Maintenance of Clear Brine Fluid Properties

This section of TETRA's Engineered Solutions Guide for Clear Brine Fluids and Filtration is concerned with the brine density families depicted in Figure 2. Our goal is to focus your search upon information that is pertinent to your project. For instance, if you are planning a completion using a 3% KCl brine, you may want to skip over information that is specific to high density, three salt fluids. Brine properties also depend on composition; as the number of salts in a fluid goes up, the response to changes in brine properties such as the weight up procedure, cutback procedure, or choice of viscosifying agent to use, will differ. If you are trying to find information quickly, make use of the icons that indicate whether a section applies to one, two, or three salt brines to help narrow your search.

Objectives

• Maintain or adjust fluid density
• Control formation pressure
• Minimize fluid loss
• Maintain adequate volume

Factors Affecting

• TVD and BHP
• Sources of dilution
• Density adjustment options
• Operational constraints

Discussion

Maintaining a consistent fluid density is of primary importance in pressure control. Formation pressure is usually estimated within a narrow range. Occasionally, BHP will be different from that anticipated, and the fluid density will have to be altered to fit the actual well conditions. Adjustments may also be required due to dilution. Lower than anticipated formation pressure and fluid loss may necessitate a cutback or downward density adjustment. Conversely, higher pressure will dictate the addition of weight material to increase density.

Responding to variations in density means that a weight up or cutback procedure may be necessary. Weight material, either a concentrated liquid blending stock spike fluid or dry salt weight material, should be available on location in order to allow you to respond quickly and maintain safe working conditions.

Single Salt Brine Density Maintenance

With densities from 8.4 lb/gal to 11.6 lb/gal and gradients less than about 0.6 psi/ft, single salt brines are employed in lower pressure wells. Maintaining pressure control in these wells is no less important than in any others.

Unintentional Dilution. Well pressure control can be jeopardized by the unintentional dilution of a CBF with any source of lower density fluid such as rain water. The effects of this unintentional dilution will depend on the density difference between the working fluid and the diluting fluid. The greater the density difference, the more sensitive the working fluid will be to dilution.

Weight Up. A single salt brine can be reconstituted by adding dry salt weight material in the correct proportions or by adding liquid weight material or spike fluid.

• Dry Salt Weight Material can be used to increase the density of a single salt fluid that has been diluted, as long as adequate mixing equipment is available. Dry salt additives can also be used if additional hydrostatic pressure is needed to control the well. (See Equation 12.) Weight up tables for the single salt fluids are provided in the "Single Salt Fluid Composition and Blending Tables" section.

 

When adding dry salt to increase density in a fluid, care should always be taken to monitor TCT with respect to environmental considerations.

 

• Spike Fluid is a fluid of higher density that can be added to a working fluid to raise its density. Figure 6, "Selecting and Using Spike Fluids," illustrates the potential for increasing fluid density using a spike fluid. As the density of the working fluid approaches the density of the spike fluid, the volume of spike required to raise the density increases rapidly.

 

Changes in brine density, either by dilution or weight up, will change the TCT of a single salt brine. Make sure you have consulted the "Single Salt Fluid Composition and Blending Tables" section before making any change to the fluid density.

Adding dry salts to a fluid can result in a substantial increase in brine temperature. A quick pilot test should be run to get a sense of the temperature rise that may occur. ALWAYS ADD DRY CHEMICALS SLOWLY, AND FREQUENTLY CHECK THE TEMPERATURE INCREASE.

Make an estimate of the quantity of weight material required to recover from a 0.2 lb/gal drop in density, and have at least that amount available on location.

Cutback.

To reduce the hydrostatic pressure on a formation, in order to slow fluid loss for example, the density of a fluid can be cut back by adding water. Cutback tables have been provided for each of the common single salt brines in Chapter 6. (See "Single Salt Fluid Composition and Blending Tables.") The values in the tables are decimal fractions of a barrel of starting brine needed to make one barrel of final density brine when diluted with water. Alternatively, Equation 15, used for cutting a fluid back using weight percent salts, has also been provided in Chapter 4, "Field Applications and Brine Maintenance."

Seawater, due to the dissolved minerals it contains, is not recommended for use in brine cutbacks.

 

Two Salt Brine Density Maintenance

Maintaining the density of a two salt brine is more complicated than for that of a single salt fluid. Standard two salt fluids are blends of calcium chloride (CaCl₂), calcium bromide (CaBr₂), and water. The relative proportion of each component determines the density and TCT of the fluid. Any changes you make to weight a fluid up or cut a fluid back must be made carefully or your TCT will be altered.

Density adjustment is most commonly required as a result of a fluid's dilution by rainwater, field brine, or simply by absorption of water from the air.

Protecting CBFs from dilution is extremely important. It takes only 3.4 bbl of fresh water to drop the density of 100 bbl of a 14.5 lb/gal CBF by 0.2 lb/gal, which represents a 50 psi drop in bottomhole hydrostatic pressure in a 5,000 ft well.

Dry Salt Weight Up.

Under certain circumstances, dry salt may be used to increase fluid density; however, caution should be exercised to closely monitor TCT with respect to environmental considerations. Weight up of a two salt fluid by adding dry calcium chloride (CaCl2) is not recommended, as doing so will alter the ratio of CaCl

₂

to CaBr

₂

and increase the fluid's TCT. It is also difficult to mix dry salts into nearly saturated brines without using specialized mixing equipment. In the event of a severe limit on fluid volume, addition of dry calcium bromide (CaBr

₂

) can be used as weight material in two salt fluids. Any composition changes should be verified using one of TETRA's proprietary fluid blending programs.

 

Spike Fluid Weight Up. For midrange densities, a liquid 14.2 lb/gal spike fluid can be sent to location for any necessary weight up applications. Calcium bromide 14.2 lb/gal blending stock is an effective spike fluid that can be used to weight fluids up from about 11.7 lb/gal to 13.5 lb/gal; however, if the target density is greater than 13.5 lb/gal, the volume increase with a 14.2 blending stock is more than 50%. A 15.1 lb/gal spike fluid can be used to achieve a weight up in the range of 13.6 lb/gal to 14.8 lb/gal; above that range, the volume increase would be more than 50%.

_Care should be taken when utilizing a 15.1 lb/gal calcium chloride/calcium bromide (CaCl2/CaBr2) spike fluid, as it may raise the TCT of the resulting brine. _

Finally, in extreme cases where the working fluid lies on the boundary of two and three salt fluids as shown on

Figure 2

, a zinc containing fluid can be used to achieve density increases. In such cases, a very small amount of 19.2 lb/gal spike fluid would be required; however, it would change the nature of the working brine; it would also change the environmental regulations regarding conducting disposal activities and reporting and reacting to spills. Before making this decision, weigh the pros and cons carefully.

Estimate the type and quantity of weight material to be used on the job based on raising the density of the working fluid by 0.2 lb/gal. Make sure this amount of material is available on location. It should be kept in sealed pill tanks to prevent any contamination and/or absorption of water from the atmosphere.

 

Three Salt Brine Density Maintenance

High density three salt fluids are blended with precision to maximize performance and minimize the cost to the operator. This special attention to composition means equal care and attention should be paid to maintaining fluid properties like density, TCT, and brine clarity.

It is especially important to protect three salt fluids from dilution. To prevent contamination and absorption of water from the atmosphere, this material should be kept in closed top tanks.

There is a considerable difference between the density of a three salt fluid and most types of dilution water. Rainwater, for example, has a density of 8.3 lb/gal, and formation water has a density ranging from 8.5 lb/gal to 10 lb/gal. A small amount of contamination from either of these sources can result in a large change in the density of the working fluid. For example, it takes only 2.6 bbl of fresh water to decrease the density of 100 bbl of a 16.5 lb/gal three salt fluid to 16.3 lb/gal (a 0.2 lb/gal decrease). In a 5,000 ft well, this dilution would reduce hydrostatic pressure by more than 50 psi.

 

The most economical means of maintaining the prescribed density in three salt fluids is by the addition of 19.2 lb/gal zinc/calcium bromide (ZnBr2/CaBr2), a common blending stock used in formulating CBFs. For working fluids with densities higher than 16.5 lb/gal, an even more concentrated zinc bromide (ZnBr2) fluid will have real economic advantages. This fluid has a density of 20.5 lb/gal. The effects are clearly shown in Figure 6, "Selecting and Using Spike Fluids."

The advantages of using a higher density spike fluid are:

1. a much smaller volume is required to achieve the same density increase,
2. less storage volume is needed for spike material on the rig, and
3. less volume increase occurs in the working fluid when adjustments are made.

Density adjustment using solid chemicals such as dry calcium chloride is possible, but is not recommended, as it will usually result in a higher TCT. Calcium bromide dry salt can be used as weight material, especially when volume increases are not practical. Dry salts will also require vigorous agitation to achieve complete solubility in highly concentrated, heavy fluids.

The quantity of 19.2 lb/gal zinc/calcium bromide spike fluid should be estimated based on raising the working fluid density by 0.2 lb/gal. As a minimum, this amount should be maintained on the rig. To prevent contamination and absorption of water from the atmosphere, this material should be kept in sealed pill tanks.

This section is intended to briefly touch on brine maintenance. See Chapter 4, "Field Applications and Brine Maintenance," for a more in depth discussion on the subject.