Page 251 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 227

2.5.6
Drilling Fluids Program [ ]

Formation damage due to insoluble solids. Solids from the
drilling fluid may plug the face of the wellbore or the perforations, or
migrate into the pore spaces of the formation. These solids may be
weighting agents from the mud, low-gravity drilled solids and clays,
mud makeup materials such as insoluble polymer, corrosion by-prod-
ucts and scale, cement solids, excess drillpipe dope, or insoluble
hydrocarbon material from the reservoir.
The larger solids tend to bridge off against the wellbore face. In
some cases, they can be removed by back flowing the well but this may
not always be the case. If back flow only causes movement in a small
part of the zone, there may not be enough differential pressure along
the rest of the zone for the solids to be removed.
If particles are small enough, they may enter the formation pores
and block them off. Blockage of pores inside of the reservoir does not
only come from migration of fine solids from the fluid in the annu-
lus. Disturbed interstitial material in the pores may become free to
migrate further into a sand and plug off. Some reservoir liquids may
provide precipitates when contacted by mud filtrate or when subject-
ed to pressure change. These precipitates may become another source
of plugging material.
Formation damage due to the liquid fraction. In some cases, the
liquid fraction, usually in the form of filtrate (but in some cases as
whole mud invasion) can contribute to formation damage. The fluid
can cause hydration of interstitial clays. It may disperse or flocculate
these clays which can move further into the reservoir and plug pores.
It may dissolve matrix cement material, allowing fine particles to
become mobile, and migrate further in to block pore constrictions. It
may cause precipitates to be generated by reacting with connate water
or formation crude. It can be appreciated that the liquid fraction of the
mud, as with the solids phase, can set up solids blocking mechanisms.
The liquid fraction, however, can cause damage in other ways as
well as initiating solids blockage. Substantial fluid invasion can cause
an increase in water saturation, which will affect the relative perme-
ability to oil. It will affect the flow of oil.
To really understand the effect of change in water saturation, the
reader is directed to reservoir engineering and production operations
manuals. An attempt is made here to condense and simplify the con-
tent (if such a thing is possible).

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