Experiment And Mechanism Study On Spontaneous Imbibition Of Fracturing Fluid To Shale Minerals

Large-scale horizontal well hydraulic fracturing is a key technology for shale gas exploitation.Engineering shows that the backflow rate of fracturing fluid in different regions is generally low and different.Due to the complex composition of fracturing fluid,long-term existence in underground may pollute underground water.Previous studies have shown that spontaneous imbibition is the main reason for the low backflow rate of fracturing fluid,but the spontaneous imbibition mechanism and influencing factors are not clear at present.In order to explore the spontaneous imbibition mechanism and influencing factors of shale reservoir to fracturing fluid,this paper carried out spontaneous imbibition experiments of rock blocks with different sizes and mineral types under different experimental conditions.According to the experimental results,combined with the analysis of mineral surface characteristics of rock blocks and the testing of solution chemical components,the main influencing factors of spontaneous imbibition of rock blocks were obtained,and the spontaneous imbibition mechanism of different shale mineral components to fracturing fluid solution was discussed.The main conclusion are as follows:（1）The influence of different factors on the spontaneous imbibition of the rock block is as follows:The influence of size on the spontaneous imbibition rate of the rock block shows that larger specific surface area lead to higher spontaneous imbibition rate.The effect of mineral species on the spontaneous imbibition rate of the rock block is that montmorillonite has the largest spontaneous imbibition rate,which is higher than chlorite,kaolinite,SiO₂ and feldspar.The reason is that montmorillonite has higher expansibility.The effect of temperature on the spontaneous imbibition of the rock block shows that higher temperature lead to higher spontaneous imbibition rate and velocity.The reason is that when the temperature is higher,the interlayer spacing between mineral particle is larger and the kinetic energy of water molecules is greater.The effect of the solution on the spontaneous imbibition of the rock block is that when the concentration of KCl solution is higher,the sponta neous imbibition rate and velocity of the rock block is lower.The reason is that K⁺inhibits hydration of the rock block.（2）Selection and establishment of spontaneous imbibition model are as follows:After fitting and verification,the spontaneous imbibition modelV（28）Kt can effectively reflect the change of rock blocks’spontaneous imbibition rate with time.The change of spontaneous imbibition rate of type III dominated by montmorillonite shows three stages:ascending,slowly ascending and descending.The reason is that the ascending stage is dominated by water spontaneous imbibition in fracture-pore,while the slowly ascending stage is dominated by water spontaneous imbibition in montmorillonite mineral and water spontaneous imbibition in fracture-pore is reducing,the reason of descending stage is that the expandtion of montmorillonite squeezes the fracture and pore.（3）Water-rock changes in the spontaneous imbibition process are as follows:The fracture and pore tend to decrease and even close because of the expansion of montmorillonite.At the same time,dissolution pores are observed on the surface of the rock block due to dissolution of minerals.When the temperature is higher,the dissolved amount of Na⁺,Al³⁺,Ca²⁺,Mg²⁺is larger;KCl inhibited the dissolution of Na⁺,Ca²⁺,Mg²⁺,Fe³⁺;the K⁺,Na⁺,Al³⁺,Ca²⁺,Mg²⁺dissolved more when the mass of the rock block is larger.Through the analysis of hydrogeochemistry,it is inferred that Na⁺in the solution comes from albite,K⁺comes from plagioclase,Ca²⁺and Mg²⁺come from montmorillonite,and Al³⁺come from albite,plagioclase and montmorillonite.The mechanism of shale spontaneous imbibition,fracturing fluid backflow and water-rock reaction during shale gas exploitation are analysed in the experiment,which provides theoretical basis for guiding shale gas exploitation and preventing groundwater pollution in the future.