Analysis of Development Methods for Gravel Envelope Wells - 5
Results depicted in Figures 9,
10 and 11 are typical of all the test
results with both 6 10 and 6 14 sands. Initial development of motion depends upon
either the existence or creation of free space in which particles of pack material can
move. Once particles are free to move, the scale of motion depends upon jet velocity,
overburden pressure, the degree of compaction of the filter pack, and intergranular
friction. The influence of each of these factors was apparent in the tests.
Considering only the effect of jet velocity, it was clear that
reduction in flow velocity reduced the depth of penetration once the system was in
motion. A second increase in jet velocity would not generate penetration depth equal to
that attained initially, a somewhat surprising result. The explanation lies in the
greater degree of compaction attained in the pack material by the jetting action over
what was initially present. This phenomenon is illustrated in
Figure 11. This was also confirmed in another test, in which
the overburden pressure was suddenly released to allow the filter pack material to move.
The whirling mass of fluid and filter pack subsided to about half of its initial extent
as the pack material compacted. In general, penetration depth attained with a smooth
rounded sand (6 10) appeared to be marginally greater than that with a sharp angular
sand (6 14).
All tests were performed with a 0.167 inch diameter jet. In order to
simulate the effect of a larger diameter jet, the jet, originally ¾ inch from the
screen, was moved back to provide a larger impact area. This test, which was performed
on a gravel pack that had been in motion, indicated that the effect of the jet impact
area was not significant. As can be seen in Figure 12,
where the jet is 20 diameters from the screen, motion of the pack material and depth of
penetration increased as overburden pressure was released. This result confirms that it
is jet power which controls the dimension of the filter pack movement and not jet
diameter.
Figure 12
To summarize, the key element in getting pack material into motion is
the ability of the jet to create moving space by washing out fine materials from the
filter pack in progressive fashion described. If the filter pack is compacted, and of a
size distribution that none will pass through the well screen, it appears no motion is
possible. Once the pack material is in motion it can be sustained with lower velocities
than are required to initiate motion. The size of the moving region is a function of the
power available in the jet relative to the power necessary to keep a given volume of
fluid and pack material in motion. The relationship of these conclusions to actual well
development will be reviewed in Section 3. No discussion is presented here of the
potential deleterious effects of creating filter pack cavities. This consideration,
however, is important in filter pack selection when jetting development methods are to
be used.
Line swabbing is the term given to hauling a close fitting rubber
flanged scow through the well. As shown in Figure 2, its purpose
is to develop a pressure differential across the section of well containing the swab.
This pressure differential results in flow being forced into the well screen on the high
pressure side and out of the screen on the low pressure side. Because the scow is
equipped with a foot valve, it drops easily and the process can be repeated.
Modeling the flows developed by this process must be done in two
stages. First there is radial production flow generated by placing the swab in motion.
This is accompanied by a steady state motion about the swab when seen from coordinates
moving with the swab. We look at each of these separately, since analysis of the system
over the complete range of motion is very difficult.
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