Page 20 - Shale Shakers Drilling Fluid Systems

HISTORICAL PERSPECTIVE 3

and went through several modifications before ar- compounds. The shaker removed the larger cut-
riving at a basic type and size for drilling. The first tings (larger than 541 microns, or 30 mesh), and
modification reduced the size and weight of the the desanders and desilters removed the smaller
unit for transporting between locations. The name particles (60 to 15 microns). However, the inter-
"shale shaker" was adopted to distinguish the dif- mediate-size particles (from 541 to 60 microns)
ference between shakers (classifiers) used in min- were still left in the drilling fluid.
ing and shale shakers used in oil well drilling since Intermediate-size particle removal led to the
both were obtained from the same suppliers. development of circular motion, or tandem, shak-
Other modifications included a 4' x 5' hook strip ers. Development was slow for these fine-screen,
screen that tensioned from the sides with tension high-speed shakers for two reasons. First, screen
bolts. Motion was elliptical, which made a down- technology was insufficiently developed for screen
slope necessary to move cuttings off the screen. strength, so screen life was short. There was in-
Screen mesh was limited to 20- to 30-square mesh sufficient mass in the screen wires to properly
(838 to 541 microns). This unit was the workhorse secure the screens without tearing. Second, the
of the industry until the late 1960s. Even though screening basket required greater development ex-
superseded by circular motion and linear motion pertise than required for earlier modifications in
shakers, standard shale shakers are still in de- solids removal equipment.
mand and being manufactured today. During this time, major oil company research
In the late 1920s and early 1930s, larger oil recognized the problems associated with ultra-
companies organized research laboratories and fines (colloidals) in sizes of 10 microns or less.
began exploring oil well drilling problems. They These ultra-fines "tied up" large amounts of liquid
began to understand that the smaller cuttings, or and created viscosity problems that could only be
particles, left in drilling fluid were also detrimen- solved by adding water (dilution). Centrifuges had
tal to the drilling process and another ore dress- been used in many industries for years and were
ing machine was introduced from the mining indus- adapted to drilling operations in the early 1950s.
try—the cone classifier. This machine, combined They were first used on weighted muds to remove
with the concept of a centrifugal separator taken and discard colloidals—fine particles smaller than
from the dairy industry, became the hydrocyclone 2 to 4 microns—and to save larger particle-size
desander. The basic principle behind separating barite (weighting material) and some drilled solids.
heavier and coarser materials from the drilling fluid In recent years, a centrifuge was applied to
is the centrifugal action of rotating the volume of the unweighted drilling fluids to reduce and discharge
sand-laden mud to the outer limit, or periphery, of fine solids in the active mud system. This appli-
the cone. The heavier particles exit the bottom of the cation saves the more expensive liquid phase of
cone, and the cleaner drilling fluid rises to the top the mud for reuse. Dilution is minimized, thereby
and exits as the effluent. The desander, ranging in reducing mud cost; however, these machines are
size from 6 to 12 inches in diameter, removes most quite expensive and require a great deal of care.
solids larger than 30 to 60 microns. Desanders have In the early 1970s, the mud cleaner was devel-
been considerably refined through the use of more oped as an addition to the desander and desilter
abrasion-resistant materials and more accurately for reducing loss in the expensive liquid phase.
defined body geometry and are an integral part of Hydrocyclones discard a slurry, including the liquid
most solids separation systems today. phase, which can be expensive over time. The
After the development of the oil-field desander, mud cleaner takes the underflow from a bank of
it became apparent that side wall sticking of the hydrocyclones and introduces the slurry to a very
drill string on the borehole wall was generally as- fine, pretentioned vibrating screen. The expensive
sociated with soft, thick filter cakes. Using the al- liquid phase and most of the barite pass through
ready existing desander design, a 4-inch desander the screen and back into the system while the
was introduced in the early 1960s. Results were larger solids are discarded. This was the first suc-
better than anticipated and included longer bit life, cessful application of a screen, bonded to a rigid
reduced pump repair costs, increased penetration frame, using very fine screens. Many mud clean-
rates, and lower mud costs. These smaller hydro- ers have screen cleaners, or sliders, which are
cyclones became known as "desilters" since they circular plastic pieces that vibrate against the
remove a much smaller particle, called silt (15 to bottom of the screen to prevent screen blinding.
30 microns), which is smaller than "API sand." In weighted muds, screens of 200 mesh (74 mi-
As barite and other compounds were developed crons) can be used, which is the upper size
to improve drilling, drilling fluid became very com- for commerical barite. For unweighted muds, the
plex. Also, the liquid phase for carrying solids was smallest practical size is 250 mesh (58 microns)
being reduced by the addition of barite and other for economical operation.