Page 112 - Shale Shakers Drilling Fluid Systems
P. 112
INTRODUCTION 95
can be separated by the screen) that is dependent Fluid rheological properties—Literature indi-
on the characteristics of the shaker ("G"-factor, vi- cates that the liquid capacity of a shale shaker
brational frequency, type of motion, and angle of screen decreases as the plastic viscosity of a
the screen deck), the screen (area and conduc- drilling fluid increases. Plastic viscosity (PV)
tance), and the fluid properties (density, rheology, is the viscosity that the fluid possesses at an
1
additives, and fluid type). The fluid-only capacity infinite shear rate. Drilling fluid viscosity is
is the fluid limit with zero removable solids. Al- usually dependent on the shear rate applied
though not true in many instances, the following to the fluid. The shear rate through a shale
assumes the drilling fluid to be a fluid with solids shaker screen depends on the opening size
no larger than the openings in the shaker screen. and how fast the fluid is moving relative to
The screen cloth can be considered a permeable the shaker screen wires. Generally, shear
media with a permeability and thickness (conduc- rates through the shaker screen vary signifi-
tance) and an effective filtration area. The fluid cantly. The exact capacity limit, therefore,
capacity will decrease as the fluid viscosity in- will depend on the actual viscosity of the
creases (plastic viscosity is most important but fluid. This will certainly change with plastic
yield and gel strengths can have a significant im- viscosity and yield point.
pact as well). Capacity will also increase as the Fluid surface tension—Although drilling fluid
fluid density increases due to increased pressure surface tensions are seldom measured, high
on the screen surface acting as a force to drive surface tensions decrease the ability of the
fluid through the screen. drilling fluid to pass through a shale shaker
The fluid-only capacity will generally be re-
duced when certain polymers are present in the screen, particularly fine mesh.
fluid. Partially-hydrolyzed polyacrylamide (PHPA) is Wire wettability—Shale shaker wire screens
most notable in this respect as it can exhibit an must be oil-wet when drilling with oil-base
effective solution viscosity in a permeable media drilling fluids. Water adhering to a screen
higher than that measured in a standard viscom- wire decreases the effective opening size for
eter. At one time, this effective viscosity of PHPA oil to pass through. Frequently, this results in
solutions was determined by flowing the solution the shaker screens being incapable of han-
through a set of 100-mesh screens mounted in a dling the flow of an oil-base drilling fluid.
standard capillary viscometer. PHPA drilling fluids This is called "sheeting" across the shaker
typically have a lower fluid-only capacity for a screen and often results in discharging large
given shaker/screen combination than similar quantities of drilling fluid.
drilling fluids without PHPA because of this higher
Fluid density—Drilling fluid density is usually
effective viscosity. This decrease in fluids-only
capacity can be as much as 50% compared to a increased by adding a weighting agent to the
gel/water slurry. Adsorption of PHPA polymer drilling fluid. This increases the number of
may decrease effective opening sizes (as it does in solids in the fluid and makes it more difficult
porous media), thereby increasing the pressure to screen the drilling fluid.
drop required to maintain constant flow. This makes Solids: type, size, and shape—The shape of
the PHPA appear to be much more viscous than solids frequently make screening difficult. In
it actually is. This effect also occurs with high con- single-layer screens, particles that are only
centrations of XC in water-base fluids, drilling flu- slightly larger than the opening size and can
ids with high concentrations of starch, newly pre- become wedged in the openings. This effec-
pared oil-base drilling fluids, and polymer-treated tively plugs the screen openings and decreases
viscosifiers in mineral oil-base fluids. the area available to pass fluid. Solids that
The solids limit can be encountered at any time, tend to cling together, such as gumbo, are
but occurs most often when drilling large diam- also difficult to screen. Particle size has a
eter holes, soft, sticky formations, or during peri-
ods of high penetration rates. A relationship ex-
ists between the fluid limit and solids limit. As the 1 The Bingham Plastic Rheological Model may be represented
fluid flow rate increases, the solids limit decreases. by the equation: Shear Stress = (PV) Shear Rate + YP, where YP
As the solids loading increases, the fluid limit is the Yield Point. By definition, viscosity is the ratio of shear
decreases. Internal factors that affect the fluid and stress to shear rate. Using the Bingham Plastic expression for
solids limits are discussed in Chapter 3. shear stress: Viscosity = [(PV) Shear Rate + YP]/Shear Rate.
Performing the division indicated, the term for viscosity becomes
Major external factors that affect the solids and (PV) + (YP/Shear Rate). As shear rate approaches infinity, vis-
fluid limits are: cosity becomes PV.
