Page 294 - gas transport in porous media

Rossabi
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Because the demand for subsurface flow measurements is relatively low compared
with the demand for flow measurements in manufacturing processes (e.g., chemical
production and mixing), manufactured applications (e.g., aircraft speed indicators),
product distribution (e.g., oil and natural gas distribution) and weather prediction,
many of the techniques used for measuring subsurface gas flow were adapted from
other applications. Several techniques employ pressure sensors and the application of
Bernoulli’s equation. These tools include Pitot and Prandtl tubes, venturis, nozzles,
and orifice plates and have primarily been advanced by aeronautics and manufacturing
needs. Other engineering process monitoring, and weather monitoring needs have
advanced additional techniques, such as thermal flow sensors, and ultrasonic flow
sensors. The range and suitability of these sensors is often designed for a specific
application. Since subsurface flow rate can vary over several orders of magnitude,
it is often necessary to have several different types of flow measuring techniques
available in a characterization tool kit.
Under field conditions gas flow is most often measured through points accessing
some portion of the subsurface, i.e., wells. Wells sample the subsurface flow through
the screened portion of the tube contacting the flowing zone. The soil gas flow primar-
ily enters the well screen through angles that are perpendicular or oblique to the axis
of the well tube and is then conducted to the surface. The loss due to the angle change
of the flow is generally ignored in subsurface gas flow applications as is the loss upon
entering a slotted or other screened section. Once gas is flowing through the well
tube, flow-measuring methods developed from other disciplines and applications can
be employed.

17.2 FLOW METERS
Flow of a fluid that is normally indistinguishable from its background medium is
generally measured by its effect on physical objects. People have made observation
scales for wind velocity based on deflection of a rising smoke column, the behavior
of flags or trees, or other common materials. Weather scientists have commonly
used vanes or cups mounted on low friction rotational points. These methods must
always account for the effect of the resistance to flow inherent in the measurement
system (e.g., the frictional resistance of the bearings of the rotational pivot in cup
and vane anemometers) and try to minimize the effects of the measurement (physical
resistance and flow obstruction) on the flow system. The resistance represents a loss
in the system. If the flow’s driving force is large, the effect of the measurement
will usually be small but the smaller the flow the more critical is the importance of
minimizing obstruction and losses. The ideal flow meter is completely noninvasive,
and it uses none of the flow’s energy when it measures the flow.
Flow measurement devices generally must balance several conflicting forces. The
ideal flowmeter should accurately measure the total flow of the entire flowing system
(at the desired time resolution) yet not affect the flow field. Generally gas flow does
not occur as a uniform planar front. Over a particular volume, the magnitude of the
flow velocity can vary significantly.Although the ideal tool can measure the total flow