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Simulations to Verify Horizontal Flow Measurementsfrom a Borehole Flowmeterby Scott C. James1, Richard A. Jepsen2, Richard L. Beauheim3, William H. Pedler 4, and Wayne A. Mandell5

AbstractThis paper reports on experiments and simulations of subsurface flow from a slotted acrylic tube deployed

in a sand-tank flow chamber for two different purposes. In the first instance, the slotted tube is used to representa single fracture intersected by an uncased well. In the second instance, the slotted tube is used to representa multislot well screen within a porous medium. In both cases, the scanning colloidal borescope flowmeter(SCBFM) measures ground water velocity within the well by imaging colloids traveling through a well to mea-sure their speed and direction. Measurements are compared against model simulations. For the case of a slottedtube representing a single fracture, SCBFM and model results agree with respect to the flow direction and towithin a factor of 1.5 for the speed near the well’s center. Model and experimental agreement lend confidencethat for an uncased well drilled in a fractured-rock medium, a calibrated SCBFM could be used to identify andquantify flowing features. Next, the SCBFM was deployed in a four-column multislotted casing with slotsaligned with the flow direction. Another numerical model was developed to estimate the flow field within thiswell screen to evaluate the potential usefulness of employing the SCBFM in a screened well to estimate flowspeed and direction in the surrounding porous medium. Results indicate that if the slots are not aligned with theflow, the SCBFM may only provide order-of-magnitude speed measurements and direction measurements withan uncertainty of approximately ±25�.

IntroductionHydraulic properties of aquifers traditionally have

been estimated from laboratory measurements of coresamples or from field estimates such as slug tests orpumping tests for composite sections of the aquifer.Although these approaches provide data that are valuable

for site assessment, they may undesirably average zonesof preferential flow that are chief conduits of groundwater contaminants. Similarly, horizontal velocity has tra-ditionally been estimated on the basis of Darcy’s equation(using measured gradients and hydraulic conductivity) orfrom transport of tracers under natural or forced-gradientconditions. Borehole tools capable of directly measuringhorizontal ground water flow and direction in narrowintervals of the aquifer may provide vital information tohydrogeologists. That is, once calibrated in the laboratory,they can be effectively deployed in the field. Boreholeflowmeter data indicate where ground water is enteringand exiting the borehole and can assist in estimation ofsubsurface transport. Cross-borehole flow-logging testscan indicate the degree of connectivity of fracturesbeyond the wellbore, and transient tests can be used toestimate hydraulic properties (for example, transmissivityand storativity) of fractured media, but these data are spa-tially averaged and not locally detailed.

Drost et al. (1968) developed and tested a scintillation-counter probe able to fit down a borehole that determinedflow direction by tracing radioisotopic elements injected

1Corresponding author: Sandia National Laboratories, Geo-hydrology Department, P.O. Box 5800, Albuquerque, NM 87185-0735; [email protected]

2Sandia National Laboratories, Mechanical Environments,P.O. Box 5800, Albuquerque, NM 87185-1135; [email protected]

3Sandia National Laboratories, Repository PerformanceDepartment, 4100 National Parks Highway, Carlsbad, NM 88220;[email protected]

4RAS Inc., 311 Rock Avenue, Golden, CO 80401; [email protected]

5U.S. Army Environment Center, SFIM-AEC-ERA, 5179Hoadley Road, APG-EA, Aberdeen, MD 21010; [email protected]

Received March 2005, accepted July 2005.Journal compilationª 2006 National Ground Water Association.No claim to original US government works.doi: 10.1111/j.1745-6584.2005.00140.x

394 Vol. 44, No. 3—GROUND WATER—May–June 2006 (pages 394–405)

into the borehole. More recently, interest has focused ondirectly measuring ground water speed and directionusing downhole instrumentation that requires no tracer(e.g., Cronk and Kearl 1990; Kerfoot et al. 1991; Kearlet al. 1992, 1999; Kearl 1997; Ferriz and Pedler 1999;Kearl and Roemer 1998; Wilson et al. 2001; Beauheim2000; Korte et al. 2000; Pedler and Jepsen 2003). Exam-ples of these types of instruments include the horizontalheat-pulse flowmeter (KVA flowmeter) (Kerfoot 1982;Kearl et al. 1999; Melville et al. 1985), the acousticDoppler velocimeter (Kraus et al. 1994), the laser Dopp-ler velocimeter (Momii et al. 1993), the fixed-pointcolloidal borescope (Kearl and Case 1992), and hydro-physical logging (Anderson et al. 1993), which directlymeasure the horizontal component of velocity insidea wellbore.

Applications of flowmeters to field assessments andcontrolled laboratory evaluations have led to a betterunderstanding of the distribution of ground water flow ina borehole and the operational mechanics of various flow-meters. Nevertheless, the applicability of these instrumentsis uncertain when they are used in existing wells with stan-dard slotted well casings. Borehole and well casing effectscan negatively impact the measured flow speed and direc-tion (Kerfoot 1988). For example, Dinwiddie et al. (1999)and Halford (2000) show that the presence of the flowme-ter itself can significantly influence calculations of aquifercharacteristics because of flow redistribution, especially ingravel-packed wells. Boman et al. (1997) provide an excel-lent overview of borehole flowmeter applications for aqui-fer characterization.

Sandia National Laboratories, RAS Inc., and theU.S. Army Environment Center developed a sand-tanklaminar flow test chamber to facilitate testing and calibra-tion of the scanning colloidal borescope. In this work,a computational fluid dynamics code is used to qualita-tively assess the likelihood that correct estimates of flowspeed and direction in the surrounding medium can be in-ferred from speed and direction measurements madewithin a well screen using a colloidal borescope. First,the results from a numerical and experimental investiga-tion for flow through a single slot in a well (approximat-ing a fracture) are presented. Then, the borescope isdeployed in a standard, slotted, fiberglass well screen anddata are compared to a second model developed for thisapplication. Further, the model is extended to examinedifferent orientations of the well-screen slots with respectto the flow direction.

The Colloidal BorescopeThe first colloidal borescope was developed at Oak

Ridge National Laboratories (ORNL). Kearl and Case(1992), Kearl (1997), and Kearl and Roemer (1998) havedescribed the tool, its applications, and associated dataanalysis. The ORNL borescope had a fixed focal plane,requiring that the tool be repositioned whenever ob-servations at different depths were desired. An improvedcolloidal borescope was developed at Lawrence Liver-more National Laboratory (LLNL) (Ferry et al. 1995;

Wood et al. 1997). The LLNL scanning colloidal bore-scope flowmeter (SCBFM) has a variable focal lengthwith a 0.5-m range. The SCBFM is used to evaluate hori-zontal ground water speed and direction in a well. It com-prises a charged-couple device (CCD) video camera,magnetometer, light source, and a remotely controlled, var-iable focal point lens mechanism to track colloidal-sizedparticles (1 to 5 lm) passing through a 1.6 3 2.2 mm2

field of view. Figure 1 is a schematic of the SCBFM usedin this study.

By recording the output of the CCD video cameraand using advanced particle-tracking software, the speedand compass direction of colloidal particles advected byhorizontal ground water flow within a well can be mea-sured. The scanning feature allows a 0.5-m interval to beinvestigated without relocating the instrument, while facil-itating a three-dimensional evaluation of the flow fieldwithin the well that can detect and characterize fast path-ways, swirling, and secondary flow cells. The upper limiton speeds measurable by the SCBFM is defined by thewidth of the field of view (1.6 3 2.2 mm2) and the timeinterval between consecutive video frames (1/30 s). Ifa particle is traveling so fast that it appears in only a sin-gle video frame, its speed cannot be measured (the

Figure 1. Schematic of the SCBFM.

S.C. James et al. GROUND WATER 44, no. 3: 394–405 395

theoretical upper limit is ~48 mm/s). Typically, a particlemoves less than half the distance of the field betweenframe grabs, and the best results are for particles thatmove less than a quarter of the field (or 0.4 mm). It is notuncommon for a particle to disappear before reaching theedge of the field, indicating that it has some verticalspeed. Thus, if a particle appears in only two grabbedframes and only moves 0.4 mm horizontally, then its ver-tical speed must have been no greater than ~25% of itshorizontal speed, and no less than 12.5%. The absolutelimit on vertical speed (assuming that a particle is countedon successive frames) is 0.1 mm/(1/30) s, or 3 mm/s. Inpractice, the vertical speed should be less than half thehorizontal speed to allow a particle to cross an eighth ofthe horizontal field while passing vertically through thefocal plane.

Experimental Set-Up

The Test ChamberBased on the work of Drost et al. (1968), a laboratory-

scale horizontal flow test chamber was deployed atthe Sandia Soil and Sediment Transport Laboratory inCarlsbad, NewMexico. This test chamber establishes a sta-ble, horizontal flow in a porous medium with an observ-able gradient and pore velocities ranging from 8.8 3 1025

to 8.83 1024 m/s. The dimensions of the test chamber are0.9 m (3.0 feet) wide, 1.2 m (4.0 feet) deep, and 2.1 m(7.0 feet) long. Two 15-cm-long (6 inch long) reservoirsare located at each end, yielding an overall length of 2.4 m(8.0 feet). A pump connecting the two reservoirs recir-culates water through the test chamber.

Once the test chamber was filled with sand to a depthof 1.2 m, the hydraulic conductivity of the porous mediumwas measured. The well-rounded, well-sorted, coarse sand(8/16) has a measured bulk porosity of 0.345. Hydraulicconductivity was calculated by pumping 5.7 3 1024 m3/s(9.0 gal/min) through the tank, yielding a 0.074-m headdifference between the reservoirs. This is still within thelaminar flow regime. Darcy’s law states that

K ¼ QL

�y�z�hð1Þ

where K is the hydraulic conductivity (m/s), Q is the dis-charge (m3/s), �y is the width of the flow simulator (m),�z is the saturated depth (m), and �h is the head differ-ence (m). L is the length of the test chamber (cm).

The test chamber is 0.9 m wide with an average satu-rated depth of 0.9 m. Therefore, for L ¼ 2.1 m and �h ¼0.074 m, the hydraulic conductivity, K, is 1.96 3 1022 m/s.Hydraulic conductivity was calculated for a secondpumping rate of 2.5 3 1024 m3/s (4.0 gal/min), alsoresulting in K ¼ 1.96 3 1022 m/s. The correspondingpermeability is 2.0 3 1029 m2.

To simulate a simple horizontal fracture, two slotswere cut in a 10.2-cm (4 inch)-O.D. acrylic tube, whichwas placed in the center of the test chamber as it wasfilled with sand. These diametrically opposed slots were

1.5 mm wide with 90� cutouts. The openings were ori-ented perpendicularly to the long axis (flowing direction)of the chamber. This was designed to simulate a simplehorizontal fracture with a partial (50%) opening as shownin Figure 2.

Three recirculation rates were employed duringhorizontal flow simulations, and the corresponding porevelocities were used in the numerical modeling.

Single-Slot Well Casing

Numerical ModelThe numerical models presented here were devel-

oped with CFD2000 (Adaptive Research 2002a). Thisgeneral-purpose computational fluid dynamics softwareoffers an integrated environment to build the modelgeometry, generate the computational mesh, specifyboundary conditions and fluid properties, stipulate thesolution method, and visualize the solution. CFD2000solves the three-dimensional Reynolds-averaged Navier-Stokes equations, which provide for conservation ofmass, momentum, energy, reacting species, and turbu-lence if necessary (Adaptive Research 2002b). The modelprovides procedures to optimize mesh spacing andorthogonality.

The first model is of the single-slot, 10.2-cm-diameterwell in porous media that is intended to simulate flow inan unscreened well through a single, water-conducting

Figure 2. Schematic of the simulated fracture.

396 S.C. James et al. GROUND WATER 44, no. 3: 394–405

fracture. This model is not perfectly representative offlow through a single fracture because flow through theporous medium will be focused on the single slot fromall three dimensions, rather than just within the plane ofthe fracture. The model does, however, accurately simu-late flow conditions in the laboratory test chamber,thereby facilitating calibration of the SCBFM. Fractureinlet velocities were estimated with a preliminary, coarselymeshed model using Cartesian coordinates with a total of24, 19, and 18 cells in the x-, y-, and z-directions, respec-tively, yielding 8208 cells that model a 0.4-3 0.4-3 0.2-m3

domain within the test chamber. Boundary conditionsinclude frictionless walls (that effectively model the muchlarger test chamber by removing model-domain no-slipboundary layer effects) on all sides except for the inlet,which has specified pore velocity, and outlet, which al-lows free flow dictated by conservation of mass. Outsidethe well screen is a porous medium with a specified per-meability of 2 3 1029 m2. The well casing is defined by(no-slip) walls everywhere except at the single slot. Slotinlet velocities are recorded for each modeledpore velocity (velocity specified at the inlet slots shownin Figure 2) and used in the more refined model of thesingle-slot well containing the borescope.

A more detailed model of the interior of the wellscreen using a cylindrical coordinate system with a finelygridded mesh is shown in Figure 3. Boundary conditionsfor the cylindrical model include no-slip walls at the wellcasing, constant inlet velocity at the slot as specifiedby output from the preliminary model, and a free-flow,mass-conserving outlet slot. This ‘‘model within a model’’

decreases computational time because the refined gridnecessary to model the borescope geometry accuratelyneed not be carried throughout the entire flow simulatordomain. The well slot (fracture) was centered in the testchamber, with the focal point of the borescope set atthe middle of its 0.5-m range as shown in Figure 3. Atotal of 89, 14, and 36 cells were used in the z-, r-, andh-directions, respectively, yielding a model with 44,856cells. The grid was refined in the vicinity of the fractureto increase model precision. Although the model wasdeveloped in cylindrical coordinates, the postprocessingtools for CFD2000 provide results in a Cartesian coordi-nate system. Nevertheless, because of the geometry of thetest chamber, results presented in the Cartesian coordinatesystem make intuitive sense and can easily be comparedto experimental data.

Single-Slot Experimental and Model ResultsSeveral simulations were run at various inlet veloci-

ties with and without the borescope inside the single-slotwell casing. Figure 4 is an example of the model resultsusing the single-slot acrylic tubing. It shows a contourplot of the x-direction velocities (u velocities) along ahorizontal plane, coplanar with the center of the slot(fracture). The fracture inlet is indicated in blue and theoutlet in red. The inlet and outlet slots were simulatedwith a single cell in the z-direction, and those cells haveboth a flow condition and a wall condition associatedwith them. The postprocessing software gives precedenceto the wall condition, and the red rim inside the well

Figure 3. An isometric view of the model geometry and grid cells for flow in the well interior. The SCBFM is included in themodel by inactivating (blocking) the model cells that it physically occupies. The plan view is shown in the lower right corner.


that crosses the inlet and outlet boundaries is a post-processing artifact and thus displays a zero velocity forcells adjacent to the wall. Although significant dissipationof the u velocity is seen as the flow proceeds toward thecenter of the well, this does not imply a violation of con-servation of mass.

The lower left inset in Figure 4 is a vertical crosssection of the velocity (vector) field that reveals circula-tion cells that appear both above and below the fracturehorizon and slightly enhance the velocity in the fractureplane because both cells converge in that region. Towardthe center of the well, flow is principally a narrow jetwhere the u velocity is progressively reduced in this jetbecause the flow begins to turn toward the vertical,thereby dissipating momentum in the horizontal direction.Furthermore, comparatively weak circulation cells are

observed in the horizontal plane of the inlet on eitherside of the jet as shown in the lower right inset ofFigure 4. Flow is focused (relative to the inlet velocity)just inside the central portion of the inlet by these circula-tion cells converging on this region. As shown in Fig-ure 5, in the vertical direction, circulation cells areobserved extending up to 15 cm above and below theplane of the slot.

Model results indicate that the three support rodsthat connect the light source to the body of the bore-scope do not significantly interfere with the flow fieldnear the center of the casing and, therefore, do notimpact borescope measurements. Model runs were con-tinued until steady state was achieved, typically at ~20 sof simulated time (~4 CPU hours on a Pentium 4 1.6-GHz PC).

Figure 4. The color scale is linear for velocity in the x-direction in m/s. The fracture inlet is on the left (indicated in blue) andflow is from left to right (negative x-direction) with the outlet indicated in red. The three support rods are shown. The inset inthe lower left corner is a vertical cross section of the velocity field and that in the lower right corner is the horizontal cross sec-tion. Here, the slot inlet velocity is 1.653 1022 m/s.


In addition to measuring flow at the plane of thefracture, the scanning capability (i.e., focusing at differ-ent depths in the borehole) of the SCBFM was used toinvestigate the flow field above and below the fracture ata flow rate yielding a pore velocity of 5.47 3 1024 m/s.Virtually no horizontal flow was measured by theSCBFM ~1 cm above and below the fracture horizon nearthe center of the well. Although there may have beena horizontal component to the flow in this region asshown by the model results, the vertical component waslarge enough to force colloids through the vertical view-ing range of the SCBFM so quickly that a horizontal

velocity was not detected. However, as shown in Fig-ure 5, at distances between 10 and 15 cm above andbelow the fracture horizon, both the SCBFM and themodel observed low-speed flow of a lesser magnitude ina direction opposite to that at the fracture. This delineatesthe outer limit of a circulation cell within the well causedby the relatively high flow velocity entering the well atthe fracture inlet. Note that the top and bottom of the fig-ure do not correspond to model-domain boundaries andthat flow circulates beyond the range shown in Figure 5.Also, while model simulations ultimately yield a steadyflow field, both laboratory and field measurement always

Figure 5. The x-z planar view is tilted 15� in the y-direction with 1.0 3 1022 m/s slot inlet velocity. Color scale is for velocity inthe x-direction and flow is from right to left (negative x-direction in this figure).


show some level of transients (chaotic flows). Thesecharacteristics are inherent to the systems under study, andadditional investigation is beyond the scope of this work.

As expected, the SCBFM showed that the flow direc-tion through the well was consistent with the flow direc-tion through the test chamber as a whole. The velocitiesmeasured by the SCBFM near the center of the well andthe modeled velocities near the center of the well arecompared in Table 1. The model generally predictedvelocities 67% to 82% of those measured by the SCBFM.This suggests that the SCBFM provides a reasonablyaccurate measurement of the flow in a well near an inter-section with a single, water-bearing fracture.

Table 1 also shows that flow is focused through thefracture inlet because the inlet velocity calculated by themodel is over an order of magnitude greater than the porevelocity in the test chamber. (Note that because of thethree-dimensional flow focusing discussed previously,comparing velocities in the well to pore velocities in the

test chamber may not provide information relevant toflow in a single fracture.) By the time the flow reachesthe center of the well, however, it has slowed significantlyfrom the inlet velocity. Unfortunately, neither the velocitymeasured by the SCBFM nor the velocity predicted bythe model near the center of the well appears to vary line-arly from the inlet velocity (or the pore velocity). Conse-quently, fluid velocities in the fracture some distancefrom the well may prove difficult to quantify based onlyon SCBFM measurements in the well.

Overall, for a single-slot acrylic tube aligned withthe flow direction, the SCBFM provides accurate meas-urements of flow speed and direction in the well. Know-ing how that flow velocity relates to the speed ina fracture, however, is problematic. Additional calibrationand modeling may yield estimates of fracture flow ratesthrough inverse modeling. The SCBFM may be bettersuited to indicate differences in flow velocities in differ-ent fractures than to determine absolute velocities.

Table 1Comparison of Model and Experimental Results for Inlet Flow Speeds and Flow Speeds

Near the Center of the Well for the Single-Slot Case

Pore Velocity inTest Chamber (m/s)

Modeled FractureInlet Speed (m/s)

Modeled Speed atCenter of Well (m/s)

Measured Speed atCenter of Well (m/s)

8.75 3 1025 2.1 3 1023 1.03 1024 1.5 3 1024

5.47 3 1024 1.0 3 1022 3.13 1023 3.8 3 1023

1.97 3 1023 1.7 3 1022 5.23 1023 Not measured1

1Rate exceeded the capability of SCBFM software.

12.58 cm (4.954")10 Slots:Aperture = 0.005 cm (0.002")Width = 6.35 cm (2.5")Vertical spacing = 0.8 cm (0.3125")Rib width = 3.5 cm (1.378")

Solid pipe to bottom10 cm (3.9")

Figure 6. Schematic of the multislot well casing used in this work.


Multislot Well Casing

Numerical ModelUnlike the single-slot modeling that used the pre-

liminary model shown in Figure 3 to define the inletvelocities for a second, more detailed model of the well,multislot modeling was performed using a refined versionof the preliminary grid. The multislot well screen modelgeometry is of a 12.58-cm-O.D. fiberglass tube with fourcolumns of 58� slots as shown in Figure 6. Although awell screen can have several hundred slots along its length,computational limitations allowed only 10 rows of slots tobe modeled. The overall model geometry is similar to thatshown in Figure 3, with the well casing placed withina 0.5- 3 0.5- 3 0.181-m3 porous medium with a perme-ability of 2 3 1029 m2. Frictionless walls are used on allsides of the model except at the specified velocity inletand free-flow outlet to approximate an infinite mediumsurrounding the casing. Moreover, the frictionless upperboundary condition at z ¼ 0.181 m effectively createsa plane of symmetry about the 10 rows of slots (only0.4 cm of blank casing were modeled between the top slotand the boundary as opposed to 0.8 cm between consecu-tive slots), yielding a model equivalent to one that esti-mates flow inside a slotted well screen with 20 rows ofslots. Because large aspect ratio cells are numericallyunstable, 24, 24, and 104 cells were required in the x-,y-, and z-directions, respectively, yielding 59,904 cells.Grid refinement is included inside the well screen toimprove model accuracy. Uniform pore velocity boundaryconditions of 8.52 3 1025 and 9.92 3 1024 m/s wereapplied in the x-direction at the inlet (refer to Figure 3),corresponding to 2.52 3 1025 and 2.52 3 1024 m3/spumping rates in the flow chamber, respectively. Theorientation of the casing is described more usefully byreferring to the directions as east, north, and high instead

of x, y, and z, respectively. The model was run with: (1)the slots oriented with the flow direction (east); (2) theribs oriented with the flow direction; and (3) an interme-diate case. Model simulation was continued until steadystate was achieved, typically at ~20 s of simulated time(~6 CPU hours on a Pentium 4 1.6-GHz PC). The mod-eled flow fields inside the well screen are described sub-sequently.

Multislot Experimental and Model Results

Flow Field When Slots Align with the Flow Direction

Figure 7 shows model results in a centered verticalcross section parallel to the imposed flow direction in themultislot well. The u velocity contours reveal ‘‘hot spots’’at the lowest two slots. These hot spots represent end ef-fects due to flow focusing where fluid in the porousmedium below the slots has an upward velocity compo-nent as it approaches the well casing (path of least resis-tance), thereby increasing the flow into these slots anddirecting it slightly upward. This flow focusing is consis-tent with what was observed in the single-slot model aswell. In addition, the superimposed velocity field showsa small recirculation eddy below the lowest slot thatserves to enhance the u velocity near the bottom slot andalso direct it slightly upward. These results suggest thatthe SCBFM should focus on points in the well casing atleast eight slots from the ends of the screened intervalwhere the magnitude of the z-component of the velocity is<10% of the x-component.

A vector plot of the flow field inside the well screenat a horizontal cross section corresponding with theeighth slot (z ¼ 0.15975 m) when the slots are orientedwith the flow is shown in Figure 8 for a pore velocity of9.92 3 1024 m/s. The color of the arrows represents the

Figure 7. Contour plot of the u velocity (x-direction) along a vertical cross section at y = 0. Velocity vectors are overlain andindicate that by the eighth slot from the bottom, nearly uniform flow in the x-direction is established within the casing.


magnitude of the velocity in the x-direction (u velocity).For this orientation, the model results indicate maximumcenterline velocities on the order of the pore velocity inthe surrounding medium. Note the circulation cells at thenorth and south slots where flow both enters and exits thecasing.

A limited number of experiments was performedwith the SCBFM in a multislot well screen with the slotsaligned with the direction of flow. One measurement wasmade with a flow rate of 2.5 3 1025 m3/s (pore velocityof 8.5 3 1025 m/s), and six measurements (at differentdepths) were made with a 2.5 3 1024 m3/s flow rate(pore velocity of 9.9 3 1024 m/s). These results are com-pared to model results for the same pore velocities inTable 2.

These results indicate that an SCBFM can be used tomeasure the speed and direction of ground water flowthrough a screened well if it is positioned a sufficient dis-tance from well screen end effects and if the slots are

aligned normal to the principal flow direction. Althoughthe results in Table 2 show good agreement between thepore velocity in the test chamber and the velocitiesmodeled and measured in the well, quantifying therelationship between the velocity observed by anSCBFM and the pore velocity in the surrounding forma-tion may be problematic. The amount of flow focusingthrough individual slots would be a function of boreholesize, ratio of formation permeability to sand/gravel packpermeability, and the particular screen geometry (slotwidth and aperture). Careful calibration using a labo-ratory flow chamber designed as closely to the fieldconfiguration as possible would be necessary to buildconfidence that SCBFM results can be extrapolated toactual pore velocities.

Flow Field When Slots Do Not Align with theFlow Direction

No experimental results were obtained with theSCBFM when the well screen ribs were aligned withthe flow direction because we were unable to maintainthe flow of colloids through the test chamber. The num-ber of visible colloids decreased significantly during theexperiments with the slots aligned with flow, and no col-loids were visible by the time the experiments with theribs aligned with flow were attempted. We believe thatthe sand in the test chamber progressively filtered col-loids out of the water.

When the ribs are aligned with the flow direction,significantly different modeled flow conditions are estab-lished inside the well casing. Modeled flow speeds nearthe center of the well are about one-third of those foundin the pores of the surrounding medium with u velocities

Figure 8. Vector plot of the flow field in a horizontal z–cross section at the eighth slot from the bottom for a pore velocity of9.92 3 1024 m/s when the slots are aligned with the flow direction. The arrows indicate flow direction and the color indicatesthe magnitude of the x-component of the velocity (u velocity). Note that near the center of the well and away from end effects,the u velocities are fairly uniform at ~9.6 3 10–4 m/s.

Table 2Comparison of Model and Experimental Resultsfor Flow Speed Near the Center of the Well for the

Multislot Well Screen

Pore Velocityin Test Chamber(m/s)

Modeled Speedat Center of Well

(m/s)

Measured Speedat Center of Well

(m/s)

8.53 1025 7.93 1025 5.93 1025

9.93 1024 9.63 1024 5.73 1024 to1.13 1023


of ~3 3 1024 m/s. Recirculation eddies are established inthe northern and southern quadrants of the well. In addi-tion, the velocity estimated near the center of the well cas-ing, while uniformly flowing in the positive x-direction,is approximately half the magnitude when the slots arealigned with the flow. Only if it is ascertained a priorithat the ribs are aligned with the flow could the SCBFMbe properly calibrated to measure both flow speed anddirection. Again, correlations between the pore velocityand measured and modeled well velocities could be in-ferred given sufficient calibration and known slotorientation.

One additional configuration with the slot/rib inter-face aligned with the principal flow direction is investi-gated (the well screen is rotated 16� counterclockwisefrom the case when the ribs are aligned with the flow). Inthis case, flow near the center of the well screen at theeighth slot is directed 23� south of east, which is not sur-prising considering the configuration. Modeled flowspeeds at the center of the well are about two-thirds asfast as the pore velocity in the surrounding medium, withu velocities near 5 3 1024 m/s and v velocities about22 3 1024 m/s. Flow enters primarily through the north-west slots and exits through the southeast slots. Severalsmall horizontal recirculation cells are present. What ismost notable is that the flow direction near the center ofthe cell is not aligned with the principal flow direction butis shifted toward the axis of the slots. While the u velocityis between that seen in the previous cases, there is anadditional v velocity in the negative y-direction (south)approximately half the magnitude of that in the x-direc-tion (east). It is evident that the orientation of the slotswith respect to the flow can significantly alter the mod-eled (and likely the measured) flow direction inside thewell casing. Thus, not knowing the orientation of the wellscreen and the principal flow direction could lead toerrors in the estimated flow direction of ±23� for this ori-entation of the slots.

As was the case with the ribs aligned with the flowdirection, no SCBFM results could be obtained when theslots and ribs were misaligned with the flow directionbecause of the apparent absence of colloids in the flowchamber.

Results and ConclusionsNumerical modeling demonstrates that the flow

fields established within either single-slot (fracture) ormultislot well screens is complex. Recirculation eddiesare observed in different locations depending upon theorientation of the well screen with the principal flowdirection. Also, within the well screen, flow may decreaseby up to an order of magnitude from just inside the slotsto the well center. Hence, flow in a screened well isfocused both by the dimensions and properties of thesand/gravel pack and by the specific configuration of thewell screen. Therefore, to relate flow velocity measure-ments made within a screened well to the pore velocity inthe surrounding formation, calibration under laboratoryconditions that are as close to the field conditions as

possible is required. Fortunately, in a well calibrated sys-tem, numerical modeling may help to estimate both theslot inlet velocity as well as velocity in the surroundingmedium.

For the case of a well with a simulated single frac-ture in the test chamber, the SCBFM measures the correctflow direction. The speeds measured in the center of thewell are as much as 33% higher than those modeled. Thisdifference may be attributed to imprecise centering of theSCBFM in the borehole and/or differences between themodeled inlet speed and the actual speed in the testchamber, which are likely somewhat variable. Both theSCBFM and the model indicate that the flow speed mea-sured at the center of the well is less than the inlet veloc-ity, and this measured speed is sensitive to the centeringof the SCBFM in the well. Care must be taken that theSCBFM is properly placed adjacent to a single fracturebecause misplacement could result in measurement ofa vertical recirculation eddy that would yield lowerspeeds directed opposite to the local fracture flow. Futuredevelopment could focus on improving and subsequentlycalibrating the SCBFM by incorporating scanning in thehorizontal plane in addition to the vertical. This would beparticularly important for single-slot or fracture caseswhen flow through the center of the well is concentratedin a narrow jet (Figure 4).

Although a variety of screens is commercially avail-able, only the five-slot type was examined. In practice,wirewrap-type screens would be the best to use in con-junction with an SCBFM. All field investigations forhorizontal flow characterization should include a videosurvey to evaluate the type and condition of the screen.For a multislot well screen, when the slots are alignedwith the flow direction and assuming sufficient calibra-tion, SCBFM measurements near the center of the wellcan be used to estimate both flow speed and direction inthe surrounding medium. However, based on model re-sults only, not knowing the rib orientation with respect tothe principal flow direction could lead to errors in thespeed estimate of more than a factor of 2. Local flowdirection could be estimated to within ±23� for the par-ticular rib-slot configuration studied here. A multislotwell casing may have small eddies above the uppermostand below the lowest slots, suggesting that the SCBFMshould be deployed at least eight slots from the top orbottom of the screen. It should also be noted that recircu-lation eddies form in the horizontal plane and theSCBFM should be located (centered) to avoid them.Unfortunately, horizontal eddies develop in differentmagnitudes and locations depending upon the orientationof the ribs with respect to the flow direction, which couldyield erroneous SCBFM approximations of the local flowvelocity. Again, horizontal scanning might improve cali-bration.

The SCBFM is a useful tool for evaluating flow speedand direction from individual fractures intersecting anunscreened well. One should keep in mind, however, thatthe speed and direction of flow within a single fracture ata single location is not likely to be representative of theaverage overall flow in the surrounding medium. In addi-tion, the SCBFM can also be a useful velocimeter for


multislot well screens. In this case, knowledge of the typeof screen construction and how the slots align with theprincipal flow direction is important. Without this knowl-edge, which is difficult to obtain, the SCBFM can be reliedon for order-of-magnitude speed measurements and direc-tion measurements with an uncertainty of approximately±25�. Additional modeling and experimentation would beneeded to assess the utility of the SCBFM in well screenswith more columns of slots or wider slots than were usedfor the experiments described herein.

The results presented here should underscore the neces-sity of performing computational fluid dynamics modelingfor any device intended to measure flow inside a wellbore.It is imperative that this type of modeling be conducted forscreened wells because the screen may significantly impactthe internal flow field. Without understanding how the wellscreen, and potentially the measurement device, is affectingthe flow in the well, measurements of flow speed and direc-tion cannot be related to flow in the surrounding formation.Controlled laboratory experiments and detailed modelingshould be used to calibrate any device intended to measureflow within a well.

AcknowledgmentsSandia is a multiprogram laboratory operated by

Sandia Corporation, a Lockheed Martin Company, for theUnited States Department of Energy’s National NuclearSecurity Administration under contract DE-AC04-94AL85000. Funding for this project was provided by theU.S. Army Environmental Center, DOE and RAS Inc.The authors acknowledge LLNL for contributing theSCBFM for this and other horizontal flow studies. Themanuscript was improved by the helpful comments ofKeith Halford and two anonymous reviewers.

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