Screen Blinding

Screen blinding occurs when grains of solids being screened lodge in a screen hole. This often occurs when drilling fine sands, such as in the Gulf of Mexico. The following sequence is often observed during screen blinding:

  1. When a new screen is installed, the circulation drilling fluid falls through the screen in a short distance.
  2. After a time, the fluid endpoint travels to the end of the shaker.
  3. Once this occurs, the screens are changed to eliminate the rapid discharge of drilling mud off the end of the shaker.
  4. After the screens have been washed, fine grains of sand that are lodged in the screen surface can be observed. The surface of the screen will resemble fine sandpaper because of the sand particles lodged in the openings.

Most every screen used in the oil field is blinded to some extent by the time it needs replacing. For this reason, when the same screen size is reinstalled, the fluid falls through the screen closer to the feed.

One common solution to screen blinding is to change to a finer or coarser screen than the one being blinded. This tactic is successful if the sand that is being drilled has a narrow size distribution. Another solution is to change to a rectangular screen, although rectangular screens can also blind with multiple grains of sand. Unfortunately, the process of finding a screen size that will not blind is expensive.

In the late 1970s, the layered screen was introduced to avoid screen blinding. This hook strip screen was mounted on a downhill sloping, unbalanced elliptical motion shale shaker vibrating at 3600 rpm. The two fine layers of screening cloth, supported at 4-inch intervals, tended to dislodge fine grains of sand and would only blind about 25% in severe laboratory tests (Figure 6-10), leaving 75% of the screen unblinded. The non-blinding feature is assumed to be the result of the deceleration of the two screens. The wire diameter is in the range of 0.002 inch and the opening sizes

DX 110, unbonded

DX 110, bonded in 1" squares i 15

Time, in minutes

FIGURE 6-10

are in the range of 0.004 inch. During the upward thrust of a layered screen, the screens must come to a stop at the upward end of the motion. The screens tended to have inertia that prevented them from stopping at exactly the same time. This created an opening size that was slightly larger than the original opening size of the layered screen during the upward thrust. Solids were then expelled from the screen. On the downward thrust of the motion, the two layers remained together until the screen began decelerating. At the bottom of the stroke, again, the inertial forces caused the screens to slightly separate allowing larger solids to pass through. This may explain why the separation cut point curve shows poorer separation characteristics for a layered screen than for a single, square mesh screen. Many particles larger and smaller than the median opening size are found in the discard from a layered screen.

Unfortunately the downhill sloping basket and high frequency limited the amount of liquid that could pass through the screen. Furthermore, lost circulation material had a high propensity to become lodged in the screen due to the high-frequency, short-stroke vibration. These problems were reduced by limiting the vibration to 1800 rpm and flattening the basket slope. In the early 1980s, linear motion was introduced so that solids could travel up an incline out of a pool of liquid. This fluid pool provided additional pressure to force fluid through the screen. Unfortunately, linear motion, combined with marginal support, tore layered screens apart. The only way to obtain satisfactory screen life on a linear motion machine was to support the layered screen in one-inch squares.

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