Studies On Mechanism Of Acid Dissolving Minerals And Acidizing Fluid System Of Viscoelastic Surfactant

Acid fracturing is an important technique to develop low permeability and tight carbonate reservoir,but during the acidification there are two problems:(1)acid cannot reach deep reservoir due to fast acid-rock reaction,(2)acid-sensitive damage to the reservoir.To solve first problem,various acid-fracturing techniques(i.e.,polymer thickening,surfactant thickening,emulsifying and foaming etc.)and citric acid as iron ion stabilizer were employed to delay the acid-rock reactions.However,these reactions would generate additional problems.For example,polymer is difficult to break and decompose,incomplete backflow is easy to damage the reservoir.The high concentration of surfactant in gelled acid needs to high cost.The stability of emulsified acid and foamed acid is poor.Regarding second problem,the metal ion coordinating citric acid and EDTA is commonly used complex.Citric acid has better stability to iron ions than calcium ions.EDTA is unstable in acid solution and precipitates could be formed.Other metal ion complex agents are insoluble in acids and/or expensive.In addition,in the study and application of reservoir acidification,hydrochloric acid is often used in carbonate reservoir,but the mechanism of hydrochloric acid reacting with zeolite minerals to form gel is unclear.During soil acid acidizing mudstone or sandstone reservoir,sand production and precipitation are formed.Moreover,there are less studies on the relation between acidizing fluid and crude oil.Therefore,in this work,we focus on fill above these gaps.In this study,different concentrations of hydrochloric acid(HCl),acetic acid(HAC)and mixture acid(MC)(combination of hydrochloric acid and acetic acid)were used to dissolve common interstitial materials in reservoirs,to explain the acid-rock reaction mechanism from the reaction of single acid and mineral,and proposing a method of acid selection for reservoir acidification.For the fast reaction of acid-rock,a clean "multi-purpose" viscoelastic surfactant(VES-c)was synthesized,which can not only effectively delay the acid-rock reaction rate and the corrosion rate of acid to steel,but also reduce the interfacial tension of oil and water.For acid-sensitive damage to the reservoir,an acid-resistant complex agent(NEDTA)was synthesized.As the above,the acidizing fluid of viscoelastic surfactant is prepared to study interaction with rocks and oil.The main contents are as follows:(1)Various concentrations of HCl or HAC(0.1%,0.5%,1.0%,3.0%,6.0%,9.0%,12.0%),and MC(3.0%,6.0%,9.0%)were used to dissolve chlorite,montmorillonite,kaolinite,illite,laumontite,calcite and dolomite,respectively.The composition of those minerals and the solid residues after reaction was determined by XRD,and the main ions and their concentrations in the residual acid solutions after reaction were analyzed by ICP-OES.The microscopic morphology of rock minerals after reaction and the element composition of the precipitations were observed by SEM-EDS.FT-IR was used to analyze the chemical structure of the precipitations.Also,we analyzed the mechanism of acidrock reaction according to the acid dissolving rock minerals and the resulting precipitates.Our experimental results show that the reaction between high concentrations of HCl and chlorite can generate the bischite-precipitation.The reaction of HCl and laumontite can form the gel due to the polycondensation reaction of the generated aluminosilicate(Si5Al2O16H6).HAC dissolving dolomite can produce calcium acetate precipitate.It is found that HAC in the mixture acid can react with Mg2+ to inhibit the catalytic effect on polycondensation of aluminosilicate,thereby promoting the aggregation and precipitation of aluminosilicate and reducing the amount of gel formation.HCl in the mixture acid can effectively reduce the formation of "fine fibers" calcium acetate.The ratio of HCl and HAC during acidification is mainly determined based on the content of chlorite,zeolite,dolomite,etc.Besides,HCl or HAC solution with a concentration higher than 1.0%has anti-swelling and shrinkage-swelling effects on montmorillonite.(2)The epichlorohydrin,glycine and erucamidopropyl dimethylamine were used to synthesiz a viscoelastic surfactant(VES-c),which has acid resistance,salt resistance,temperature resistance and viscosity enhancement.VES-c can delay the reaction time of mixture acid with dolomite by 4.5 times and the reaction time with core fragments by 1.6 times.The reason of VES-c retarding the reaction rate is that VES-c increases the viscosity of the acid solution,which reduces the mass transfer rate of H+,prevents CO2 from "escaping" the rock surface,reduces the solid-liquid contact area,and prevents CO2 from "escaping" liquid to increase liquid volume.In addition,VES-c can reduce the oil-water interfacial tension to 0.25 mN/m.(3)An acid-resistant and temperature-resistant metal ion complex agent NEDTA was synthesized by EDTA-2K and sodium chloroacetate.Precipitation titration was used to compare the effect of complex agents(i.e.,NEDTA and EDTA-2K,oxalic acid,citric acid,HEDTA,tetrasodium N,N-bis(carboxymethyl)-L-glutamate,MGDA-3Na)to prevent the formation of precipitation of ions(i.e.,Ca2+,Mg2+,Ba2+,Sr2+,Fe2+,Fe3+).It shows that NEDTA has a good effect on preventing the above metal ions precipitation.Also,the mechanism of NEDTA coordinating metal ions is analyzed,and it is proved that cationic complex agent can also coordinate with metal ions.(4)Analyzing the properties of core and crude oil in block Y of X oilfield:(a)the determination of the acid concentration used in acidizing fluid system depends on the composition of core from block Y of X oilfield;(b)analyzing the properties of the crude oil and its relationship with the surfactant VES-c can determine the concentration of VES-c used in acidizing fluid.Considering the economy,the concentration of NEDTA is 1.0%.In order to prevent the water-sensitive expansion of the reservoir,the laboratory-synthesized anti-swelling agent-Y and KCl were used.The acidizing fluid is 3.0%HCl+5.0%HAC+0.3%VES-c+1.0%NEDTA+1.0%anti-swelling agent-Y+2.0%KCl.The VES-c acidizing fluid system has good stability,which inhibits the corrosion rate for N80 steel sheet of 94.82%.Moreover,the VES-c acidizing fluid system after reacting with N80 steel sheet has anti-swelling rate and shrinkage rate for montmorillonite 99.57%and 50.68%,respectively.The emulsification of this system on crude oil is not good,but the static oil washing efficiency can reach about 27.0%.(5)The full-diameter cores and a triaxial physical simulation test were applied to compare and analyze the fracturing effects of water,acid solution,and VES-c acidizing fluid system.10.0 mL of deionized water(sample 1),3.0%HCl+5.0%HAC acid solution(sample 2)and VES-c acidizing fluid system(sample 3)were injected into the core samples,respectively,and then sealed to react for 5 days.Process of fracturing operation is design fracturing→temporary plugging→secondary fracturing→secondary temporary plugging→tertiary to simulate deep acid fracturing.The acoustic emission instrument was used to monitor the fracture position,and X-CT was used to observe the sample’s crack form after fracturing.The physical simulation of oil displacement test was used to verify that the VES-c acidizing fluid system can effectively improve the oil recovery.The results show that the effect of the VES-c acidizing fluid system is better,which reduces the core fracturing pressure by 11.6%,and generating more micro-fractures.The VES-c can increase the oil recovery by 12.7%.Combined with the results of the on-site acid fracturing,the effect of the VES-c acidizing fluid system is predicted.We performed the reactions of different concentrations of acids and minerals and analyzed the reaction mechanism of acid-rock.According to the content of chlorite,zeolite and dolomite in reservoir,a method was established to determine the type and concentration of acid solution.A "multi-purpose" VES-c and acidresistant metal ion complex agent NEDTA were synthesized.Based on the mechanism of acid-rock reaction and the synthesized chemical reagents,the VESc acidizing fluid system with good performance was prepared.The triaxial physical simulation test and physical simulation oil displacement test was used to verify the system has good acidification and fracturing property.Hopefully,this thesis provides a helpful guidance for acidification damage of reservoirs and the study on the relationship between acidified liquid and crude oil.