Drilling in the Anadarko Basin in southern Oklahoma can present a number of distinctive challenges including fractured formations, conglomerates and other eccentric geologic phenomena. In this unpredictable drilling environment, it is difficult to optimize/refine the bit/tool and BHA selections, resulting in unacceptable or inconsistent drilling performance. To maximize on-bottom time and cut costs, operator and service-company engineers conducted an in-depth analysis in an attempt to improve section economics. They selected a consistently troublesome application where inconsistencies in the use, performance and dull conditions of bits in close offsets were driving up field development costs. From this group, a specific interval was selected for analysis and the data compared with direct offset runs. In the study, engineers considered bit design/materials, turbodrill and BHA stabilization and drilling parameter optimization. Finally, turbodrill/impreg limitations were considered. The analysis determined that turbodrilling with impregnated bits was a viable option to create savings and could effectively optimize drilling in applications where:specific lithology/formation tops are difficult to define/predictdeviation is caused by hard formations encountered at severe dip angleshigh probability of bit/tool damage from impact/abrasionexcessive fishing/tripping issueshole stability/quality may be compromised by frequent tripsroller-cone and fixed cutter bit performance is inconsistent The new-style turbine/impreg BHA was run with outstanding results. The test interval was successfully drilled, eliminating multiple bit trips and reducing overall days. Cost savings were substantial versus a direct offset well that experienced 38 days of trouble time through the same formations. This trial run gives considerable strength to the hypothesis that impreg/turbine drilling can significantly reduce risk compared to traditional BHAs used in direct offsets. The operator plans to further explore the turbodrill/impreg BHA in the near future. Introduction The Anadarko Basin in Southern Oklahoma is bordered by three major geological uplifts and four major geological basins (see figure A-1). Fracturing, conglomerates, and other geologic phenomena that are associated with these transitions make the drilling environment very problematic. Figure A-2 illustrates some the drilling environment between the Wichita Uplift and Anadarko Basin across the A-A strike plane. Here, overlapping, repeating, and overturned formations, along with extreme dip angles are among the several unique features encountered that make the drilling environment difficult to predict, but also have great potential payouts.

Technology Update 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. Well Plan 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.

The encroachment of PDC drilling upon previously roller cone dominated fields continues to occur at an ever-increasing rate. In many fields, the mere introduction of standard PDC technology can result in monumental improvements in performance. However, in southwest Washita County, Oklahoma the conversion to a PDC dominated drilling program has proven difficult utilizing standard PDC technology and operating procedures due to the shallow showing (5,000 - 6,000ft TVD) of the highly abrasive weathered granite conglomerate, known as the Granite Wash formation. Recently, the combination of new bit technology and operational practices has resulted in performance improvements allowing for a 41% reduction in drilling time. In much of the surrounding Anadarko Basin, the interval to 11,000ft (where the Granite Wash is typically encountered), is dominated almost exclusively by PDC bits. However, as recently as nine months, many operators in the SW Canute area were forced to utilize a drilling program comprised solely of roller cones due to the shallow arrival of the Granite Wash and the inability of standard PDC technology and operating parameters to drill this formation. This paper will document how a major operator in the area utilized the concept of aggressive high WOB/low RPM drilling parameters, typically reserved for roller cone bits, coupled with innovative PDC bit frames designed to increase the load/cutter ratio, and new cutter technology to replace the typical 6–8 roller cones required to drill to 10,700ft with just two PDC's thereby realizing an approximate cost savings of $201,000. Introduction Southwest Washita County is part of the "deep" Anadarko Basin. In this area, exploratory wells are either drilled to approximately 13,500ft in order to produce the Atoka Wash and Red Fork sands, or are drilled to approximately 17,500ft in order to produce the Morrow sands. The surface string in both types of well is set in or below the Brown Dolomite formation (˜ 3500 - 4500ft). The lithology below the surface casing consists of a long sequence of sand/shale with interbedded granite wash. The granite wash is a conglomerate of limestone, dolomite and shale mixture that was washed in from the mountain front. The varying amounts of wash have long been a limiting factor on PDC success in the upper-hole section. Deeper in the well (˜ 9000ft.), the lithology gives way to a series of more consolidated sand intervals and granite wash formations. This transition is often marked by the showing of the Tonkawa (˜9,000ft) and Prue sands (˜ 11,000ft.) While most of the Anadarko basin has readily converted to PDC drilling, especially in the upper hole (above 11,000ft), much of the SW Canute area remained roller cone dominated, often requiring 3–8 bits to reach the Prue formation. As recently as nine months ago, many operators were employing an all roller cone approach to drill wells in the area. It was the goal of the operator to reach the Prue sand utilizing two PDC's on their Atoka Wash/ Red Fork wells, thereby eliminating multiple bit trips, as well as tremendously increasing ROP. To achieve this goal, the operator required both the use of unconventional PDC drilling parameters as well as innovative PDC designs. This paper will discuss both the drilling parameter changes and bit technology that allowed the operator to achieve the goal, as well as two additional case histories. Well Plan For the operator, a typical Atoka Wash/Red Fork well involves a 12.25″ section drilled into the Brown Dolomite and 7.875″ then drilled to TD as seen in the attached well bore schematic (Figure 1). For the 12.25″ section, casing points differ in the area according to overall well TD. Typically the operator chooses to set 9.625″ casing at ˜3500ft, midway into the Brown Dolomite. The 12.25″ interval is easily drilled with a single PDC.

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