit the scope of the present invention. Further, grooves may be machined into surfaces onto either or both the upper section 202 or lower section 201 to provide a fluid “passageway” that moves the fluid away from the junction.
In one embodiment, different cutting element types may be used in the lower section 201 and the upper section 202, to improve the drilling performance of the split cone bit. For example, PDC cutting elements may be brazed into pockets on the upper or lower surfaces 201, 202. In other embodiments, only portions of the upper portion 202 and lower section 201 may be coated with a hardfacing material. In yet other embodiments, either or both of the upper section 202 and lower section 201 may be formed from diamond impregnated material.
In one embodiment, the split roller cone may be divided based on cutting rows. For example, the rotational speed of a roller cone is determined by the rotational speed of the bit and the effective radius of the “drive row” of the roller cone. The effective radius is generally related to the radial extent of the cutting elements that extend axially the farthest from the axis of rotation of the cone, these cutting elements generally being located on a so-called “drive row.” With reference to
In one or more embodiments, a split roller cone may be designed for a drill bit by performing a drilling simulation. The drilling simulation may be performed using one or more of the methods set forth in U.S. patent application Ser. No. 09/524,088 (now U.S. Pat. No. 6,516,293), Ser. No. 09/635,116 (now U.S. Pat. No. 6,873,947), Ser. Nos. 10/749,019, 09/689,299 (now U.S. Pat. No. 6,785,641), Ser. Nos. 10/852,574, 10/851,677, 10/888,358, and 10/888,446, all of which are expressly incorporated by reference in their entirety. The drilling simulation may be used to identify the appropriate location for the intersection of the upper section and the lower section by allowing a designer to locate the wall contact zone and the bottom contact zone. For example, by performing a drilling simulation, cutting elements on the lower section may be arranged such that a selected amount of the cutting elements are in the wall contact zone. In another embodiment, a drilling simulation may be used to balance the work between the upper section and the lower section, such as by adjusting the relative cutting area between the upper section and the lower section.
In one or more embodiments, a split roller cone may be designed for a drill bit by performing drilling tests in a lab environment. For example, in one embodiment, a test sample to be drilled may include two materials having different strengths to simulate a roller cone drill bit drilling through a stringer. Such a test could show at whether the roller cone drill bit stalls at certain drilling parameters (e.g. weight on bit or revolutions per minute). Test data may also be used to improve the location of the intersection(s) between sections of the split roller cone.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
