Analysis of Development Methods for Gravel Envelope Wells - 1
1.0 INTRODUCTION
Constructing of gravel envelope wells by the hydraulic rotary drilling
system requires circulation of a drilling fluid in order to remove drilled cuttings from
the borehole, lubricate the drill string and bit, provide sufficient hydrostatic pressure
to overbalance formation pressures and deposit a thin impervious filter cake on the
borehole wall. This mud system along with residual solids remaining in the borehole will
reduce well capacity significantly unless removed prior to production.
The process of development first requires that these drilling
byproducts be removed from the interface between the borehole and the filter pack in
order that final development, which consists of removal of fine materials from the
aquifers and stabilization of pack-aquifer materials around the well screen, can be
carried out. Preliminary development of gravel envelope wells normally consists of
various combinations of circulating, swabbing, jetting, and other methods of conditioning
filter pack prior to installation of a turbine development pump.
This report provides a scientific basis for evaluation of some of the
most commonly used preliminary development methods. It describes in general terms
mathematical models of these methods. The results of a laboratory study of a test model
of the jetting method will be included. Also included are techniques used in common well
configurations. From the results presented it will be concluded that development methods
vary considerably in their effect and should be selected carefully to match project
requirements.
The five basic methods to be analyzed will be described briefly. The
first considered is jet development. As described in The Johnson Driller's Journal,
January- February 1979* , this method was developed in an attempt to provide high levels
of flow energy to any wall cake on a borehole wall. Figure 1
shows schematically a typical jetting operation. Typical recommended jet velocities are
150-190 ft/ sec with a jet orifice of ¼ inch to ½ inch.
Figure 1
The second method, line swabbing (Figure 2),
involves successively raising and lowering a rubber-flanged scow. Typical haul
velocities will be on the order of 3 ft/ sec. The scow is equipped with a flapper valve
at the foot to facilitate the down motion of the swab.
Figure 2
The third method, a variation of the previous technique, uses a
rocker arm to provide an oscillatory motion of the swab as it is hauled. Typical
oscillation will be 30 strokes/ minute with a 3 ft stroke.
The fourth method involves pumping below a single swab mounted on
drill pipe so that the swab causes return flow to enter the gravel envelope and bypass
the swab, as shown in Figure 3. The swab may be hauled and
dropped simultaneously with pumping. A typical fall velocity would be 8 ft/ sec.
Figure 3
The fifth method considered uses a double swab mounted on a drill
pipe. As depicted in Figure 4, fluid is pumped out between
the swabs into the gravel envelope. In an alternative version, the swab is equipped
with a bypass to allow flow from below the lower swab to pass to the well region above
the upper swab (Figure 5).
Figure 4
Figure 5
* See References
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