The WSS will occasionally need to build completion or workover fluids. Many workovers require a very simple fluid, such as NaCl brine, 2% KCl, or a
conventional drilling mud, and the workover program may call for the fluid to be mixed on the rig. At other times, the WSS will have to change the density of commercially prepared brines. For example, if an order of 10.2 ppg brine proves to be only 9.7 ppg when delivered to the rig and weighed, the crew will have to increase its density. The equations in Fig. 5-3 and Fig. 5-5 can be used to calculate the amount of salt required to increase fluid density.
Solids-Laden Fluids
The equation in Fig. 5-3 is used to determine the amount of weight material required to change the density of a solids-based fluid in which the solid does not dissolve in the base fluid (e.g., barite, calcium carbonate). Note that there are separate equations for brines.
Figure 5-3 Increasing density in solids-laden fluids
To decrease the density of a solids-laden fluid by adding water, use the equation in Fig. 5-4.
Weight of Material Required (lbs per bbl base fluid) = ppb solid × fw2 - fw1 ÷ (ppg solid - fw2)
ppb solid = pounds per barrel of weight material (barite = 1,470) ppg solid = pounds per gallon (ppg) of weight material (barite = 35) fw2 = desired fluid weight, ppg
fw1 = initial fluid weight, ppg Example:
Given: 500 bbl weighted barite-based fluid with a density of 9.6 ppg Find: Pounds of barite required to increase density to 10.2 ppg Solution:
Pounds per barrel = 1,470 × (10.2 - 9.6) ÷ (35 - 10.2) = 1,470 × 0.6 ÷ (24.8) = 35.56 ppb
Figure 5-4 Decreasing density of solids-laden fluids
Single-Salt Brines
Building a single-salt brine to a specified density is a simple procedure that involves using commonly available brine tables (see Table 5-4 through Table 5-7). The tables show the required salt and water volumes to build one barrel of the required density. Multiply those numbers by the desired total volume to determine the total salt and total water required.
The density of an existing brine can be increased by adding salt, using the brine tables along with the equation in Fig. 5-5.
Brine density is decreased by dilution—i.e., by adding fluid (generally fresh water). Fig. 5-6 provides the required calculations.
Liquid Volume Required to Reduce Density of Solids-Laden Fluids
where:
W1 = Original fluid density (ppg)
W2 = Desired reduced fluid density (ppg) Dw = Density of liquid used to dilute (ppg)
Example:
Given: 100 bbls of 14.0 ppg oil-based fluid; diesel with density 7.0 ppg Find: Barrels of diesel to dilute fluid to 12.0 ppg
Solution:
Liquid Required (bbls) Initial Fluid Volume (bbls)×(W1–W2) W2–Dw ( ) --- = Liquid Required 100×(14.0–12.0) 12.0–7.0 ( ) --- 200 5.0 ---- 40 bbls diesel = = =
Figure 5-5 Increasing density in single-salt brines
Pounds of Salt Required (per bbl existing brine) = Wi × Sf ÷ Wf - Si Final Brine Volume = Initial Volume × Wi ÷ Wf
Wi = water per bbl (from table) at initial density Wf = water per bbl (from table) at desired density Si = salt per bbl (from table) at initial density Sf = salt per bbl (from table) at desired density Example:
Given: 200 bbl of 10.4 ppg CaCl2 brine
Find: Pounds CaCl2 to increase density to 11.0 ppg and final volume Solution:
From Table 5-7: Wi = 0.909, Wf = 0.879, Si = 119.0, Sf = 155.0 Added salt per bbl = 0.909 × 155.0 ÷ 0.879 - 119.0 = 41.29 lbs/bbl Total salt required = 41.29 × 200 = 8,258 lbs
Figure 5-6 Decreasing density by dilution
Decreasing brine density will change the crystallization temperature (as shown in Table 5-4 through Table 5-7), so the brine may freeze at a higher temperature. Always check the appropriate brine table for the fluid you are using. Look up the crystallization temperature at the final density. If the ambient temperature in the mixing and storage area is likely to be lower than that, take precautions. If the mixing and storage area is enclosed, use space heaters to raise and maintain the brine temperature above the crystallization temperature before changing the density.
Multiple-Salt Brines
Two- and three-salt brines are needed to achieve the higher densities required to control higher formation pressures. Some of these brines can be mixed to 19.2 ppg density (see Table 5-3). However, changing these brine weights in the field must be done carefully to avoid salt precipitation. Water additions and exact salt proportions are required when increasing density, or the least soluble salt(s) can precipitate out. Equations for calculating salt and water additions are more complex than for single
Barrels of Fluid Required to Decrease Density (per bbl existing brine) = (Di - Df) ÷ (Df - Da)
Di = initial brine density, ppg Df = desired brine density, ppg
Da = density of fluid added to dilute, ppg (fresh water = 8.33 ppg) Example:
Given: 150 barrels of 9.9 ppg NaCl brine Find: Barrels of water to dilute to 9.4 ppg Solution:
Di = 9.9 ppg, Df = 9.4 ppg, Da = 8.33 ppg
Water required per barrel = (9.9 - 9.4) ÷ (9.4 - 8.33) = 0.467 bbl per bbl Total water required = 0.467 × 150 = 70.05 bbl
salts. When working with these heavyweight fluids in the field, experienced fluid personnel are required on location to maintain correct fluid properties.
For an example of a two-salt brine density calculation, see “Increasing Density in Multiple-Salt Brines” on page A-8 in the Appendix.
Thermal and Crystallization Effects on Density
Commercially premixed brines are used in many workovers where formation pressure requires greater than about 11.5 ppg fluid. Generally, the completion engineer who developed the workover procedure will specify the brine properties. The brine plant will need to know the minimum and maximum expected
temperatures so that the brines are optimally mixed to yield required densities under downhole temperature conditions and still prevent crystallization in the coldest expected ambient conditions for the geographic location and the season. For example, Wyoming, USA, might have a range of -20°F air temperature to 160°F bottomhole temperature in the winter.
Brines can be ordered in “summer” or “winter” blends, which allows for seasonal temperature ranges.
When weighing a brine on the rig, use a brine hydrometer instead of a mud balance, which can produce an error of up to 0.5 ppg. The hydrometer’s temperature
correction capability was discussed earlier (see “Density” on page 5-5). The equation for correcting temperature with a hydrometer is shown in Fig. 5-7.
Figure 5-7 Temperature correction with a hydrometer
Remember that the density of the crude oil in the hole is also temperature sensitive. If a mud balance is used to weigh the oil, the actual downhole density will be less and must be corrected with the appropriate equations (see “Crude Oil Hydrostatic Pressure” on page 2-11). If a hydrometer is used to measure the density of the oil, a temperature correction for the oil can also be calculated with the equation in Fig. 5-7.
Unintentional Brine Dilution
Brines can lose density when they become inadvertently diluted with water, as when rainwater or rig washdown water enters the brine tank. Brines (especially the heavy multiple-salt brines) can also become diluted by absorbing water directly out of the air. For these reasons, brines (like oil-based muds) should be stored in covered tanks on the rig.
Corrected Brine Density (ppg) = Specific Gravity × 8.33 ×
Temp. Conversion Example:
Given: Air temperature = 92°F, s.g. = 1.2; temperature conversion table value = 1.110.
Find: Brine weight in ppg, corrected for standard temperature
Solution:
Corrected Brine Density =