Nico Foekema

Revised Drilling Practices, VSS-MWD Tool Successfully Addresses Catastrophic Bit/Drillstring Vibrations

To date, several wells have been drilled in the Friesland/Groningen area of northern Holland. Historically, catastrophic cutter damage/failure in the 12 1/4 Triassic section resulted in inconsistent and unacceptable bit performance. Here, the Triassic is an interbedded formation containing hard dolomite and anhydrite stringers and abrasive sands. The transition from a softer to harder formation at high inclination evidently initiated bit and drill string vibrations, thus causing premature cutter failure of the PDC bits used in the program. This paper presents a detailed analysis of the 12 1/4 section conducted after the drilling of Well No. 1. The recommendations arising from the study concentrated on the use of a real-time downhole vibration monitoring or VSS-MWD tool (Vibration Stick Slip) in combination with revised drilling practices to reduce vibrations when encountering the interbedded formation. Thus, on Well No, 2, nearly all of the 2358-m Rijnland and the hard and abrasive Triassic formations were drilled with one PDC bit on a rotary assembly, where previously six bits were required. The authors will illustrate the benefits of a detailed interval analysis and discuss the role of the VSS system and the drilling crew in addressing catastrophic drillstring and bit vibrations.

Unique drilling system solves perennial stuck pipe problems in squeezing Zechstein salts

In the Upper Permian Zechstein Salts, previous attempts to drill a gauge hole to facilitate cementation and prevent the frequent incidents of casing collapse caused by salt movement habitually resulted in stuck pipe. A variety of different mud systems, drill bits and bottom hole assemblies have been used in attempts to achieve gauge hole and overcome the expensive problems associated with stuck pipe. A potassium-magnesium mud system (KMG) resulted in an average of 0.6 stuck pipe incidents per well. When this system was heated and supersaturated (heated KMG), the stuck pipe incidence increased to 1.25 occurrences per well. Use of a synthetic oil-based mud intensified the problem with stuck pipe incidents increasing to an average of 2.5 occurrences per well. Meanwhile, field trials with eccentric bits proved unsuccessful, as it was impossible to drill both the float equipment of the previous casing and the succeeding formation. Further, problems surrounding the unpredictable directional behavior of eccentric bits necessitated costly correction runs. The majority of wells the operator drills employs 53.5 lb/ft of 9 5/8-in. casing set at the top of the Zechstein. Standard drift sizes for this casing is 8.378 in., which historically resulted in the drilling of 8 3/8-in. holes. This paper describes the development and application of a redesigned 8 1/2-in. IADC M332 PDC bit, featuring an undersized (8.375-in.) gauge pad that when run on standard 8 3/8-in. assemblies overcame the costly stuck pipe problems resulting from attempts to drill gauge hole through the Zechstein Salts. With the new bit/BHA combination a special drift specification of 8.5 in. was allowed at no additional cost, which subsequently permitted an 8 in. cutting structure to be run on standard 8 3/8 in. assemblies. The combination has been used to drill more than 16,400 ft (5000 m) of deviated hole in the Zechstein squeezing salts with no incidents of stuck pipe. This approach was used to drill the longest well in The Netherlands, where 8,200 ft (2500 m) of salt was drilled at 80° to a depth of 20,404 ft (6221 m) at an average of 50.5 ft/hr (15.4 m/hr). In a performance analysis, the authors will show that while mud weights gradually reduce, shoetracks are being drilled successfully and directional objectives are being achieved with predictable performance.