This invention relates to an improved roller cone core bit used to cut a core of a subterranian formation. More particularly, the improved roller cone core drill bit includes a plurality of cone cutters facing inboard and a single cone cutter facing outboard.
Conventional roller cone core bits include a plurality of inboard facing cone cutters. The cone cutters are frusto-conical in shape with rows of cutting elements, such as milled steel teeth or wear resistant tungsten carbide inserts. During drilling operations, the cone cutters cut a cylindrical core in addition to cutting the bottom and outer diameter of the borehole. With insert roller cone core bits, the nose inserts, located about the apex of the cone cutter, cut the cylindrical core. There are fewer inserts in the nose area than on the remainder of the core cutter. Thus, relatively few inserts actually cut the core. The remaining inserts cut the bottom of the borehole. When the bit is rotated, the nose inserts move slowly relative to the formation in comparison with the outer row of inserts that cut the gage diameter of the borehole. The slower moving nose inserts tend to drag across the formation which leads to accelerated wear on the inserts, especially in abrasive formations. Increased wear on the nose inserts leads to nose area failure which is one of the most common modes of failure in rotary cone core bits.
To overcome the problem of nose area failures, practitioners in the coring industry increased the number of inserts cutting the core by reversing a plurality of cone cutters to face outboard. The outboard facing cone cutters, or reversed cones, had the rows with the most inserts cutting the core. The prior art reversed cone core bits were symmetrically balanced and did not impart a continuous lateral outward load on the borehole while coring. Conventional reverse cone core bits were symmetrically arranged with, for example, four cone cutters facing inboard and two cone cutters opposite each other facing outboard; with three cone cutters facing inboard alternating with three cone cutters facing outboard; or with two cone cutters facing inboard alternating with two cone cutters facing outboard. Due to the symmetrical arrangement, there was negligible continuous lateral loading against the borehole while coring.
Conventional reversed cone core bits reduced the number of nose area failures by providing more inserts for cutting the core. However, reversing 2 of 4 cones or even 2 of 6 cones places a higher percentage of the available inserts than is necessary to the inner area of the borehole. While this improves the life of the bit adjacent the core, it diminishes the life of the bit at the outer areas by removing too many inserts from the zone where the largest volume of rock is being removed. Accordingly, conventional reversed cone cutters have an inadequate percentage of inserts, or cutting elements, to cut the remainder of the borehole. The useful life of the conventional reversed cone core bits suffered because of the insufficient number of inserts for cutting the remainder of the borehole.
The present invention overcomes the problems associated with conventional roller cone core bits and conventional reversed cone core bits. The present invention includes a plurality of inboard facing cone cutters and a single outboard facing cone cutter. The single outboard facing cone cutter provides additional inserts for cutting the core, thereby reducing the potential for nose area failure of the core bit. At the same time, the present invention leaves more inserts for cutting the remainder of the hole than conventional reversed cone core bits. Unlike conventional reversed cone core bits, the bit of the present invention is not starved for cutters for cutting the remainder of the borehole.
In addition to the above mentioned benefits, the present invention creates an unbalanced load on the bit, with the net result being a continuous lateral load imparted on the borehole by the bit. This unbalanced loading is created by the asymmetrical arrangement of the plurality of the inboard facing cones and the single outboard facing cone. As a result of the unbalanced load, one side of the bit will always be forced against the borehole wall causing the bit to run smoother than conventional reversed cone core bits. The unbalanced loading reduces the natural whirling action of the bit causing it to drill much smoother. A smoother drilling core bit reduces the likelihood of breaking and jamming the core in the core barrel and thus increases the likelihood of recovering a longer core section. The reduction of the whirling action of the bit is also beneficial because the whirling action tends to reduce the diameter of the recovered core. Thus, the roller cone core bit of the present invention also increases an operator's ability to recover a full gage core. Reduction in whirling also contributes to more efficient cutting action and longer bit life. The energy which would normally fuel the whirling action is now available for drilling. Whiffing causes lateral shock loading on rock bit bearings and stresses the seals. It also causes shock loading on the inserts which can result in early failure due to breakage. The lateral motion caused by whirling also causes more wear on the inserts.