BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Turbidimeter (nephelometer)
The measurement of brine clarity (solids content) is not a direct measurement of the concentration
of suspended particles but a measurement of the scattering effect that such particles have on light.
On-site measurement of brine turbidity with a portable nephelometer is quick and easy. Field
testing has shown that this turbidity method is a useful way to evaluate brine quality and is most
often used to decide when to stop displacement to brine after clean up.
The procedure only places a relative value on the presence or absence of solids in a particular
sample. The unit of measurement for turbidity is called a nephelometer turbidity unit (NTU). The
absolute NTU value of brine is specific to each fluid and can be only used for comparison values to
the same fluid. The absolute NTU reading is dependent on fluid color, solids distribution, air
entrapment, etc. Each operator or completion procedure will have specific requirements for NTU
reduction. Typical requirements are 50 – 100 NTU.
The field use of a turbidimeter is recommended as a guide only. For example, during filtration,
when the NTU values are no longer falling, no further gains in fluid quality will be made without
making some change to the filtration process.
Solids Content
Measurements of solids content can be taken using a portable electrical centrifuge. For most
operations, a solids content of < 0.05% is acceptable. For gravel packing this should be reduced to
0.02%
NTU and solids content are not directly related. Solids content will only assess materials collected
at the bottom of the test tube during centrifuging.
These tests use sample sizes of a few mls and it is difficult to draw conclusions about a well with
an annular volume of 500 bbl. 0.1% vol/vol solids is equal to more than one ft 3 of solids deposited
for tubing contents of 200 bbl.
When clear fluids are pumped without adding oxygen scavenger, rusty coloured fluid will, after
some circulation, return to surface. Corrosion products in return fluids give high NTU values. To
deal with the problem of rust (as a result of oxidation of water wet steel surfaces) impacting
turbidity readings, a little acid (HCl) should be added after the initial reading. When the well is
displaced to packer fluid, it should always be inhibited with oxygen scavenger.
Where the string is rotated during clean up the increase in torque can be used as an indicator of
clean well. The coefficient of friction in sea water/brine is more than twice that of OBM. Typical
friction factors for OBM is 0.12-0,15 and 0.20-0.29 for seawater/brine.
For critical wells, e.g. high angle wells/where milling has occurred/previous problems with debris
etc, it is recommended to use ‘Wellbore Custodian’ or similar junk basket. If the junk basket is
pulled full it should be re-run.
Visual inspection of the clean up string after pulling out of the hole will give a good indication of
the cleanliness of the well. If it is mud free and water wet, the mud displacement and clean out has
been successful. If the string is mud coated, a junk basket should be run.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-95