Screen blinding occurs when grains of solids being screened find a hole in the screen just large enough to get stuck in. This often occurs during the drilling of fine sands in the Gulf of Mexico. The following sequence is often observed during screen blinding:
- When a new screen is installed, the circulating drilling fluid falls through the screen a short distance.
- After anywhere from a few minutes to even several hours, the fluid end point slowly or even quickly travels to the end of the shaker.
- Once this occurs, the screens must be changed to eliminate the rapid discharge of drilling mud off the end of the shaker.
- After the screens have been washed, fine grains of sand are observed stuck in the screen.
- The surface of the screen will feel like fine sandpaper because of the sand particles stuck in the openings.
Most every shaker screen used in the oilfield is blinded to some extent by the time it has worn out. This is the reason that when the same screen size is reinstalled, the fluid falls through the screen closer to the feed.
A 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 that will not blind is expensive.
In the late seventies the layered screen was introduced to avoid screen blinding. This hook-strip type of 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 blind only about 25% of the screen in severe laboratory tests, leaving 75% of the screen nonblinded.
The nonblinding 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 are in the range of 0.004 inch. In the upward thrust of a layered screen, the screens must come to a stop at the upward end of the motion. They would tend to each have an inertia that would prevent them from stopping at exactly the same time.
This would create an opening size slightly larger than the original opening size of the layered screen during the upward part of the thrust. Solids would be expelled from the screen. On the downward thrust of the motion, the two layers remain together until the screen starts deceleration.
At the bottom of the stroke, again the inertial forces could cause the screens to slightly separate, allowing larger solids to pass through the screen. This probably also explains 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 limitsthe amount of liquid that can pass through the screen. Furthermore,lost circulation material has a high propensity to get stuck in the screen due to the high-frequency, short-stroke vibration.
These problems have been ameliorated by reducing the vibration to 1800 rpm and flattening the basket slope. In the early 1980s, linear motion was introduced so that solids could march 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 ona linear motion machine was to support the layered screen in 1-inch squares.