Development Of Nano Bentonite Drilling Fluid And Its Effect On Shale Sidewall Stability

Since the increasing demand for fossil energy in the economic development of human society,the production degree of non-renewable resources such as oil and natural gas is gradually weakening,which makes the energy security of large countries seriously threatened.With the continuous improvement of fossil energy development technology,unconventional energy resources such as shale gas have gradually come into people's vision.At present,shale gas exploration and development has become the mainstream of energy and mineral exploitation in most countries of the world,and has been booming in North America,Europe,Asia,Oceania and other regions.In view of the abundant reserves of shale gas,its wide exploitation and utilization,and its economic value,a"shale gas revolution"has been launched around the world,which has also changed the current global energy supply and demand pattern.In this paper,nano-bentonite was synthesized by two step method,and natural calcium-based bentonite?Ca-Bt?was used as raw material.First,Ca-Bt was acidified with acetic acid by microwave-assisted treatment;then,under the condition of water bath energy,nano-bentonite was modified by NaCl,Na₂CO₃,sodium lignosulfonate and polyquaternary ammonium salt-7,and the influence of each factor on the synthesis was analyzed by the control variable method;The composite was characterized by Fourier far infrared?FT-IR?,X-ray diffraction?XRD?and electron scanning electron microscopy?SEM?.Finally,the formula of drilling fluid for shale wells was optimized.Apparent viscosity,plastic viscosity,dynamic shear force,dynamic plastic ratio and filtration loss at high temperature and high pressure were tested respectively,and the optimum additive dosage was obtained.In addition,mineral analysis of shale samples was carried out by XRD to obtain the optimum additive dosage.The expansibility and recovery of shale under nano-bentonite system and drilling bentonite system are studied in order to study the influence of nano-bentonite system on shale wellbore stability.The results show that the acidification effect of bentonite acetic acid is the best,when the concentration of acetic acid is 2mol/l,the initial microwave temperature is 50?,the power is 400W and the irradiation time is 4min.The swelling volume of bentonite acetic acid can reach 34 ml/g,which is doubled than that of original bentonite.Na₂CO₃ and polyquaternary ammonium salt-7are selected as modifiers under the conditions of water bath temperature 50 C and stirring rate 300 r/min,and the mass ratio of them is 1:2.The colloid value of the nanocomposites was 430ml/15g,the apparent viscosity was 29.6mPa.s,and the dynamic-plastic ratio was 0.6.According to the current formulation of shale drilling fluid,the viscosifier xanthocellulose,filtrate reducer sulfonated asphalt and shale inhibitor potassium silicate are selected.The formulation of the system is 4% nano-bentonite+0.4%xanthocellulose+0.5%sulfonated asphalt+2%potassium silicate.The rheological property of the high-efficiency water-based drilling fluid is good,and the filtrate loss at high temperature and high pressure is nearly double that of the drilling bentonite system.Through the experiments of expansion rate and recovery rate,the nano-bentonite system has excellent stability for shale wellbore.