Abstract
A single-well push–pull test is one of the most effective ways to estimate aquifer transport parameters. However, a patchy aquifer of finite thickness may surround the test well due to a gravel pack, mud invasion and stress redistribution during the well construction, which is usually neglected for single-well push–pull tests. In such a case, the aquifer should be regarded as a radial two-zone system, i.e., patchy zone and aquifer formation zone. In this study, a numerical model of a single-well push–pull test for a radial two-zone confined aquifer was developed using finite-element COMSOL Multiphysics. Two special cases, i.e., uniform patchy aquifer and non-uniform patchy aquifer, were considered. For the uniform patchy aquifer, results indicate that larger values of effective porosity and dispersivity in the patchy zone results in larger values of the breakthrough curves in the pumping phase. Patchy-aquifer thickness was also noted to have considerable impact on the breakthrough curves. As for the nonuniform patchy aquifer, the variation of hydraulic conductivity, dispersivity and effective porosity in the patchy zone leads to a change of concentration along the z-direction around the well screen, and as such, a smaller dispersivity of patchy regions results in more fluctuation in the concentration curve, while little impact can be found for a larger dispersivity. In addition, results showed that the average concentration over the entire screened section with an equivalent uniform patchy aquifer can be applied to interpret the data of a single-well push–pull test for the case of a nonuniform patchy aquifer.
Résumé
Un essai push–pull en puits unique est. un des moyens les plus efficace pour estimer les paramètres de transport d’un aquifère. Cependant, une partie d’aquifère colmatée d’épaisseur finie peut encercler le puits à cause d’un massif filtrant, d’une invasion de boue et de la redistribution des contraintes pendant la construction du puits, ce qui est. généralement négligé pour les essais push–pull en puits unique. Dans ce cas, l’aquifère devrait être considéré comme un système radial à deux zones, c’est-à-dire une zone colmatée et la zone de formation aquifère. Dans cette étude, un modèle numérique d’un essai push–pull en puits unique dans un aquifère captif radial à deux zones a été développé en utilisant le code à éléments finis COMSOL Multiphysics. Deux cas spéciaux, par ex., un aquifère colmaté uniformément et un aquifère colmaté non uniformément, ont été considérés. Pour l’aquifère colmaté uniformément, les résultats indiquent que des valeurs de porosité effective et de dispersivité plus élevées dans la zone colmatée impliquent des valeurs plus élevées des courbes de temps d’arrivée pendant la phase de pompage. Il faut aussi remarquer que l’épaisseur de colmatage de l’aquifère a un impact considérable sur les courbes de temps d’arrivée. Pour les aquifères colmatés de manière non uniforme, la variation de conductivité hydraulique, la dispersivité et la porosité efficace dans la zone colmatée entrainent un changement des concentrations selon la direction z autour du tubage du puits, et de ce fait, une dispersivité plus petite des régions colmatées a pour conséquence plus de fluctuations des courbes de concentration, alors qu’une dispersivité plus élevée a moins d’impact. De plus, les résultats ont montré que la concentration moyenne sur toute la section crépinée avec un aquifère colmaté uniformément peut être appliquée pour l’interprétation de données d’essais push–pull menés sur un puits unique pour le cas d’un aquifère colmaté nonuniformément.
Resumen
Una de las formas más eficaces de estimar los parámetros de transporte del acuífero es la prueba push–pull en un solo pozo. Sin embargo, un acuífero irregular de espesor finito puede envolver el pozo de prueba debido a un paquete de grava, invasión de lodo y redistribución de la tensión durante la construcción del pozo, lo cual generalmente se descuida en las pruebas push–pull de un solo pozo. En tal caso, el acuífero debe considerarse un sistema radial de dos zonas, es decir, una zona irregular y una zona de formación de acuíferos. En este estudio, se desarrolló un modelo numérico de una prueba push–pull de un solo pozo para un acuífero confinado radial de dos zonas usando elementos finitos COMSOL Multiphysics. Se examinaron dos casos especiales, a saber, el acuífero irregular uniforme y el acuífero irregular no uniforme. En el caso del acuífero irregular uniforme, los resultados indican que los valores mayores de porosidad y dispersión efectivas en la zona irregular dan como resultado valores mayores de las curvas de ruptura en la fase de bombeo. También se observó que el espesor del acuífero colgado tiene un impacto considerable en las curvas de ruptura. En cuanto al acuífero irregular no uniforme, la variación de la conductividad hidráulica, la dispersión y la porosidad efectiva en la zona irregular conduce a un cambio de concentración a lo largo de la dirección z alrededor de los filtros del pozo, y como tal, una menor dispersión de regiones irregulares resulta en una mayor fluctuación en la curva de concentración, mientras que se puede encontrar poco impacto para una mayor dispersión. Además, los resultados mostraron que la concentración media en toda la sección examinada con un acuífero irregular uniforme equivalente puede aplicarse para interpretar los datos de una prueba push–pull de un solo pozo en el caso de un acuífero irregular no uniforme.
摘要
单井注-抽试验是一种非常有效获取水文地质参数的方法之一。然而,通常我们忽略抽水井附近的补丁含水层对单井注-抽试验的影响,所谓的补丁含水层是由于抽水井砾石充填、泥浆入侵以及应力的变化导致的水文地质特征异常的一个特殊含水层。在这种情况下,抽水井附近承压含水层应该被认为是的两个区域组成的含水层系统,分别为抽水井附近的补丁区域以及外部的含水层区域。本文利用有限元COMSOL Multiphysics软件建立了抽水井附近补丁含水层的影响下的单井注-抽试验的数值模型,并且分别讨论了均匀补丁含水层和非均匀补丁含水层对单井注-抽试验的影响。对于均匀的补丁含水层,研究结果表明较大的有效孔隙度以及弥散度导致了抽水阶段的穿透曲线值的增大,并且均匀补丁含水层的厚度变化也引起穿透曲线的较大的变化。对于非均匀补丁含水层来说,补丁含水层的渗透系数、弥散度以及有效孔隙的变化均会导致抽水井滤管在垂向上溶质浓度分布曲线发生变化,并且较小的弥散度会导致较大的浓度分布曲线的波动,然而较大的弥散度对浓度分布曲线波动影响较小。另外,研究结果也表明对于非均匀补丁含水层的单井注-抽试验数据,可以用等效均匀补丁含水层并利用平均浓度来解译该数据。
Resumo
Um teste push–pull de poço único é uma das formas mais eficazes de estimar parâmetros de transporte em aquíferos. No entanto, um aquífero irregular de espessura finita pode cercar o poço de teste devido a um pacote de cascalho, invasão de lama e redistribuição de estresse durante a construção do poço, o que é normalmente negligenciado para testes push–pull de poço único. Em tal caso, o aquífero deve ser considerado como um sistema radial de duas zonas, ou seja, zona irregular e zona de formação do aquífero. Nesse estudo, um modelo numérico de um teste push–pull de poço único para um aquífero confinado de duas zonas radiais foi desenvolvido usando o COMSOL Multiphysics de elementos finitos. Foram considerados dois casos especiais, isto é, um aquífero irregular e um aquífero irregular não uniforme. Para o aquífero uniforme e irregular, os resultados indicam que valores maiores de porosidade e dispersividade efetiva na zona irregular resultam em maiores valores das curvas de ruptura na fase de bombeamento. Também foi notado que a espessura irregular do aquífero, tem um impacto considerável nas curvas de identificação. Quanto ao aquífero irregular não uniforme, a variação da condutividade hidráulica, dispersividade e porosidade efetiva na zona irregular levam a uma mudança de concentração ao longo da direção z ao redor da tela do poço e, como tal, uma menor dispersividade nas regiões irregulares resultou em maiores flutuações na curva de concentração, enquanto que para uma maior dispersividade um pequeno impacto pode ser encontrado. Além disso, os resultados mostraram que a concentração média em toda a seção filtrada com um aquífero irregular uniforme equivalente pode ser aplicada para interpretar os dados de um teste push–pull de poço único para o caso de um aquífero irregular não uniforme.
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Acknowledgements
We greatly appreciate the constructive comments from the Associate Editor (Dr. Willem Zaadnoordijk), which significantly helped to improve the quality of the paper. We would also like to thank the two anonymous reviewers for their useful comments and observations.
Funding
This research was partially supported by the National Natural Science Foundation of China (Grant Numbers: 41772259, 41830862, 41372253, 41521001), the Natural Science Foundation of Hubei Province, China (2018CFA085, 2018CFA028), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan).
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Li, X., Wen, Z., Zhu, Q. et al. Numerical simulation of single-well push–pull tests in a radial two-zone confined aquifer. Hydrogeol J 27, 2645–2658 (2019). https://doi.org/10.1007/s10040-019-02014-y
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DOI: https://doi.org/10.1007/s10040-019-02014-y