Unique Drilling System Solves Perennial Stuck Pipe Problems in Squeezing Zechstein Salts

Abstract 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 1/2 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 800 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. Introduction In the early 1990's, it was estimated that stuck pipe incidents were costing the worldwide industry more than $250 million a year. The majority of stuck pipe problems occur in the so-called squeezing salts found in the Zechstein group of Upper Permian evaporites. The Zechstein group is deposited throughout much of The Netherlands, onshore Great Britain, the North Sea and extending into Germany and Poland. The group comprises four major and basically identical stratigraphic sequences - Zechstein 1 through Zechstein 4. The order of precipitation is carbonates (calcite and dolomite), sulfates (anhydrite and gypsum), chlorides (halite, "rock salt") and lastly a mixture of chlorides and sulfates (carnalite, sylvite, kieserite and bischofite). The latter sequence is generally referred to as the squeezing salts. As Swale amplified, the global problems associated with salt squeeze occur during the drilling process as primary deformation. Secondary deformation occurs over a period of months after the well has been cased and cemented. Drilling a production well through massive salt sections that can be as thick as 900 m frequently results in a tight hole, and ultimately stuck pipe. P. 815^