Page 165 - Drilling Technology in Nontechnical Language

156 Drilling Technology in Nontechnical Language Second Edition

▪ Carry drilled cuttings to the surface while circulating.
▪ Suspend the cuttings to prevent them falling back down the hole
when pumping stops.
▪ Release the drilled solids at the surface so that clean mud can be
returned downhole.
▪ Keep the bit cool.
▪ Provide lubrication to the bit and drillstring.

▪ Allow circulation and pipe movement without causing formations
to fracture.

▪ Absorb contaminants from downhole formations and handle
the difference between surface and downhole temperatures, all
without causing serious degradation of mud properties.
To perform all these functions requires characteristics that may
sometimes be contradictory. This requires the best overall compromise to
balance the various needs.

Basic Mud Classi cations

Drilling fluids can be divided into seven major classifications,

depending on the continuous phase fluid and the type and condition of
the major additive within the continuous phase. (As defined in the Dowell

Drilling Fluids Technical Manual [1994], the continuous phase of a drilling

fluid is the main component of the system, the carrying phase into which
everything else is mixed.)
1. Fluids with water as the continuous phase (fig. 7–1), and with

clays present dispersed throughout the water (“dispersed water-
based muds”).
2. Fluids with water as the continuous phase and with clays present
inhibited from dispersing throughout the water (“nondispersed
water-based muds”).

3. Clear fluid systems based on water with soluble salts used to

control density (“solids free” systems or “brines”). Brines may
include acid soluble solids that can be removed from the reservoir
face by circulating acid past the reservoir.

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