Study Of Improved Chemical Equilibrium Equation Module In The ASP-Flooding Numerical Simulator

The improved equilibrium model of chemical reaction in ASPflooding numerical simulator is based on original UTCHEMsoftware .Through the dissection and analysis of various problemsoccurred in pilot application of former software, this paper discussesbuilt-up foundation and resolution method based on mathematical modelof chemical equilibrium in this system as well as its practicalproblems, especially hypothesis condition for core part of chemicalreaction equilibrium model and chemical systems of chemical flooding.Focusing on problems such as uncertainty of parameters, resolutionprocess that may affect the model, incremental iterations,computation time, and even unusual errors causing interruption ofcalculation, the study revises model and defines basic resolutionmethod more fit for actual running requests. Actual computationcomparison in test area indicates this improved model has acquiredsuccess for its simulation rate enhances at 2-5 times, coincidencerate of precision and actual condition increase greatly and practicalefficiency is reinforced too, providing an important direction forindustrialized application in the future.1 .Important role of chemical equilibrium model in numericalsimulator of ASP floodingSimulation of chemical reaction system plays a very importantrole in the whole numerical simulation progress of chemical flooding.It describes chemical environment required by numerical simulationof chemical flooding drawn from analysis and simulation for chemicalreaction process of chemical flooding. In EQBATCH program, chemicalreaction system is described by chemical equilibrium model. Thetarget of model is chemical system of chemical flooding, involvedwith chemical reaction of multi-species of groundwater, oil in situ,injected water, injected surfactant and rocks.As an essential part of restricting computation rate andstability, EQBATCH, equilibrium model of chemical reaction, is usedto calculate chemical reaction equilibrium of various chemicalmaterials under reservoir conditions, with which physicochemicalmechanisms of alkali in chemical flooding progress may be describedscientifically and completely. The study of EQBATCH model aims atchemical system of chemical flooding, covering chemical reaction ofvarious species of ground water, in situ, injected water, injectedsurfactant and rocks.2 .Chemical systems suitable for chemical equilibrium modelChemical flooding progress is a complex system composed byheterogeneous multi-species. This paper introduces two aspects ofchemical reaction between chemical species and other species,universal chemical flooding system being an example.Firstly, the paper analyzes elements constituting variousspecies within chemical system, and studies composition of fluidspecies, solid species, rock adsorption species and surfactant. Italso classifies chemical reactions closely related with the model.(1)Partition equilibrium of oil acid constituents between waterphase and oil phase(2)Chemical equilibrium of various species in water solution(3)Precipitation equilibrium of solid body(4)Ion-exchange equilibrium on rock surface(5)Ion-exchange equilibrium of surfactant adionIn which, chemical equilibrium among different species in watersolution involves widely, including multi-acid base balance anddissociation equilibrium ect.3. Analysis of basin resolution method for chemical equilibriummodelThrough analysis of chemical reaction system, basic model ofchemical reaction equilibrium model is described.mass conservation equationequilibrium relationship of liquid reactionconstraint equation of solubility productcorrelated ion-exchange equilibrium equation of surfactantDuring resolution process of model, one major problem is dividedinto two small ones in EQBATCH: first is to compute concentrationof fluid species, adion on rock surface and correlated ions ofsurfactant;second is to calculate concentration of solid body.After completion of calculation, solubility product limitationof solid body is re-judged, assuring whether solid production ordissolution occurred within reaction system according to solubilityproduct limitation. If the above phenomenon occurs, basic equationmay be changed to re-describe whole reaction system, and compute thetwo equations again, so as to work out an acute materialconcentration.4. Actual work problems● Iterative process may be run unable of converging equationsolutionAt actual work, we find un-convergence occurred as initial valueof species concentration is improper, in which local convergence ofNewton is the basic reason. According to Newton local convergenceprinciple, only selecting initial value x0 is close to x* could makeconvergence of Newton. But it is very difficult to choose an initialvalue close to x0 for meeting Newton convergence condition. Duringnumerical simulation procedure of chemical flooding, simulator ofchemical equilibrium model is used with other simulators. Initialconcentration assumption of various values in the model, namely x0initial value of Newton, is provided by other simulators. Therebyif distance between x0 and x* is large, it surely would lead to un-convergence of Newton method.●When iterative process could not be run, and Jacobbi resolutionmatrix appears strange, abnormal mistakes will occur.Under some particular conditions, nonsingular may not occur inJacobbi matrix computed with Newton iteration method. Thensingularity appears in the matrix, relevant method of Gauss forsolving linear equations could not be used, leading to iteration stopand abnormal mistakes in program report.The occurred mistakes show singularity of Jacobbi matrix forsolving equation group is the error root.5 .Improved resolution method of modelAccording to problems found in actual run, this paper firstmodify chemical equation for solving EQBATCH program, and omit effectof solid body and surfactant related species in modifying resolutionprogress. Study focuses on fluid species and absorbing species,reaction equation of fluid reaction and adion exchange reaction arealso revised respectively.Next, on the basis of actual field needs following resolution methodsare selected:First, concentration of independent fluid species being asresolution variant.Under this condition, concentration of otherfluid species and addition may be re-expressed by independentspecies, by which rank of equation group declines sufficiently.Independent species concentrations ofC1 , C2 , C3 , C4 , C5 areregarded as resolution variations.Second, concentration of independent fluid species and adionbeing as resolution variation.Therefore other fluid species concentration may be defined byindependent fluid species. Adion concentration as resolutionvariation may avoid complex of resolution variation .However it willcause rank decline of equation group less obvious than that of thefirst method.