Experimental Research On Oil-based Drilling Fluid Lost Circulation Prevention For Shale Gas Formation

Shale gas is a kind of clean and strategic energy resource,and it has important development and utilization values.It has become the popular topic of research in the area of petroleum engineering around the world.At present,oil-based drilling fluid is mainly applied in the horizontal well drilling in shale gas formations.The filed application indicates that the lost circulation of oil-based drilling fluid in shale gas formation is prominent,which greatly influences the comprehensive benefit of drilling.Therefore,optimized research on lost circulation prevention of oil-based drilling fluid in shale gas formation is urgently required.The shale samples of Longmaxi formation from Fuling block was taken as the research object in the present study.The mineralogical composition and the physical-chemical properties of the shale samples were tested.And the interaction between shale and oil-based drilling fluid was investigated.The main cause of lost circulation of oil-based drilling fluid in shale gas formation was then analyzed.Shale samples of Longmaxi formation was mainly composed by brittle shales.Micro-fractures and Nano-pores were observed in the shale samples.The content of organic matter was relatively high,and the shale surface exhibited mixed the wettability properties with both water affinity and oil affinity.Under the pressure difference and the capillary force,oil-based drilling fluid would invade into the formation along the micro-fractures,and erode the organic matter in the formation.Several factors such as reducing the fracture restart pressure,the stress concentration at the tip of microfracture,and difficult retention and sealing for the lost circulation materials,etc.,promoted the extension,development,and penetration of micro-fractures,resulting in the lost circulation of brittle shale formation.Therefore,it is critical to effectively seal micro-pores and fractures for preventing lost circulation in the shale gas formations.Micro-and Nano-materials sealing formula with favorable sealing properties were optimized by particle size grade distribution and compatibility test.Meanwhile,novel swellable lost circulation material SD-OS for oil-based drilling fluid was prepared by suspension polymerization method with acrylate monomers as raw materials.The structure of SD-OS was characterized by FT-IR,SEM and thermal stability analysis and so on.The properties of SD-OS was evaluated through oil adsorption test,compatibility test,temperature resistant test,sand disc filtration test and sand bed filtration test.The results indicate that,SD-OS showed excellent properties of sealing and reducing filtration,which was tolerant to high temperature of 200°C.The particulate sealing agent SD-OS could deform and invade in the surrounding rock,and swell after adsorbing oil continuously,which could realize dense sealing.Based on the shale gas formation properties of target block and the principle of intensified dense sealing,oil-based drilling fluid formulas with dense sealing were obtained by incorporating with SD-OS after optimization experiments.Optimized sealing formula containing 1.5%of SD-OS,1%of hydrophobic TiO₂ and 1.5%of elastic graphite was obtained when the lost circulation medium was 400mD permeable sand disc.Optimized sealing formula containing 1%of rigid particles,1.5%of elastic particles,1.5%of micro-fibers,1%of softening particles,0.5%of high friction particles and 1.5%of SD-OS were obtained when the lost circulation medium was 10D high permeable sand disc.Furthermore,after selecting sealing agents with different types,different sizes,and different concentration,oil-based drilling fluids with intensified dense sealing capacity were established for wedge fracture with different openings.The formulas have a high adaptive plugging ability.The pressurization capacity was higher than 6 Mpa for fractures with openings ranging from 1 to 4 mm.Meanwhile,the lost circulation volume in the process of sealing was relatively low.