Page 104 - Shale Shakers Drilling Fluid Systems

CHAPTE R ON E

Introduction

This book is primarily about shale shakers and type of drilling fluid used, and the characteristics
their role in a complete drilling fluid system. A of the solids. There are some wells in which sol-
shale shaker is a device that removes undesirable ids seem to have very little effect. These cases,
solids from a drilling fluid. Drill bit cuttings and which allow high drilled solids content levels to
pieces of formation that have sloughed into the be tolerated, are the exception rather than the rule.
wellbore (collectively called "drilled solids") Many work-over rigs that occasionally do some
are brought to the surface by the drilling fluid. The drilling, still do not employ solids removal equip-
fluid flows across a shale shaker before entering ment. This seems to work satisfactorily only as
the mud pits. Most shale shakers impart a vibra- long as drilling is confined to short intervals. In
tory motion to a wire or plastic mesh screen. This any event, rather than experience increased drill-
motion allows the drilling fluid to pass through the ing costs, it is best to plan for solids control.
screen and removes particles larger than the open- Drilling without removing solids or liquid, the
ings in the screen. Usually, drilled solids must be circulating system will contain a constant volume
maintained at some relatively low concentration. of liquid phase. The pit levels will also remain at
The reason for this control is explained in the next a constant level even though the circulating vol-
section. The shale shaker is the initial and primary ume increases as an additional hole is drilled. (Pit
drilled solids removal device and usually works in levels will actually rise by the volume of the drill
conjunction with other solids removal equipment. string added to the hole.) These additional solids
Drilled solids may also be controlled by adding a from drilling will increase the mud weight. Usu-
fluid containing no drilled solids or a lower drilled- ally, many problems will arise if too many drilled
solids concentration. This reduces the solids con- solids are allowed to remain in the drilling fluid.
centration L, [he circulating system and is called These effects frequently do not immediately appear
"dilution" (see Chapter 8). after the drilled solids concentrations reach exces-
Solids control equipment, also called solids sive levels. For this reason, many drillers accus-
removal equipment, is designed to remove drilled tomed to drilling shallow wells with unweighted
solids from the circulating drilling fluid. This equip- drilling fluids, find it hard to believe that drilled sol-
ment includes a gumbo remover, scalper shak- ids are the cause of their problems. Experience has
ers, shale shakers, dryer shakers, desanders, shown them otherwise. Therefore, it is beneficial
desilters, mud cleaners, and centrifuges. Vari- to take the detrimental effects of drilled solids
ous arrangements of these components are used seriously and remove them from the drilling fluid.
to remove specific size particles from drilling fluid. Some of the detrimental effects are discussed below.
The scalper, shale, and dryer shakers are the main
subjects of this book. The other solids removal
equipment (gumbo remover, desanders, desilters,
mud cleaners, and centrifuges) are discussed in FILTER CAKES
Chapter 7.
As drilled solids are added to a drilling fluid,
fluid loss decreases but filter cakes become thicker
WHY CONTROL DRILLED SOLIDS? and less compressible. Thick filter cakes can cause
many problems:
Usually, a well has a maximum level of drilled
solids that can be tolerated before problems develop. 1. The most common problem caused by poor-
This level depends on the formations drilled, the quality filter cakes is torque and drag, which
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