Research On Plugging Technology Of Anti-collapse Drilling Fluid In Hard And Brittle Shale Formation

Wellbore instability is encountered frequently during drilling in the hard and brittle shale formation,the sealing technology of anti-collapse drilling fluid aimed at this kind of shale formation is studied in this paper.Based on the analysis of the physical and chemical properties,and the microstructure characteristics of Longmaxi shale by XRD and SEM,the causes of the collapse of Longmaxi shale are revealed.It is suggested that the efficient sealing and inhibiting abilities of drilling fluids are needed to tackle the wellbore stability problem,which provide the theoretical basis for the design of water based drilling fluid in Longmaxi shale.The range which the drilling fluid inhibitors and sealing agents can affect on the water in shale was analyzed under the basis of the theory of soil science and soil mechanics.The inhibitors are effective for bound water on the inside of clay particles while the sealing agents mainly playing a part on free water and partial bound water,which deepened the understanding of the mechanism of inhibiting and sealing.There is no universal standard for evaluating the sealing performance of drilling fluids.Besides,the traditional evaluation method of the sealing performance of drilling fluid neither can accurately reflect the characteristics of pore structure nor the physical and chemical properties of shale.Therefore,combining with the macroscopic observation of rock specimens,the test of filtrate permeation and rock’s hardness and the analysis of microscopic morphology of rock samples,a new evaluation method using a modified filtration instrument for shale samples under high temperature and high pressure was established.The sealing performance of conventional rigid sealing materials,deformation sealing materials,nanometer size sealing materials,and wetting inversion sealing materials were evaluated by the new evaluation method.The results showed that the micro-sized sealing agents can enter into the inner of the shale,which also indicates that there are micron-sized pores or cracks within shale.Accordingly,sealing agents in the micron scale as well as that in nanometer size are required in water-based drilling fluids for shale.In addition,the titanium dioxide under nanometer size is excellent in maintaining its original size in drilling fluids,can be used as a kind of nanoscale sealing materials.According to the mechanism of sealing,three sealing types were presented,i.e.,particle size fill,hydrophobic barrier and film-forming reverse osmosis.Moreover,the acting modes,objects and the locations of sealing were analyzed,respectively.The new classification method makes up the drawbacks of the traditional classify system which is based on physical properties of sealing agents.Aiming at the requirements of drilling fluid properties for Longmaxi shale,the key sealing agents,inhibitors,coating agents,filtrate reducers and lubricants of drilling fluids were optimized by a series of experiments.The formulation of water-based drilling fluids for horizontal wells in Longmaxi shale is 4%bentonite slurry + 0.2%Na2C03 + 0.5%FA-367 +0.5%XY-27 + 3%ultrafine calcium carbonate + 2%emulsified asphalt + 3%nanoscale titanium dioxide emulsion + 7%KCl + 2%polyamine + 3%SMP-2 + 3%SPNH + 5%RH-220 + barite.A sequence of experiments manifest that the water-based drilling fluid performs well in sealing,thereby as well as in maintaining stability of shale.Meanwhile,the effects of lubrication,inhibition and anti-pollution of this drilling fluid are satisfying.