Page 253 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 229

2.5.6
Drilling Fluids Program [ ]

adsorption of one phase onto the rock). In any two phase reservoir,
there will always be a “wetting” phase and a “nonwetting” phase. In
an oil/water system, water will normally be the “wetting” phase and
in the case of a gas/oil system, it is oil. Changes in oil or water satu-
ration can alter wettability of a reservoir. However, some surfactants
in the filtrate of a drilling fluid can also cause changes in wettability.
A change in wettability in a reservoir from water wet to oil wet can
reduce the permeability to oil. This is particularly the case in low-
permeability reservoirs.
The surfactants that will cause a change in wettability from water
to oil are usually cationic, but some nonionic surfactants can also have
this effect.
Proactive measures to be applied to drilling fluids to minimize
formation damage. There are some reservoirs that are not at all sus-
ceptible to damage by most drilling fluids. The damage that occurs is
easily cleaned up. Heavily fractured carbonates would be one example.
However, listed below are some considerations for minimizing forma-
tion damage.
Whatever mud is used, minimize on all insoluble solids and par-
ticularly low gravity solids. The cation exchange capacity of the mud
should be as low as possible. If necessary, consider displacing at the top
of the reservoir (if there is a casing point) to a drill-in fluid. In this
drill-in fluid, do not use any clay to get a filter cake. (In a polymer
mud, around 6 ppb of clay is necessary to get filtration control irre-
spective of how much fluid loss control additives are added). Calcium
carbonate bridging agent might be used to aid filtration control along
with HEC for viscosity and appropriate fluid loss control additives.
The bridging agent’s particle sizes should be designed around the
pore throat diameter, if known. The best way to determine the opti-
mum bridging agents is by return permeability tests on core samples.
The fluid needs to be designed such that a thin filter cake is formed
with low spurt loss invasion that will come off the formation when
flow starts. This will require a combination of inert bridging solids
(e.g., CaCO ) of a size range from one-third pore size upwards, togeth-
3
er with hydrocolloids such as starch. The coarser particles bridge first,
then progressively finer particles, and finally the hydrocolloids pro-
gressively block the remaining spaces. Spurt loss takes place at this

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