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A well generalization method for multiple production wells with variable discharge applied to a coal pit in An-Hui province of China

  • Liu–II coal pit is a typical example of China’s deep coal mines which is seriously threatened by groundwater inrush from the underlying carboniferous Taiyuan limestone formation. An exhaustive data set of this confined aquifer exists. The aquifer lies 45 m∼ 60 m below the major coal seam. A traditional artesian aquifer test has been performed in order to assess the hydraulic properties, e.g. transmissivity (T) and storage coefficient (S). This artesian aquifer test is conducted with four simultaneously operating production wells while the discharge of each production well varied with time. The results of this test suggest that the aquifer is heterogeneous. Therefore, the according problems are: (1) how to analyze the artesian aquifer test with linearly declining discharge; (2) how to deal with multiple production wells in an aquifer test; (3) how to adequately consider aquifer heterogeneity. Thus, the objective of this thesis is to solve these problems. 1) As opposed to classical above-ground pumping tests, it is difficult to control the discharge rate of the production well in a deep mine artesian aquifer test since the hydraulic pressure is extraordinary high. Moreover the discharge rate won’t descend rapidly to zero, thus the analytical solution of Jacob and Lohman (1952) type curve for the artesian aquifer test will not be applicable. It is more reasonable to analyze the test as a pumping test with variable discharge. It is considered to rebuild a hydrogeological conceptual model which is similar with Theis (1935) model but with the variable discharge. A general equation for any discharge variability is given. Its application for the linearly declining discharge is presented subsequently, and a type curve of this equation with linearly declining discharge is given as well. After that, a simple numerical model is built by FEFLOW to simulate an artificial pumping test with the linearly declining discharge by assigning different parameter sets for transmissivity and storage coefficient. The type curve method is applied to evaluate transmissivity and storage coefficient for the linearly declining discharge well. The deviation between the given values of transmissivity and storage coefficient in FEFLOW and the values of those calculated by matching point are sufficiently small. Thus, when the discharge of production well declines linearly, a type curve method as an empirical method is reasonable and gives satisfactory values of these hydrogeological parameters. 2) In some cases, it is necessary to conduct a pumping test (or an artesian aquifer test) with several pumping wells (or production wells) which work simultaneously in order to discharge maximum quantity of groundwater. Normally, the superposition method or numerical simulation is applied to analyze the test result. However, a new approach called “Well Generalization Method” is defined and analyzed in this thesis. It is an easy–to–use approach for hydrogeologist to estimate the aquifer parameters while conducting an aquifer test. Since the key point of this approach is using a generalization well to substitute the pumping (or production) wells, it is obvious that this approach will generate the estimated error of parameters. Accordingly, several scenarios are analyzed and discussed based on the artificial type aquifer designed in FEFLOW. A homogeneous aquifer and a heterogeneous aquifer which is generated by geostatistical stochastic simulation technique (see 3)) are discussed separately. As a result, this approach is feasible and applicable under some conditions when the calculated observation well is arranged more than about 2.5 times the scale of the multi–pumping–wells field away from the center of the multi–pumping–wells field, furthermore, the maximum deviation of drawdown resulting from these observation wells will be less than 0.5 m, and the estimated value of transmissivity will be 0.44% smaller than real value. 3) Finally aquifer heterogeneity is addressed, in order to check the introduced method for applicability under realistic conditions. It has been described that aquifer heterogeneity plays a major role in hydrodynamic processes (e.g. de Marsily et al., 1998). Geostatistics which is considered as a useful tool for characterizing the spatial variability of transmissivity is applied to solve this problem. Based on the results of the artesian aquifer test conducted in Liu–II coal pit, a model of spatial variability of transmissivity is developed. Sequentially, the variogram model is applied in ordinary kriging to interpolate the transmissivity distribution, and in sequential Gaussian simulation to simulate a random field of transmissivity data in order to reflect its small scale variability. A comparison of the results of estimation and simulation of transmissivity indicates that the simulated values better reflect the spatial variability, reversely, the estimated values are much smoother.
  • 刘二矿作为中国深部开采煤矿正受到来自石炭纪太原组灰岩含水层的威胁。该含水层与煤层之间只有45米到60米的距离。在该煤矿进行了一次放水试验,意在评价该含水层的倒水系数以及储水系数。 我们布置了四个放水孔,而且每个放水孔的流量不是常数,根据试验结果得知该含水层是非均质的。因此我们提出三个研究问题。首先是如何分析流量现行下降的问题,其次是如何解决多放水孔的问题,最后是如何分析含水层的非均质性。 1)相对于抽水试验,煤矿深部放水试验很难控制放水流量,而且出水速度不能很快降到零,所以标准曲线法不再适用。为此我们重新分析地下水向井运动的水文地质概念模型,得到一个基本公式,根据这个基本公式分析流量线性下降的问题,最后得到一个降深公式,并应用配线法来评价倒水系数以及储水系数。为了分析这种解析法的精确度,我们建立了简单的数值模型来比较估计值与实际值的误差。经过分析,误差较小,所以得到的解析法适合解决流量线性下降的问题。 2)对于多个抽水孔的问题,我们采用一个概化孔来代替所有抽水孔,并用这个概化孔来评价含水层参数。很显然这个方法会产生误差,为了分析误差,我们设计了三种情况。通过分析当选用的观测孔距离抽水孔区大于2.5倍的抽水区距离时,概化法产生的降深误差将小于0.5米,而且估计的导水系数误差小于0.44%。 3)最后我们分析了含水层的非均质性以及概化法在非均质含水层中的应用情况。首先应用地质统计的方法来分析含水层的非均质性,本文采用了高斯后处理的方法模拟了含水层导水系数的分布情况。

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Author: Guowei Zhang
Title Additional (English):A well generalization method for multiple production wells with variable discharge applied to a coal pit in An-Hui province of China
Title Additional (German):Eine verallgemeinerte Methode zur Auswertung von Pumptests an multiplen Förderbrunnen mit variabler Abflussmenge am Beispiel einer untertägigen Kohlemine in der Provinz An-Hui, China
Advisor:Prof. Dr. Maria-Theresia Schafmeister
Document Type:Doctoral Thesis
Date of Publication (online):2015/06/15
Granting Institution:Ernst-Moritz-Arndt-Universität, Mathematisch-Naturwissenschaftliche Fakultät (bis 31.05.2018)
Date of final exam:2015/06/10
Release Date:2015/06/15
Tag:Förderbrunnen, Pumptests
Pumptests, wells
GND Keyword:Abflussmenge, Pumpe
Faculties:Mathematisch-Naturwissenschaftliche Fakultät / Institut für Geographie und Geologie
DDC class:500 Naturwissenschaften und Mathematik / 550 Geowissenschaften, Geologie
MSC-Classification:62-XX STATISTICS / 62Pxx Applications [See also 90-XX, 91-XX, 92-XX] / 62P99 None of the above, but in this section