Numerical Simulation And Kinetic Study On Dissolution Of Feldspar And Quartz Complex Minerals

The secondary pores formed by dissolution of feldspar and quartz are the main way to improve porosity and permeability in the late diagenetic stage of reservoir.Feldspar minerals are sensitive to acidic environment and are more likely to dissolve in acidic environment,while quartz has the opposite sensitivity to environmental pH.In view of the complex fluid environment of the formation,it is of great significance to clarify the dissolution law of feldspar and quartz composite minerals in different environments of sandstone reservoir for the evaluation and prediction of reservoir pore development.This experiment through the laboratory simulation conditions for the development of reservoir for the mixture of feldspar and quartz mineral dissolution reaction under different conditions factors,through a variety of modern analysis means the mineral after reaction were characterized,using experimental data in combination with software simulation analysis under different conditions to determine the system changes of the mineral ion concentration and the reaction liquid,The mineral transformation law and the development law of secondary pores in the process of mineral reaction are summarized to provide data support for the prediction of high quality reservoir.Taking feldspar and quartz composite minerals as the experimental object,this paper simulated the reservoir environment to carry out the simulation experiment of organic acid dissolution of composite minerals.The experiment mainly explored the following aspects:（1）The change of ion concentration in the dissolution process of feldspar and quartz composite minerals in acetic acid system under different pH conditions（pH=3,7,9）,the correlation kinetic analysis,the effect of increasing pore,the effect of pH on the dissolution process,the dissolution characteristics and the dissolution mechanism;（2）the change of ion concentration,the correlation kinetic analysis and the relationship between the rate of feldspar quartz composite minerals and organic acids during the dissolution process under the same pH and different acid systems（acetic acid and oxalic acid）;（3）The changes of dissolution rate,ion concentration,pH value and saturation index of potassium feldspar and quartz composite minerals were simulated and calculated by GWB geochemistry software under reaction time and equilibrium time,and were compared with the results of laboratory experiments.The mineral dissolution process was analyzed by combining experiment with theoretical simulation.The experimental results of acetate dissolution of feldspar and quartz composite minerals at different pH values show that the dissolution of feldspar in composite minerals weakens and the dissolution of quartz increases with the increase of pH value.The dissolution process of the composite minerals under acidic conditions showed strong dissolution of feldspar and weak dissolution of quartz.In the dissolution process,the cluster kaolinite was the main secondary mineral,which completely covered the surface of feldspar at the later stage of the reaction.At the same time,the formation of granular boehmite and flaky muscovite was observed.Under neutral conditions,feldspar was strongly dissolved in the early stage,and quartz was strongly dissolved in the late stage.The dissolution of feldspar and quartz was similar in the whole reaction process.In the reaction,the hemispherical metastable analcime secondary minerals were observed,and no kaolinite was found.Under alkaline condition,the dissolution is strong quartz dissolution and weak feldspar dissolution.The main secondary mineral in the reaction is the new hexagonal autogenous crystal of K-feldspar and albite.Compared with the dissolution of single mineral,the presence of quartz in the composite mineral inhibits the dissolution of feldspar and reduces the dissolution rate of feldspar.The dissolution rate and solubility of quartz are obviously higher than that of single ore.The pore increasing effect of composite minerals increases first and then decreases with the increase of pH.Under acidic conditions,the pore increasing effect is mainly caused by feldspar dissolution,while under alkaline conditions,it is mainly caused by quartz dissolution.Then,it can be seen that compared with acetic acid,oxalic acid greatly promotes the dissolution of feldspar in complex minerals by comparing the experimental results of different acid systems,but has no promotion effect on the dissolution of quartz basically.Because of oxalic acid on feldspar dissolution,high concentrations of Al³⁺in the reaction process as well as secondary minerals in the middle of the reaction of Si O₂in the liquid phase generated,mainly authigenic albite crystal,scanning electron microscopy（SEM）show the conditions on the surface of the feldspar dissolution is much stronger than acetic acid dissolution,but on the surface of the quartz dissolution and acetic acid corrosion conditions.Compared with acetic acid dissolution,the dissolution rate of feldspar under oxalic acid dissolution increased by 2-10 times,but the dissolution rate of quartz basically remained unchanged.Oxalate increased the release rate of Al in the solid phase,leading to an increase in the dissolution rate of feldspar.Finally,the dissolution process of the composite minerals at pH 3,7 and 9 was analyzed in detail by GWB software,and it was found that the simulated results were in good agreement with the experimental results.The simulation results show that the composite minerals are far from reaching the equilibrium state during the experiment time,and with the increase of pH,the dissolution time of each stage of the composite minerals is obviously prolonged,and the system needs a longer time to reach the equilibrium state.Because the complexation of organic acids on minerals was not considered in the software simulation,and it was difficult to test the nucleation density and surface area data of secondary minerals,and there was a lack of corresponding literature support,the peak of Al³⁺concentration appeared late,and the decline was slow in the later period.At the same time,with the increase of pH,the dissolution of feldspar gradually weakened,and the dissolution of quartz gradually strengthened,increasing pore efficiency first decreased and then increased,and basically remained unchanged under neutral conditions.The results indicate that secondary pores are more easily developed in acidic and alkaline environments.