Study On Shale Gas-water Drilling Fluid System In Changning And Weiyuan Areas

During the development of shale gas,oil-based drilling fluid is dominant in shale gaswell drilling.However,with the continuous improvement of people's awareness of environmental protection and in order to increase economic efficiency,water-based drillingfluid for shale gradually attracts more attention.Nevertheless,using water-based drilling fluid has a few disadvantages and seriously restricts the application in shale gas well drilling.For example,the lubricating property and cleaning ability of water-based drilling fluid are poor,which will lead to a high friction and high torque phenomenon prone segment in kickoff section and long horizontal section.At the same time,the aqueous phase invasion of water-based drilling fluid will cause hydration,structure damage and well collapse.Therefore,it is very important to research and resolve these issues of water-based drilling fluid for thedevelopment of shale gas.This article analyzed fabric characteristics,drilled geological data and engineering data of Changning and Weiyuan Longmaxi shale.Based on these analysis,the assessments and optimizations of sealing,fluid loss,inhibitor and lubricant have been studied.Through a lot of the compatibility tests,a water-based drilling fluid system is initially developed which can be used in horizontal shale well.Following understandings and the results are obtained:(1)The studies showed that Changning and Weiyuan Longmaxi shale mainly contains quartz and clay.The clay minerals are mainly illite,with a little illite/smectite formation and chlorite.According to SEM analysis,there are well developed micron-sized pore throats and micro-fractures in the shale.Therefore,effective seal and hydration inhibition are very important for designing and optimizing the water-based drilling fluid.(2)The API and HTHP fluid loss experiments optimized the nano-blocking agents LAT and ReduSH,which have the best fluid loss reduction effects,as plugging agents and fluid loss additives for shale water-based drilling fluids respectively.The hot rolling test preferably uses the surface hydration inhibitor UHIB as a shale water-based drilling fluid inhibitor;after determining the correlation coefficient by the viscosity coefficient meter,the bonding type lubricant is selected as the lubricant for the shale water-based drilling fluid.A shale water-based drilling fluid system was established through a large number of compatibility experiments:3%soil slurry+0.5%NaOH+5%PEG+3%LAT+3%UHIB+ 7%KCl+3%ReduSH+3%bonded lubricant+0.8%SP-80+barite.(3)According to the performance evaluation experiment,the shale water-based drilling fluid system developed has good sealing capacity and inhibition performance,and has good lubricating performance.Its well wall stability is stronger than that of polysulfone and polymerization currently used in Changning.The drilling fluid system is weaker than the oil-based drilling fluid system,and has good anti-pollution ability and no pollution to the environment.