Analysis of Development Methods for Gravel Envelope Wells   -   9

2.5   Double Swabs Mounted On Drill Pipe With Simultaneous Injection
        Pumping Below the Swab

In this method of development, flow is injected into the filter pack from between two adjacent swabs (Figure 4). A flow Q is forced into the filter pack and reenters the well above and below the swabs. There is a variation of this method which incorporates a bypass connecting the sections of well above and below the swabs.

Consider the first case without a bypass. Flow forced into the filter pack has two routes. It moves upward, reenters the well and flows to the surface, or it moves downward to the lower section of the well and must flow into the formation as recharge. In both cases, it must pass through the filter pack.

If the formation has any resistance to recharge, the section below the swabs will be pressurized very quickly and flow will pass upward through the filter pack to the surface. The net result will be similar to that described in section 2.4. Peak tangential velocities will be given by the pumping head, corresponding to injection with a single swab, as shown in Figures 13 and 14.

If there is a flow bypass from the lower section of well to the upper section, the flow configuration is different. Flow will move in both directions within the filter pack, and even if significant recharge occurs in the well all flow must exit between the swabs via the filter pack. Geometry for the mathematical model and numerical results obtained from the model described below are depicted in Figure 15.

Figure 15

The scaling velocity in this case is



which is the mean velocity for flow exiting between the swabs. For swabs spaced a distance equal tot he well diameter, peak tangential velocity at the filter pack / formation interface is found to be equal to v₀ when b/a = 1.5, as shown in Figure 15. When b/a is increased to 2.0 peak tangential velocity drops to 0.36 v₀, as shown by the dotted curves in Figure 15.

Peak tangential velocity appears to depend rather weakly on the ratio of k₂/k₁ (the hydraulic conductivity ratio). Changing k₂/k₁ from 0.001 to 0.1 reduces peak tangential velocity by about 10%, because more flow will leak into the formation.

Radial velocities are also induced by the pumping operation. There is a weak inflow into the formation, adjacent to the swabs, as depicted by the streamlines in Figure 15. This flow reverses to become a flow out of the formation within a distance from the swabs about equal to half the swab spacing. The magnitude of these velocities is small relative to the tangential velocities in the filter pack and probably does not contribute much toward flushing.

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