Page 105 - Shale Shakers Drilling Fluid Systems
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88 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
may result in stuck pipe. When drill collars reach the surface because they will never again be
are buried in a thick filter cake, the differ- that large. The following points are important when
ential pressure between the wellbore and the discussing plastic viscosity:
formation fluid pressure pushes the drill col-
lar against the formation. This is called "dif- 1. Plastic viscosity of a drilling fluid impacts the
ferential sticking." Drilled solids create a effectiveness of cuttings removal from the
high friction force that can exceed the strength bottom of a borehole. Fluids with low plas-
of the drill string. tic viscosity can remove cuttings better. This
2. Thick filter cakes can cause high surge raises the founder point of a drill bit and
and swab pressures. When a drill bit moves allows for greater bit loading. Higher drill
through the thick filter cake, it resembles the rates are now possible.
plunger in a syringe. As the drill bit is raised, 2. High plastic viscosities diminish the effective-
formation fluid can be pulled into the well- ness of yield point for hole cleaning in nearly
bore. This can result in a kick. A hole cannot vertical holes. Carrying capacity of a drilling
be made while the well is shut-in handling fluid is dependent on the viscosity of the
a kick. If the drill bit is lowered rapidly, the fluid in the annulus. Decreasing the plastic
pressure below the bit increases dramati- viscosity of a drilling fluid while maintaining
cally. This can result in lost circulation. a constant yield point increases the low-
3. Poor filter cakes can create formation dam- shear-rate viscosity of a drilling fluid. This is
age. Formations that are sensitive to the known as the "K" value of the Power Law
drilling fluid filtrate should be exposed to as Rheological Model.
small amount as possible. Fluid invasion 3. High plastic viscosities have a detrimental
should be minimized. effect on the performance of all solids re-
4. Large invasion zones may make interpreting moval equipment.
electric logs difficult. This is particularly true 4. High plastic viscosity increases fluid pressure
in thin beds. Shale barriers create a problem losses in the circulating system, which de-
in thin beds because the depth of electric creases drilling performance.
investigation is greatly diminished. Deep in-
vasion of the filtrate means that hydrocar- WEAR
bons may go undetected by conventional
logging techniques. Solids, particularly sand-size particles (greater
5. Cement will not effectively displace thick than 74 microns), can cause significant wear of
filter cakes. If this occurs, cement cannot fill expendables. The effect is similar to grinding pump
the annulus between the casing and the for- parts and elastomer seals against coarse sand-
mation. In this case, several problems may paper. Seals in drill bits, pump liners, swabs, valves,
occur: (a) the casing seat will not pressure- swivels, and so forth, will deteriorate much faster
test and will require a squeeze job, or sev- if the large drilled solids, such as sand, are not re-
eral squeeze jobs, delaying drilling; (b) for- moved from the drilling fluid. Bearing life of roller
mations will not seal, thereby allowing fluid cone bits with no seals is strongly dependent on
to enter the well behind the casing; and the quantity and type of drilled solids in the drill-
(c) this fluid could charge an upper forma- ing fluid. The life of conventional diamond bits is
tion or could eventually broach the surface. also adversely affected by an increase in the quan-
tity of sand-size particles in the drilling fluid.
PLASTIC VISCOSITY
CARRYING CAPACITY
Plastic viscosity is determined by four factors:
the viscosity of the liquid phase, the size, the One important component of a good solids man-
shape, and the number of particles in the fluid. agement philosophy is the drilling fluid carrying
Drilled solids increase the plastic viscosity of a capacity. As drilled solids (sloughings and drill
drilling fluid. A pound of colloidal particles (less bit-generated) enter the wellbore, they should be
than 1 to 2 microns) will have a greater detri- brought to the surface as soon as possible. If these
mental effect on drilling fluid performance than a drilled solids are tumbled and slowly brought to the
pound of 30 to 40 micron-size particles. The shale surface, they have time to disintegrate and increase
shaker removes many large particles that will the low-gravity solids content of a drilling fluid.
eventually disintegrate into colloidal size. It is im- Suprisingly, drilled solids also affect the drilling
portant to remove drilled solids as soon as they fluid properties that control carrying capacity in
