Mixed-well model of the relation between drawdown and water inflow in a pumping well with variable-diameter
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摘要:
抽水试验是确定含水层水文地质参数和评价地下水资源的重要手段。对于单井抽水试验分析, 现有井流模型假定抽水井的直径不随深度变化、含水层为单一的潜水或承压含水层, 未考虑抽水井变径及穿越多个含水层的情况。建立多含水层异径抽水的稳定态混合井流模型, 假设地下水仅发生水平流动, 推导出了抽水井涌水量与降深关系的理论解析公式, 探讨根据变径抽水井的单井稳定流抽水试验获取含水层参数的方法, 提出了承压含水层段抽水井等效半径精确解和替代半径计算公式。把混合井流模型用于分析淄河源区的傍河抽水试验, 根据3个流量的阶梯式抽水试验数据确定了涌水量
Q w与降深s w的抛物线型关系, 预测允许降深s w为25 m时抽水井涌水量为4 093.8 m3/d, 反算得到潜水含水层渗透系数为1.88 m/d, 用替代半径计算的承压含水层渗透系数约为0.43 m/d, 相对误差小于5%。混合井模型为多含水层抽水井的涌水量预报提供了理论依据, 其适用性也受到假设条件的限制。在完整河情景下忽略三维流将导致反求的渗透系数偏大, 在非完整河情景下解析解反求的渗透系数则偏小。Abstract:Objective Pumping tests are an important method for determining the hydrogeological parameters of aquifers and evaluating groundwater resources. For the analysis of single-well pumping tests, existing models assume that the well diameter remains constant with depth and that the aquifer is either unconfined or confined. They do not consider situations where the well diameter varies or when multiple aquifers are encountered.
Methods In this study, we developed a steady-state mixed-well flow model to account for a pumping well with a variable diameter that penetrates multiple aquifers with the assumption of horizontal flow within the aquifers. The analytical solutions for the relation between pumping discharge and drawdown were derived. This study explores the methodology of using single-well steady-state pumping tests with variable-diameter wells to obtain aquifer parameters. Additionally, accurate solutions for the equivalent radius of a confined aquifer segment and alternative radius calculation formulas are proposed.
Results The mixed-well flow model was applied to analyze pumping tests near the Zihe River. Based on the data from three stepped pumping tests, a parabolic relationship between pumping discharge
Q w and drawdowns w was established.The model predicted a pumping discharge of 4 093.8 m3/d when the maximum drawdowns w was 25 m. The hydraulic conductivity of the unconfined aquifer was estimated to be 1.88 m/d, and the hydraulic conductivity of the confined aquifer was estimated as 0.43 m/d, with a relative error of less than 5%.Conclusion The mixed-well model serves as a theoretical foundation for predicting water yield from pumping wells in multiple aquifers system. However, it is important to acknowledge that the model's applicability is constrained by certain assumptions. In situations where rivers fully penetrate aquifers, there is a possibility of overestimating the hydraulic conductivity. On the other hand, when rivers only partially penetrate aquifers, the analytical solution may underestimate the hydraulic conductivity.
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图 1 2种混合井抽水稳定流模型
r0, r1, r2, …, rn分别为抽水井所揭露第1, 第2, …, 第n+1含水层进水段井半径(m);K0, K1, K2, …, Kn分别为抽水井所揭露第1, 第2, …, 第n+1含水层水平渗透系数(m/d);z1, z2, z3, …, zn, zb分别为抽水井所揭露第1, 第2, …, 第n+1含水层底部标高(m);M1, M2, M3, …, Mn分别为抽水井所揭露第2, 第3, …, 第n+1含水层厚度(m);Qw为混合抽水井抽水流量(m3/d);hR为圆岛外边界定水头水位(m);hw为开采井中水位(m);R为开采井中心距圆岛外边界水平距离(m);L为开采井中心与定水头边界的水平距离(m); 下同
a.圆岛中心抽水;b.定水头边界附近抽水Figure 1. Two types of steady state flow models for a multilayer mixed pumping well
表 1 抽水井分段特征
Table 1. Sectional characteristics of the pumping well
编号 底面深度/m 井孔直径/mm 揭露地层 含水层岩性 有效厚度/m 分段体积/m3 分段侧面积m2 顶层 16.3 426 Q、O2b 砂层、白云岩 套管阻隔、侧面不进水 Ⅰ 26.1 377 O2b 白云岩 潜水含水层段 Ⅱ 105.9 377 O2b、O2d、∈4O1s、∈4O1ĉ 白云岩、灰岩 79.8 8.91 94.5 Ⅲ 250.2 325 ∈4O1s、∈4O1ĉ 白云岩、灰岩 144.3 11.97 147.3 Ⅳ 353.2 273 ∈4O1ĉ 灰岩为主 74.2 4.34 63.6 注:Q.第四系;O2b.北奄庄组;O2d.东黄山组;∈4O1s.三山子组;∈4O1ĉ.炒米店组 表 2 不同解析模型涌水量计算值相对数值模拟结果的误差
Table 2. Errors of the flow rate estimated from different analytical models with respect to numerical simulations
抽水井降深sw/m 单井涌水量 数值模拟Qw/(m3·d-1) 传统解析模型(定半径均质含水层) 混合井解析模型 取rw=136.5 mm 取rw=188.5 mm 涌水量Qw/ (m3·d-1) 相对误差/ % Qw/(m3·d-1) 相对误差/% Qw/(m3·d-1) 相对误差/% 5 939.0 789.9 -15.9 835.6 -11.0 924.4 -1.6 10 1 877.0 1 568.4 -16.4 1 659.1 -11.6 1 795.9 -4.3 15 2 813.0 2 335.4 -17.0 2 470.5 -12.2 2 614.7 -7.1 20 3 679.2 3 091.0 -16.0 3 269.8 -11.1 3 380.6 -8.1 25 4 154.5 3 835.2 -7.7 4 057.0 -2.3 4 093.8 -1.5 30 4 985.8 4 567.9 -8.4 4 832.1 -3.1 4 754.2 -4.6 35 5 816.1 5 289.2 -9.1 5 595.1 -3.8 5 361.8 -7.8 -
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