Improved ROP in Ultrahard Rock

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 99045, "ROP Enhancement in Ultrahard Rock," by M. Roehrlich and K.-U. Belohlavek, ExxonMobil Development Co., prepared for the 2006 IADC/SPE Drilling Conference, Miami, Florida, 21–23 February. Drilling the hard and very abrasive Bunter formation in northwest Germany has been a challenge for 40 years. Low rates of penetration (ROPs) and high tool wear are common. The full-length paper details the analysis of historical data and the combination of bits, motors, and other bottomhole-assembly (BHA) components to improve ROP. The combination of new impregnated bits, new downhole motors, and improved hydraulics was the basis for the improvement. Introduction The Bunter, a hard and very abrasive formation, typically at a depth of 8,000 to 12,500 ft, consists of layers of quartzitic sandstone and silificated claystone with a compressive strength of 30 to 50 ksi. In the past, typical ROPs in 12 1/4-in. sections were 3 to 6 ft/hr. In the 1970s and 1980s, 15 to 20 insert bits were required to drill the section and reaming was necessary as a result of undergauge bits. In the 1990s, a research project was initiated and funded by the German drilling industry to improve ROP in ultrahard and abrasive formations. In the past, the Bunter was characterized by the lowest ROP, shortest bit life, and highest cost per foot. Increasing the depth of cut (DOC) of impregnated bits in the Middle and Lower Bunter was identified as having potential for improvement. DOC means the length that is drilled with one revolution of the bit. An increase in DOC can be achieved by a decrease in overbalance or an increase in the amount of power available for drilling the rock. To analyze the key parameters, drilloff tests or drilling tests at fixed drilling parameters were conducted and plotted into Bingham diagrams. Within the German drilling industry, two approaches to increase ROP have been chosen—use of downhole turbines and use of high-speed downhole motors in combination with improved hydraulics made possible by larger-inside-diameter drillpipe. The full-length paper details results from downhole-motor runs with impregnated bits. All results are from 12 1/4-in. bits because this is the most common bit size used in Germany for the Bunter formation. Rotary Speed. Until 1997, the standard for high-speed downhole motors was 400 rev/min at 790 gal/min (360 hp). In 1997, a 9 1/2-in. downhole motor was tested in Germany at 780 rev/min at 849 gal/min (515 hp). In 2004, a new 9 1/2-in. downhole motor was developed that delivers 975 rev/min at 1,055 gal/min (1,025 hp). Weight on Bit (WOB). In 2004, the maximum WOB for high-speed downhole motors was approximately 48,100 lbm. With the 975-rev/min downhole motor, maximum WOB increased to 60,700 lbm because of improvements in bearings.