Study On Etching Mechanism Of Acid Fracturing Inhomogeneity In Dolomite Reservoir

For carbonate reservoirs,acid fracturing technology is often used to transform,but for dolomite and limestone reservoirs,there is a huge difference in the effect of acid fracturing.In order to improve the acid fracturing effect of porous dolomite reservoir,the mechanism of non-uniform etching of dolomite is studied in this paper.Through the experiment of acid rock reaction kinetics,the boundary of dolomite reaction control mode is defined.Through orthogonal test and single factor analysis,the influence degree of controlling factors of acid rock reaction was ranked.The effect of crystal structure on etching morphology was studied by scanning electron microscope（SEM）.3D laser scanning and digital analysis were used to characterize the etching morphology,and the non-uniform analysis and evaluation method of etching morphology was established.Through the experiment of acid fracture conductivity,taking the conductivity and macro etching shape as indexes,the influencing factors of conductivity were compared,and the main controlling factors of dolomite acid fracture conductivity were identified.Based on the two-scale continuous model,considering the mineral composition and dissolution degree,a two-component acid fracturing model suitable for complex dolomite reservoir is established,and the transformation direction is optimized by acid fracturing numerical simulation.The results show that the acid rock reaction rate of dolomite is one order of magnitude lower than that of limestone.Dolomite content of 75%,temperature of90℃,acid concentration of 20%and rotation speed of 500 r·min^-1 are the limits of dolomite surface reaction control mode.The low acid rock reaction rate is an important reason for the low degree of non-uniform etching of dolomite wall,and the injection mode affects the non-uniform etching morphology of fracture wall.The surface reaction rate constant of dolomite is k_{s Ca Mg}=0.02mm/s,and the control mode of acid rock reaction is limited by k_s/k_c=0.1 and 10.The composite measures of properly reducing the acid flow rate(U₀=1cm·min^-1)and thickener concentration(D_mgel1=10×10^-11 m²·s^-1,D_mgel2=20×10^-11 m²·s^-1)and injecting two kinds of acid with different viscosity into the formation alternately are suitable for the acid fracturing of dolomite reservoir.