A combination of new drill-bit technology and new operational practices has enabled operator Marathon Oil to use polycrystalline-diamond-compact (PDC) drill bits, and reduce drilling time by 24%, on wells in a part of the Anadarko basin where abrasive conditions at shallow depths previously have required the use of roller-cone bits.
Much of the Anadarko basin is dominated by PDC-bit drilling. However, in the southwest Canute area of Washita County, Oklahoma, the highly abrasive Granite Wash formation is encountered at depths of 5,000−6,000 ft, compared with its more typical 11,000-ft point of emergence in most of the Anadarko basin.
In the deep Anadarko, of which the southwest Canute area is a part, exploratory wells are drilled either to approximately 13,500 ft to produce the Atoka Wash and Red Fork sands or to approximately 17,500 ft to pro-duce the Morrow sands. The surface string in either type of well is set in or below the Brown dolomite formation (≈3,500−4,500 ft). The lithology below the surface casing consists of a long sequence of sand/shale with interbedded granite wash. This gives way to more-consolidated sand intervals and granite-wash formations, a transition often marked by the showing of the Tonkawa and Prue sands at approximately 9,000 and 11,000 ft, respectively.
In drilling Atoka Wash/Red Fork wells in the southwest Canute area, the operator set a goal of reaching the Prue sands by use of two PDC bits, eliminating the use of three to eight roller-cone bits that previously would have been required. Aggressive drilling parameters typically reserved for roller-cone bits, employing high weight on bit (WOB) and low revolutions per minute (rev/min), were combined with innovative PDC-bit frames, designed to increase the load/cutter ratio, and with new cutter technology to enable the operator to achieve the goal. A depth of 10,700 ft was reached with two PDC bits, and cost savings of approximately USD 200,000 were realized.
For the operator, a typical Atoka Wash/Red Fork well involves a 12.25-in. section drilled into the Brown dolomite and a 7.875-in. section then drilled to total depth (TD). For the 12.25-in. section, casing points differ according to TD. Typically, 9.625-in. casing is set at ≈3,500 ft. The 12.25-in. interval is easily drilled with a single PDC bit.
A 7.875-in. hole is drilled below the 9.625-in. casing and extends to TD. Historically, this interval has been drilled with multiple IADC 5–2–7 or 5–3–7 roller-cone bits. More recently, the use of PDC bits at drillout has become common. However, the ability to make the interval to the Prue sands has proved very elusive.
