Research On Water-Based Drilling Fluid For Shale Gas Drainage And Strong Blocking

During the drilling of shale formations in the Longmaxi Formation in Sichuan,borehole collapse occurred frequently,which severely restricted the efficient development of shale gas.At present,drilling fluids for shale gas are mainly oil-based drilling fluids,but due to environmental protection and cost pressures,the use of water-based drilling fluids is the future development direction of shale gas drilling,and water-based drilling fluids are often encountered during use.Problems such as leakage,shale hydration,and well wall collapse caused by the development of cracks and micro-fractures.In view of the above problems,a hydrophobically modified silica plugging agent is selected,and its performance is evaluated through experiments such as scanning electron microscopy,expansion rate,recovery rate,wettability,plugging property,and nuclear magnetic field.The results show that it can effectively inhibit shale hydration expansion and dispersion,and it can also block pores,micro-cracks and cracks,and at the same time make the shale surface hydrophobic.The shale recovery rate of 2.5wt%plugging agent solution at 120℃is81.3%（water recovery rate is 3.9%）,and the bentonite base slurry（4wt%）added with 2.5wt%plugging agent has a fluid loss of 40.2 m L.The contact angle of shale water droplets after 2.5wt%plugging agent soaking reaches 121.1°.With the plugging agent as the core,preferably fluid loss agents,lubricants,etc.,a hydrophobic and strong plugging water-based drilling fluid has been developed.Experiments on expansion rate,recovery rate,wettability,plugging properties,compressive strength and anti-pollution properties show that the drilling fluid not only has good rheology,fluid loss reduction properties,but also has excellent inhibition,plugging,and hydrophobic properties.And anti-pollution ability,while effectively slowing down the reduction of shale compressive strength.The shale recovery rate of the drilling fluid at 120℃is 90.2%,the penetration depth into the 40-60 mesh sand bed is only 15 mm,and the high temperature and high pressure filtration loss is 7.2 m L when the density is 2.2 g·cm^-3.The drilling fluid provides a new technical way to solve the well wall collapse of shale gas formation.