In soft to medium-hard clays and shales, polycrystalline diamond compact (PDC) bits have proved economically successful in the Kuparuk River field, AK. Through the redesign and modification of PDC bits and rig equipment, the necessary operating parameters have been achieved, and the use of PDC bits has become routine. These bits are typically run with a bit weight of 30,000 to 40,000 lbf [133 to 178 kN], a standpipe pressure of 4,000 psi [27 MPa], a pump rate of 400 to 450 gal/min [1.5 to 1.7 m /min], and a rotary speed of 150 to 200 rev/min. Use of these high operating parameters saves about $50,000 per PDC bit when compared with roller-cone bits. per PDC bit when compared with roller-cone bits.
Development drilling in the Kuparuk River field of Alaska began in late 1980. Since that time, 115 wells have been drilled. ARCO Alaska Inc. has been the operator of the Kuparuk River Unit since field development began. The Unit is located approximately 35 miles [56 km] northwest of the Prudhoe Bay field on the North Slope. The early (1980) drilling programs differed slightly from those used today. In many wells, oil-based muds were used. Casing programs were flexible and were varied from well to well to allow for individual well objectives and potential hole problems. Today's drilling programs are fairly well standardized. Typically, surface casing is set at 2,500 ft [762 m] true vertical depth (TVD) and an 8 1/2- or 8 3/4-in. [21.6- or 22.2-cm] bit is used to drill to a total depth (TD) of 6,500 to 7,000 ft [1981 to 2133 m] TVD with a water-based mud. Because of high drilling costs, drilling must progress as efficiently as possible. PDC bits have helped progress as efficiently as possible. PDC bits have helped reduce drilling costs. Most wells in the Kuparuk River field are directionally drilled. Economic and environmental concerns make it necessary to drill a number of wells from one gravel pad. The kickoff point for directional holes varies from pad. The kickoff point for directional holes varies from 500 to 4,000 ft [152 to 1219 m]; after the proper angle has been achieved there is a thick section of soft to medium-hard shales and clays with some thin layers of siltstones and sandstones. PDC bits have proved most successful in this section of the hole, which is in the depth range of 3,500 to 6,500 ft [1067 to 1981 m] TVD. Early successes in the North Sea and south Texas areas with PDC bits used oil-based muds drilling firm to medium-hard shales at penetration rates of 30 to 40 ft/hr [9.1 to 12.2 m/h]. Where soft to medium-hard shales were being drilled in these areas at relatively high penetration rates with water-based muds, roller-cone bits were more economical. However, these types of formations are now being drilled economically in the Kuparuk River field with about 15 hhp/sq in. of bit face area and rotary speeds from 150 to 200 rev/min. The first PDC bit run in the Kuparuk River field (Sept. 16, 1981) drilled 2,590 ft [789 m] in 44 hours for an average penetration rate of 59 ft/hr [ 17.9 m/h]. The cost per foot of this first run ($59/ft [$18/m]) is comparable per foot of this first run ($59/ft [$18/m]) is comparable with the current performance of roller-cone bits ($56/ft [$17/m]). Two years of design and application development have improved PDC bit performance in the Kuparuk River field. PDC bits now are being used routinely to help reduce drilling time.
The lithology in the Kuparuk River field consists of (from surface down) permafrost, shallow Cretaceous sands, clays and shales, the Kuparuk River sands, and Jurassic Kingak shale. The permafrost section consists of frozen sands and gravels. The base of the pennafrost section is at 1,500 ft [457 m] TVD. Below the permafrost lie shallow Cretaceous sands with interbedded clay intervals. These extend as far as 3,000 to 3,500 ft [914 to 1067 m] TVD. A thick section of soft to medium-hard clays and shales is encountered from +/-3,500 ft [1067 m] TVD to between 6,000 and 6,500 ft [1828 and 1981 m] TVD. This is the interval where PDC bits have been used most successfully. The lowermost shale in this series contains siderite, and bit weight and rotary speed must be reduced to prevent damage to the bit. prevent damage to the bit. Below the clay and shale series are the Kuparuk River sands, normally 300 ft [91 ml thick. Several of the sand units are cemented with siderite, which again requires reduced bit weight and rotary speed to prevent bit damage. This interval is drilled with PDC bits; however, on occasion, roller-cone bits are required after a PDC bit is dulled first. Underlying the Kuparuk River sands is the Kingak shale. It is a hard, brittle, fractured, and sometimes sensitive shale normally drilled with roller-cone bits. PDC bits have been used occasionally to drill this interval. Normally, 200 to 600 ft [61 to 183 m] TVD of this interval is drilled to provide sufficient "rathole" to accommodate completion accessories.
JPT
p. 1220
