| The extended reach well drilling (ERD) technology has wide use in development of marginal oilfield on offshore. 112 oilfields and/or structures have been found on offshore in China, in which there 67 small or marginal oilfields whose reserves are below 10 million tons. The existed subsea equipments and pipe lines can be fully utilized by using ERD technology. Therefore some small fault and/or marginal oilfields on offshore can be exploited and developed efficiently and economically.During study for my doctoral degree, as a General Chief Engineer of Bohai oilfield of CNOOC and a coordinator of 863 project of offshore ERD technology, I took part in 863 project of offshore ERD technology form very beginning. CNOOC used oversea advanced ERD technology and experiences for reference, probed, practiced, developed key ERD technology in China, formed a set of ERD technology. We have fulfilled 6 extended reach wells on Bohai offshore oilfield whose departures are about 4000m and the ratios of departure to true vertical depth are close to 2 by using present rigs and water based drilling fluid system.This paper discussed and summarized fully key techniques related to ERD, those key techniques are as folio wings: reservoir engineering research: finding traps suited for using ERD technology and setting down developing plan, rig evaluation and upgrade: according to special requirements of ERD, all equipments of rig should be evaluated, and be upgraded if necessary, wellbore stability technology: safe window of drilling fluid density was calculated, Inclination and azimuth along which wellbore of extended reach well may be stable by using sonic logging data, prediction of analysis on torque and drag of drill string: theoretical models for analysis on torque and drag of drill string with extended reach well were developed, in which drill string buckling is taken into account. The model can be used to predict and analyze hook load and torque on rotary table when drill string is under any working conditions with well bore, model also can be used to predict and analyze hook load when casing string with floatation collar is run into hole, rotary steerable drilling and trajectory control technology: hydraulically drived variable gauge stabilizer was developed, mechanical performance of rotary steerable BHA was calculated. the optimum drill string design: calculations of critical WOB for preventing drill string from buckling and torsional strength, hole cleaning technology: some factors affecting hole cleaning were taken into account, minimum transporting velocity (MTV) within annulus was presented, drilling fluid technology: drilling fluid system withgood lubricity and inhibitivity was developed and presented corresponding evaluation methods. (9) cementing and well completion technology: well completion fluid system was developed, running casing string into hole and cementing procedures were discussed, producing capability was analyzed according different well completion method, including perforating completion grabble packed completion.A set of ERD technology has formed in Bohai oilfield, all ERD techniques were applied in extended reach wells drilled in QK17-2 and QHD32-6 oilfield.The main research achievements and innovations are:(1) The technical difficulties related to rig position settling, upgrading have been set down.(2) Wellbore stability prediction model took shale hydrated stress into account, the research results have important theoretical significance.(3) Torque and drag analysis model provides a cogent tool in optimizing well profile and rig capability evaluation. Torque and drag monitoring technique plays a key role in finding downhole abnormity, preventing downhole accidents.(4) The extended reach well with 4000m departure was drilled in Bohai oilfield by using water based drilling fluid at first time in China. The developed drilling fluid system have good lubricity, inhibitivity and cutting carrying capability, its low toxicity be propitious to diminishing environmental pollu...
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