Laboratory and numerical evaluation of borehole methods for subsurface horizontal flow characterization.

Abstract

The requirement to accurately measure subsurface groundwater flow at contaminated sites, as part of a time and cost effective remediation program, has spawned a variety of flow evaluation technologies. Validation of the accuracy and knowledge regarding the limitations of these technologies are critical for data quality and application confidence. Leading the way in the effo rt to validate and better understand these methodologies, the US Army Environmental Center has funded a multi-year program to compare and evaluate all viable horizontal flow measurement technologies. This multi-year program has included a field comparison phase, an application of selected methods as part of an integrated site characterization program phase, and most recently, a laboratory and numerical simulator phase.As part of this most recent phase, numerical modeling predictions and laboratory measurements were made in a simulated fracture borehole set-up within a controlled flow simulator. The scanning colloidal borescope flowmeter (SCBFM) and advanced hydrophysical 4 logging (NxHpL™) tool were used to measure velocities and flow rate in a simulated fractured borehole in the flow simulator. Particle tracking and mass flux measurements were observed and recorded under a range of flow conditions in the simulator. Numerical models were developed to aid in the design of the flow simulator and predict the flow conditions inside the borehole.Results demonstrated that the flow simulator allowed for predictable, easily controlled, and stable flow rates both inside and outside the well. The measurement tools agreed well with each other over a wide range of flow conditions. The model results demonstrate that the Scanning Colloidal Borescope did not interfere with the flow in the borehole in any of the tests. The model is capable of predicting flow conditions and agreed well with the measurements and observations in the flow simulator and borehole. Both laboratory and model results showed a lower limit of fracture velocity in which inflow occurs, but horizontal flow does not establish itself in the center of the borehole. In addition, both laboratory and model results showed circulation cells in the borehole above and below the fracture horizon. The length of the interval over which the circulating cells occurred was much larger than the interval of actual horizontal flow. These results suggest that for the simple fracture geometry simulated in this study, horizontal flow can be predictable and measurable, and that this flow is representative of the larger, near-field flow system. Additional numerical refinements and laboratory simulations of more robust, life-like fracture geometries should be considered.The preliminary conclusions of this work suggest the following: 1) horizontal flow in the fractured medium which is representative of the near-field flow conditions can be established in a wellbore; 2) this horizontal flow can be accurately measured and numerically predicted; 3) the establishment of directionally quantifiable horizontal fl...

Accuracy of Flowmeters Measuring Horizontal Groundwater Flow in an Unconsolidated Aquifer Simulator

“…The SCBFM employs a charge‐coupled device camera, an optical magnification lens, and an illumination source to visually track neutrally buoyant colloids (1 to 5 μm diameter) moving in groundwater (Wood et al 1997; Pedler and Jepsen 2003; James et al 2006). Digital renderings of the visual recordings are analyzed with computer algorithms to compute the advective transport velocity and flow direction of the colloids.…”

“…This scanning capability is possible by use of a variable focus lens. The SCBFM has been solo tested in a fractured‐rock simulator but until this study, it had not been tested in an unconsolidated aquifer simulator (Pedler and Jepsen 2003; James et al 2006). A colloidal borescope, with no scanning capability, has been solo tested in an unconsolidated aquifer simulator by the manufacturer (Kearl 1997).…”

“…The FCLS can identify multiple flowing zones in a single flowing well. The FCLS has not been previously tested in an unconsolidated aquifer simulator but has been examined in a fractured‐rock simulator (Pedler and Jepsen 2003).…”

“…James et al (2006) used the same simulator set‐up as Pedler and Jepsen (2003) but extended the numerical simulations and SCBFM measurements to examine single‐ and multislotted screens. Their study simulated a 5‐inch (0.13‐m) outside diameter fiberglass tube with ten rows of 58° slots, vertically separated by 0.31 inch (0.008 m), and aligned in four columns.…”

“…Based on laboratory and field experience, Kerfoot (1988) concluded that the accuracy of velocity measurements improved with slot thickness, wellscreen diameter, use of mud-free drilling methods, use of annular fills with fine grain-size distribution (i.e., sand is preferred over gravel), well centralizing, and negligible vertical flow in the borehole. James et al (2006) used the same simulator set-up as Pedler and Jepsen (2003) but extended the numerical simulations and SCBFM measurements to examine single-and multislotted screens. Their study simulated a 5-inch (0.13-m) outside diameter fiberglass tube with ten rows of 58° slots, vertically separated by 0.31 inch (0.008 m), and aligned in four columns.…”