Discussion On Mechanism Of Alkyl Glycoside Drilling Fluid In Shale Gas Formation

Shale gas formation is characterized by high clay mineral content,low porosity,low permeability and compact physical properties,and is usually developed by extended reach horizontal well groups.During the drilling process,shale hydration causes changes such as decreased rock strength and increased pore pressure,which easily leads to complex downhole situations such as well collapse and lost circulation,which seriously hinders the drilling and development progress of shale gas formation.Oil-based drilling fluid is often the first choice for shale gas formation drilling due to its strong anti-collapse ability and good wellbore stability,but it has poor environmental performance and high preparation cost.Therefore,it is urgent to develop high performance water-based drilling fluids similar to oil-based drilling fluids.After the introduction of alkyl glycoside derivatives into water-based drilling fluids,some properties of water-based drilling fluids are close to that of oil-based drilling fluids,which are non-toxic,environmental friendly and biodegradable,and have good field application effects at home and abroad.However,the mechanism of alkyl glycosides is still unclear.Based on the analysis of the molecular structure of alkyl glycosides and their interaction with shale,this paper discusses the anti-collapse and lubrication mechanism of alkyl glycosides drilling fluid.Firstly,the properties of alkyl glycoside based liquid were analyzed by viscosity test and surface tension test;the performance of alkyl glycoside drilling fluid in inhibiting hydration expansion and dispersion of shale was evaluated by means of shale rolling dispersion and pressure transmission,and the anti-collapse mechanism of alkyl glycoside drilling fluid was discussed.The results show that the alkyl glycosidyl based liquid is a low molecular weight polymer with various functional groups such as amine group,hydroxyl group,glycosidic bond and hydrophobic group.When the based liquid concentration is low,it can effectively inhibit the shale hydration dispersion and expansion,and can be absorbed into the clay hydration layers through forces such as electrostatic attraction and hydrogen bonding,forming multi-point single layer adsorption on the surface of clay,reducing the clay hydration repulsion and extruding part of the interlayer water;hydrophobic groups cover the surface of clay particles to form a membrane structure,which retarded liquid water molecules from entering the interior of the shale particles.When the based fluid is in high concentration,the stability of shale borehole wall can be improved by activity regulation.The lubrication characteristics of alkyl glycoside drilling fluid were tested by extreme pressure lubrication and mud cake adhesion experiment;the lubrication mechanism of alkyl glycoside drilling fluid was discussed by using four-ball friction test,scanning electron microscope and energy dispersive spectrometer to analyze the friction surface.The results show that the lubrication coefficient decreases by 41%after adding 1%alkyl glycoside based liquid into bentonite basd slurry.The alkyl glycoside base fluid is adsorbed on the metal surface,and the contact between the metals is converted into the contact between the hydrophobic groups,thereby exerting a better lubricating effect;alkyl glycoside based liquid is adsorbed on the surface of barite,which helps to reduce the resistance of collision flow between barite particles,and then plays the role of lubrication and drag reduction.Four-ball friction experiments showed that alkyl glycosid based liquid was adsorbed on the friction surface and formed a lubricating film inside the abrasion mark of steel ball,which significantly reduced the friction coefficient and the diameter of the wear spot,and increased the maximum non-sticking load,thus played a role in reducing wear and abrasion.The rheological properties of alkyl glycoside base fluid and alkyl glycoside drilling fluid were analyzed with the help of high temperature and high pressure rheometer.The experimental results show that the rheological properties of alkyl glycoside based liquid and alkyl glycoside based drilling fluid are obviously affected by high temperature,Herba rheological model has the best fitting effect when fitting the rheological curves.Alkyl glycosides drilling fluid had excellent plugging and loss performance,API filter loss is only 0.2 m L after 140℃ hot rolling and 4.0 m L at 150℃/3.5 MPa at HTHP.The reservoir protection performance of alkyl glycoside drilling fluid was evaluated by multifunctional oil layer protection instrument and core self-priming experiment.The experiments show that the permeability recovery values of the alkyl glycoside drilling fluid are more than 90%after static/dynamic contamination.Alkyl glycoside drilling fluid can reduce the water activity,effectively reduce the surface tension,reduce the water locking effect,and contribute to the filtrate flowback.Based on the mechanism analysis of alkyl glycoside drilling fluid,a set of water-based drilling fluid formula suitable for shale gas formation was optimized:15%NAPG+2%sodium bentonite+0.2%Na OH+0.3%XC+0.3%PAC-HV+1%SPNH+2%nano-Ca CO₃.It has good rheological property,filtration property,lubrication property and reservoir protection property,can resist temperature of 160℃,and the cost is low,can better meet the basic requirements of shale gas formation water-based drilling fluid technology.