Analysis of Development Methods for Gravel Envelope Wells   -   4

2.1.2   Laboratory Model of Jetting

A laboratory test model of a section of a well with an artificial filter pack was constructed as shown in Figure 8. The test section could be filled with a selected pack, the top and diaphragm bolted own and an overburden pressure as high as 60 psi imposed via the diaphragm. A test jet with internal diameter 0.167 inches was located in the model such that the flow from the jet directly impacted the section of well screen. The distance of the jet from the screen was adjustable, allowing simulation of a jet of larger diameter. Maximum jet flow possible was 12 gpm, corresponding to a jet velocity of close to 180 ft/ sec and a stagnation pressure within the gravel of about 220 psi.

Figure 8

The purpose of this test facility was to determine the mode of jet operation and evaluate the influences of pack material size distribution, jet flow rate, and overburden pressure on the jetting operation. Tests were performed with three pack material sizes commonly employed in gravel envelope wells. Details of the materials are given in Appendix B. the well screen used was the continuous wire wrap type, constructed with 0.060 inch aperture size.

The fundamental result of the tests performed shows that pack material will not move under jet action unless there is sufficient free space in the filter pack. In other words, there must be "elbow room" for the particles. In the tests this could be provided either by release of the pressuring diaphragm or by waiting for the jet to wash enough fine particles through the screen to create the space. No motion occurred in the test using 1/8 1/4 gravel filter pack (Appendix B).

Operation of the test apparatus with finer pack material, such as a 6 14 Crystal Silica gravel (Appendix B), disclosed that pack motion would develop only after a cavity was formed by elutriation of the finer size fraction of pack material through the screen. The progression of the pack motion was systematic. First, a few grains would move in the initial cavity formed. The motion of these grains in turn allowed the jet to penetrate deeper and was out more fine material, thereby increasing the free space. The flow of jet fluid and pack material developed a vortex structure which advanced into the filter pack until an equilibrium penetration depth was attained in about two minutes (Figure 9).

Figure 9

At equilibrium it appears that total jet power is consumed in keeping a ball of fluid and pack material in motion, and none is available to generate new particle motion. It is to be expected that reducing the intergranular stress in the motionless pack should allow the depth of penetration of the jet to increase. This argument is confirmed in the test results. In Figures 10 and 11 overburden pressure, as represented by the test cell diaphragm, has been released, thus providing more free volume and a reduction in the intergranular stress. The result is an increase in jet penetration from about 2 ½ inches to 4 ½ inches.

Figure 10                       Figure 11

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