New mud chemistry is shown to improve the efficiency of directional drilling with a rotary steerable drilling system. Full-scale (8 1/2-in. bit) drilling results using a rotary steerable drilling system drilling to horizontal at an experimental test site and an application in the Huizhou 21–1 field, offshore Hong Kong in the South China Sea are presented documenting the effectiveness of this technology.
Introduction
The ability to drill directionally has proven to be one of the most important developments in our industry and can be expected to remain popular in the near future. Development of reserves in remote offshore areas, environmentally sensitive areas and areas with restricted surface access; re-entry and multilateral drilling to extend asset life; and horizontal completions to increase production rates and recovery will all keep interest in directional drilling high. Fluctuating oil and gas prices have also put a premium on efficient directional drilling techniques and for complex 3-dimensional well paths, so-called designer wells, rotary steerable drilling systems appear to be the most efficient. The choice of the drilling fluid can either assist or hinder the performance of the tools.
Directional Drilling Tools
Steerable Motors
Steerable motor systems using positive displacement motors became the method of choice for demanding directional drilling applications starting in the late 1980's1. These systems rely on a bend in the motor housing to provide bit offset that is oriented to turn the well. Steerable motors are operated in either the slide drilling mode or rotational-drilling mode. The drillstring does not rotate in the slide-drilling mode but is simply pushed along the well path with bit rotation provided strictly by the motor. The drillstring and entire drilling assembly is rotated from the surface in rotational-drilling mode drilling a slightly over-gauge hole due to the bend in the motor housing. Measurement-while-drilling (MWD) systems monitor the well path and bent motor orientation (tool face) as well as transmit the information to the surface via mud pulse telemetry. Alternating between the two drilling modes give directional drillers the ability to alter build rates without having to trip the pipe.
Despite the tremendous advance in directional drilling technology steerable motors provide, they suffer from a number of problems. These include:Stabilizer hang-ups on ledges formed from rotational-drilling mode to slide-drilling mode transitionsIncreased torque, reactive torque, drag and probability of stuck pipe due to friction from cuttings accumulation caused by poor hole cleaning during slide-mode drillingPoor transfer of weight to the bit and bottom hole assembly (BHA)/bit damage from axial stick slip also caused by poor hole cleaningPoor ROP due to the above as well as use of less aggressive bits due to the high reactive torqueLimited total displacement due to high friction from spiral well paths drilled by bent motor assemblies as well as poor hole cleaning
Rotary Steerable Drilling Systems
Effective rotary steerable drilling systems overcome most of the problems with steerable motors by providing continuos pipe rotation while drilling. This is accomplished, with the tool reported on in this paper, with a non-rotating steerable sleeve containing three extendable pads that push against the borehole wall2. The drillstring, which is connected to the bit, rotates inside the sleeve. Independently varying the force on each of the pads steers the tool along the desired well path. The tool can be programmed for inclination or steering force and deviations are automatically adjusted for by a closed loop control. Any adjustments needed can be downlinked to the tool without interrupting drilling.
