Aquifer Storage and Recovery : Physical and Geochemical Modelling (SWIFTPHREEQC)of British Aquifers for Aquifer Storage and Recovery Purposes.udPart 1, physical modelling and geochemical model calibration

摘要

This report describes the progress that has been made in developing models simulating both theudphysical and geochemical aspects of Aquifer Storage Recovery (ASR) schemes. This work is part ofuda 30 month project, entitled ASR-UK, which started in April 1999. The report describes progress toudthe end of 1999.udSWIFT is a fully transient 3-Dimensional model that simulates flow and transport of fluids. It isudideally suited to modelling ASR and has been used to simulate the response of the four major aquifersudin UK – Chalk, Lincolnshire Limestone, Sherwood Sandstone and Lower Greensand.udRanges of parameters applicable to these aquifers, which encompass dual-porosity, fractured andudporous aquifers, were used to predict their response to a hypothetical, but realistic, cycle testingudprogramme and an annual operational cycle over five years.udSensitivity analyses of the model’s responses to changes in matrix porosity, fracture porosity, fractureudpermeability and thickness were carried out for each aquifer for the operational cycles. In addition,udsensitivity analyses to changes in matrix diffusivity and dispersivity were undertaken for the Chalkudaquifer.udThe results showed low sensitivity to parameters in the estimated ranges with only matrix porosityudhaving a significant effect in the Chalk, the Lincolnshire Limestone and the Sherwood Sandstone.udThe main conclusion is that dual-porosity aquifers will need more conditioning than single porosityudaquifers. However, the suitability of any aquifer depends on the dispersive and diffusive parametersudwhich can generally only be obtained from field trials.udSWIFT cannot incorporate geochemical reactions, both water/water and water/rock, which can beudimportant in determining recovered water quality. PHREEQC is a reactive model which can quantifyudprocesses likely to occur in ASR schemes. In a non-reactive mode, PHREEQC was successfullyudcalibrated to the output from SWIFT and sensitivity analysis on the physical aspects of the model wasudexecuted in four stages of increasing complexity.udPHREEQC has several limitations in describing the physical aspects of an ASR-cycle including:ud• a simplified conceptualisation of dual porosity.ud• inability to define injection or recovery rate.ud• conceptualisation of radial flow in a one-dimensional model.udA variety of techniques were used to overcome these limitations including a thorough calibration toudthe results of the SWIFT model.udLinking the two models successfully should enable both the physical and geochemical aspects of ASRudto be simulated. This will provide a valuable tool to assist site selection and design of testing andudoperational schemes. The programme of continued research and development is identified in order toudachieve the objectives of the project.