Any application of new technology requires proper preparation to ensure a safe and efficient field program. Slim hole technology requires different preparation and procedures than conventional rotary drilling techniques. Application of the mining industry's coring techniques, small wellbore annuli, precise fluid flow measurement for kick detection, and specialized drilling fluids requires significant technical understanding and preparation. For Mobil in 1991, to prepare for a first application of Slim Hole Drilling Technology, the preparations included drilling a test well with Long year Company's Oil and Gas Division at Mobil's Dallas Research Lab and using the information to prepare for two slim hole oil and gas exploration wells to be drilled in Bolivia, South America. The test well gave Mobil drilling personnel practical hands on experience with the technology and provided a slim hole test laboratory to perform extensive borehole hydraulics tests and practice well control techniques. The information gained from the test well was used to plan two exploration wells in the Madre De Dios Block in Northern Bolivia. Industry has proven the economic advantages of utilizing this emerging new technology for Oil and Gas Exploration, especially in the remote frontier areas of the world. The planning, technical, and operational points of interest are highlighted in the paper. The advantages and disadvantages of the technology observed during the wells are illustrated. Methods available to allow the technology to adapt to Oil and Gas and API standards are discussed. Many technical aspects of the application of the technology are discussed which will give an insight to the minimum technical preparation a company needs to perform to be comfortable with a first application. Areas of technology are highlighted where additional development of the technology is needed. Introduction The drilling industry has applied many definitions to the term "Slim Hole Drilling." As applied to conventional drilling, slim hole typically indicates a well is designed to reach total depth (TD) at the next smaller hole diameter than would be normal. For the purposes of this paper, the definition will reflect the more common definition relating to mining slim hole technology. Most of all slim holes drilled to date have used the mining industry technology. The mining industry has been drilling slim hole for mineral and precious metal exploration for decades. A complete and separate infrastructure from the oil field has grown to support this industry. Therefore, while many same requirements exist between the mining and oil and gas industry, they have used different methods to accomplish the same tasks. This requires an operator to study the mining industry technology carefully so as to adequately transfer the technology to its exploration needs. Figure 1 illustrates the generic differences between the drilled hole diameters and sizes of casing typically run in conventional and slim hole wells. P. 235^
HIGH CURRENT and DEEPWATER are two topics which each require very special considerations and equipment to allow trouble free drilling throughout the waters of the world. There has been a substantial increase in the interest and activity of drilling in deepwater, high current, and deepwater high current locations. This paper will focus on the planning and equipment required for drilling in those areas where deepwater and high currents exist. The combination of the two requirements requires each have its own stand-alone considerations as well as a special merging of the separate considerations.
Summary An unprecedented intrusion of the Loop Current into the northern Gulf of Mexico severely affected expiration drilling in areas where water depths are asshallow as 450 ft. At Mobil E and P U.S. Inc.'s well, drilling was discontinuedowing to rig offset and resultant high riser angle. Safe disconnect of theriser required modeling of the mooring system and riser, building necessaryequipment, and proper planning. Introduction In Aug. 1989, the Loop Current unexpectedly moved into the northern Gulf of Mexico, severely affecting exploration drilling in several lease block areas. At least six operators were forced to shut down operations. The affected blockswere primarily in deep water; however, strong currents were found at sites withwater depths as shallow as 450 ft. At Mobil's well in 770 ft of water at EwingBank 871, drilling from the Sedco Forex 601 rig was discontinued owing to rigoffset, vortex-induced vibrations, and resultant high riser angle on Aug. 11. The riser was safely disconnected on Aug. 19 and was not reconnected until Aug.27, when current speeds had diminished greatly. Data on currents measured atthe rig were entered into models of the mooring system and riser that were usedat the rig to ensure a safe disconnect. The Loop Current and Eddies The Loop Current is a portion of the Gulf Stream system that enters the Gulfof Mexico between Cuba and the Yucatan, loops through the eastern Gulf of Mexico, and exits between Florida and Cuba. The path of the Loop Current variesconsiderably. At intervals of 6 to 17 months, large clockwisecirculating eddiesbreak from the current, as shown in Fig. 1. Eddies generally translate to thewest and southwest, but occasionally move northwest onto the Louisiana shelf. The Loop Current and its eddies have the potential to produce currents inexcess of 4 knots near the surface and up to 1 knot at depths to 1,000 ft. Tracking the Loop Current and eddies is possible from November through May bypossible from November through May by use of satellite thermal images availableat no cost from government agencies. The images show the strong temperaturegradients that occur at the edge of the Loop Current and eddies. Certainfactors affect the accuracy of thermal images. Cloud cover often obscures thesea surface, and the location of currents can vary with respect to surfacetemperature gradients. If the edge of the Loop Current is near a site ofinterest, detailed tracking must be undertaken from ships. During June through October, the entire Gulf of Mexico becomes uniformly warm at the surface. Therefore, surface contrasts disappear, making satellite imagery of littlevalue and more costly in-situ measurements necessary. Strategic seeding of the Gulf of Mexico with satellite-tracked drifting buoys is used to infer grossmovements, and ship-deployed instruments are required to delineate the edge ofthe highcurrent features. Time Series of Data at the Drillsite. Measurements on the rig, shown in Fig. 2, began on Aug. 17 and continueduntil Sept. 7. Owing to instrument failures, two major data breaks convenientlyseparate the record into three segments for discussion:Aug. 17–19,Aug. 21–28, andSept. 1–7. Currents during Period 1 were strong with aminimum speed of 1.75 knots and a maximum of 2.68 knots, with the directionrelatively steady at about 70 deg. At the beginning of Period 2, currentscontinued to be strong; however, the direction was almost due north. Thisstrong northward flow is questionable because steerage by the bottom topographyis typically in a more eastward direction. Visual estimates from the rig, however, support the northward direction; the direction change is similar to ameander passage. Current speeds decreased linearly from 2.25 knots on Aug. 24at 5 p.m. to a speed of 0.26 knots on Aug. 26 at 7 p.m., a decrease of about0.25 knots every 6 hours. The current direction change was much more abrupt. Direction changed nearly 90 deg., from 356 to 77 deg., in 4 hours (the changemay have been quicker, but data are not available between the two readingsseparated by hours). The reduction in speeds allowed the riser to bereconnected on Aug. 27. The measured current data in Period 3 are indicative ofcurrents normally experienced at the site. Speeds are generally less than 0.5knot, and direction is continually changing in response to tides and/orinertial oscillations. The vector plots indicate a slow westward flowsuperimposed on the rotational currents during this period. JPT P. 1038
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