Active-Line Source (ALS) Logging
Basic Concept
Active line source (ALS) logging monitors the borehole-fluid temperature over time as it is heated to a state of thermal disequilibrium and/or returns to its equilibrium state after being heated. ALS logging can be used to identify low-volume groundwater flow within and estimate thermal conductivity (Kf) of formations intersected by open and cased boreholes. Though a novel technique, ALS logs have proven vital for identifying hydraulically active features that are overlooked by core analysis or standard flowmeter methods.
Theory
The ALS method uses a line source to uniformly add thermal energy (i.e., heat) into the fluid within a borehole or an isolated portion of a borehole. Fluid temperature is measured over time as the borehole fluid is heated and/or returns to thermal equilibrium, which generally represents the local geothermal gradient, after heating. The ALS method exploits the thermal-energy transport (i.e., heat flow) mechanisms that occur due to or are accentuated by fluid flow to characterize local groundwater-flow patterns.
Groundwater flow that occurs within open (i.e., uncased) boreholes and around cased boreholes can be evidenced by temperature anomalies that result from heat being influenced by fluid movement. If no flow exists within or near a borehole, heating the fluid column would result in a proportional rise in fluid temperature and anomalous values would be unlikely. However, when fluid flow is influencing the borehole system, fluid temperature over time is no longer a simple function of heating.
Because ALS methods heat borehole fluid with a line source, the theory of heat dissipation around a vertical-line source in a cylindrically symmetric, vertically uniform medium can be applied. The rate of temperature rise during and/or the decay of temperature after heating can be used to estimate values of formation thermal conductivity (Kf) and diffusivity (kf). Assuming constant borehole properties, Kf and kf are estimated using the heating and cooling data (Beck and others,1971; Shen and Beck, 1986; Pehme and others, 2007).
It is suspected that the greatest amplification of thermal-parameter contrasts would be observed in the ALS log during the establishment of thermal disequilibrium (i.e., heating). However, Pehme and others (2007) suggest that, though lengthier in logging time, ALS logs collected during the transition back to thermal equilibrium more effectively provide formation thermal parameter information and, the data tend to be smoother and more consistent.
Applications
The FLUTe sleeve liner (Cherry and others, 2007) and other liners allow sections of the borehole to be isolated and analyzed independently. The temperature anomalies exhibited in lined boreholes can be assumed to reflect the heat from the stationary water column being dissipated by the fluid surrounding the borehole. When applied accordingly, ALS methods can distinguish between vertical-fluid flow that exploits the borehole and horizontal-fluid flow that occurs in borehole-intersecting fractures, for example (Pehme and others, 2007).
In open boreholes, the zones of fluid inflow or outflow exhibit lower temperatures, as they are not easily heated due to the continuous transport of thermal energy fluid flow. Such flow patterns can often be resolved with other logging methods such as the suite of vertical flowmeters. The unique benefits of the ALS method is its potential to detect low volume and/or horizontal flows, which are often not detected by the other methods (Pehme and others, 2007).
Especially when interpreted in conjunction with other methods, ALS logs allow for the identification and characterization hydrologically active features within the formations intersected by a borehole. ALS logging may be a useable substitute for water-content estimation without having to deal with the procedures associated with nuclear methods (see neutron porosity and gamma density). Though more testing is required to widen the application and useability of this technique, the ALS method has been successfully applied to the following:
- Detection of horizontal and sub-horizontal fluid flow through casing
- Estimation of apparent formation thermal conductivity and diffusivity
- Identification and comparison of hydrogeologically active features
- Identification of very low volume and horizontal groundwater flow
- Reconnaissance to guide the implementation of other flow techniques
Examples/Case studies
Coleman, T.I., Parker, B.L., Maldaner, C.H., and Mondanos, M.J., 2015, Groundwater flow characterization in a fractured bedrock aquifer using active DTS tests in sealed boreholes: Journal of Hydrology, v. 528, p. 449-462, doi: 10.1016/j.jhydrol.2015.06.061.
Abstract: In recent years, wireline temperature profiling methods have evolved to offer new insight into fractured rock hydrogeology. Important advances in wireline temperature logging in boreholes make use of active line source heating alone and then in combination with temporary borehole sealing with flexible impervious fabric liners to eliminate the effects of borehole cross-connection and recreate natural flow conditions. Here, a characterization technique was developed based on combining fiber optic distributed temperature sensing (DTS) with active heating within boreholes sealed with flexible borehole liners. DTS systems provide a temperature profiling method that offers significantly enhanced temporal resolution when compared with conventional wireline trolling-based techniques that obtain a temperature–depth profile every few hours. The ability to rapidly and continuously collect temperature profiles can better our understanding of transient processes, allowing for improved identification of hydraulically active fractures and determination of relative rates of groundwater flow. The advantage of a sealed borehole environment for DTS-based investigations is demonstrated through a comparison of DTS data from open and lined conditions for the same borehole. Evidence for many depth-discrete active groundwater flow features under natural gradient conditions using active DTS heat pulse testing is presented along with high resolution geologic and geophysical logging and hydraulic datasets. Implications for field implementation are discussed.
Moscoso Lembcke, L.G., Roubinet, D., Gidel, F., Irving, J., Pehme, P.E., and Parker, B.L, 2016, Analytical analysis of borehole experiments for the estimation of subsurface thermal properties: Advances in Water Resources, v. 91, p. 88-103, doi:10.1016/j.advwatres.2016.02.011.
Abstract: Estimating subsurface thermal properties is required in many research fields and applications. To this end, borehole experiments such as the thermal response test (TRT) and active-line-source (ALS) method are of significant interest because they allow us to determine thermal property estimates in situ. With these methods, the subsurface thermal conductivity and diffusivity are typically estimated using asymptotic analytical expressions, whose simplifying assumptions have an impact on the accuracy of the values obtained. In this paper, we develop new analytical tools for interpreting borehole thermal experiments, and we use these tools to assess the impact of such assumptions on thermal property estimates. Quite importantly, our results show that the simplifying assumptions of currently used analytical models can result in errors in the estimated thermal conductivity and diffusivity of up to 60% and 40%, respectively. We also show that these errors are more important for short-term analysis and can be reduced with an appropriate choice of experimental duration. Our results demonstrate the need for cautious interpretation of the data collected during TRT and ALS experiments as well as for improvement of the existing in-situ experimental methods.
Pehme, P.E., Greenhouse, J.P., and Parker, B.L., 2007, The Active Line Source Temperature Logging Technique and its Application in Fractured Rock Hydrogeology: Journal of Environmental and Engineering Geophysics, v. 12, no. 4, p. 293-356, doi:10.2113/JEEG12.4.307.
Abstract: We present a technique for placing a borehole into thermal dis-equilibrium, and thereby interpreting groundwater flow through fractures where it may have been previously undetected. Denoted as Active Line Source (ALS) logging, the method consists of temperature logging while a borehole is heated by the cable and∕or during cooling after the heating. With two or more logs collected during either heating or cooling, an estimate of thermal conductivity is obtained. The basic theory, widely used for such things as thermal conductivity probes, is shown to fit the recorded data well. The mechanics of ALS logging are described, and the practical challenges are outlined. In the absence of groundwater flow in or around the borehole, variations in the thermal conductivity of the rock are largely due to variable water content and the ALS log provides a reasonable surrogate for the neutron log. When groundwater flow dominates the dissipation of thermal energy from the borehole, however, the apparent thermal conductivity is increased. In open boreholes this flow can be both ambient (within the formation itself) and connecting (vertical flow between fractures intersected by the borehole). In cased or lined holes with no connecting flow, ALS logs are particularly useful as detectors of ambient groundwater flow. Alternative methods for flow detection, such as chemical dilution or flow-meters, require an open borehole and either have poor vertical resolution or require multiple stationary measurements, often with packers to minimize the effects of connecting flow. The ALS technique is a comparatively simple tool, useful in both open and cased or lined boreholes, run continuously down the length of the borehole, with fracture resolution on the order of a few centimeters. We describe ALS logging of a 75‐meter section of a borehole through fractured dolomite which has been lined with a FLUTe sleeve. The ALS results are compared to the geologic units encountered, conventional geophysical logging techniques, time-lapse passive temperature logging, heat pulse flowmeter data and packer testing.
Pehme, P.E., Parker, B.L., Cherry, J.A., and Blohm, D., 2014, Detailed measurement of the magnitude and orientation of thermal gradients in lined boreholes for characterizing groundwater flow in fractured rock: Journal of Hydrology, v. 513, p. 101-114, doi:10.1016/j.jhydrol.2014.03.015.
Abstract: Recent developments have led to revitalization of the use of temperature logging for characterizing flow through fractured rock. The sealing of boreholes using water-filled, flexible impermeable liners prevents vertical cross connection between fractures intersecting the hole and establishes a static water column with a temperature stratification that mimics that in the surrounding formation. Measurement of the temperature profile of the lined-hole, water column (using a high sensitivity single-point probe achieving resolution on the order of 0.001 °C) has identified fractures with active flow under ambient groundwater conditions (without cross connecting flow along the borehole). Detection of flow in fractures was further improved with the use of a heater to create thermal disequilibrium in the active line source (ALS) technique and eliminate normal depth limitations in the process. This paper presents another advancement; detailed measurement of the magnitude and direction of the thermal gradient to characterize flow through fractured rock. The temperature within the water column is measured along the length of the lined hole using a temperature vector probe (TVP): four high sensitivity sensors arranged in a tetrahedral pattern oriented using three directional magnetometers. Based on these data, the horizontal and vertical components of the thermal field, as well as the direction of temperature gradient are determined, typically at depth intervals of less than 0.01 m. This probe was assessed and refined by trials in over 30 lined boreholes; the results from two holes through a fractured dolostone aquifer in Guelph, Ontario are used as exampled. Since no other device exists for measuring flow magnitude and direction under the ambient flow condition created by lined holes, the performance of the TVP is assessed by examining the reproducibility of the temperature measurements through an ALS test, and by the consistency of the results relative to other types of larger-scale information from the study area. Temperature profiles were measured in lined holes under both ambient thermal conditions and subject to ALS heating of the entire length of the holes to demonstrate resolution and reproducibility. The hydraulic gradient in three-dimensional space, based on pressure measurements from three depth discrete, multilevel monitoring systems in nearby holes, was used to independently estimate variations in groundwater flow directions. The characteristics of the hydraulic and thermal regimes are compared to assess response to changes in flow in a fractured rock system. When used in the lined holes, the level of detail provided by this multi-sensor probe is much greater than that provided by a single-sensor probe and this detail strongly supports inferences concerning the relative magnitude and direction of the flow. The results of this study indicate that the details of the thermal gradient can be measured and provides superior results compared to a conventional one dimensional temperature profile, thereby substantially enhancing the characterization of groundwater flow in fractured rock.
Pehme, P.E., Parker, B.L., Cherry, J.A., Molson, J.W., and Greenhouse, J.P., 2013, Enhanced detection of hydraulically active fractures by temperature profiling in lined heated bedrock boreholes: Journal of Hydrology, v. 484, p. 1-15, doi:10.1016/j.jhydrol.2012.12.048.
Abstract: The effectiveness of borehole profiling using a temperature probe for identifying hydraulically active fractures in rock has improved due to the combination of two advances: improved temperature sensors, with resolution on the order of 0.001 °C, and temperature profiling within water inflated flexible impermeable liners used to temporarily seal boreholes from hydraulic cross-connection. The open-hole cross-connection effects dissipate after inflation, so that both the groundwater flow regime and the temperature distribution return to the ambient (background) condition. This paper introduces a third advancement: the use of an electrical heating cable that quickly increases the temperature of the entire static water column within the lined hole and thus places the entire borehole and its immediate vicinity into thermal disequilibrium with the broader rock mass. After heating for 4–6 h, profiling is conducted several times over a 24 h period as the temperature returns to background conditions. This procedure, referred to as the Active Line Source (ALS) method, offers two key improvements over prior methods. First, there is no depth limit for detection of fractures with flow. Second, both identification and qualitative comparison of evidence for ambient groundwater flow in fractures is improved throughout the entire test interval. The benefits of the ALS method are demonstrated by comparing results from two boreholes tested to depths of 90 and 120 m in a dolostone aquifer used for municipal water supply and in which most groundwater flow occurs in fractures. Temperature logging in the lined holes shows many fractures in the heterothermic zone both with and without heating, but only the ALS method shows many hydraulically active fractures in the deeper homothermic portion of the hole. The identification of discrete groundwater flow at many depths is supported by additional evidence concerning fracture occurrence, including continuous core visual inspection, acoustic televiewer logs, and tests for hydraulic conductivity using straddle packers as well as rock core VOC data, where available, that show deep penetration and many migration pathways. Confidence in the use of temperature profiles and the conceptual model is provided by numerical simulation and the demonstrated reproducibility of the evolution of the temperature signal measured in the lined holes with and without heating. This approach for using temperature profiling in lined holes with heating is a practical advance in fractured rock hydrogeology because the liners are readily available, the equipment needed for heating is low cost and rugged, and the time needed to obtain the profiles is not excessive for most projects.
References
Beck, A.E., Anglin, F.M., and Sass, J.H., 1971, Analysis of heatflow data – in situ thermal conductivity measurements: Canadian Journal of Earth Sciences, v. 8, no. 1, p. 1-19, doi:10.1139/e71-001.
Cherry, J.A., Parker, B.L., and Keller, C.E., 2007, A New Depth‐Discrete Multilevel Monitoring Approach for Fractured Rock: Ground Water Monitoring and Remediation, v. 27, no. 2, p. 57-70, doi:10.1111/j.1745-6592.2007.00137.x.
Coleman, T.I., Parker, B.L., Maldaner, C.H., and Mondanos, M.J., 2015, Groundwater flow characterization in a fractured bedrock aquifer using active DTS tests in sealed boreholes: Journal of Hydrology, v. 528, p. 449-462, doi: 10.1016/j.jhydrol.2015.06.061.
Conaway, J.G., and Beck, A.E., 1997, Fine-scale correlation between temperature gradient logs and lithology: Geophysics, v. 42, no. 7, p. 1401-1410, doi:10.1190/1.1440801.
Drury, M.J., 1984, Borehole temperature logging for the detection of water flow: Geoexploration, v. 22, no. 3-4, p. 231-243, doi:10.1016/0016-7142(84)90014-0.
Moscoso Lembcke, L.G., Roubinet, D., Gidel, F., Irving, J., Pehme, P.E., and Parker, B.L, 2016, Analytical analysis of borehole experiments for the estimation of subsurface thermal properties: Advances in Water Resources, v. 91, p. 88-103, doi:10.1016/j.advwatres.2016.02.011.
Pehme, P.E., Greenhouse, J.P., and Parker, B.L., 2007, The Active Line Source Temperature Logging Technique and its Application in Fractured Rock Hydrogeology: Journal of Environmental and Engineering Geophysics, v. 12, no. 4, p. 293-356, doi:10.2113/JEEG12.4.307.
Pehme, P.E., Parker, B.L., Cherry, J.A., and Blohm, D., 2014, Detailed measurement of the magnitude and orientation of thermal gradients in lined boreholes for characterizing groundwater flow in fractured rock: Journal of Hydrology, v. 513, p. 101-114, doi:10.1016/j.jhydrol.2014.03.015.
Pehme, P.E., Parker, B.L., Cherry, J.A., Molson, J.W., and Greenhouse, J.P., 2013, Enhanced detection of hydraulically active fractures by temperature profiling in lined heated bedrock boreholes: Journal of Hydrology, v. 484, p. 1-15, doi:10.1016/j.jhydrol.2012.12.048.
Shen, P.Y., and Beck, A.E., 1986, Stabilization of bottom hole temperature with finite circulation time and fluid flow: Geophysical Journal International, v. 86, no. 1, p. 63-90, doi:10.1111/j.1365-246X.1986.tb01073.x.