Page 203 - Well Control for Completions and Interventions

196 Well Control for Completions and Interventions

Adjusted brine density is therefore:
d c 5 9:5 1 :0:3474 2 0:0463 5 9:8

The difference of 0.5 ppg between the two example calculations is a
result of using different expansion and compression coefficients. In a
10,000 foot well (as per the example), a 0.5 ppg difference in brine
weight is equivalent to 260 psi; in many cases more than the required
overbalance. These differing results only go to emphasize the importance
of obtaining accurate expansion and compression data directly from the
fluids vendors.
Deepwater wells require the calculations to be performed twice.
• Fluid density in the marine riser (from surface to seabed) where cool-
ing will increase density.
• Fluid density in the wellbore (from mudline to TD).
Having made the density corrections for both sections of the well, the
values are combined to give an overall density for the well.

5.4 CRYSTALLIZATION TEMPERATURE

Brine crystallizes at low temperature. The crystallization temperature
of a brine is the temperature at which solids start to form. Those solids will
either be ice crystals formed by water freezing, or salt crystals forming as
brine salts come out of solution. Allowing brine temperature to fall to the
crystallization temperature is to be avoided. Salt crystals can plug lines and
interfere with pumping operations. They can also plug filtration units. Of
greater concern is the potential for crystallization to result in a reduction of
brine hydrostatic pressure, leading to a well control incident. Since it is the
salts dissolved in water that gives brine the required density, crystallization
of those salts cause the density to be reduced. If reduced density fluid is
pumped into the well, the hydrostatic overbalance is reduced or lost alto-
gether, and a kick can occur. Conversely, if ice crystals form, the fluid
being pumped into the well will be denser than expected, increasing the
potential for fluid loss, perhaps even fracturing the formation. Knowing
the brine crystallization temperature is therefore essential.

Freshwater freezes at 32 F(0 C). Adding salt to freshwater lowers the
freezing point. For example, dissolved sodium chloride (NaCl) reduces