Drilling across two sections (10,000-ft laterals) in a horizontal well in the Middle Bakken formation of North Dakota offers challenges and opportunities for improved well-construction performance. The wells in this area were drilled with three different hole sections after setting the 16-in. conductor at 60 ft.
The first hole section was drilled 13 1/2-in. vertical hole with fresh mud to the 9 5/8-in. casing set at +/- 2,000 ft and cemented to the surface. The second hole section was drilled 8 3/4-in. vertical hole with oil-based mud and built the curve to the 7-in. casing set at +/- 11,000 ft and cemented to a point above the Dakota formation. The curve was built at a 12 to 14° BUR to a landing point in the Middle Bakken target zone at approximately 90°. The last interval drilled was a 6-in. lateral-hole section, geosteering with mud motors, using two different bend settings of 1.15° ABH and 1.5° ABH. Wellbore stability in the Middle Bakken is not an issue when using a cut brine-produced fluid. Most of the wells were completed by installing 4 1/2-in. liner with swell packers and ball-actuated stimulation ports without cementation.
The lesson learned on three offset wells in this area found that a bend setting of 1.15° has contributed to reduce tortuous well paths, controlled dogleg severity, and stayed in a very tight target interval while drilling the lateral section of 10,000 ft along the hole. The reduced spiraling saved countless hours of reaming requirements and allowed the Swellpacker® liners to be run to TD without significant working. Drillstrings, bottomhole assembly (BHA), bits, the length of the lateral section of horizontal and drilling fluid were all considered.
Many operators are finding success drilling horizontal wells to develop the Middle Bakken in the Williston Basin, which is relatively stable tectonically as compared to other intra-cratonic sections. The Bakken formation is comprised of the lower, upper, and middle member; the middle member which is the target of the current development. The middle member typically consisted of silty sandstone and very fine-grained sandstone, which is very calcareous and slightly dolomitic in the zones with the best oil and gas shows. This paper will focus on drilling and the performance of mud motors with two different bent housings on three different wells in North Dakota.
The bent housing tilts the axis of the bit relative to the axis of the hole. Significant bit side-force is achieved for a relatively small bit offset (Gaynor et al. 2001; Gaynor et al. 2002). This allows the bit to build inclination and/or change the hole direction when the rotary table-top-drive is locked and to drill straight ahead when the drillstring is being rotated. The number of round trips is thus drastically reduced, provided the correct bit (PDC bit of six-bladed aggressive designs with durable cutting structure) and BHA are chosen. The BHA is provided by using vibration tools with sufficient weight to be run to the shoe and drill to section TD in slide or rotate modes of drilling (Dugas et al. 1994; Anderson et al. 2006). The BHA also prove that excessive torque is not generated in the drillstring when drilling to section TD.
The surface-adjustable bent housing allows the incremental adjustment of the bend in the motor from 0 up to approximately 3° (Allison et al. 1988; Reich et al. 2000). These change the magnitude of the side force generated at the bit while slide-drilling and, as such, the achievable build and turn rates. As the bent-housing offset angle is increased, bit side-force increases. It becomes more difficult to slide. In the rotary mode, it becomes more difficult to rotate. There is also more stress on the motor.