Vertical Flowmeter Logging

Basic Concept

Fluid flow is often directly related to aquifer properties, and the determination of fluid flow throughout a borehole can help better understand the subsurface hydrology. Vertical-flowmeter logging measures the direction and magnitude of the vertical component of flow that occurs at depths throughout a borehole. Flowmeter data can aid the estimation of hydraulic parameters (e.g. transmissivity, hydraulic-head gradients) that are used in conceptual site models.

Vertical-flowmeter logging can be used to identify the locations of fluid production and loss in boreholes that intersect fractured rock and/or unconsolidated aquifers. Additionally, vertical-flowmeter data can aid the determination of the fluid velocity between transmissive fractures and/or permeable units and the calculation of volumetric flow rates. Flowmeter data are collected using a heat pulse-, electromagnetic-, and/or spinner flowmeter, and the tool employed is based on the expected borehole environment and flow velocities.

Theory

Groundwater typically travels through the subsurface via unconsolidated and/or fractured earth materials. Groundwater movement is driven by differences in hydraulic head, which is the total energy available to move fluid at a given point in an aquifer. A borehole that intersects an aquifer with a hydraulic-head gradient will contain fluid that experiences vertical flow along the borehole. Fluid will enter the borehole at zones of higher head and exit at zones of lower head.

Throughout the length of the borehole, fluid can enter and exit through various transmissive zones (i.e., those through which fluid can pass). The direction and magnitude of fluid flow between two transmissive zones primarily depends on their difference in hydraulic head and magnitudes of transmissivity. The net vertical flow within a borehole is a composite of each individual transmissive zone and the sum of each transmissivity-weighted hydraulic heads.

Net flow is dominated by the head of the most transmissive zone, which may hinder the detection of smaller flows induced by zones with lower transmissivity. However, stressing the system (e.g., pumping or injecting) can accentuate smaller flows and allow transmissive zones with similar or smaller ambient-head differences to be identified. Additionally, stressing the aquifer may help identify two transmissive fractures that have the same ambient head (i.e., no ambient vertical flow exists between them).

Vertical-flowmeter logs often involve collecting point measurements at specific depths. To increase the measurement potential of fluid-production and -loss zones, point data are often collected above and below the estimated locations of fractures or permeable units. Such locations are usually based on the results of other borehole methods. Additionally, flowmeters can be equipped with a diverter that partially or fully diverts fluid into a measurement chamber to improve sensitivity to low flows or in large-diameter wells.

Vertical flowmeters measure vertical-fluid velocity in distance per time, and these data can be converted to volumetric flow rate (i.e., volume per time). Volumetric flow rate considers borehole-diameter data (see caliper log) or, if used, the diameter of the diverter through which fluid is channeled. The rate of fluid flow from one zone to another can be used to estimate aquifer parameters such as transmissivity, vertical-hydraulic head, and hydraulic conductivity.

Applications

Like most methods, vertical-flowmeter tools have limitations. Vertical flowmeters encounter the issue of dynamic range, whereby smaller velocity features may go undetected in the presence of high velocity features. Additionally, the borehole construction can affect flowmeter-data results, and successful flowmeter logging in cased holes requires the absence flow in the annular space of the well. Furthermore, the resolution of each tool differs, and, therefore, the minimum and maximum detectable-flow rates are tool specific and should be considered.

Using stationary measurements, the heat-pulse flowmeter (HPFM) is used in low-flow boreholes and can detect flow between 0.01-1.5 gallons per minute (gpm) with a full diverter. A HPFM contains a wire-heating grid situated vertically between two thermistors that measure temperature over time. The logger activates a pulse that heats the grid-adjacent borehole fluid. Any vertical flow occurring at the measurement point will influence the heated fluid to move up or down toward a thermistor.

The arrival of the heat pulse at a thermistor is represented as the peak-temperature value in the recorded fluid temperature over time curve. The volumetric fluid-flow rate at that time and position in the borehole is calculated using the heat-pulse travel time and distance between the heating grid and thermistor. A properly calibrated HPFM system resolves the asymmetry that is caused by the natural rising of heat (i.e., the buoyancy effect that occurs in the absence of flow).

The electromagnetic flowmeter (EMFM) can measure fluid-flow rates spanning from 0.05-15 gpm with a fully fit diverter to more than 100 gpm without one. Exploiting Faraday's Law of Induction, the EMFM generates a primary magnetic field that intersects a hollow chamber within the tool. Electrically conductive-borehole fluid that flows through the chamber passes through the magnetic field at a right angle, which induces a voltage within an internal-wire coil.

The induced voltage is proportional to the fluid velocity through the chamber. The volumetric flow rate is calculated by multiplying flow velocity by the cross-sectional area of the chamber.

EMFM data can be collected either as stationary point measurements or continuously in tool-trolling mode. As the tool trolls (i.e., moves) through the borehole, flow is induced in the direction opposite of logging. Artificial flow is calculated using logging speed and subtracted from data to render naturally occurring flow.

The conventional spinner flowmeter is commonly used and can measure a wide range of vertical flow rates. However, because the tool often stalls if flow velocity is less than 5 feet per minute, it is not ideal for very low flow rates under most conditions. The spinner tool contains an interchangeable lightweight impeller (i.e., fan-bladed rotary with well-specific diameter) that is housed within a protective cage through which fluid can pass.

Any vertical fluid flow through the cage applies force to the impeller blades, which respond by revolving. The rotation rate (i.e., revolutions per time) of the impellor is recorded by the tool and used to calculate the fluid velocity. Spinner flowmeter measurements can be collected while trolling to produce a continuous flow profile over a depth range or while stationary to investigate flow at a specific depth.

Vertical-flowmeter data can provide qualitative and/or quantitative information for subsurface-hydraulic analyses. Flowmeter logs can identify the locations of transmissive features and determine relative hydraulic gradients. Flowmeter data can help identify vertical-flow directions, measure vertical-flow velocities, and estimate volumetric flow rates. Estimates of flow-zone transmissivity and hydraulic head can be achieved using proportion-, analytical solution-, and numerical-modelling methods. Such critical information provided by vertical flowmeter logging can aid numerous groundwater studies and applications such as the following:

Design and interpretation of hydraulic testing
Determination of chemical sampling strategies
Understanding of historical chemical results
Conceptual modeling of a site
Completion of a well
Monitoring of hydraulic head

Examples/Case studies

Barahona-Palomo, M., Riva, M., Sanchez-Vila, X., Vazquez-Sune, E., and Guadagnini, A., 2011, Quantitative comparison of impeller-flowmeter and particle-size-distribution techniques for the characterization of hydraulic conductivity variability: Hydrogeology Journal, v. 19, p. 603-612, doi:10.1007/s10040-011-0706-5.

Abstract: Hydraulic conductivities associated with measurement scale of the order of 10–1 m and collected during an extensive field campaign near Tübingen, Germany, are analyzed. Estimates are provided at coinciding locations in the system using: (1) the empirical Kozeny-Carman formulation, providing conductivity values, K GS, based on particle-size distribution, and (2) borehole impeller-type flowmeter tests, which infer conductivity, K FM, from measurements of vertical flows within a borehole. Correlation between the two sets of estimates is virtually absent. However, statistics of the natural logarithm of K GS and K FM at the site are similar in terms of mean values (averages of ln K GS being slightly smaller) and differ in terms of variogram ranges and sample variances. This is consistent with the fact that the two types of estimates can be associated with different (albeit comparable) measurement (support) scales. It also matches published results on interpretations of variability of geostatistical descriptors of hydraulic parameters on multiple observation scales. The analysis strengthens the idea that hydraulic conductivity values and associated key geostatistical descriptors inferred from different methodologies and at similar observation scales (of the order of tens of cm) are not readily comparable and should not be embedded blindly into a flow (and eventually transport) prediction model.

Basiricò, S., Crosta, G.B., Frattini, P., Villa, A., and Godio, A., 2015, Borehole Flowmeter Logging for the Accurate Design and Analysis of Tracer Tests: Groundwater, v. 53, no. S1, p. 3-9, doi:10.1111/gwat.12293.

Abstract: Tracer tests often give ambiguous interpretations that may be due to the erroneous location of sampling points and/or the lack of flow rate measurements through the sampler. To obtain more reliable tracer test results, we propose a methodology that optimizes the design and analysis of tracer tests in a cross borehole mode by using vertical borehole flow rate measurements. Experiments using this approach, herein defined as the Bh‐flow tracer test, have been performed by implementing three sequential steps: (1) single‐hole flowmeter test, (2) cross‐hole flowmeter test, and (3) tracer test. At the experimental site, core logging, pumping tests, and static water‐level measurements were previously carried out to determine stratigraphy, fracture characteristics, and bulk hydraulic conductivity. Single‐hole flowmeter testing makes it possible to detect the presence of vertical flows as well as inflow and outflow zones, whereas cross‐hole flowmeter testing detects the presence of connections along sets of flow conduits or discontinuities intercepted by boreholes. Finally, the specific pathways and rates of groundwater flow through selected flowpaths are determined by tracer testing. We conclude that the combined use of single and cross‐borehole flowmeter tests is fundamental to the formulation of the tracer test strategy and interpretation of the tracer test results.

Bomana, G.K., Molz, F.J., and Boonec, K.D., 1997, Borehole Flowmeter Application in Fluvial Sediments: Methodology, Results, and Assessment: Groundwater, v. 35, no. 3, p. 443-450, doi:10.1111/j.1745-6584.1997.tb00104.x.

Abstract: In many situations, inadequate design or performance of ground‐water remediation systems is the result of underestimation of aquifer hydraulic heterogeneity, and in particular, the vertical variation of hydraulic conductivity which plays an important role in contaminant migration. Described herein are applications of the electromagnetic (EM) borehole flowmeter to fluvial sediments in Louisiana and South Carolina. The direction of natural vertical flow in the test aquifers was defined easily, and short pumping tests enabled the calculation of hydraulic conductivity profiles for each test well. The results correlated well with other information obtained independently, including natural gamma logs, driller's logs and a hydraulic conductivity profile based on grain size analysis. Large variations in hydraulic conductivity over short vertical and horizontal distances were documented. Tests in gravel‐packed wells suggested that flowmeters produce misleading data for a variety of reasons in such situations. Among other things, an annulus of high permeability around a well screen allows flow to bypass the meter, and the phenomenon is amplified by high pumping rates. The resulting error is displayed as an erroneous high permeability zone at the top of the well screen. This observation deserves further study. In its present form the EM flowmeter is awkward to handle on a routine basis. However, none of the present design flaws preclude its effective use.

Crisman, S.A., Molz, F.J., Dunn, D.L., and Sappington, F.C., 2001, Application Procedures for the Electromagnetic Borehole Flowmeter in Shallow Unconfined Aquifers: Groundwater Monitoring and Remediation, v. 21, no. 4, p. 96-100, doi:10.1111/j.1745-6592.2001.tb00645.x.

Abstract: It is increasingly common for the electromagnetic borehole flowmeter (EBF) to he used to measure hydraulic conductivity (K) distributions in subsurface flow systems. Past applications involving the EBF have been made mostly in confined aquifers (Kabala 1994; Boman et al. 1997; Podgorney and Ritzi 1997; Ruud and Kabala 1997a, 1997b; Flach et al. 2000), and it has been common to set up a flow field around a test well using a small pump that is located near the top of the well screen (Mob, and Young 1993). In thin, unconfined aquifers that exhibit ground water tables near the ground surface and that undergo drawdown during pumping, such a configuration can be problematical because pumping and associated drawdown may effectively isolate the upper portion of the aquifer from the flowmeter. In these instances, a steady‐state flow field in the vicinity of the test well may be created using injection rather than pumping, allowing for testing in the otherwise isolated upper portion of the aquifer located near the initial water table position. Using procedures developed by Molz and Young (1993), which were modified for an injection mode application, testing was conducted to determine whether or not the injection mode would provide useful information in a shallow, unconfined aquifer that required the collection of data near the initial water table position. Results indicated that the injection mode for the EBF was well suited for this objective.

Genereux, D. and Guardiario Jr., J., 2001, A borehole flowmeter investigation of small‐scale hydraulic conductivity variation in the Biscayne Aquifer, Florida: Water Resources Research, v. 37, no. 5, p. 1511-1517, doi:10.1029/2001WR900023.

Abstract: Geostatistical analysis of closely spaced borehole flowmeter measurements was used to estimate the variance (2.53) and vertical and horizontal correlation lengths of ln K(0.57 and 7.3 m) in the Biscayne Aquifer, a limestone aquifer critical for Florida's water supply and for Everglades restoration efforts. The variance and correlation lengths of the Biscayne Aquifer are similar to some of the values for unconsolidated siliciclastic sediments (especially those at Columbus, Mississippi). The larger λh, for the Biscayne Aquifer (7.3 m) is thought to be due at least in part to the lower lateral variability of the carbonate platform depositional environment, compared to the fluvial environments in which the siliciclastic sediments were deposited. An improved down hole packer would allow for data with finer vertical resolution; the current system is adequate for work inside well screens but cannot adequately seal many spots in open, irregular rock boreholes. Research in rock presents additional logistical difficulties but is important for addressing fundamental questions about solute transport in a wider range of geological media, beyond unconsolidated siliciclastic deposits.

Lo, H.C., Chen, P.J., Chou, P.Y., and Hsu, S.M., 2014, The combined use of heat-pulse flowmeter logging and packer testing for transmissive fracture recognition: Journal of Applied Geophysics, v. 105, p. 248-258, doi:10.1016/j.jappgeo.2014.03.025.

Abstract: This paper presents an improved borehole prospecting methodology based on a combination of techniques in the hydrogeological characterization of fractured rock aquifers. The approach is demonstrated by on-site tests carried out in the Hoshe Experimental Forest site and the Tailuge National Park, Taiwan. Borehole televiewer logs are used to obtain fracture location and distribution along boreholes. The heat-pulse flow meter log is used to measure vertical velocity flow profiles which can be analyzed to estimate fracture transmissivity and to indicate hydraulic connectivity between fractures. Double-packer hydraulic tests are performed to determine the rock mass transmissivity. The computer program FLASH is used to analyze the data from the flowmeter logs. The FLASH program is confirmed as a useful tool which quantitatively predicts the fracture transmissivity in comparison to the hydraulic properties obtained from packer tests. The location of conductive fractures and their transmissivity is identified, after which the preferential flow paths through the fracture network are precisely delineated from a cross-borehole test. The results provide robust confirmation of the use of combined flowmeter and packer methods in the characterization of fractured-rock aquifers, particularly in reference to the investigation of groundwater resource and contaminant transport dynamics.

Molz, F.L. and Young, S.C., 1993, Development And Application Of Borehole Flowmeters For Environmental Assessment: The Log Analyst, v. 34, no. 1, 11 p.

Abstract: In order to understand the origin of contaminant plumes and infer their hture migration, one requires a knowledge of the hydraulic conductivity (0 distribution. In many aquifers, the borehole flowmeter offers the most direct technique available for developing a log of hydraulic conductivity in the horizontal direction. A new electromagnetic flowmeter developed by the Tennessee Valley Authority (TVA) is based on Faraday''s law and produces a voltage that is proportional to the velocity of the water passing through the central cylindrical channel of the meter. The threshold velocity of a prototype instrument is less than 8.8 + 0.9 cm/min. Calculation of a K distribution (granular aquifer) or flowpath distribution (fracture flow) based on flowmeter data is a straightforward process as described herein. Applications of both spinner and electromagnetic flowmeters to granular and fractured-rock aquifers are described. Results indicate the electromagnetic meter is capable of supplying a new level of detail concerning the K distribution in granular aquifers and flowpath delineation in f?actured-rock aquifers. At one site flowmeter data were used to bridge the gap between geologic field data and hydraulic conductivity. Geomorphic features such as ancient river meanders and buried channels were tentatively identified in the flowmeter data. At a second site, natural flow to or from individual fractures or narrow fracture zones was located and measured. Future work should be devoted to applying borehole flowmeters designed for groundwater and environmental applications to as many different hydrologic environments as possible so that the capabilities and limitations of the method can be more completely evaluated and the instrument transferred to commercial use. A patent application is pending.

Paillet, F.L., 2001, Hydraulic Head Applications of Flow Logs in the Study of Heterogeneous Aquifers: Groundwater, v. 39, no. 5, p. 667-6758, doi:10.1111/j.1745-6584.2001.tb02356.x.

Abstract: Permeability profiles derived from high‐resolution flow logs in heterogeneous aquifers provide a limited sample of the most permeable beds or fractures determining the hydraulic properties of those aquifers. This paper demonstrates that flow logs can also be used to infer the large‐scale properties of aquifers surrounding boreholes. The analysis is based on the interpretation of the hydraulic head values estimated from the flow log analysis. Pairs of quasi‐steady flow profiles obtained under ambient conditions and while either pumping or injecting are used to estimate the hydraulic head in each water‐producing zone. Although the analysis yields localized estimates of transmissivity for a few water‐producing zones, the hydraulic head estimates apply to the far‐field aquifers to which these zones are connected. The hydraulic head data are combined with information from other sources to identify the large‐scale structure of heterogeneous aquifers. More complicated cross‐borehole flow experiments are used to characterize the pattern of connection between large‐scale aquifer units inferred from the hydraulic head estimates. The interpretation of hydraulic heads in situ under steady and transient conditions is illustrated by several case studies, including an example with heterogeneous permeable beds in an unconsolidated aquifer, and four examples with heterogeneous distributions of bedding planes and/or fractures in bedrock aquifers.

Paillet, F.L., Crowder, R., and Hess, A., 1996, High‐Resolution Flowmeter Logging Applications with the Heat‐Pulse Flowmeter: Journal of Environmental and Engineering Geophysics, v. 1, no. 1, p. 1-84, doi:10.4133/JEEG1.1.1.

Abstract: A number of recently‐developed high‐resolution flowmeter logging techniques are described in the well logging literature. These techniques are now or soon will be capable of measuring flows corresponding to less than 0.01 gallons per minute of borehole discharge. Such measurements have the potential to significantly improve the interpretation of geophysical logs in groundwater studies by showing how geological structure indicated on logs and tomographs is related to hydraulic properties of beds, solution openings and fractures. The potential benefits of such borehole measurements are illustrated by the results obtained with the U.S. Geological Survey heat‐pulse flowmeter. One useful application of high‐resolution borehole flow measurements is associated with the disturbance to aquifer hydraulics induced by the presence of an open borehole. This effect can be modeled and analyzed to provide a controlled test of aquifer response to disturbance. Flow measurements can also indicate the character of groundwater flow in the vicinity of the well bore during short, low‐capacity aquifer tests. A number of researchers have shown how such flowmeter profiles can be analyzed to give rigorous profiles of permeability and specific storage in‐situ. At the same time, monitoring of flow transients in cross‐borehole pumping tests may be formulated as a hydraulic tomography study. One useful application of flow logging is the quick and definitive assessment of flow connections between and around boreholes within hours after the completion of drilling. Such measurements could greatly reduce the time and effort involved in the otherwise expensive and labor intensive procedures of conventional hydraulic testing and tracer studies.

References