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.
