The use of straight hole turbodrills in the North Sea, in vertical and deviated well applications, is discussed with reference to the optimisation of drilling performance and to the techniques employed for directional control. The adaptation of turbo drilling to maximize the benefits of STRATAPAX* bits is examined, with specific reference to well examples in the Northern North Sea. Improvements in drilling performance due to the development of the turbodrill/STRATAPAX* bit combination are analysed. INTRODUCTION Straight hole turbodrills have played an active role in North Sea drilling since their introduction to the area in 1968. They have been subject to an ever widening sphere of applications, helped by increased knowledge of North Seageology, good rig hydraulics, improved diamond bit design and the introduction of STRATAPAX* bits. Operations cover vertical and deviated drilling in hole diameters ranging from 5 7/8" to 12 1/4", and mud specific gravity as high as 2,0 in the smaller hole sizes. Current turbodrilling activities which are equally divided between development work and exploration/ appraisal drilling are described. TURBODRILLING IN VERTICAL HOLES Neyrfor straight hole turbodrills have been used with considerable success for vertical 12 1/4", 8 1/2" and 6" hole sections in exploration and appraisal wells drilled from semi-submersibles, which are generally accepted by the oil industry as the best adapted drilling unit for the North Sea area. The daily cost of such units is very high and the economic advantages of turbodrilling are evidenced by the regularity of its application. Experience has shown that the use of packed hole assemblies and extended gauge diamond bits maximizes turbodrilling performance, gives an excellent gauge hole and eliminates holespiralling. String rotation is used to ensure smooth transmission of weight to the bit and to minimise the danger of sticking problems.
Since 1985, Dome Petroleum Ltd. has developed slant-well drilling in the Lindbergh heavy-oil field in east-central Alberta. The oil-bearing sands occur in the lower Mannville formations of the Cretaceous period. To date, four pads, each consisting of 15 or 16 wells, have been completed. The experience gained in completing these projects has allowed us to make optimum use of slant-well technology in multiwell pad applications.The heavy-oil reservoirs in the Lindbergh field contain oil with a gravity of 0.98 g/cm3 [12.5° API]. The reservoir is at a vertical depth of 600 m [1,970 ft]. The wells are drilled on 4-ha [IO-acre] spacing and placed on primary production for 2 years, after which steam stimulation is used for EOR. This primary production of cold oil has significant economic impact on the development of this field because capital recovery is achieved for drilling and completion before the massive investment into steam generation and distribution facilities. Maximizing this primary recovery requires that low-dogleg-severity (DLS), "straight" holes be drilled.The Lindbergh area is characterized by high agricultural land use. With slant-well technology, the required spacing unit is achieved with minimum surface land usage, optimum hole quality, and minimum environmental impact. In this application, slant-well drilling provides an attractive alternative to conventional directional multiwell pad projects.In the initial stages of slant-well development, drilling times of 5.5 days/well and costs** of $315/m [$811ft] were experienced. Through an optimization process involving pad design, drilling equipment, casing design, bits and hydraulics, and directional/survey assemblies, significant savings in drilling time and cost were realized. To date, 71 slant wells have been drilled. In the latest project, drilling time averaged approximately 2 days/well and costs were reduced to $177/m [$45/ft]. These savings were achieved with an improvement in directional control and overall hole qUality. This paper describes the optimization process and the techniques used to realize these savings.
Summary Dome Petroleum Ltd.'s experience with a new mobile offshore drilling unit - the single steel drilling caisson (SSDC) - is described. The SSDC's design enables offshore exploratory drilling operations in the Beaufort Sea to continue beyond the short open-water season. The SSDC proved to be well-suited to offshore arctic operations. The operator's requirements for storage facilities and rig equipment are discussed and the drilling and testing of the first well are described to illustrate the success of this innovative drilling unit. Problems associated with Beaufort Sea operations are discussed with specific reference to ice management and drilling problems. A brief update describes the relocation of the SSDC to a second wellsite and its successful operation through a second winter. Introduction Dome has been drilling offshore wells in the Canadian Beaufort Sea with drilling units owned by its subsidiary Canadian Marine Drilling Ltd. (Canmar) since 1976. Initially, drillships modified for arctic conditions were used for offshore operations in this area during the short open-water season. Ice breakup typically occurs between mid-June and early July. During the summer, drilling operations are interrupted occasionally by incursions of multiyear ice. New ice usually begins to form in October. As freeze-up progresses, ice breakers are used to prevent interference with drilling operations. Drilling operations may be suspended at any time from October to late November, depending on government regulations, drilling program, anticipated ice conditions, and well progress. Occasionally, operations have continued to the limit of operational capability in severe ice conditions. For the last 9 years, season length, from first to last operating day, has ranged from 85 to 150 days. The average number of operating days on location is 100. An average of 13 days per year waiting on ice or weather reduces the effective number of operating days to 87. Restriction of floating drilling operations to this brief period means that it is not possible to drill and to test a typical deep Beaufort well in one summer season when a drillship is used. Because of this limitation, we considered a variety of alternative drilling systems. Modifications to the most recently acquired drillship, Explorer 4, extended its operating ability beyond that of the other three drillships. It was evident, however, that the drilling season could not be extended throughout the winter with existing floating rigs. The limiting factor in the case of a floating rig was station-keeping ability. Existing mooring or dynamic-positioning systems could not resist the forces generated by moving first-year ice. A much heavier mooring system and higher-class icebreaker support would be required to operate a floating rig beyond the existing season. The alternative was a bottom-founded structure that was designed to operate in ice conditions. Artificial, dredged islands have been built for drilling in the Beaufort Sea. These islands are built with "sacrificial" beaches and sandbag protection. The berm slope is relatively low, and berm volume is high. These islands are restricted to relatively shallow water on economic grounds and are susceptible to summer storm damage. Generally, the accepted economical water-depth limit for these islands is about 20 m [66 ft]. To provide a platform for winter drilling operations in deeper water outside the landfast ice zone, we determined that a bottom-founded structure that rested on a subsea berm would be most appropriate and cost-effective. Canmar's first arctic winter drilling system was a sandfilled, concrete multiple-caisson structure placed on a subsea berm - Tarsiut artificial island. Tarsiut island was built in 1981. Gulf Canada Resources Inc. operated Tarsiut during the winter of 1981–82 and the spring and summer of 1982.1 This caisson-retained island provided a stable drilling platform throughout, and two wells were drilled. The system had disadvantages, however, including limited mobility and "deck" space. Also, relocation of the structure would require considerable time because of the need to rig down completely, to excavate the caissons, and then to reconstruct the structure and rig up completely at the new location. Canmar's second-generation system, the SSDC, overcame many of the disadvantages of the Tarsiut system and has proved to be an excellent solutions to the problems of arctic drilling operations.
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