Study Of The Mechanism And Control Methods Of Formation Damage For Longmaxi Shale Formation In Eastern Sichuan

The success of shale gas industry in North America has largely stimulated the exploration and development of such unconventional source in China.However,considering complex domestic geological conditions and less advanced technologies,mechanical imitation from overseas methods is not recommended.According to previous studies,fracture development,micro-crack and bedding fracture exist in gas-bearing layers of shale reservoir.In this case,it is very likely to cause leakage or filtration of oil-based drilling and completion fluids while drilling in horizontal sections and thus bring extensive damage to the reservoir.Meanwhile,after fracturing,the majority of fracture fluid will remain in the reservoir which often generates water phase trap of shale substrate porosity.Besides,when free gas in cracks escapes faster than the substrate can offer,stress sensitivity damage of fracture permeability may be easily induced.In this paper,damage mechanism and evaluation method of conventional reservoir are used for reference.Based on core analysis and well data,including drilling and logging data,of gas-bearing shale,the potential damage type is determined as well as its severity and timing to explore how the shale reservoir is impaired by drilling and fracturing fluids.The damage mechanism of lithologic trap induced by invasion of working fluids under in-situ conditions and corresponding control measures are studied.What's more,on the basis of Nano-temporary plugging technology,the protection method that employs nanoparticles to block formation pores is proposed laying the foundation for the safe development of shale gas.The major research contents and relevant results include:(1)The potential damage factors of shale reservoir are analyzed.The minerals in Longmaxi formation are mainly sensitive to water,alkali and acid.And the high lipophilic property and modest hydrophilic property of reservoir enable filtrate of oil-based drilling fluids and fracturing fluids to permeate into pore throat in shale spontaneously,which usually leads to phase trap.With severe fluid leakage,oil-based drilling and completion fluids usually flow into formation through natural or induced fractures.This may results in solid plugging for cracks,water trap,fluid sensitivity and etc.In addition,fracturing in shale formation features large scale,long backflow time and low backflow rate which is commonly 10%?20%.These factors will bring about fracturing fluid retention and make the formation extremely vulnerable to aqueous phase trap and solid plugging.(2)The sensitivity damage mechanism of shale reservoir is defined.Experimental results show weak to moderate velocity sensitivity,strong water sensitivity and alkali sensitivity.As for Stress Sensitivity,along with the increase of effective stress,crack width shrinks and permeability reduce at a rate of over 80%.Longmaxi formation is fracture-developed,rich in sensitive minerals and vulnerable to working fluids.These characteristics make stress sensitivity damage more complicated as rock samples display a more serious damage degree than average.Likewise,under the negative influence of oil-based drilling fluid,stress sensitivity damage of fracture permeability gets more prominent.And fracturing fluid will also take a toll on stress sensitivity to an even higher degree than oil-based drilling fluid does.(3)The damage type and mechanism of working fluids within gas-bearing layers are figured out.The results of indoor experiment show weak to moderate velocity sensitivity,strong water sensitivity and alkali sensitivity.The static impairment test for drilling fluid filtrate indicates a moderate to strong damage degree and an impairment rate of over 60%to permeability of formation samples.Some even reflects a complete loss in that.In dynamic experiment,after the invasion of drilling fluid leakage,the loss ratio of permeability for cracks with a width of less than 50?m is between 40%?90%,while for other cracks the number is 20%~85%during simultaneous flow-back.And other experiments tell us that fracturing outlet liquid will cause a damage,generally at a rate of 70%?90%,for various permeability levels(1.6?848mD).(4)The damaging mechanism and control method of phase trap in shale reservoir are determined.The harm processes of water phase trap,under onsite temperature and pressure conditions and normal conditions respectively,due to filter liquid intrusion are analyzed through test.The research finds that intrusions of both oil and water will increase the level of liquid saturation.But under the same inlet liquid condition,the oil invasion amount outweighs that of water distinctly so as to produce higher liquid saturation degree.This is because shale is more readily moistened by oil.Additionally,shale invaded by oil has a lower permeability recovery rate than that of shale invaded by water,since oil is less likely to flow back than water.Along with the concentration increase of FW-134 fluoride surfactant,which has low surface tension,high hydrophobic property and good temperature resistance,the permeation decreases while the permeability recovery rate of shale goes up conversely.When the concentration of FW-134 fluoride surfactant is 0.05%,namely the optimal situation,the flow-back liquid can recover 112.4%?135.3%displaying an extraordinary inhibitory effect on water trapping.(5)In hope of reducing damage generated by working fluid,protection method based on Nano-temporary plugging technology is put forward.The existence of nanometer materials will result in rise of apparent viscosity and plastic viscosity and reduction of yield value and API filtration loss.Higher concentration of nanomaterial corresponds to lower filtration loss.Nano-CaCO3 turns out to be most effective in diminishing filtration loss for Longmaxi formation.The same amount of oil based drill-in fluid with nanomaterial has lower penetration ability than that of water based drill-in fluid.Nanoparticles plug rock pores in two ways,namely throat plugging and bridge building.Throat plugging mainly aims at macro-pore whereas bridge building generally targets macro-pore.Besides,nanoparticles can work with solid particles in drilling and completion fluid to block micro-crack which has bigger size.Among other things,acid dissolution can effectively unleash the plug caused by acid soluble nanomaterial and thereby empower permeability to restore at a rate of 90.3%?13.1%,but it fails to work on insoluble nanomaterial in the same way.