Micromechanism And Main Controlling Factors Of Imbibition Of Fracturing Fluid In Tight Sandstone

Tight sandstone has the characteristics of fine pores,strong microscopic heterogeneity,and difficult to find the law of fluid occurrence.External fluid is easy to imbibition and thus form water lock effect,which puts forward higher requirements for the selection of fracturing fluid during process reform.Therefore,it is of great significance to strengthen the main controlling factors of fracturing fluid imbibition in tight sandstone reservoir and clarify the microscopic imbibition mechanism for the selection of fracturing conditions and the enhancement of oil recovery.In this paper,the typical tight gas intervals of Linxing Block:Hehe 8 member of Shihezi Formation and Tai2 member of Taiyuan Formation are taken as the study area.The tight sandstone samples in the study area are selected for laboratory experiments,and the main influencing factors and microscopic mechanisms of reservoir physical properties,rock fabric,pore structure and physical and chemical characteristics of fracturing fluid on imbibition of fracturing fluid are analyzed.The following conclusions and understandings are obtained:（1）The rock type of Hehe 8 member is mainly feldspar lithic sandstone,with an average porosity of 8.4%and permeability of 0.642×10^-3μm².The rock type of Tai2 member is mainly lithic sandstone,with an average porosity of 6.2%and permeability of 0.169×10^-3μm².Pore and permeability are positively correlated and well correlated.（2）There are a variety of combination pore types in the reservoir of Hehe 8 and Tai2member in the study area,which are mainly interdissolved particle pores and dissolved particle pores.The reservoir samples of Hehe 8 Member and Tai2 Member are divided into classⅠ～ClassⅣ,and the corresponding reservoir and seepage capacity gradually weaken.The distribution of pore-throat size ranged from 0.21 nm to 63000 nm,and the distribution of pore-throat size was dominated by micropores,followed by mesoporous pores,and the least by macropores.（3）Optimization of 0.3%of discharge aid K and 0.1%of a waterproof locking agent,combined with several fracturing fluid formula main agent obtained from the field practice in the study area,it is concluded that a formula configured by field water is the optimal fracturing fluid formula.（4）The imbibition effect of the sandstone in Hehe 8 member is better than that in Tai2member.In the development of tight sandstone gas reservoirs in Hehe 8 member,water should be avoided as far as possible to prevent the occurrence of water lock effect.The porosity of Hehe8 member and Tai2 member is positively correlated with imbibition efficiency,but the correlation is small.Permeability is positively correlated with imbibition efficiency,but the correlation is small.Imbibition efficiency is negatively correlated with illite content.The interfacial tension and contact Angle of fracturing fluid are negatively correlated with imbibition efficiency,and the interfacial tension is more correlated with the contact Angle.The imbibition efficiency is negatively correlated with the relative sorting coefficient,and the R² is larger.Imbibition efficiency is positively correlated with average pore size,and R² is larger.（5）The final average R² of pore diameter and imbibition efficiency was 0.8597,which was the main factor affecting imbibition efficiency.The imbibition mechanism is mainly controlled by microscopic factors.The contribution rate of mesoporous pores to imbibition is larger.The higher the proportion of micropores and mesoporous pores,the better the imbibition effect,and the smaller the interfacial tension,illite content and relative sorting coefficient,the better the imbibition effect.