Page 152 - Shale Shakers Drilling Fluid Systems

SHALE SHAKER SCREENS 135

liquid wetting phase on the solids. Frequently, a solids discarded from a shale shaker may or may
mist or a spray of drilling fluid will cause the ag- not have the same distribution as the potential
glomeration (or patties) to disperse with better separation curve. The variables listed above deter-
separation of solids, but also returns fine solids to mine the results of the actual shaker performance.
the drilling fluid. The potential separation curve is designed to elimi-
As previously discussed, the openings in a square nate all variables except those relating to the
mesh screen are fairly uniform and vary with a shaker screen openings or screen cloth.
layered mesh screen. Some particles of a given
size pass through the screen and some are dis-
carded. If the amount (% by weight) of discarded Screen Blinding
particles is plotted against the size of the particles,
a curve is obtained that is similar to the one shown Screen blinding occurs when grains of solids
in Figure 6-8. This curve is called the percent being screened lodge in a screen hole. This often
separated curve, cut point curve, tromp curve, or occurs when drilling fine sands, such as in the
partition curve. As stated earlier, the percent sepa- Gulf of Mexico. The following sequence is often
rated curve for common oil field screens has the observed during screen blinding:
same shape as the potential separation curves.
The cut point (also known as the median cut 1. When a new screen is installed, the circula-
point) is the point on the curve where 50% of the tion drilling fluid falls through the screen in
particles of this size are discarded. The area be- a short distance.
neath and to the right of the curve represents the 2. After a time, the fluid endpoint travels to the
mass of solids removed by the shaker. The area end of the shaker.
to the left of the curve represents the mass of sol- 3. Once this occurs, the screens are changed to
ids retained in the mud. Chapter 9 discusses a eliminate the rapid discharge of drilling mud
method to determine cut points. off the end of the shaker.
The separation potential curves are reported in 4. After the screens have been washed, fine
terms of the d so, d, 6, and d 84. These sizes repre- grains of sand that are lodged in the screen
sent 16%, 50%, and 84% of the opening sizes in surface can be observed. The surface of the
the screen and are used to compare various shaker screen will resemble fine sandpaper because
screens. One half of the openings are larger than of the sand particles lodged in the openings.
the d 50, or median point, and one half of the open-
ings are smaller. The d 16 size indicates that 16% Most every screen used in the oil field is blinded
of the openings are smaller than this size; whereas to some extent by the time it needs replacing. For
the d 84 size indicates that 84% of the openings are this reason, when the same screen size is rein-
smaller than that size. These points are statisti- stalled, the fluid falls through the screen closer to
cally significant for Gaussian distributions because the feed.
they each represent one standard deviation from One common solution to screen blinding is to
the d 50, or median, point. In a Gaussian distribu- change to a finer or coarser screen than the one
tion, the openings sizes would be uniformly dis- being blinded. This tactic is successful if the sand
tributed around the d so median value. that is being drilled has a narrow size distribution.
The percent separated curve, which is deter- Another solution is to change to a rectangular
mined experimentally, incorporates all of the test screen, although rectangular screens can also blind
conditions. It is the result of the following variables: with multiple grains of sand. Unfortunately, the
process of finding a screen size that will not blind
• Size and shape of the openings in the screen is expensive.
• Solids shape In the late 1970s, the layered screen was intro-
• Characteristics of solids duced to avoid screen blinding. This hook strip
• Drilling fluid properties screen was mounted on a downhill sloping, unbal-
• Screen wettability anced elliptical motion shale shaker vibrating at
• Solids and liquid loading 3600 rpm. The two fine layers of screening cloth,
• Agglomeration of solids supported at 4-inch intervals, tended to dislodge
• Shaker vibration pattern, screen angle, screen fine grains of sand and would only blind about
tightness, etc. 25% in severe laboratory tests (Figure 6-10), leaving
75% of the screen unblinded. The non-blinding
The separation potential curve, which is deter- feature is assumed to be the result of the decel-
mined by examining the screen, is designed to eration of the two screens. The wire diameter is
identify only screen parameters. Experimentally, in the range of 0.002 inch and the opening sizes