Study On Mechanism Of Fracturing Fluid Retention And Reservoir Action

During the exploitation of low permeability and ultra-low permeability oil and gas fields,volume fracturing of reservoir is needed to improve recovery by pumping a large amount of fracturing fluid into reservoir.Retention effect and liquid phase build effect result in flow back slow or difficult.A large number of fracturing fluids can not be flowed back,and ultimately remain in the reservoir.Retained fracturing fluid undergoes physical and chemical changes under the influence of reservoir temperature,pressure,formation mineralized water,retention time,lithologic composition and other factors.For example,degradation,adsorption,change of pore throat morphology and so on,ultimately affect reservoir oil and gas migration.Based on the above background,polyacrylamide(PAM),guar gum and octadecyl trimethyl ammonium chloride(1831)were the main components of fracturing fluid,the existing form of retained fracturing fluid,the adsorption law and the influence on fluid flow resistance were studied.(1)The effects of temperature,salinity,oxygen and degradation time on PAM degradation were studied,the results show that:PAM hardly degraded in anaerobic environment,and its viscosity retention rate was still about 97%for 28 days.The process of aerobic degradation was greatly influenced by temperature,time and ion type.When the temperature rised,the degradation rate accelerated.The longer the degradation time was,the greater the degradation degree was,and the viscosity retention rate was about 7%for 28 days.Cationic polyacrylamide degraded more slowly than other ion types under the same conditions.(2)The adsorption characteristics of fracturing fluid were studied using natural mica as template,the results show that:(a)Anionic polyacrylamide was mainly monolayer adsorbed with a small amount of bilayer adsorbed and the thickness of monolayer was about 4 nm;Cationic and non-ionic polyacrylamide adsorbed on the surface of mica with large amount and no obvious adsorption layer;The adsorption state of three ion types of polyacrylamide on mica surface after degradation was similar to that of acrylamide monomer,and the adsorption layer was mainly bilayer,and the thickness of the adsorption layer was about 1-2 nm.(b)The low concentration(200 mg/L)of hydroxypropyl guar gum was dominated by monolayer adsorption,while the high concentration(2000 mg/L)was dominated by bilayer adsorption,and the thickness of the adsorption layer was about 0.6 nm;Carboxymethyl hydroxypropyl guar gum and cationic guar gum were mainly adsorbed by monolayer,and the thickness of adsorbed layer is about 0.7 nm and 1.5-2 nm.(c)In 1831 low concentration(200 mg/L),monolayer adsorption was dominant,while in high concentration(2000 mg/L),bilayer adsorption was dominant,and the thickness of adsorption layer was about 3 nm.(3)Resistance reduction experiments show that the fracturing fluid had good resistance reduction performance:The concentration of APAM-2(Molecular weight 6.08 X 106)1500mg/L drag reduction rate was 71.1%.The drag reduction rate decreases rapidly when the concentration exceeded 1500 mg/L,and within a certain concentration range(<1500 mg/L),it conformed to Virk effective slip hypothesis.The drag reduction rate before and after crosslinking was 2%and more than 50%.The drag reduction rate of guar gum before and after crosslinking was about 70%and-27.3%.Under the same conditions,drag reduction performance was cationic>nonionic>anionic.