Page 214 - Well Control for Completions and Interventions

Completion, Workover, and Intervention Fluids 207

A centrifuge can be used to separate solids from the liquid and a sim-
ple measurement of percentage solids by volume recorded. This is a
coarse measurement, but quick and easy to obtain at the wellsite. As with
the NTU value, there is no measurement of particle size or distribution.
A maximum solids-by-volume content of 0.05% is widely used across the
industry as an acceptable measure of fluid cleanliness.
Particle size and particle distribution (if required) can be measured
using a Coulter counter or laser particle size analyzer.

5.8 BRINE FILTRATION

Removal of unwanted solids from brine requires the fluid to be fil-
tered. Filtration is normally a two-stage process.
High volume coarse filtering is accomplished using large press filters.
2
Press filters come in a range of size and throughput capacity. Large (1500 ft )
units can filter up to 22 bbls/min, and will filter out any solids larger than
8 10 µm (nominal) using diatomaceous earth as the filter medium.
Where a very high degree of fluid cleanliness is required, for example,
brines used with low permeability reservoirs, brines used to mix lost cir-
culation polymers and brines used for gravel packing, cartridge filters are
used. These can remove all particulate matter larger than 2 µm.

5.9 FLUID LOSS CONTROL

Where brine is used as a well control fluid barrier, overbalance pres-
sure is typically 200 300 psi. This pressure, acting against a permeable for-
mation, means fluid will continually seep away to the formation, unless
preventative action is taken. In low permeability reservoirs, fluid loss is man-
ageable and no additional action is necessary. In more permeable reservoirs,
measures will have to be taken to slow or stop fluid loss. In some reservoirs
controlling fluid loss can be difficult; for instance low pressure, naturally
fractured, and highly permeable reservoirs. Losses can also be triggered, or
made worse, through some mechanical actions such as surging when run-
ning packers and bridge plugs. In some cases, fluid can be lost to one layer
in the formation while a gas influx is taking place at a different layer.