6 - Kuwait Oil Company

Insoluble low gravity solids are drilled cuttings that
are ground up so fine that cannot be removed by
mechanical means. LGS are the worst enemy of
the drilling fluids. In any mud system, the problems
associated with solids are mainly due to low gravity
solids. Controlling the low gravity solids are vital
for the success. If the particle size is less than 5
microns, then the drilled solids remain in the mud.
Indications of High Solids in the Drilling Fluid
There are several indications from which one can
monitor the solid build-up in the drilling mud.
- The first visible indication of solids building up on
the mud is an increase in mud viscosity without the
addition of a viscosifier. This takes place gradually and
it is understood that most significant contribution to
the fluid viscosity comes from the low gravity solids.
- Progressive increase in the plastic viscosity is
another main indicator of building up of LGS
in the mud. PV primarily depends on the size,
shape and the number of solids in the mud.
- Sometimes in the mud, it can be observed that
phenomenal increases occur in the YP and gel strengths.
This happens due to high LGS in the mud system. In
this case any mud treatment will be less effective.
- Increase in fluid loss and deteriorated filter cake.
Effects of Drilled Solids on Mud Properties
For years the drilling industry was not concerned with
the buildup of drilled solids in the mud system and the
mud was circulated as long as it could be pumped.
Problems associated with higher solids, like funnel
viscosity and plastic viscosity were countered by thinners
and dilution. Action should be focused on eliminating
the drilled solids instead of taking counter measures
to treat the mud to be tolerant to the higher solids.
Removal of drilled solids will decrease the plastic
viscosity and thus increase the low shear rate viscosity.
This will bring larger, more easily removable solids
to the surface. On the other hand, failure to remove
the cuttings will increase the plastic viscosity and that
will decrease the ability to bring up the cuttings to the
surface and allow them to grind into finer particles.
When it is suspected that cuttings are not removed
completely from the well, normal reaction is to increase
the yield point and pumping of high viscosity pills.
Sometimes increasing in YP to the significant level
also increases the overall rheology of the mud. This
develops another situation where shakers are not
able to handle the flow which needs to be changed
to coarser mesh size to handle the flow. Again
this increases the finer solids in the mud system.
Solids problem happens several times cyclically.
Any increase in drilled solids concentration on the drilling
fluids between 4-5% has a direct effect on the quality of
the filter cake. As the concentration of the drilled solids
increase further, the filter cake becomes sticky which
results in other drilling fluids related complications.
The situation of very fine particles arises where
drilling fluid fails to remove all the drilled cuttings
from the bottom of the hole due to which the drill
bit grinds/drill the cuttings instead of the formation.
Insufficient hole cleaning is another important reason
for building up of solids in the drilling mud.
Effect of Drilled Solids on Drilling Operations
The drilled solids in the mud system impede every aspect
of drilling operations. They directly affect operations by
increasing pump pressure, wear, high fuel consumption,
and increased wear on all rotating and circulating parts.
The significance of the solids control is that rate of
penetration is closely related to the total solids in the
mud. The graph shows the relation between the solids
content and the drilling rate. The effects are drastic,
in general, studies shows that over a range from 0 to
14% of solids in the mud, the required rig days increase
by the factor of 3 and on the other hand if 14% of the
solids were removed entirely the drilling rate would be
increased by 400%. From the studies it is also observed
that the average gain in drilling rate is around 10% for
removing each 1% volume of solids from the mud.
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