Drilling deep exploratory wells in Kuwait presents a number of distinctive challenges. The 16" hole section is particularly demanding because it must be drilled through hard interbedded formations including the Zubair sandstone, Ratawi shale, Ratawi limestone, Minagish limestone/dolomitic limestone, Makhul limestone/shale and Hith anhydrite.1
Historically, 16" roller cone bits were used to drill the float equipment and several hundred feet of Zubair in order to get the stabilizers out of the casing. Several such wells have been drilled in North Kuwait with the best case requiring three PDC bits to complete the section subsequent to the roller cone run. The main challenge has been the highly abrasive Zubair formation with UCS in the 12,000–15,000 psi range.
Traditionally, drilling abrasive formations with PDC has been challenging even in smaller hole sizes (8-1/2" etc).2 However, as hole size increases torque response is magnified increasing vibration issues. In this application, vibration is further amplified by variations in formation hardness. These factors make efficiently drilling the Zubair with PDC a serious challenge. The operator required new PDC bit and cutter technology.
A team was assembled that analyzed PDC technology using numerical models, laboratory and field tests. Engineers focused on the bit frame/cutting structure, and cutter design. Cutters diamond table properties were evaluated in an effort to increase durability and wear resistance. The result is a tougher, more abrasion-resistant cutter capable of maintaining an efficient edge in the hard/abrasive Zubair sand.
Patented depth-of-cut control (DOCC) technology provided smoother, more stable drilling through the interbedded formations. By limiting the unnecessary depth-of-cut of PDC cutters, the amount of torque generated can be effectively controlled. Penetration is managed by adjusting cutter exposure from the bit body and providing a load-bearing surface. Laboratory testing proved how DOCC technology maintains bit aggressiveness needed and limits reactive torque as more weight is applied or as softer formations are encountered. The threshold for bearing engagement is precisely adjusted for each application. This minimized vibration when transitioning between formations while optimizing the ROP advantage of a PDC bit.
Utilizing the new bit design, the operator drilled the entire Zubair 16" interval in a single run for the first time and was able to cut drilling costs by eliminating several trips for new bits and the up-front expense of the extra downhole tools. The authors will discuss how the new technology cut costs and increased drilling efficiencies.