Wytch Farm was at the forefront of extended-reach drilling (ERD) during the 1990's. This was achieved through opportunity, innovation, technology and team work. Late life management of the field has now required that cost-effective sidetrack wells be drilled to sweep the remaining oil in place in the offshore part of the field. As with many ERD developments, one specific challenge associated with the original Wytch Farm wells was the ability to control torque and drag within manageable levels. While drilling the original ERD wells, it was noticed, by chance, that the addition of particular loss circulation material (LCM) had a profound impact on torque and drag. The mechanism by which this was happening was not fully understood at the time, but considered to be a combination of hole cleaning improvements and mechanical friction reduction. In an attempt to quantify and better understand the friction reducing mechanism, a series of controlled field experiments have been undertaken. This paper describes the basis for the field experiments, implementation plan, analysis techniques and results.Results correlate the amount of LCM addition with a marked reduction in friction factor. The effectiveness of LCM sweeps for torque and drag reduction is also discussed. Understanding the relationship between LCM addition and torque and drag reduction will offer cost savings during the drilling of more demanding ERD wells. Explanations of the torque and drag reduction mechanism are also discussed in the paper. Introduction BP's Wytch Farm oil field is located on the South Coast of England in an area of outstanding natural beauty and special scientific interest. Oil has been produced from the area for the past 25 years. The ERD programme commenced in 1993 to access offshore reserves under Poole Harbour from land based well-sites. The economic incentive for the ERD campaign was reserves access whilst avoiding the need to build an artificial island from which conventional wells would be drilled. Since the start of the ERD campaign, the step-out of wells increased incrementally over the life of the project. The first well F18 in 1993 had a departure of 3.8km; M05 in 1995 a departure of 8km; M11 in 1997 broke the 10km departure milestone setting a new world record. The ERD campaign finished in 2000 with M16, which set another world record. This last well had an equivalent (unwrapped) departure of 10,727m and measured depth of 11,278m [1]. The 2004 Infill Drilling Programme In 2004 a new ERD infill drilling programme was initiated. This targetted reserves in the upper Sherwood reservoir, to access oil that was unswept by the original ERD wells. A new rig was designed and built with the ultimate aim of being able to drill out to 11km. The Boldon B92 rig has a 5,000psi surface pressure system, a top drive capable of delivering 44 kft.lbs of continuous torque and enough derrick capacity to rack 9,000m of 5½" drill pipe. Figure 1 shows an equivalent departure plot and a plan view of the first four sidetracks drilled in the infill programme between April 2004 and April 2005. Equivalent departures for these first four sidetrack wells ranged from 4122m to 5423m. Table 1 shows the details of these sidetracks listed in the order in which they were drilled. All wells are 8½" sidetracks with windows milled from the 9–5/8" casing. The first three wells (F22wx, F20zy and F19zy) are oil producers consisting of a primary lateral geosteered through the upper Sherwood sands and a secondary lateral drilled as an open hole sidetrack through other productive sands. The reservoir is sufficiently competent to allow the laterals to be left barefoot. The upper completion design includes variable flow control equipment in order to retain production from the mother bore and combine it with that from the new laterals. The fourth and longest infill well, M01y, was drilled as a water injector and consisted of a single lateral. In this well a 7" pre-drilled liner with non expanding open hole packers were installed in order to preferentially inject water into selected formations.

The Captain Field which lies off the coast of Scotland is a shallow sandstone reservoir (3000 ft) comprising clean, unconsolidated sand with high permeability (up to 5D). The oil is heavy and bottomhole temperature very low (30o C). Throughout the development of this field (14 years) two of the main challenges have been control of unconsolidated sand and maximising production of the oil by water injection to maintain reservoir pressure. Particular attention has been paid to drilling and completion of the water injection wells. The drill-in fluid used was initially oil based mud but changing to water based drill-in fluid facilitated use of faster completion procedures. Initially, when using a water based drill-in fluid, displacement of the openhole to clear brine was always troublesome. This issue was resolved by the introduction of a new low temperature starch into the drilling operation. Adoption of the new formulation has facilitated a simpler, faster displacement operation and made it easier to test various techniques that are offered for filtercake clean up. Treatments, involving acetic acid released in situ, enzymes, sequestering agents, etc., provided questionable results. However, a breaker system that provides a delayed release of formic acid has recently been introduced and has led to a significant improvement in performance. New techniques have introduced significant benefits, for example: the improved starch shortened the completion process by at least several hours of rig time,the four most recently completed wells which were all treated with the formic acid system had an average initial Specific Injectivity Index that was about 50% better than the average achieved for the first five wells that were completed with oil based mud. The paper will present important aspects of the learning process on the Captain Field with particular emphasis on application of the new starch, and drilling/clean-up of the water injection wells.