Multiscalc Model Of Heterogeneous Porous Medium And Its Application In Groundwater Modeling

Numerical simulation of groundwater flow and solute transport based on relative high densitydrilling lithology data of a region, which is jointly driven by multi-scale, different accuracyrequirements and data, can provide theoretical support for solving issues of regional groundwaterresources, environmental problems, ecological problems, and groundwater disasters. However, dueto the heterogeneity of aquifer medium, hydrogeological parameters of aquifer arespatial variabilityand scale effect, which result in there is a large deviation between groundwater flow and solutetransport simulation results with the actual situation. Therefore, it is very meaningful thatestablishing multi-scale heterogeneous porous medium model based on high density drillinglithology data to obtain hydrogeological parameters field, and applying parameter field to numericalsimulation of groundwater.This paper carried out the following researches. The heterogeneity of aquifer medium wasanalysed based on high density drillings lithology data. Voronoi diagram method was employed toanalyze the density and distribution of drilling data. A scale generalization approach for high densityboreholes data was proposed. The stochastic simulation of transition probabilities geostatistics wasresearched in this paper, and a method for computing lateral transition probabilities was proposed.The relationship of lateral transition probabilities and simulation result was analyzed, and theestablished three-dimensional stochastic models with scale generalization virtual boreholes andoriginal boreholes were compared. Simulation of multiscale porous medium model based on scalegeneralization virtual drillings and real drillings and deterministic-stochastic coupled simulationwere realized. Hydrogeological parameters were extracted from stochastic model of porous medium.The relationship of the stability of the extracted parameters with realization number in transitionprobabilities geostatistical simulation was analyzed. Porous medium model with numerical model ofgroundwater flow and solute transport was combined, which can provide support for regionalgroundwater management and multi-scale numerical simulation of groundwater. The mainconclusions and results are as follows:(1) Based on the high density drilling lithology data, the spatial correlation distance of the twomain directions for the instance study area of the aquifer were obtained; A scale generalizationapproach for high density boreholes data was proposed to generate virtual drillings, and thegenerated virtual drillings can better reflect the large-scale structural characteristics of aquifermedium. Stochastic model based on scale generalization virtual drillings can shows the overalldistribution characteristics of aquifer medium, and the simulation result in strong heterogeneousregion are shown as continuous equivalent material. (2) The analysis of lateral transition probabilities shows if the ratio of mean lens length of thelateral to vertical is estimated too small, it will lead to a strong random materials distribution in thesimulation result, which is uncontinuity. If the estimated ratio is too big, it will lead to too strongcontinuity materials in horizontal direction.The proposed method based on vertical stratification andthe hypothesis of continuous geological type in plane for computing lateral transition probabilitiescan effectively obtain lateral transition rate matrix, and can get better simulation result. A programbased on GIS which can manage drilling data and conduct transition probabilities geostatistics wasdeveloped.(3) The simulation result of transition probabilities geostatistics was assessed, which shows themodel can better restore materials distribution of aquifer medium in weak heterogeneous orhomogeneous area, but difficult to restore the true materials distribution in strong heterogeneousregion. Thus the simulation method based on multi-scale virtual boreholes data was proposed, whichis combining the normal scale generalization boreholes in weak heterogeneous area and finer scalegeneralization boreholes in strong heterogeneous area to simulate.The simulation results can betterrestore the actual distribution of aquifer medium in a strong heterogeneous region.(4) Multi-scale porous medium model based on scale generalization virtual drillings and actualdrillings can effectively established with transition probabilities geostatistics. Hydrogeologicalstructure model and the stochastic models in the structures were combined to establishdeterministic-stochastic porous medium model. The final model has large-scale structuralcharacteristic and small-scale random characteristic.(5) The stabilities of nodes in transition probabilities geostatistical model are related withdrillings’ density and distribution, the strength of heterogeneity of the simulation area, and otherfactors. In weak heterogeneity area the result of realizations with increasing number is faster to tendto be stable, and the probability of the same result in the same node in different realization is higher,but slower to tend to be stable and random of that in strong heterogenous area. There is a morerandomness simulation results in the area of sparse data. However, the proportion of each material ata node tends to be stable with the increase in realization number.(6) The instances of combining multi-scale porous medium model with groundwater flow andsolute transport numerical simulation show it is feasible that numerical simulation of large-scalegroundwater flow uses hydraulic conductivity values extracted from transition probabilitiesgeostatistical model based on scale generalization virtual boreholes, and small-scale solute transportsimulation uses hydraulic conductivity values extracted from the model based on actual boreholes.The conditional stochastic model reflects the spatial variability of hydraulic conductivity, which maychange the local head distribution and flow direction, and make the simulation results often closer tothe actual situation. This innovations are reflected in:(1) proposing scale generalization method for generatingvirtual boreholes based on high density boreholes lithology data;(2) proposing the method forcomputing lateral transition probabilities based on vertical stratification and the hypothesis ofcontinuous geological type in plane;(3) realizing multi-scale porous medium simulation withtransition probabilities geostatistics based on scale generalization virtual drillings and actualdrillings.