Numerical Simulation Of Flow And Distribution Characteristics Of Acid In Fracture

It is as extraordinary important as to improve acid fracture conductivity and increase acid fracture effective length for stimulation of deep and HTHP carbonate reservoir.Acid flow and distribution situation will directly determine the etching of fracture surface,and further affect conductivity and effective length,which makes the characterization of acid flow and distribution properties have practical significance for guiding high-efficiency acid fracturing.Acid physical properties would change with acid etching process,which could affect acid flow state.Simultaneously,fracture surface morphology directly determines acid flow state,but current research has not taken into account.Therefore,it is necessary to deepen discuss the influence of acid physical properties and fracture surface morphology on acid flow and distribution characteristics.On the basis of molecular structure and acid-rock reaction dynamics,acid density and viscosity change mechanism of acid flow process in fracture was analyzed.The results show that acid density change is mainly due to ions diffusion,while acid viscosity dynamic change is caused by shearing action and the breaking of chemical bonds during acid flow.Taking advantage of equivalence relation of chemical reaction,the change rule of gelled acid density with flow time was defined.Combined with rheological properties of gelling acid,the variation rule of gelling acid viscosity with flow time was revealed.Based on conservation principle,the fracture was meshed and mathematical model of acid flow in fracture was established.Based on actual fracture scale,a macro-scale acid fracturing fracture model that characterizes fracture length-height profile was established,and the "VOF+UDF" method was used to simulate flow and distribution characteristics of acid under the conditions of changes in different physical properties between different working fluids and acid injection parameters.The simulation results show that viscosity difference between acid and pad is key factor controlling acid flow pattern.As viscosity difference increases,viscous fingering is obvious.Density difference between the two controls acid distribution position.The greater the density difference,the greater the acid flow tendency to the upper or lower part of fracture.Acid displacement mainly affects acid distribution range along fracture height direction and flow distance along fracture length direction.Acid dosage is closely related to the acid flow distance.Using digital method to reconstruct real fracture rough surface,the tortuosity range was calculated from 1 to 1.3 and micro-scale acid fracturing fracture models of different tortuosity was established.The simulation results show that acid streamline distribution is relatively stable,and etching degree of fracture surface is more uniform when tortuosity is small.When tortuosity is large,the streamline of convex part of fracture surface is concentrated,while the streamline of the concave part is relatively sparse,and velocity in the fracture middle is greater than velocity on fracture surface.The acid flow manifests as a sudden forward flow in the middle of fracture.As tortuosity further increases,vortex is easily formed in the undercut of fracture.