Analysis of Development Methods for Gravel Envelope Wells   -   14

3.5   Double Swabs Mounted on Drill Pipe With Injection Pumping Between
        Swabs

Without a bypass through the swabs the system acts as a single swab, or can be considered as one half of a bypass-equipped double swab. The analysis is therefore subject to a degree of uncertainty in that some flow will enter the formation as recharge. Hauling and dropping the swab creates the same flows as a single swab. In an example illustrating this development method, 2200 gpm of flushing water was pumped into a newly constructed well with 18-inch diameter casing and 30-inch diameter filter pack. Water was pumped out between double swabs which in one case were spaced 5 ft apart and in another case 18 inches apart. Peak tangential velocity in both cases is approximately the same since, as we have seen, the ratio b/a is the predominantly controlling mechanism. If we assume all flow passes upward through the filter pack and there is essentially no recharge, then tangential velocity cannot exceed

v_e = Q/(b² - ^a2)

   

    = 0.93 ft/ sec.

As shown, actual tangential velocities are, for b/a = 1.7, about 70% of this estimate, giving tangential velocity of about 0.65 ft/ sec. Swab spacing will not influence this significantly.

When there is a flow bypass as described in Section 2.5, and when b/a = 1.5, peak tangential velocity at the formation/ filter pack interface is equal to mean velocity of flow exiting between the swabs. When b/a = 2.0 peak tangential velocity drops 0.36 v₀ as shown by results in Figure 15. Thus to obtain a peak tangential velocity of 1.5 inches/ sec in a 14-inch well with a 7-inch filter pack (k₁ = 10,000 gpd/ ft₂)

Q = 1.5 cfs = 666 gpm

The pressure difference necessary to produce this flow is

 

    = 17.4 ft.

For the actual case considered above with a flow of 2200 gpm and a swab bypass then, from Figure 16, with a b/a = 1.67 and a 5-foot swab spacing (L/a = 3.33), peak tangential velocity is predicted to be 0.55 ft/sec. For b/a = 1.67 and L/a = 1.0 (18-inch swab spacing) tangential velocity is 0.45 ft/ sec. The slight reduction that is predicted in the peak tangential flushing velocity at the formation/ filter pack interface is a consequence of a less uniform flow distribution in the filter pack as the swabs are moved closer together (Section 2.5).

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