Page 55 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:55 PM Page 31

1.3.5
Precompletion and Completion Design [ ]

1. Density. Different brines have different ranges of possible densities.
Downhole density can be significantly different to surface density
due to the effects of pressure and temperature. This difference is
greater with heavier brines. The desired density will restrict the
choice of brine to use.
2. Compatibility. The brine system must be compatible with the reser-
voir solids and fluids to ensure that solid additives, precipitates, or
emulsions do not form and block the reservoir; and to minimize
problems with the well (e.g., corrosion).
3. Cost. Different brine configurations are possible to meet the two
criteria previously listed, but the cost can vary significantly
depending on the salt(s) used.

Additives can be used in the base brine system to control other
properties such as fluid loss.

Salts used in brines. The general salts used in the oilfield for brine
formulation include sodium chloride, potassium chloride, calcium
chloride, sodium bromide, calcium bromide, and zinc bromide. Other
less commonly used salts include magnesium chloride, ammonium
chloride, sodium formate, and potassium formate.
Some of these salts can be blended together to produce the most
cost-effective recipe at a certain density. This is commonly the case
when mixing high-density brines using expensive bromides.
Many salts, especially calcium chloride, are manufactured at a vari-
ety of purities. When comparing costs of salts for formulating brines,
base the calculations on the salt purity that will actually be supplied.
The cost per unit of actual chemical is what should be compared.
Effects of temperature and pressure on brines. Often overlooked
during brine planning is the effect of downhole temperature and pres-
sure on the density of a column of brine. When calculating the required
density this must be considered, especially when using high-density
brines. This depends on several factors: brine type, brine density, well
depth, ambient temperature, and bottom hole temperature.
Specialist companies can run a program for your specific well, plot-
ting the temperature effect on the brine density against depth (See Fig.
1-4). This allows the average density at any depth in the hole to be cal-
culated. This will determine the density that must be achieved at sur-
face temperature to produce the desired density in the hole.

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