Milling tools are known for use in removing a section or window of existing casing from a well bore. In a known implementation, a whipstock-mill combination tool is run into the wellbore. This combination tool typically comprises a milling bit which is secured to a top of the whipstock. The milling bit is in some instances a starting mill. At a desired wellbore depth, typically set with a packer, the whipstock (which is hanging from the milling bit) is set at a correct orientation and secured in place. The milling bit is then freed from the whipstock and rotated with applied pressure. The concave face and taper of the whipstock forces the milling bit against the inside of the casing to cut an initial casing window (this initial milling operation may also remove the mechanism which attached the top of the whipstock to the milling bit). The starting mill is then removed from the wellbore and a window mill bit, attached to a more flexible drill string, is lowered into the wellbore. This window mill bit is then rotated to mill down from the initial casing window as the window mill bit rides further down the concave face and taper of the whipstock. This forms a casing window opening. The window mill bit is then removed. Additional mill bits may be run in the wellbore to define the size and shape of the opening in the casing. To the extent additional formation drilling is desired after opening the casing window, for example in connection with directional drilling operations, a drill bit would then be run into the wellbore and out through the formed casing window to engage the formation.
It will be recognized by those skilled in the art that the operation for milling a casing window is quite time consuming and expensive since it requires several different tools and multiple trips down the wellbore. If further formation drilling is required after opening the casing window, yet another trip is needed to run the formation drilling equipment back into the wellbore. There would be an advantage if more efficient milling and drilling methods and tools were available for use in connection with the milling of a casing window and the drilling of formations through that casing window.
In connection with known one trip mill-drill systems, which utilize a whipstock hung under the mill or mill drill for whipstock run in, orientation, and placement, the typical method for attaching the whipstock to the mill or mill drill utilizes one or more threaded sockets or a threaded nozzle port in the lower portion of the bit. An intervening component is then bolted to the threaded socket(s) and to the upper portion of the whipstock assembly. Operation of the system involves shearing off the bolted connection with the application of weight and rotational force. This weight and rotational force, through the concave face and taper of the whipstock, further urges the mill drill into the casing wall so as to commence window milling.
For efficiency and trip savings reasons, tool designers try to design the mill drill to be capable of both milling out the casing window and effectively drilling ahead into and through the rock formations beyond the casing. This design goal has not generally been satisfactorily achieved. One reason for this is that the diamond layers of PDC drill bit cutters which tool designers prefer for use in effectively removing formation material are not as effective for use in milling steel casing material. It is known that other materials, such as tungsten carbide, or cubic boron nitride (CBN), are better at cutting ferrous materials, like well casings, but are not as effective at cutting rock that is encountered for instance after the casing has been penetrated. To address this issue, it is known in the art to provide PDC drill bits with tungsten carbide, or cubic boron nitride (CBN), cutting features to assist in milling operations, while the PDC cutters of the bit function in connection with subsequent formation drilling. The performance of these combination mill/drill bits has not been entirely satisfactory.
An issue also arises with respect to connecting the whipstock to a PDC drill bit. Most PDC drill bits used in oilfield applications today are manufactured with bodies cast from a tungsten carbide matrix. It is extremely difficult and time consuming to attempt to retrofit a matrix PDC bit design with a threaded socket for whipstock attachment because the matrix material is too hard for standard machining. Alternatively, the threaded socket can be included in the original casting design. However, the design and integrity of the bit can be compromised by the inclusion of the threaded socket (either machined in, or cast in). These problems exist as well for bits whose bit bodies are made from material other than tungsten carbide. The incorporation of threaded sockets for whipstock attachment in steel bit bodies, for example, may also compromise optimum design or bit body integrity.
There accordingly exists a need in the art for an attachment mechanism for hanging a whipstock from a drag-type PDC drill bit. Preferably, a non-invasive way of attaching a whipstock to a standard drag-type PDC drill bit would be provided. This solution could also be applied to application specific mills or mill drills so as to eliminate the necessity for an invasive threaded socket to attach the whipstock.
Reference is made to prior art U.S. Pat. Nos. 7,178,609, 5,887,655, 3,652,138 and 5,069,297, the disclosures of which are hereby incorporated by reference in their entirety. Reference is also made to the Knight Fishing Services, X-1 Single Trip Whipstock; the Smith Services Trackmaster Plus Wellbore Departure System, the Weatherford Quickcut Casing Exit System, and the Western Well Tool Non-Rotating Protectors, prior art devices (incorporated by reference).