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Vertical Flowmeter Logging

Introduction

Vertical flowmeter logging measures vertical movement of fluid in a borehole. Flowmeter data can provide critical information for the design and interpretation of hydraulic testing and chemical sampling in groundwater investigations, helping to refine site conceptual models. The results of flowmeter logging can also be used to design well completions used to prevent cross-contamination and to monitor head and (or) water chemistry.

Vertical flowmeter logging can be conducted in boreholes in fractured-rock aquifers and in wells in unconsolidated sand and gravel aquifers. For more information on flowmeter logging applications in unconsolidated overburden, please see Molz and others (1989) or Molz and Melville (1996).

USGS scientists can view more in-depth flowmeter logging information and related resources.

What causes vertical fluid flow in a borehole?

Differences in hydraulic head between two transmissive geologic units or fractures produce vertical fluid flow in a borehole. Water enters the borehole at the unit with the higher head and flows toward and out of the unit with the lower head. The vertical flow rate is limited by the geologic unit or fracture with the lower transmissivity. If the heads of the different transmissive zones are the same, no vertical flow will occur in the well or borehole.

Applications:

Single-hole flowmeter logging can be used to:

• measure the rate of vertical flow;
• identify the direction of vertical flow;
• establish relative hydraulic gradients; and
• identify transmissive fractures or geologic units open to boreholes that are the source(s) of water or that could act as conduits for cross-flow in an open hole.

In cross-hole flowmeter profiling, a flowmeter is placed in one borehole while fluid is pumped out of or into another borehole at a constant rate. Cross-hole flowmeter profiling can be used to:

• identify cross-hole connections and
• provide data that can be used to solve for
• transmissivity,
• head, and (or)
• the storage coefficient of the geologic unit or fractures between the boreholes.

Flowmeter Logging in Fractured-Rock Aquifers

In fractured-rock investigations it is important to conduct flowmeter logging under ambient and stressed conditions. The composite head in an open hole is the sum of the transmissivity-weighted heads of the individual fracture zones and is dominated by the head of the most transmissive zone. Flowmeter logging conducted under pumping conditions can be used to identify transmissive fracture zones with similar ambient heads that would not be identified without stressing the aquifer.

Vertical Flowmeter Tools

The three main types of vertical flowmeters are characterized by the method used to measure flow and the range of flow that they can measure. These three types of flowmeters are:

• heat-pulse (fig. 2a),
• electromagnetic (fig. 2b), and
• spinner (fig. 2c).

(a)	(b)	(c)
Figure 2. Photos of three main types of flowmeters. (a) Heat-pulse flowmeter tool heat grid and sensor area fitted with diverter. (b) Electromagnetic flowmeter tool sensor area. (c) Spinner flowmeter cage and sensor area, with impellar blades displayed next to tool (spinner photo from Colog, 1990).

Heat-pulse flowmeter:

Heat-pulse flowmeter (HPFM) measurements are made with the tool at a stationary position in the borehole. A heat grid in the tool is activated to heat a packet of water in the borehole. If there is flow in the borehole, the heated packet of water will move with the flow toward the upper or lower sensor in the tool. The difference in temperature between the sensors is monitored. The equipment measures the time from when the heat grid was first activated to the moment when the greatest temperature change is detected by one of the sensors. This information is then used to calculate a rate of flow and direction of flow at that particular location and time. An HPFM with a fully fitted diverter can usually measure flow of 0.01 to 1.5 gallons per minute.

Electromagnetic flowmeter:

The design of the electromagnetic flowmeter (EMFM) is based on Faraday's Law of Induction: voltage induced by a conductor moving at right angles through a magnetic field is directly proportional to the velocity of the moving conductor. In an EMFM, an electromagnet is used to generate a magnetic field in a hollow cylinder within the tool. The flow of water (a conductor) through this magnetic field at a right angle (90 degrees) to the field induces voltage that is measured by electrodes within the tool. This voltage is then used to calculate the velocity of the water through a fixed-diameter chamber, which is used to calculate a volumetric flow rate.

EMFM measurements can be made while trolling (moving) in order to generate a continuous flow profile over a depth range or while stationary in order to measure flow at a specific location in the borehole. Under trolling conditions the movement of a flowmeter tool induces a measured flow in the direction opposite the direction of trolling. Measurements of ambient or stressed flow within the borehole are superimposed on the induced flow rates. An EMFM with a fully fitted diverter can generally measure flow from 0.05 to 10 gallons per minute.

Spinner flowmeter:

The impeller or spinner flowmeter (spinner) uses an impeller that revolves in response to fluid flow. As fluid moves through the impeller blades, rotating them, the number of impeller revolutions per second is automatically recorded and used to calculate the velocity of the fluid. Spinner measurements can be made while trolling in order to generate a continuous flow profile over a certain depth or while stationary in order to measure flow at a specific location in the borehole. Spinners can make measurements over a wide range of flow rates, although the tool often has poor resolution at very low flow rates. The minimum velocity that can be measured by a typical spinner flowmeter is about 5 feet per minute, limiting its use to higher flow conditions.

Analysis of Flowmeter Logs

Vertical flowmeter measurements can be analyzed to provide qualitative and quantitative aquifer characteristics. Qualitative measurement can (and should) be made in the field, with inflow and outflow are attributed to individual geologic units or fractures in the borehole. Methods commonly used for quantitative hydraulic analysis of flow zones from flowmeter log data include proportion, analytical solution, and numerical modeling techniques. Proportion and analytical solution methods provide estimates of transmissivity for the flow zones, while numerical model methods provide estimates of transmissivity and hydraulic head for the flow zones.

References

For more information about vertical flowmeter logging theory, see:

Molz, F. J., Morin, R. H., Hess, A. E., Melville, J. G., and Guven, O., 1989, The impeller meter for measuring aquifer permeability variations - Evaluations and comparisons with other tests: Water Resources Research, v. 25, p.1677-1683.

Molz, F. J. and Melville, J. G., 1996, Discussion of combined use of flowmeter and time-drawdown data to estimate hydraulic conductivities in layered aquifer systems: Ground Water, v. 34, no. 5, p. 770.

Paillet, F.L., Hess, A.E., Cheng, C.H., and Hardin, E., 1987, Characterization of fracture permeability with high-resolution vertical flow measurements during borehole pumping: Ground Water, v. 25, no. 1, p. 28-40.

Paillet, F.L., 1998, Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations: Water Resources Research, v. 34, no. 5, p. 997-1010.

Paillet, F.L., 2000, A field technique for estimating aquifer parameters using flow log data: Ground Water, v. 38, no. 4, p. 510-521.

Paillet, F.L., and Reese, R.S., 2000, Integrating borehole logs and aquifer tests in aquifer characterization: Ground Water, v. 38, no. 5, p. 713-725.

Paillet, F.L., 2001, Hydraulic head applications of flow logs in the study of heterogeneous aquifers: Ground Water, v. 39, no. 5, p. 667-675.

USGS WRD National Research Program Borehole Geophysics Group's online bibliography of flowmeter logging references.

For more information about the results and applications of vertical flowmeter logging, see:

Colog, 1990, Hydraulic conductivity from a borehole flowmeter: Colog, Inc., Golden, Coloradao, Technical Notes, vol.1, no.3, p. 1-2.

Day-Lewis, F.D., Johnson, C.D., Paillet, F.L., Halford,K.J., 2011, A Computer Program for Flow-Log Analysis of Single Holes (FLASH): Ground Water, DOI: 10.1111/j.1745-6584.2011.00798.x

Johnson, C.D., Haeni, F.P., Lane, J.W., Jr., and White, E.A., 2002, Borehole-geophysical investigation of the University of Connecticut landfill, Storrs, Connecticut: U.S. Geological Survey, Water-Resources Investigations Report 01-4033, 187 p.

Lane, J.W. Jr., Williams, J.H., Johnson, C.D., Savino, Sr. D.-M., and Haeni, F.P., 2002, An integrated geophysical and hydraulic investigation to characterize a fractured-rock aquifer, Norwalk, Connecticut: U.S. Geological Survey Water-Resources Investigation Report 01-4133, 97 p.

Morin, R. H., Hess, A. E., and Paillet, F. L., 1988, Determining the distribution of hydraulic conductivity in a fractured limestone by simultaneous injection and geophysical logging: Ground Water, v. 26, p. 587-595.

Paillet, F.L., 1999, Geophysical reconnaissance in bedrock boreholes--finding and characterizing the hydraulically active fractures, in Morganwalp, D.W. and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, v. 3, p. 725-733.

Paillet, F.L., 2000, Flow logging in difficult boreholes - making the best of a bad deal, in 7th International Symposium on Borehole Geophysics for Minerals, Geotechnical, and Groundwater Applications, , Denver, Colo., 2000, Proceedings: Houston, Tex., The Minerals and Geotechnical Logging Society, A Chapter at Large of the Society of Professional Well Log Analysts, p. 125-135.

Williams, J. H., and Paillet, F.L., 2002a, Crosshole-flowmeter method for the characterization of hydraulic connections in fractured rock, in Fractured Rock 2002, Denver,Colo., March 13-15, 2002, Proceedings [abs.]: Westerville, Ohio, National Ground Water Association.

Williams, J. H., and Paillet, F. L., 2002b, Using flowmeter pulse tests to define hydraulic connections in the subsurface - A fractured shale example: Journal of Hydrology, v. 265, p. 100-117.

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