Fluid-Temperature and Fluid-Conductivity Logging

Basic Concept

Fluid-temperature and fluid-conductivity logging are standard borehole techniques commonly applied to hydrogeological environments wherein groundwater flows predominately through distinct transmissive zones (e.g., fractured rock). Fluid-temperature logging measures the temperature of the borehole fluid with depth, whereas fluid-conductivity logging measures variations in fluid-electrical conductivity (or its inverse, resistivity) with depth. When interpreted concurrently, fluid-temperature and fluid-conductivity logs can be used to estimate the specific conductance of the borehole fluid with depth.

Chemical leachates resulting from many surface operations often produces groundwater with anomalously high specific conductance. As such, fluid-temperature and fluid-conductivity logs are most often used to monitor the flow of contaminated groundwater. Additionally, both techniques can be used to identify potential locations of inflow and outflow within the borehole as well as vertical-flow zones through the borehole. As such, fluid-temperature and fluid-conductivity logs are effective for determining the locations appropriate for stationary vertical flowmeter measurements.

Fluid-temperature and fluid-conductivity logs can be collected under non-pumping (i.e., ambient), pumping, and injection conditions. However, under ambient conditions, neither fluid-temperature nor fluid-conductivity logs can indicate the direction of borehole-fluid flow, which, itself, is not necessarily representative of formation-fluid flow. Flow-direction determination requires a stress to the system (e.g., borehole tracer tests) or use a flowmeter. Though not discussed further, the three above-mentioned (i.e., ambient, pumping and injection) conditions are included in the referenced case studies.

Theory

In the absence of fluid flow, the temperature in the borehole is in equilibrium with the formation and reflects the typically increasing geothermal gradient. Local geothermal gradients depend on the thermal conductivity of the borehole-intersected formations as well as the characteristics of the thermal energy originating at depth. Thus, when stationary borehole fluid and adjacent formations are in thermal equilibrium, temperature logs can be used to infer certain subsurface conditions and material properties (Keys, 1990).

A fluid-temperature gradient that deviates from the expected rate of change (i.e., frequently 1°F per 100 feet) most often occurs as a result of fluid flow within the borehole. Zones of vertical fluid flow can be indicated by a temperature gradient lower than expected and characterized by negligible temperature change over the zone. Temperature gradients significantly higher than expected may indicate inflow or outflow boundaries at transmissive features or formations intersected by the borehole.

add FIGURE showing abrupt changes in the FEC/T profile-CJ

Fluid-electrical conductivity is the capacity of a fluid to conduct electrical current and varies with fluctuations in temperature and/or dissolved solids. Pure water is electrically resistive (i.e., resists the flow of electrical current), but water residing within natural environments contain numerous ions that increase electrical conductivity (i.e., decrease electrical resistivity). The contribution made to fluid conductivity by each ion species depends upon its concentration and chemical properties (e.g., valence, mobility) (Wood and Bennecke, 1994).

The chemical composition of earth materials can vary significantly with space, and the chemical- and electrical properties of fluid are often directly related to specific source environments (Hem, 1989). Anomalous electrical-conductivity values may indicate the presence of a transmissive zone that allows fluid to flow into and/or out of the borehole. Thus, fluid conductivity can be used as a proxy to help identify specific groundwater-flow paths within the subsurface, especially when combined with other logs and information.

Temperature can significantly affect electrical conductivity, and, for a given ionic solution, conductivity increases proportionally with temperature. Thus, conductivity data can be more accurately compared when temperature corrected using a composition-dependent coefficient. Specific conductance is the value of electrical conductivity at a standard temperature and is usually reported in microSiemens per centimeter (μS/cm) at 25° C. Though not absolute and requiring ionic-specific corrections, specific conductance allows for total dissolved solid (e.g. salt) concentration estimations (Keys, 1990).

Applications

Fluid-temperature and -conductivity are usually measured simultaneously using a single tool that is the first run in a logging suite so that data most closely represent ambient conditions. The sensors, which respond to nearby fluid, are mounted in a tubular housing that channels the fluid as the tool is moved through the borehole. Though highest quality data require time for measurement readings equilibrate with the fluid, reliable data can be collected at many standard logging speeds (e.g., 10-40 ft/min).

Fluid-temperature data are typically collected with a glass-bead thermistor, which is an electrical resistor with a resistance related to heat exposure. Using Ohm’s Law and a small electrical current, resistance across the thermistor is measured, and temperature is subsequently computed or calculated. Using four small, closely spaced ring electrodes and a low current, conductivity data are collected comparably to borehole resistivity. Depth can be measured with a winch depth encoder or manually if using a handheld reel (Keys, 1990).

Fluid-temperature data can be displayed in a temperature log or in a differential-temperature log. The differential temperature log, which displays the rate of change of temperature versus depth (i.e., instantaneous change in the temperature log), can identify changes in slope that are not always evident in the standard temperature log. A fluid-conductivity log displays electrical conductivity (or resistivity) with depth and can be used in conjunction with temperature data to calculate specific conductance with depth.

Fluid-temperature and -conductivity methods can be employed in boreholes with numerous types of construction. However, because borehole construction interferes with ambient processes, logging in open or properly screened boreholes provides data that most accurately represents the fluids within the adjacent formations. Though the fluid temperature and -conductivity are most often applied to monitor the flow of contaminated groundwater with increased electrical conductivity, both logs can complement numerous other methods. The applications for the individual and combined methods are as follows:

Fluid-temperature and fluid-conductivity logging used together can aid the following:

Identification of zones of vertical fluid flow
Location of inflow and outflow zones
Identification of locations for stationary flowmeter measurements
Plan the completion of boreholes with packers or sampling devices
Monitoring of plumes with anomalous properties (e.g., saltwater, wastewater, recharge)
Estimation of concentration of total dissolved solids (TDS)

Fluid-temperature logs can aid the following:

Correction of temperature-sensitive logs
Estimation of thermal conductivity of rocks
Calculation of water density, viscosity, and thermal conductivity
Development of heat-flow maps
Location of cement behind casing

Fluid-temperature logs can aid the following:

Interpretation of electric logs
Determination of depth intervals for chemical analysis sampling

Examples/Case studies

Doughty, C., and Tsang, C., 2005, Signatures in flowing fluid electric conductivity logs: Journal of -Hydrology, v. 310, no. 1, p. 157-180, doi:10.1016/j.jhydrol.2004.12.003.

Abstract: Flowing fluid electric conductivity logging provides a means to determine hydrologic properties of fractures, fracture zones, or other permeable layers intersecting a borehole in saturated rock. The method involves analyzing the time-evolution of fluid electric conductivity (FEC) logs obtained while the well is being pumped and yields information on the location, hydraulic transmissivity, and salinity of permeable layers. The original analysis method was restricted to the case in which flows from the permeable layers or fractures were directed into the borehole (inflow). Recently, the method was adapted to permit treatment of both inflow and outflow, including analysis of natural regional flow in the permeable layer. A numerical model simulates flow and transport in the wellbore during flowing FEC logging, and fracture properties are determined by optimizing the match between simulation results and observed FEC logs. This can be a laborious trial-and-error procedure, especially when both inflow and outflow points are present. Improved analyses methods are needed. One possible tactic would be to develop an automated inverse method, but this paper takes a more elementary approach and focuses on identifying the signatures that various inflow and outflow features create in flowing FEC logs. The physical insight obtained provides a basis for more efficient analysis of these logs, both for the present trial and error approach and for a potential future automated inverse approach. Inflow points produce distinctive signatures in the FEC logs themselves, enabling the determination of location, inflow rate, and ion concentration. Identifying outflow locations and flow rates typically requires a more complicated integral method, which is also presented in this paper.

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.

Abstract: In a fractured rock body that is penetrated by a borehole, water flow along fracture planes and water flow between fractures, the borehole providing the pathway, produce different, characteristic thermal anomalies. Consequently, closely-spaced temperature measurements in a borehole allow quantitative estimates to be made of flow rates or velocities. Temperature measurements at depth intervals of as coarse as 3 m provide very useful information. Such logs can be obtained manually with lightweight field equipment that is capable of resolving temperature differences of approximately 2 mK. Some examples of the detection of water flow in boreholes of the Canadian Shield are given.

Löw, S., Kelley, V., and Vomvoris, S., 1994, Hydraulic borehole characterization through the application of moment methods to fluid conductivity logs: Journal of Applied Geophysics, v. 31, no. 1-4, p. 117-131, doi:10.1016/0926-9851(94)90051-5.

Abstract: In any type of groundwater transport problem (contaminant solutes, heat, etc.), knowledge of the location and properties of pathways of increased hydraulic conductivity is essential. However, answering such questions in strongly heterogeneous media, such as fractured rock, can be very difficult and budget-intensive with standard geophysical or hydrogeological field investigations. We present a new testing concept and analysis procedure based on a time sequence of wellbore electric conductivity logs, which provides the exact location and the outflow parameters (transmissivity, formation fluid conductivity) of flowing features (fractures, faults, layers) intercepted by the borehole. Previously the quantitative analysis of this time sequence of electrical conductivity logs was based on a code, called BORE, used to simulate borehole fluid conductivity profiles given these parameters. The present report describes a new direct (not iterative) method for analyzing a short time series of electric conductivity logs which is based on moment quantities of the individual outflow peaks, and applies it to synthetic as well as to field data. The results of the method are promising and are discussed in terms of the method's advantages and limitations. In particular it is shown that the method is capable of reproducing hydraulic properties derived from packer tests well within a factor of three, which is below the range of what is recognized as the accuracy of packer tests themselves. Furthermore the new method is much quicker than the previously used iterative fitting procedure and is even capable of handling transient fracture outflow conditions.

Morin, R.H., Senior, L.A., and Decker, E.R., 2000, Fractured-Aquifer Hydrogeology from Geophysical Logs: Brunswick Group and Lockatong Formation, Pennsylvania: Groundwater, v. 38, no. 2, p. 182-192, doi:10.1111/j.1745-6584.2000.tb00329.x.

Abstract: The Brunswick Group and the underlying Lockatong Formation are composed of lithified Mesozoic sediments that constitute part of the Newark Basin in southeastern Pennsylvania. These fractured rocks form an important regional aquifer that consists of gradational sequences of shale, siltstone, and sandstone, with fluid transport occurring primarily in fractures. An extensive suite of geophysical logs was obtained in seven wells located at the borough of Lansdale, Pennsylvania, in order to better characterize the areal hydrogeologic system and provide guidelines for the refinement of numerical ground water models. Six of the seven wells are approximately 120 m deep and the seventh extends to a depth of 335 m. Temperature, fluid conductivity, and flowmeter logs are used to locate zones of fluid exchange and to quantify transmissivities. Electrical resistivity and natural gamma logs together yield detailed stratigraphic information, and digital acoustic televiewer data provide magnetically oriented images of the borehole wall from which almost 900 fractures are identified. Analyses of the geophysical data indicate that the aquifer penetrated by the deep well can be separated into two distinct structural domains, which may, in turn, reflect different mechanical responses to basin extension by different sedimentary units: 1. In the shallow zone (above 125 m), the dominant fracture population consists of gently dipping bedding plane partings that strike N46°E and dip to the northwest at about 11 degrees. Fluid flow is concentrated in the upper 80 m along these subhorizontal fractures, with transmissivities rapidly diminishing in magnitude with depth. 2. The zone below 125 m marks the appearance of numerous high‐angle fractures that are orthogonal to the bedding planes, striking parallel but dipping steeply southeast at 77 degrees. This secondary set of fractures is associated with a fairly thick (approximately 60 m) high‐resistivity, low‐transmissivity sandstone unit that is abruptly terminated by a thin shale bed at a depth of 190 m. This lower contact effectively delineates the aquifer's vertical extent at this location because no detectable evidence of ground water movement is found below it. Thus, fluid flow is controlled by fractures, but fracture type and orientation are related to lithology. Finally, a transient thermal‐conduction model is successfully applied to simulate observed temperature logs, thereby confirming the effects of ground‐surface warming that occurred in the area as a result of urbanization at the turn of the century. The systematic warming of the upper 120 m has increased the transmissivity of this aquifer by almost 10%, simply due to changes in fluid viscosity and density.

Williams, J.H., and Conger, R.W., 1990, Preliminary Delineation of Contaminated Water‐Bearing Fractures Intersected by Open‐Hole Bedrock Wells: Groundwater Monitoring and Remediation, v. 10, no. 4, p. 118-126, doi:10.1111/j.1745-6592.1990.tb00028.x.

Abstract: Contaminated water‐bearing fractures intersected by open‐hole bedrock wells were preliminarily delineated through a combination of geophysical logging, vertical‐flow measurements, and downhole water sampling as part of remedial site investigations in southeastern New York. The wells investigated range from 100 to 450 feet in depth, have only shallow surface casing, and intersect multiple water‐bearing zones. The distribution of water‐bearing zones that intersect the wells was determined from single‐point resistance, caliper, fluid‐resistivity, temperature, and acoustic‐televiewer logs. Measurable flow in the wells was downward from upper producing zones to lower receiving zones that are poorly connected in the aquifer and that differ in hydraulic head as a result of nearby pumping. A down hole sampler was used to collect discrete and composite water samples for analysis of volatile organic compounds from producing zones that are self‐purging as a result of flow in the wells. The results obtained at two of the study sites are presented—the Spring Valley wellfield and the Mahopac business district. At the Spring Valley wellfield, a supply well completed in Mesozoic sandstone and conglomerate intersects water‐bearing zones at depths of 204 to 245 feet that produced contaminated water that was received by a zone at 278 feet. In the same well, a deeper zone at 345 feet produced uncontaminated water that was received by a zone at 403 feet. Correlation of information from the well, geophysical logs and drill cores from nearby monitoring wells, and bedrock outcrops indicates that most of the water‐bearing zones are bedding‐plane separations that probably provide pathways for contaminant transport in the bedrock aquifer for significant distances. In the Mahopac business district, a deep test well completed in Precambrian gneiss intersected shallow waterbearing zones at 50 to 79 feet that produced contaminated water that was received by deep zones at 260 and 328 feet. The water‐bearing zones consist of single or closely spaced multiple fractures with dips of 5 to 50 degrees. By analogy with the results from this test well, deep open‐hole wells in the area may serve as “short circuits” in the ground water flow system and allow direct transport of contaminants to deeper zones in the fractured‐bedrock aquifer. The methods presented can be used to investigate ground water flow and contamination in fractured‐bedrock aquifers in advance of more focused monitoring programs. The methods can be applied in existing open‐hole wells before test drilling and monitoring well installation to provide for efficient program design. The methods also can be used during the installation of monitoring wells to help determine completion depths and open intervals and to ensure that the wells are not serving as conduits for the flow of contaminated water.

Wood, S.H., and Bennecke, W., 1994, Vertical Variation in Groundwater Chemistry Inferred from Fluid Specific-Conductance Well Logging of the Snake River Plain Basalt Aquifer, Idaho National Engineering Laboratory, Southeastern Idaho, in Proceedings, Hydrogeology, Waste Disposal, Science and Politics: 30^th Symposium on Engineering Geology and Geotechnical Engineering: Boise, Idaho, p. 267-283.

Abstract: Well logging of electrical fluid specific conductance (Cs) shows that permeable zones yielding ground water to intrawell flows and the water columns in some wells at INEL have highly different chemistry, with as much as a two-fold variation in Cs). This suggests that dedicated pump sampling of ground water in the aquifer may not be representative of the chemistry of the waste plumes migrating southwest of the nuclear facilities. Natural background Cs in basalt-aquifer ground water of this part of the Snake River Plain aquifer is less than 325µS/cm (microSiemans/cm), and total dissolved solids in mg/L units, (TDS) ≈ 0.6Cs). This relationship underestimates IDS for waters with chemical waste. when Cs) is above 800 µS/cm. At well 59 near the ICPP water of 1115 µS/cm (≈670+ mg/L TDS) enters the well from a permeable zone between 521 and 537 ft depth; the zone being 60 ft below the water level and water of 550 µS/cm. At the time of logging (9/14/93) the 1115µS/cm water was flowing down the well, mixing with less concentrated waters and exiting at 600 or 624-ft depth. Waste water disposed of down the injection well at ICPP until 1984 was estimated to have a Cs) of 1140 µS/cm, identical to the water detected in logging. At well OW2, the highest Cs) water (760µS/cm) is in the upper 30 feet of the water column: water from two flow zones below have different chemistry with lower values of Cs. The Site 14 well and USGS 83 show uniform values throughout the water column. The water column in Site 14 is dominated by a downward flow of 50 gal/min probably entering between 475 and 500 ft depth and exiting near the bottom of the well at 700 ft depth. Impeller flowmeter and precision temperature logging are used to define and quantify temperature variations and intrawell flows. At well 59 (depth=657 ft) and OW2 (depth=996 ft), are downward decreasing temperatures in the bottom zones of no flow, suggesting that major flow zones lie beneath the deepest parts of these wells.

References