Fluid Loss Control|
Controlling fluid loss should be an integral part of planning for any completion operation. Prior to bringing any CBF to the location, provisions should be made to deal with any potential fluid loss situation. This section is designed to provide a brief overview of the topic. For a more in depth discussion, see the "Reduction of Fluid Loss" section in Chapter 8.
- Maintain well control—ensure a full column of adequate density brine
- Minimize seepage losses that may contribute to formation damage
- Reduce relative permeability effects
- Stop lost circulation conditions
- Fluid density and pressure differential
- Formation permeability and porosity
- Formation pressure and temperature
- Completion fluid type
- Length of thief zone
Clear brine fluids are designed to minimize formation damage. Despite their low potential for damage, these fluids are still foreign to oil and gas producing formations; for this reason, their introduction to the wellbore may have adverse effects if large quantities of fluid escape the wellbore and enter the formation, mixing with formation fluids.
Knowledge of reservoir characteristics should give some indication as to the potential for movement of fluids from the well into the formation.
Generally there are two types of fluid loss:
- Seepage is the migration of wellbore fluids into the formation under the influence of hydrostatic pressure; it is controlled by formation permeability. With light seepage, penetration may be a matter of only a few inches.
- Lost circulation is severe fluid loss that has reached a rate at which circulation can no longer be maintained. It is wholesale loss of fluid to highly fractured or very porous formations and requires immediate action.
Between these two extremes, there is a continuum that spans the full range. Completion engineers can choose between technologies designed to address three broad categories: (1) light seepage, (2) moderate seepage due to a relatively permeable formation, and (3) lost circulation, which is severe and requires immediate and decisive action.
Light Seepage. In cases of light seepage, consider lowering the density of the fluid to reduce flow into the formation. Well control and safety considerations should both be carefully weighed. If lowering the density is not feasible, a solid free, viscosified pill should be placed across the producing zone to slow the loss. The ability of a viscosified pill to control seepage will depend on the wellbore temperature, as the viscosifying properties of most polymers are reduced at higher temperatures.
Polymer pills are generally applicable in formations with permeabilities of less than one darcy. Most common polymer pills are made using BioPol, TETRAVis, or a combination of the two. A decision as to which polymer to use should be based on temperature stability, salt system, and damage characteristics. For most general brine applications, the TETRAVis products are most widely used, since the polymer is considered less damaging and easier to clean up.
- Single Salt Fluid Polymer Pills. TETRAVis and BioPol L are both commonly used to viscosify single salt fluids. BioPol L is often chosen when bottomhole temperatures exceed 225°F.
- Two Salt Fluid Polymer Pills. The most common viscosifying agent for use in two salt calcium chloride/calcium bromide brines is TETRAVis L Plus.
- Three Salt Fluid Polymer Pills. TETRAVis L Plus is the most common viscosifying agent used for three salt systems. It will not hydrate in three salt fluids containing more than 1% ZnBr2 (0.3% Zn) and less than about 30% ZnBr2 (9% Zn). This range covers three salt fluids with densities up to 17.2 lb/gal. In order to viscosify a heavy fluid between 15.2 lb/gal and 17.3 lb/gal, a special cutback fluid must be made from 19.2 lb/gal Zn/CaBr2 and fresh water.
|A weighted pill containing zinc bromide can be formulated by using a fluid made by cutting back 19.2 lb/gal spike fluid with fresh water. This method will work throughout the range of three salt fluids. (See “Mixing Viscosified Pills.”)
Moderate Seepage. At moderate loss rates, you will want to approach the problem using a mixture of viscosifying agents and bridging material. There are four options available when dealing with moderate loss situations. They are:
- TETRACarb Sized Calcium Carbonate (CaCO3). TETRA’s sized calcium carbonate (CaCO3) pills provide a reversible means of quickly shutting off rapid fluid loss to the formation. Carefully ground and sized particles of CaCO3 are suspended in a viscosified pill and placed across the thief zone. The procedure for building a viscosified pill is given in Chapter 4. (See "Mixing Viscosified Pills.")
|When completion operations are finished, a mild acid treatment may be required to dissolve calcium carbonate solids.
- TETRA SS Sized Sodium Chloride (NaCl). Another reversible means of stopping fluid loss is by using a viscous pill with sized particles of sodium chloride (NaCl) suspended in it. Sodium chloride particles can be used to bridge formation pores and reduce fluid loss. Application is limited to situations where a saturated salt (NaCl) solution can be maintained; otherwise, the particles will dissolve. There are generally sufficient chloride ions from calcium chloride in three salt fluids to keep the salt crystals from dissolving.
|The advantage of sized sodium chloride crystals is that they will dissolve during flowback operations, provided formation fluids are not saturated with respect to sodium chloride. Salt crystals can be removed by using an undersaturated potassium or sodium chloride brine or a fresh water rinse.
- TETRAFlex FLC Seal. For moderate losses, this sized, shredded crosslinked polymer mixed with a brine can be used for fluid loss control. The treatment is completely and rapidly reversible with a mild acid treatment.
- TETRA SmartSeal. In the late stages of completion, especially after a screen and gravel pack emplacement, a TETRA SmartSeal pill will enable fluid loss control while running the final production tubing. SmartSeal is a viscosified pill with a carefully chosen blend of TETRACarb calcium carbonate bridging material. SmartSeal pills are usually small in volume, approximately five bbl, and can be designed to maintain viscosity at temperatures above 300°F. To ensure integrity of the SmartSeal pill and facilitate removal of the calcium carbonate filter cake, a TETRA SmartSeal Pad should be run in front of and behind the SmartSeal pill. In addition to guarding against dilution of the pill, the SmartSeal Pads play an active role by treating the screen to reduce adhesion of the TETRACarb particles, thus making cleanup easier and more complete.
Lost Circulation. For situations involving lost circulation, the primary objective is to seal off the thief zone. Coarse sized calcium carbonate or sodium chloride bridging materials should be used.
A TETRA fluids representative can help with these decisions. Whichever situation is anticipated, fluid loss control should be an integral part of planning for any completion operation. Provisions should be made to deal with fluid loss prior to bringing any CBF to the location. For more information regarding this topic, see the "Reduction of Fluid Loss" section in Chapter 8.