PDC Shearing Cap Technology Protects Cutters When Drilling Out Casing Bits for Increased ROP and Bit Life in the Next Hole Section
Casing drilling technology uses a specially designed casing drill bit that is attached to a traditional casing string. The technology is typically applied when hole problems such as severe formation swelling or caving cannot be controlled with drilling mud or by rig operations, making it extremely difficult to reach the planned casing setting depth. In these applications, the casing and bit are set and cemented. To drill the next section of the wellbore, the casing drill bit, as well as the float equipment, must be drilled out.
In PDC bit applications this is typically done in two ways – using a roller cone bit for the drill out, followed by a new PDC bit for the next interval; or by using a single new PDC bit and BHA to drill out the cemented casing bit and continue drilling the next interval. Each method presents inherent inefficiencies in the drilling operation. With the first, additional drilling and trip time are required to run the two bits. The second method using a single new bit has the potential to damage the PDC cutters while drilling out the casing bit such that performance is limited in the next formation interval. The cutter damage is heightened when using a bent motor assembly. This degraded cutter condition negatively affects bit durability and performance, resulting in slower rates of penetration, shorter runs, and pulling the bit before completing the intended interval.
Milling cap technology has been developed for the single-bit method that provides a shearing action during the casing bit drill out process while protecting the primary PDC face cutters. It preserves the cutters in virtually pristine condition so that performance is optimized when drilling the formation. The shearing cap technology allows flexibility in selecting the best performing bit for the interval below the casing, and it increases the opportunity to use a bent motor assembly when drill string rotation is required during the drill out.
This paper discusses the development of the shearing cap technology and examines the two initial field tests in South America and West Africa. In the first use of the technology, drilling on a well in Colombia, an 8 ¾-in. PDC drill-out bit with shearing caps completed the drill out portion of the run in less than 15 minutes. The bit then drilled the next 4,900 foot formation interval to the next casing point with an ROP 5% higher than the field average. The bit showed no signs of reduced ROP after the drill out and came out of the hole in good condition with a dull grade of 1-2-CT-G-X-I-BT-BHA. In West Africa, a 12 ¼-in. PDC bit with shearing caps produced the best performance in the field, reducing casing bit drill out time by 44% and improving ROP in the formation by 50% compared to offsets.
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