Research And Application Of Inhibitive Drilling Fluid

Artificial islands have been built to explore and develop ChengHai1, ChengHai2blocks by drilling extended reach wells and horizontal wells in DaGang oilfield. ChengHai2block is located in the intertidal zone east of Zhang Juhe village, HuangHua, HeBei province; the formation encountered during drilling there are Ming Huazhen; GuanTao, DongYing, ShaHejie. Since the lithology in Dongying and ShaHejie is rigid mudshale with high clay content which is very brittle, wellbore instability problems are likely to happen because of the hydration and expansion of mudshale. These problems may result in downhole incidents such as stickings and well caving. Severe well caving incidents were encountered in2008. Among the7completed extended reach wells, stickings and sidetrack incidents happened in3of them as a result of well caving.5sidetracks happened in Zhang21-21L and3happened in Zhang13-25L. Dongying and ShaHejie are the formations with wellbore instability problems, well inclinations of the well sections with incidents were all over40°. The average incidents rate per well in2008was even up to22%, which severely affects the development of shallowwater oilfield。The safety adjustable density range of ChengHai2formation has been determinated with the help of the research made to the wellbore instability mechanism of Chenghai2mudshale formation by Mud Service Company, BHDC based on the relevant formation characteristics as well as engineering difficulties. A kind of inhibitive drilling fluid which utilizes solid-free high density inhibitive organic salt base fluid as key mud chemical together with sealing materials has been developed based on many research and lab experiment to solve wellbore instability problems, reduce incidents rate and protect reservoir. The mudshale hydration and expansion; well caving incidents in ChengHai2block have been successfully solved, which makes safety drilling in this block truly feasible.The approved inhibition, lubricity, cuttings carrying ability, anti-caving and reservoir protection ability of inhibitive drilling fluid applied in ChengHai2block have been fully admitted. The drilling fluid remained still rather than circulated for72hours in the mudshale formation due to engineering problems and POOH went smoothly after that, which proved its approved inhibition; the returning cuttings were normal and no blocks or stickings happened during POOH and RIH, which proved its cuttings carrying ability; torque as well as pull force were normal without lubricant during drilling and directional drilling went smoothly with only3%BZ-YRH lubricant added, which proved its lubricity; BZ-YFT and BZ-YRH together with other anti-caving materials were added to help form dense mudcake on the wellbore to enhance the anti-caving and sealing ability of drilling fluid. As a result, the mudshale caving problem in ShaHe formation have been successfully solved; POOH and RIH went smoothly without reaming; wire logging and casing running were successful at the first attempt. Compared with the wells drilled by inhibitive drilling fluid, many incidents such as well caving and reaming were encountered in the offset wells drilled by other drilling fluid system.397m cement section formed in technical casing and liner because of the problems of float collar and float shoe in Zhang26-26, drilling fluid properties remained stable after running cement plug, which means this system can resist pollution effectively; since the density of BZ-WYJ-I base fluid was high, the adding of inert materials into drilling fluid could be reduced to the largest extent. As a result, the fluid was compatible with reservoir formation water and reduced the damage to the reservoir, which is helpful for oil production.The research results are as follows:(1) The reason and countermeasure for wellbore instability of extended reach wells in ChengHai2block have been studied and safety adjustable density range has been determinated.(2) Inhibitive drilling fluid system has been customized to meet the requirement of inhibition, sealing, wellbore stability, lubricity and cuttings carrying ability in ChengHai2wells based on project study. This system can be recommended to be applied in new blocks with similar lithology in future.(3) The severe variation of drilling fluid properties caused by formation fluid pollution during drilling have been successfully solved by inhibitive drilling fluid. Its properties remained stable with high concentration of (-HCO3-)(-CO32-)(CL-)(Ca2+) soluted(4) The wellbore instability problem in massive mudstone in ShaHe formation has been successfully solved.(5) The approved reservoir protection of this system is helpful for reservoir evaluation and development.