Study On Mechanism And Law Of Spontaneous Gas-water-shale Imbibition At High Temperature And High Pressure

Shale reservoir has very low permeability and requires large-scale hydraulic fracturing to achieve industrial production capacity.After fracturing fluid enters shale reservoir,it will spontaneously imbibe into pores.Spontaneous imbibition of massive fracturing fluid into shale can affect fracturing performance and cause reservoir damage.It can cause hydration,which may causing new fractures in shale and affecting reservoir permeability.Therefore,it is of great significance to correctly understand the mechanism of spontaneous gas-water-shale imbibition and study the law in the process of spontaneous imbibition.They can provide guidance for optimizing the fracturing fluid flowback system and enhancing the recovery of shale gas.At present,spontaneous gas-water-shale imbibition experiments are mostly carried out at room temperature and atmospheric pressure,and there are few reports at high temperature and high pressure.However,the actual shale gas reservoir is in the formation environment of high temperature and high pressure,and the rock pores are filled with high pressure gas rather than atmospheric air.If the air-water-shale system at room temperature and atmospheric pressure is used for spontaneous imbibition experiment,the experimental results may not reflect the real law.In order to make the experimental environment more close to the real formation conditions,a spontaneous gas-water-shale imbibition experimental device for high temperature and high pressure was developed first,and the experiments were carried out at high temperature and high pressure.Then,the effects of temperature and pressure on interfacial tension of methane-water,methane-water-shale contact angle and methane-water viscosity are studied,and finally,the spontaneous gas-water-shale imbibition models are established.The main research achievements are as follows:(1)A spontaneous gas-water-shale imbibition experimental device was developed,which can simulate spontaneous gas-water-shale imbibition in the environment of gas reservoir temperature,pore pressure and confining pressure.The corresponding experimental method was estabilished,and the reliability of the experimental device was verified by weighing rock samples.(2)Taking temperature,pore pressure,confining pressure and fracture development degree as variables,this paper carried out spontaneous methane-water-shale imbibition experiment at temperature 25 ～ 90℃ and pore pressure 1 ～ 20 MPa,and compared with the spontaneous air-water-shale imbibition experiment results measured by gravimetric method at atmospheric pressure.The experimental results show that the higher the pore pressure,the lower the spontaneous imbibition rate.The higher the temperature,the faster the spontaneous imbibition rate,the greater the total spontaneous imbibition volume,the higher the displacement efficiency of spontaneous imbibition;The smaller the confining pressure,the faster the spontaneous imbibition rate;The more the fractures,the faster the spontaneous imbibition rate.New cracks appear in shale samples with high clay content after spontaneous water imbibition.The results of spontaneous air-water-shale imbibition experiment at atmospheric pressure cannot reflect the characteristics at shale reservoir pressure,and its spontaneous imbibition rate is faster and displacement efficiency of sopontaneous imbibitoin is higher.(3)Spontaneous imbibition models of the composite capillary and plate with heterogeneous wetting were established,and further the pore and pore-fracture system of spontaneous gas-water-shale imbibition models which were verified by the numerical simulation and experiment were estabilished.It is found that the models are in good agreement with the numerical simulation and experimental data.So it indicates that the models were reliable.(4)The mechanism that the spontaneous imbibition time index of shale is less than 0.5 is revealed.The spontaneous imbibition of shale pores is non-piston flow for the heterogeneity of pore size and wetting.It leads to the different time of reaching the equilibrium of spontaneous imbibition,and the spontaneous imbibition curve deviates from Handy equation.When the double log curve of Handy equation is used to describe spontaneous imbibition of shale,the time index is less than 0.5...