Patent Application
20100101870
This invention relates generally to the use of coring bit technology in combination with a drill bit used for drilling an earth borehole, and more particularly, to a combination coring bit and drill bit using fixed cutter PDC cutters for both functions.
It is known in the art of drilling earth boreholes to combine the features of a rotary cone drill bit with a dome area cutter insert, for example, as described in U.S. Pat. No. 5,695,019 to James M. Shamburger, Jr. which issued on Dec. 9, 1997.
In the field of exploration and production of oil and gas, one type of drill bit, sometimes referred to as a rock bit, is used for drilling earth boreholes and is commonly known as a rotary cone drill bit. The typical rotary cone drill bit employs a multiplicity of rolling cone cutters rotatably mounted to extend downwardly and inwardly with respect to the central access of the drill bit. Typically, the rolling cone cutters may have milled teeth or cutter inserts disposed on each cutter in predefined patterns. The predefined teeth or insert pattern of each rolling cone cutter are typically different from one another so that the teeth or inserts of the cone cutters mesh as the cone cutters are rotated. In constructed in this manner, the teeth are inserts located on the conical sides of the cutter do not create grooves in the borehole. Further, the meshing rotating teeth or inserts on the conical sides is capable of contacting and cutting the entire bottom of the borehole.
Although the typical rotary cone drill bit has been the tool of choice for drilling earth boreholes, several undesirable phenomenon affects the efficiency and performance of the bit. It should be noted that the teeth or inserts located at or near the conical tips of the rolling cone cutters mesh and cut the center of the borehole. Because of the rotary cone drill bit configuration, the rotation of the conical tips of the rolling cone cutters travels the least amount of distance. Indeed, certain teeth or inserts may remain substantially stationary but for the rotational movement. Because the amount of cutting is directly related to the movement of the teeth or inserts, it is a poor translation of rotation energy to the cutting structures which generate little rock removal. Therefore the center of the well bore present a special challenge to the progress of the drilling.
Typically, one measure of the rotary cone drill bits performance is its ability to maintain a consistent borehole diameter. This ability to hold gauge is important to enable drill strings to be easily removed from and inserted into the borehole. Gauge holding is particularly important for directional drilling applications. A further disadvantage associated with typical rotary cone drill bits is the tendency for the conical cutters to cut under-gauge. As the drill bit advances, the rock formation tends to pinch or force the rolling cone cutters and the support arms inwardly. The results in a borehole that is under-gauge.
It was one of the primary objects of the invention disclosed in the aforementioned U.S. Pat. No. 5,695,019 to eliminate the pinching effect which would otherwise cause the rolling cone cutters and the support arms inwardly.
To overcome the pinching effect, U.S. Pat. No. 5,695,019 introduced the concept of using a coring device to create a central core in the center of the borehole which prevents the rotary cones from being pinched in the drilling process. However, the core produced in U.S. Pat. No. 5,695,019 had a really small diameter, some 0.25 inches to 1.0 inches, and would tend to fall down on their own, a concept having no particular relevance to the present invention.
In sharp contrast, the present invention provides a bit design which is based on utilizing the concept of coring as part of the drilling process. By drilling a core and then destroying the core once it is freestanding in the bottom of the hole, the stress concentrations within the core stump shape make the core more susceptible to rupture, and thus increased drilling performance in harder rock formations.
Traditionally, roller cone rock bits have been used to drill the very hard formations, essentially to the exclusion of using PDC cutters in such formations. By being able to destroy the core formed in the center of the borehole, the PDC cutters can be used to drill in very hard rock in accordance with the present invention.
In order to form a core, the outer diameter of the well bore must be drilled first. The outermost portions of the bit profile are designed similar in profile to a standard PDC core head, with both inner and outer vertical gauge sections and inner and outer gauge reliefs. The inner gauge relief allows for the core stump to stand free of support by the leading cutting structure. Blades are used to position the cutters with a core-head like terminus, as illustrated hereinafter, or thicker lug-like protuberances as are also illustrated hereinafter. In both examples, the common coring profile is exhibited where the outer portion of the borehole is drilled first.
Distil to the core/gauge section and above the inner gauge relief, traditional blade structures having PDC cutters are used in the interior of the bit to drill up the core stump. Since the profile being specified herein grossly resembles that of a roller cone bit, those skilled in the art will sometimes refer to the cutters which eradicate the core stump as “dome cutters” or “dome blades.” These structures can be concave, flat or convex, or any other profile which lends itself to eradicating the core stump. The leading edge of the cutting structure is the core-head like terminus of the blades or lugs, which create the core stump, whose form concentrates residual stress in the upper 360° peripheral top end of the stump. The inner core stump is drilled by the dome blades and associate cutters must take advantage of and even increase residual stresses to further concentrate them and initiate formation rupture at lower energy levels.
Due to the length and the intricate shape of the blades having the PDC cutters required to execute this profile, steel might seem to be an obvious choice for body material. However, in very abrasive formations, steel is problematic in that it wears rapidly. Tungsten carbide is the norm for abrasive elements but its brittle nature doesn't lend itself to thinner, more delicate designs.
In very abrasive environments, such as sharp sands and quartzite, for example, there are relatively few young, reactive shales or clays to cause bit balling. Thus, one can do with reduced junk slot volumes and still drill efficiently. In order to facilitate matrix construction, carbide matrix material is used to connect the blades at key points reinforcing the longer blades required for this profile by interconnecting them. Thus, reinforcing joints are formed to create a sort of “webbing” or “ring” to connect the thinner blades and add strength to a more brittle material.
An additional feature of the present invention involves the use of a core-head like profile as the nose of the bit, thus effectively locking the cutting structure into the bottom hole pattern, disallowing strong lateral movement and minimizing the effects of drill string vibrations. The core stump provides a great degree of lateral stability, disallowing large lateral movement. By employing a self-stabilizing cutting structure on the core/gauge terminus of the blades a very stable bit is created. This is effectively the “tool rest” for the PDC cutters, and thus they are in more of a “formation milling” mode of operation that an engrossed, less stabilized drilling mode traditionally associated with typical PDC cutter designs. This results in a longer lasting PDC cutter and reduced PDC cutter chipping.
From a directional standpoint, the bit in accordance with the present invention is very difficult to move laterally, which makes it have very stable directionally. For the bit to move laterally, one has to essentially break the core stump off. Thus it is very likely that this bit will maintain any angle built into the well very effectively. It also tends to resist following a fault or similar change in dispositional planes.
By concentrating residual rock stresses in the periphery of the core stump, the amount of energy required to rupture the rock is reduced, providing a more efficient and faster drilling bit. As an optional design modification, a frustum can be formed in the center of the bit, and thus be made to increase the residual stress in the bottom of the hole. These optional design modifications may include a series of steps, waves, ridges or any other geometric shape inherent in the bottom hole pattern which concentrates stress and weakens the rock.
Additional modifications can be made or created on a smaller scale by cutter placement. By placing cutters in a highly redundant fashion (identical Y position with respect to midline) and closely adjacent to each other a very heavily scalloped bottom hole profile is created. The scalloped bottom hole pattern induces additional stresses in the native rock, further weakening the rock. Saw tooth and other obvious patterns can be built into the design in similar fashion.
Obviously as additional options, different cutter shapes can be employed to develop stresses, such as a pointed cutter adjacent to a round cutter. An elliptical cutter next to a round cutter, or cutters set at varying heights which provide obvious alternatives to develop stresses in the rock. Even different size round cutters placed adjacently will develop stress risers which weaken formation strength and promote rupture of the rock.
Thus, the combination of the gross profile of the design and the optional cutting structure patterns, or cutter geometry modifications will result in a drill bit that significantly weakens the end-situ rock strength and will promote formation rupture at lower energy levels, resulting in a more rapid removal of formation when drilling very hard rock.
Referring now to the drawings in more detail,
Each of the blades has a row of PDC cutters 28. It should appreciated that for each of the blades, the row of PDC cutters 28 starts off from the wear pad 26, as best illustrated in
As illustrated in
Referring now to
In operation of the drill bit of drill bit 70, it is very much like the operation of the embodiment of
Referring next to 6, there is illustrated stress concentrations from the use of a prior art PDC drill bit in which the stress concentrations are all towards the lower end of the bore hole but are not concentrated as described herein in accordance with the invention.
Referring now to
