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Improved barometric and loading efficiency estimates using packers in monitoring wells

Estimations améliorées de l’efficacité barométrique et de la sensibilité à la marée en utilisant des packers au sein de piézomètres

Mejora de las estimaciones de la eficiencia barométrica y de la carga utilizando packers en pozos de monitoreo

利用观测井中的封隔器提高气压和载荷效率估算水平

Estimativas de eficiência barométrica e de carga melhoradas utilizando obturadores em poços de monitoramento

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Abstract

Measurement of barometric efficiency (BE) from open monitoring wells or loading efficiency (LE) from formation pore pressures provides valuable information about the hydraulic properties and confinement of a formation. Drained compressibility (α) can be calculated from LE (or BE) in confined and semi-confined formations and used to calculate specific storage (S s). S s and α are important for predicting the effects of groundwater extraction and therefore for sustainable extraction management. However, in low hydraulic conductivity (K) formations or large diameter monitoring wells, time lags caused by well storage may be so long that BE cannot be properly assessed in open monitoring wells in confined or unconfined settings. This study demonstrates the use of packers to reduce monitoring-well time lags and enable reliable assessments of LE. In one example from a confined, high-K formation, estimates of BE in the open monitoring well were in good agreement with shut-in LE estimates. In a second example, from a low-K confining clay layer, BE could not be adequately assessed in the open monitoring well due to time lag. Sealing the monitoring well with a packer reduced the time lag sufficiently that a reliable assessment of LE could be made from a 24-day monitoring period. The shut-in response confirmed confined conditions at the well screen and provided confidence in the assessment of hydraulic parameters. A short (time-lag-dependent) period of high-frequency shut-in monitoring can therefore enhance understanding of hydrogeological systems and potentially provide hydraulic parameters to improve conceptual/numerical groundwater models.

Résumé

La mesure de l’efficacité barométrique (BE) dans des piézomètres ouverts ou de la sensibilité à la marée (LE) à partir de pressions de pore de formation fournit une information précieuse sur les propriétés hydrodynamiques et la captivité d’une formation. La compressibilité drainée (α) peut être calculée à partir de LE (ou BE) dans des formations captives ou semi-captives et utilisée pour calculer l’emmagasinement spécifique (S s). S s et α sont importants pour prévoir les effets des pompages d’eau souterraine et, par conséquent, pour la gestion durable des prélèvements. Cependant, dans des formations de faible conductivité hydraulique (K) ou pour des piézomètres de gros diamètre, les décalages temporels causés par la capacité du puits peuvent être si longs que BE ne peut pas être évaluée correctement dans les piézomètres ouverts dans des configurations captives ou semi-captives. Cette étude démontre l’intérêt de l’utilisation de packers pour réduire le décalage temporel dans les piézomètres et permettre des évaluations fiables de LE. Dans un exemple d’une formation captive de forte K, les estimations de BE dans le piézomètre ouvert sont en bon accord avec les estimations de LE en piézomètre obstrué. Dans un second exemple d’une couche d’argile de faible K d’un aquifère captif BE ne pourrait pas être évaluée de manière satisfaisante dans le piézomètre ouvert du fait du décalage temporel. L’obstruction du piézomètre avec un packer a suffisamment réduit le décalage temporel pour qu’une évaluation fiable de LE puisse être réalisée sur une période de suivi de 24 jours. La réponse en piézomètre obstrué a confirmé l’existence de conditions captives au niveau de la crépine du piézomètre et inspire confiance dans l’évaluation des paramètres hydrodynamiques. Une courte période de suivi à haute fréquence en piézomètre obstrué (de durée dépendant du décalage temporel) peut donc améliorer la compréhension des systèmes hydrogéologiques et potentiellement fournir des paramètres hydrodynamiques pour améliorer les modèles hydrogéologiques conceptuels/numériques.

Resumen

La medición de la eficiencia barométrica (BE) de los pozos de monitoreo abiertos o la eficiencia de carga (LE) de las presiones de los poros de formación proporciona información valiosa sobre las propiedades hidráulicas y el confinamiento de una formación. La compresibilidad drenada (α) se puede calcular a partir de LE (o BE) en formaciones confinadas y semi-confinadas y se utiliza para calcular el almacenamiento específico (S s). S s y α son importantes para predecir los efectos de la extracción de aguas subterráneas y, por lo tanto, para la gestión sostenible de la explotación. Sin embargo, en las formaciones de baja conductividad hidráulica (K) o en los pozos de monitoreo de gran diámetro, los retardos causados por el almacenamiento del pozo pueden ser tan largos que no se puede evaluar adecuadamente en pozos de monitoreo abiertos en entornos confinados o no confinados. Este estudio demuestra el uso de los packers para reducir el tiempo de retardo en los pozos de monitoreo y para permitir evaluaciones confiables de LE. En un ejemplo de una formación confinada, de alto K, las estimaciones de BE en el pozo de monitoreo abierto presentaron una buena concordancia con las estimaciones de LE del pozo cerrado. En un segundo ejemplo, de bajo K a partir de una capa de arcilla confinante, la BE no se pudo evaluar adecuadamente en el pozo de monitoreo abierto debido al retardo temporal. Sellando el pozo de monitoreo con un packer el tiempo se redujo lo suficiente para que se pudiera hacer una evaluación confiable de LE a partir de un período de monitoreo de 24 días. La respuesta cerrada confirmó condiciones confinadas en los filtros del pozo y proporcionó confianza en la evaluación de los parámetros hidráulicos. Un corto periodo de monitoreo cerrado de alta frecuencia puede mejorar la comprensión de los sistemas hidrogeológicos y proporcionar potencialmente parámetros hidráulicos para mejorar los modelos conceptuales/numéricos de agua subterránea.

摘要

从开口监测井测量气压效率或根据地层孔隙压力测量载荷效率提供了地层的水力特性和承压状态有价值的信息。排过水的压缩系数(α)可通过承压及半承压地层的气压效率计算,并用来计算单位储水率(S s)。S sα对于预测地下水开采的影响非常重要,因此,对可持续开采他管理也非常重要。然而,在水力传达率低的地层或者大口井监测井中,井储水引起的时间延迟可能很长,以至于承压或非承压背景下监测井中的气压效率不能恰当地评价。本研究展示了使用封隔器能减少监测井时间延时,并能可靠地评价载荷效率。在一个承压、水力传导率很高的地层例子中,开口监测井气压效率估算值与关在里边的载荷效率估算值一致。在第二个例子中,从水力传导率低的承压粘土层中,由于时间延迟开口监测井中的气压效率不能得到合适的评价。用封隔器密封监测井充分减少了时间延迟,载荷效率可以在24小时的监测期得到可靠的评价。关闭的响应确定了井滤水管的承压条件,提供了评价水力参数的信心。因此,高频率关闭的监测短期(取决于时间延迟)可以提高对水文地质系统的了解,有可能提供水力参数,使概念/数值地下水模型变得更好。

Resumo

Medidas da eficiência barométrica (EB) em poços de monitoramento abertos ou eficiência de carga (EC) a partir de pressões de poro da formação propiciam informações valiosas sobre as propriedades hidráulicas e condições de confinamento de uma formação. A compressibilidade drenada (α) pode ser calculada a partir da EC (ou EB) em formações confinadas e semiconfinadas e utilizadas para calcular o armazenamento específico (S s). S s e α são importantes na previsão de efeitos de extração de águas subterrâneas, e portanto, para a gestão sustentável da extração. Entretanto, em formações de baixa condutividade hidráulica (K) ou poços de monitoramento de grande diâmetro, os tempos de atraso devido ao armazenamento do poço podem ser tão extensos que a EB não pode ser avaliada adequadamente em poços de monitoramento abertos em condições confinadas ou não-confinadas. Este estudo demonstra a utilização de obturadores para reduzir os tempos de atraso em poços de monitoramento, propiciando avaliações confiáveis da EC. Em um exemplo de uma formação confinada e de alta K, as estimativas de EB no poço de monitoramento aberto estiveram de acordo com as estimativas de EC em condições selantes. Num outro exemplo, este envolvendo uma camada argilosa confinante de baixa K, EB não pôde ser avaliado adequadamente no poço de monitoramente aberto devido ao tempo de atraso. O sêlo aplicado pelo obturador instalado no poço de monitoramento reduziu o tempo de atraso suficientemente para que a avaliação da EC pudesse ser realizada através de um período de 24 dias de monitoramento. A resposta típica de condições selantes confirmou a existência de características confinantes na seção filtrante e deram confiança na avaliação dos parâmetros hidráulicos. Um curto (dependente do tempo de atraso) período de alta frequência de monitoramento em condições selantes pode, portanto, melhorar o entendimento dos sistemas hidrogeologicos e potencialmente prover os parâmetros hidráulicos para contribuir com modelos conceituais/numéricos de águas subterrâneas.

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Acknowledgements

This work was financially supported by UNSW, Geoscience Australia, and the National Centre for Groundwater Research and Training (Program 1B), supported by the Australian Research Council and the National Water Commission. The Federal Government NCRIS infrastructure program funded the installation of the NMRDC1 bore and monitoring equipment. The New South Wales Office of Water provided access to their monitoring well network for the study duration. The authors are grateful for the valuable reviews of an earlier manuscript by Associate Editor Tomochika Tokunaga, reviewer Shinichi Hosoya, and a second anonymous reviewer.

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Correspondence to Scott B. Cook.

Appendix: Shut-in system compliance calculations

Appendix: Shut-in system compliance calculations

Here, an evaluation of the likely compliance/compressibility of the shut-in monitoring system at NMRDC1 to support the discussion in section “Compliance/compressibility of the shut-in system” is given. It aims to assess if the time lag evident from the BRF for shut-in NMRDC1 (Fig. 9) is likely to be solely caused by system compliance.

Equations used in analysis

If the screen radius in a monitoring well is equal to the riser radius (this is true for NMRDC1), the volume of the water column it contains is given by:

$$ V = L\uppi {r}^2 $$
(3)

Where L is the water column length and r is the monitoring well riser radius. In shut-in NMRDC1, L = 17.37 m and r = 0.025 m giving a volume of 0.034 m3 from Eq. (3).

The change in volume of a material in response to a change in pressure under isothermal conditions can be calculated from:

$$ \beta = -\frac{1}{V}\frac{\partial V}{\partial P} $$
(4)

(Fine and Millero 1973), where β is the compressibility (Pa−1), V is volume (m3), and P is pressure (Pa).

Rearranging Eq. (4):

$$ \partial V = - V\beta \partial P $$
(5)

For isotropic K and well geometries where the screen length (L sc) divided by well intake radius (R) exceeds 8, Hvorslev’s (1951) solution simplifies to:

$$ K = \frac{r^2 \ln \left({L}_{\mathrm{sc}}/ R\right)}{2{L}_{\mathrm{sc}} T} $$
(6)

where T is the basic time lag (Freeze and Cherry 1979). This is the equation used to calculate K values reported in the main text from slug test data. Equation (6) is used in the following to estimate T associated with literature estimates for compliance of the shut-in system.

Apparent total time lag observed in shut-in NMRDC1

If it is assumed that LE = 0 at time 0 on Fig. 9a for the shut-in BRF, it takes approximately 40 min (2,400 s) for LE to reach 63% of its stabilised value of 0.91. This value of 2,400 s is an approximate estimate of T for the shut-in NMRDC1 monitoring installation. It is noted that a recovery plot generated from the shut-in NMRDC1 BRF has a concave up geometry which increases the uncertainty in this estimate.

Time lag associated with system compliance based on literature values

Compliance of shut-in packer systems has been expressed in the literature as a compressibility relative to the compressibility of water, β w (e.g. Neuzil 1982). β w is 4.6 × 10−7 kPa−1 at 20 °C (e.g. Smith et al. 2013). As discussed in the main text, the maximum compressibility of a shut-in packer system (β p) identified from a literature search was six times greater than β w (Neuzil 1982), which equates to 2.8 × 10−6 kPa−1 at 20 °C. For β p = 2.8 × 10−6 kPa−1, Eq. (5) yields a volume change of 0.93 ml for a change in pressure of 1 m H2O (approximately 9,806 Pa).

From Eq. (3), an open monitoring well registering a 1-m change in water level (L = 1 m) due to 0.93 ml of water (V = 0.93 ml) entering the well would require an r of 0.55 mm. This is the equivalent riser radius of the shut-in NMRDC1 system if β p = 2.8 × 10−6 kPa−1.

From Eq. (6) (R = 0.07 m, L sc = 2 m; see NMRDC1 in main text Table 2) and assuming that T s is negligible, a monitoring well with an equivalent r of 0.55 mm would have a T of 85 s at the estimated K of 3.0 × 10−9 m/s (Table 3 in the main text). This value of 85 s is an estimate of T that would be caused by compliance of the shut-in monitoring installation at NMRDC1 if it had a compressibility six times larger than β w. It is more than 28 times (2,400 s / 85 s) smaller than the apparent T for the shut-in system estimated from the BRF in Fig. 9a.

Appendix conclusions

The effects of packer compliance on shut-in measurements cannot be conclusively determined without undertaking testing (Forster and Gale 1980). Repeating the fieldwork from this study with water-filled (lower compliance) packers may provide useful information. However, the maximum identified literature value for compliance of shut-in packer systems would result in T more than 28 times smaller than that estimated from the BRF for shut-in NMRDC1. Consequently, packer compliance seems unlikely to be the major cause of time lag in the shut-in system.

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Cook, S.B., Timms, W.A., Kelly, B.F. et al. Improved barometric and loading efficiency estimates using packers in monitoring wells. Hydrogeol J 25, 1451–1463 (2017). https://doi.org/10.1007/s10040-017-1537-9

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