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Quality HDD Support Equipment |
Monitoring Your Solids
by Frank Canon
There is more solids removal equipment used in the HDD industry today than ever
before. This is causing some problems. Some contractors are buying solids
removal equipment without realizing that there are additional properties to be
monitored.
Solids
There are two types of solids – beneficial solids and detrimental solids. The
beneficial solids are bentonite and other drilling fluid additives. The
detrimental solids are those that become entrenched in the fluid during drilling
operations. The beneficial solids are not a concern. Drilled solids are. Solids
in our fluids are like cholesterol in our blood. There is good cholesterol and
bad cholesterol. There are good solids and bad solids in our re-circulated
fluid.
There is still a belief that sand is the detrimental solid for pumps. Sand will
kill a pump. However, there is too much emphasis being put on sand content alone
in re- circulated drilling fluids. Sand is probably the easiest solid to remove
because of its size. We are not downplaying sand content. On the other hand, we
would like to re-emphasize the detrimental effects of solids smaller than sand.
Sand is not a mineral. Sand is a size, specifically 74 microns (200 mesh) and
larger. Solids 73 microns in size and smaller fall into the category of fine
solids or "fines." It is the build-up of the fines that are, in many cases,
being overlooked. Fines can do as much damage, if not more, to a pump than sand.
We make this statement because there is usually a much higher fines content in
these fluids than there is sand content.
Where do these fine solids come from? They usually result from drilling reactive
soils such as clay or shale. However, this is not always the source. They can
also occur in rock drilling, especially when drilling harder rock. They can be
the result of harder tungsten carbide insert bits that tend to make smaller
cuttings, sometimes more like dust or flour than chip-like cuttings.
How can we determine the total solids content of our fluids? Extremely simple
tests can be run with almost no training needed. A mud balance can be used to
determine the drilling fluid weight. From this determination, the solids content
can be calculated. The following formula is used to calculate the total solids
content of the fluid:
(Fluid weight, pounds per gallon – 8.33) x 8 = solids content, % of total volume
The average specific gravity of the solids at 2.5 is assumed in this formula.
There are other methods used in the oilfield to determine solids content but
they are not necessary in our industry because we are not using weighted or
oil-based fluids.
Sand content is also easily determined using a sand content test kit. Sand
content is also reported in percentage of total volume. Sand content is a part
of the total solids content that was calculated above.
Knowing the sand content and the total solids content are the two most important
properties to know when operating solids removal equipment. Without knowing
these values, we have no idea if our equipment is running properly and is doing
its job. We have no idea if adjustments, such as screen size changes, need to be
made. We have no indication that something is not working properly. An increase
in fines may indicate that we need to look for wear on the impellers in our
centrifugal pumps that would not be providing proper header or manifold pressure
to our hydro-cyclones.
We will have tens of thousands of dollars invested in our re-circulating systems
and mud pumps. For a cost of under $200, we can monitor the performance of our
solids removal equipment and protect our pumps. Having this equipment on the
rig, however, is not enough. The equipment has to be used on a regular, routine
basis. We can see trends start happening before there is a problem or major
breakdown.
Viscosity
Viscosity can also be a culprit in solids retention. Do not confuse viscosity
with gel strengths. Gel strengths are necessary to maintain suspension of the
cuttings in the borehole. However, the gel strengths are broken when the fluid
comes under agitation through the pumps. Viscosity refers to the thickness of
the fluid or slurry. Viscosity is not reduced through agitation. Thick, viscous
fluids fight the solids removal equipment to retain the solids.
One of the problems we have is that drilled solids, especially fines, raise
viscosity. A common occurrence in the field is checking viscosity and the
viscosity increases without the addition of drilling fluid additives. Where is
this viscosity increase coming from? The only source of the viscosity increase
is fine solids build-up in the fluid. This then becomes a self-sustaining
problem. We then dilute the fluid with water to reduce the viscosity. The
detrimental solids replace the beneficial solids. What happens to our other
drilling fluid properties such as filtration control, filter cake and gel
strengths? They are either gone or are detrimentally influenced and the result
is that we are left with a false sense of security.
Having pump problems? What is your solids content? If the solids content is 3 or
4 percent and has been maintained at that, you may have a pump problem. If your
solids content is 9 or 10 percent, you may need to take a closer look at
something else. The problem is probably what we are putting through the pump.
When we mix a tank of drilling fluid and sent it through the borehole, the fluid
we get back is not the same fluid.
Frank Canon, Baroid Industrial Drilling Products, is a member of the Drillmaster
Editorial Board. All reports are reviewed by the board: Canon, Ron Lowe, Myers-Aplex
Industries; Mark van Houwelingen, Vermeer Mfg.; and Jerry Watson, INROCK
Drilling Systems.