Analysis of Development Methods for Gravel Envelope Wells - 11
Now consider the situation where gravel motion can be initiated by
creation of a cavity through elutriation of finer material. In this case flow patterns
will be established such as shown in the photographs of Figures 9,
10, 11 and 12.
The question is how will the flow fields vary with jet diameter and jet flow. The answer
is relatively simple. Power in the jet is proportional to the product of the discharge
rate Q and velocity vj, or, writing this in terms of jet diameter 2c and jet
velocity vj, jet power is proportional to 
Power dissipation in the motion of pack
material will be proportional to 
and to the volume of material placed in motion. Thus, for
given jet flow velocity and pack material it would be expected that the volume of
gravel in motion will be proportional to c2, everything else held constant.
The linear dimension of volume in motion should then increase as c2/3.
Given a jet 0.167 inches in diameter, delivering water at 180 ft/sec, with a volume of
moving pack material 5 inches deep, then a jet 0.5 inches in diameter delivering water
at about the same velocity should produce a moving pack volume about 10 inches deep,
provided free volume for material to move either exists or can be created by flushing
fine material through the screen. The fact that this may not always occur helps explain
mixed field results using this method of development.
If the filter pack can be placed in motion by the jet, effective
flushing will develop. Motion of the jetting tool must necessarily be slow or there is
a strong probability of moving the jet away from the pack material in motion. In
regions with high over-burden pressures giving high intergranular stress, development
of pack motion will be very slow and the technique less effective. With low
intergranular stress and excessive fine filter material, slow jet motion may destroy
effectiveness of the filter pack through the creation of cavities.
In the swabbing analysis performed in Section 2.2, it was explained
that the process occurs in two stages. First, there is a radial flow velocity at the
borehole of
where a is the well radius
U is swab velocity
b is filter pack radius
D is depth of well screen below the swab
Second, a tangential velocity through the filter pack adjacent to the swab is set up.
A scale velocity for this process was defined, where
where H is the head drop across the swab
k1 is the filter pack conductivity
The peak tangential velocity vt was found to be
vt = 3.0v* when b/a = 1.5
vt = 1.5 v* when b/a = 2.0
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