大口径单井注抽试验求参过程中井底流和混溶效应的综合影响  

Combined Effects of Bottom Flow and Wellbore Mixing on the Single-well Push-pull Test Using Large-diameter Well

作　　者：文章[1] 朱棋 WEN Zhang;ZHU Qi(School of Environmental Studies,China University of Geosciences,Wuhan 430074,Hubei,China)

机构地区：[1]中国地质大学(武汉)环境学院,湖北武汉430074

出　　处：《华南地质》2024年第3期435-444,共10页South China Geology

基　　金：国家自然科学基金优秀青年基金项目(No.42022018);湖北省重点研发计划项目(No.2022BCA073)。

摘　　要：单井注抽(single-well push-pull,SWPP)试验因其成本低、周期短以及操作简单的优点,被广泛用于求取含水层弥散度等水文地质参数。依托干旱半干旱区常用的大口径井开展SWPP试验,常面临井底流渗漏和井筒混溶效应共同的影响。为此,本研究构建了同时考虑井底流和混溶效应的SWPP试验三阶段溶质运移模型,利用有限差分法对模型进行求解,并利用前人的渗流和溶质运移模型解析解验证其准确性,随后依托该模型探究大口径井附近渗流特征及井尺寸对SWPP试验的影响机制。研究结果表明:(1)新提出的考虑井底流的SWPP试验模型对抽水过程降深以及井壁处溶质运移过程的刻画,同前人解析解匹配良好,表明了该模型构建及求解的准确性;(2)随着井筛长度的逐渐减小,井底流对含水层整体流场的影响程度逐渐增大,流场从径向逐渐转变为球形流场;(3)粗井径相比细井径,注入阶段井中的溶质BTC(Breakthrough curve,穿透曲线)值更小,但抽出阶段的溶质BTC更大,井中混溶过程相比井底渗漏过程对SWPP试验的影响更为显著;(4)当井深小到井径的一半左右时产生类球形流场,井底的渗漏过程相比混溶过程对SWPP试验可能存在更为显著的影响;(5)对于本研究设置的模拟场景,即使不存在井底流,前人的解析解由于忽略井筒混溶,对弥散度的解译仍存在152.5%的误差,而随着井底埋深逐渐减小,井底流导致前人解析解的误差也逐渐增大。The single-well push-pull(SWPP)test is widely used for determining hydrogeological parameters such as dispersivity coefficient due to its advantages of low cost,short duration,and simple operation.Conducting SWPP experiments in large-diameter wells,commonly used in arid and semi-arid regions,often faces the combined effects of leakage at the well bottom and wellbore mixing.To address this,this study constructed a three-stage solute transport model for the SWPP test that considers both the influences of flow at the well bottom and wellbore mixing effects.The model was solved using the finite difference method,and its accuracy was verified against the analytical solutions of previous seepage and solute transport models.Subsequently,the study explored the seepage characteristics near large-diameter wells and the impact mechanism of well size on the SWPP test based on this model.The research results showed that:(1)The newly proposed SWPP test model,which considers well bottom flow,matches well with previous analytical solutions in depicting the drawdown process during pumping and the solute transport process at the well screen,indicating the accuracy of the model’s construction and solution.(2)As the length of the well screen gradually decreases,the influence of the well bottom flow on the entire flow field gradually increases,changing the flow field from radial to spherical.(3)Compared to a small well diameter,a larger well diameter results in a smaller breakthrough curve(BTC)value for solutes during the injection phase,but a larger BTC during the extraction phase,indicating that the mixing process within the well has a more significant impact on the SWPP test than the leakage process at the well bottom.(4)When the well depth is about half of the well diameter,creating a quasi-spherical flow field,the leakage process at the well bottom may have a more significant impact on the SWPP test than the mixing process.For the simulated scenarios set in this study,even in the absence of bottom flow,the previous analytical

关 键 词：单井注抽试验 大口径 井底流 混溶效应 有限差分法 

分 类 号：P641.2[天文地球—地质矿产勘探]