The Characteristics Of Spontaneous Imbibition And Ion Diffusion Of Fuling Shale Gas Reservoir

In the process of shale gas reservoir fracturing,a series of complex interactions are produced between reservoir and fracturing fluid,which have an impact on the production of natural gas.The spontaneous imbibition of the fracturing fluid by reservoir and the diffusion of the ions from the reservoir to the fracturing fluid are the important forms of the interaction between the fracturing fluid and the reservoir.Combined with the characteristics of the low flowback rate and high salinity of flowback water of Fuling shale gas wells,this paper analyzes the physical and lithologic characteristics of Fuling shale gas reservoir,carries out experimental study on spontaneous imbibition and ion exchange of reservoir,analyzes the characteristics,laws and influencing factors of spontaneous imbibition and ion diffusion of reservoir,and analyzes the mechanism preliminarily.The following conclusions are obtained:(1)The physical and electrochemical characteristics of Fuling shale gas reservoir are analyzed by means of scanning electron microscope,X-ray diffraction of whole rock,dispersion test,and electrochemical test.It is found that in Fuling area,the stratification is highly developed,and the pore connectivity is good.Under the condition of deionized water,the CST value of Fuling shale gas reservoir is mostly between 40 s and 60 s,the Zeta potential absolute value is between 12 mV and 20 mV,and the CEC is between 2.6mmol/kg and 8mmol/kg,which indicates that the hydration dispersion of Fuling shale gas reservoir is not strong.(2)The spontaneous imbibition test shows that the imbibition rate of Fuling shale gas reservoir is relatively fast and the water absorbing capacity is strong,which is related to marine sedimentary environment of Fuling shale gas reservoir.The characteristics of spontaneous imbibition in three well areas and each sub-layer are different.The imbibition capacity,imbibition rate and diffusion rate of well A are generally higher than those of well B and well C,which may be one of the important reasons that the production of well A area is higher than that of the other two areas.(3)The ion diffusion test shows that the initial conductivity of Fuling shale gas reservoir is high,the ion diffusion rate is fast,and the ion diffusion ability is strong.There are differences in ion diffusion among the three well areas and the sub-layers.The ion diffusion rate of well B is obviously higher than that of well A and well C.(4)The spontaneous imbibition characteristic of Fuling shale gas reservoir is influenced by bedding,microfracture,porosity and permeability,and it is also related to the cation exchange capacity of reservoir.While the ion diffusion characteristic is affected by porosity,mineral composition and zeta potential.(5)The mechanism of spontaneous imbibition and ion diffusion of Fuling shale gas reservoirs is preliminarily discussed.It is considered that the strong ability of spontaneous imbibition and ion diffusion of Fuling shale gas reservoir are related to the characteristics of shale gas reservoir formation Fuling shale gas.The strong capillary force caused by clay and ion hydration,micropore size and ultra-low water saturation,and the chemosmosis related to ions are the main driving forces of spontaneous imbibition and ion diffusion,while bedding and fracture strengthen the spontaneous imbibition and ion diffusion of reservoir.