A 4-year study of diamond drilling with 385 bits in Utah's Altamont field showed that, compared with common diamond-bit drilling, use of high rotary speeds and high bit weights with bits designed for larger and fewer diamonds significantly improved performance in hard rock.
Diamond bits were used to drill about 2,000 ft per well of 6 1/8-and 6 1/2-in. hole in 75 wells in the Altamont field in Utah during the 4-year period from 1970 to 1974. This interval starts at about 11,000 ft. The mud weights were increased from 11+ to 16+ lb/gal during drilling. Penetration rate tripled and bit life almost doubled during Penetration rate tripled and bit life almost doubled during this time. This improvement of performance was achieved by applying faster rotary speeds, increasing the diamond-point loading, using fewer, larger, and more wear-resistant diamonds, and using a medium pressure drop across the bit face. Operating parameters in use at the end of 1973 were a 200-rpm rotary speed, a 30,000- to 35,000-lb bit weight, and a 400-psi pressure drop with a 150-gal/min flow rate. These parameters were applied to 6 1/8-in. bits having 320 to 450 face stones of 1/2 to 1 carat. Point loading under these conditions ranged from 60 to over 100 lb per diamond. Using these drilling parameters, the average rate of penetration during the last half of 1973 (8.88 ft/hr) was penetration during the last half of 1973 (8.88 ft/hr) was three times that of 1970 (2.85 ft/hr). The diamond drilling performance over the 4-year period is summarized in Table 1. performance over the 4-year period is summarized in Table 1. The best bit run was 1,436 ft in 87.75 hours for a 16.3-ft/hr average penetration rate.
For more than 15 years before the discovery well at Altamont, diamond bits had been used in the general area. A "good" bit run had a 2- to 3-ft/hr penetration rate with a 30- to 60-hour life. Common bit designs used massive set bits with small diamonds and little exposure, while using as much drilling fluid volume as permitted to develop a high pressure drop. A typical bit would have 1,500 to 2,500 face stones of 1/8 to 1/15 carat with a 0.015- to 0.020-in. exposure. Pressure drops exceeding 800 psi with 250-gal/min flow rates were common. Operating parameters generally applied were a 50- to 60-rpm rotary speed and a 13,000- to 23,000-lb bit weight.
A low pressure drop is defined for this paper as being up to 200 psi. From 200 to 600 psi is considered a medium pressure drop, and above 600 psi is a high pressure drop. pressure drop.Bits of the preceding general design were used by Shell in 1970 and 1971 while data were being gathered. The average rotary speed was gradually increased from 59 to 71 rpm. A study of the collected data indicated the greatest potential for improvement in bit performance was with higher point loading, increased diamond size, and faster rotary speed. A blanket effort to increase the applied rotary speed and to change the bit designs by using larger and fewer diamonds with greater exposure was initiated to gain the primary consideration of increased penetration rate. Rotary-speed response tests were conducted during the last half of 1972 and the first half of 1973. Diamond sizes were increased from 1/8 to 1/2 carat during this time. The upper limit of pressure drop (Delta p) was reached when loss circulation was induced by formation fracturing while some bits with Delta p exceeding 1,000 psi were used. Weight-response tests were conducted in Feb. 1972 using a 1-carat/stone bit with a 0.044-in. exposure.
JPT
P. 785
