Filtration and Brine Clarity|
Brine filtration is another component in TETRA’s integrated productivity protection system which encompasses more than just the fluid. Workover and completion operations often generate solids that are picked up and transported by the CBF. These solids can be carried into the formation or perforations. Such solids can be difficult to remove and their presence often results in lower productivity. In order to eliminate the possibility of production impairment, brine filtration is used to remove solids from the system. Another important aspect to consider when planning a filtration operation is the potential impact that solids can have on corrosion within the annulus, especially with the use of CRAs. Effectively removing these solids is one more way to reduce the probability of corrosion.
Filtration equipment is operated continuously during all phases of completion operations as long as the CBF is in the hole. The primary goal of this section is to assist you in selecting the correctly sized filtration equipment. Chapter 8, "Special Topics," also includes a section on filtration, which provides a better understanding of the filtration process.
- Protect the producing formation
- Remove solids from completion fluid
- Remove potential contaminants for CRA applications
- Assure an uninterrupted supply of filtered, solid free completion fluid
- Ensure tool operability
- Hole volume and available tankage
- Efficiency and type of displacement
- Type of completion
- Anticipated flow rates
- Deck space
- Brine density and viscosity
Protection of the formation should always be the primary objective of filtration. To optimize filtration operations, the following factors should be considered.
Hole Volume. Simply put, the larger the hole, the larger the filtering equipment should be. Typical flow rates for plate and frame filter presses range from 0.8 bbl/min to 1.2 bbl/min per 100 square feet of filter area. A hole with a volume of 800 barrels and appropriately sized surface tankage may have a circulating volume of as much as 1,800 barrels. Using the above range of 0.8 bbl/min to 1.2 bbl/min per 100 square feet, a 1,100 square foot filter unit would operate in the range from 8.8 bbl/min to 13.2 bbl/min. Using an average value, it would take 2.7 hours to filter the entire fluid volume once.
Drilling Fluid Displacement. Brine filtration is primarily a means of removing potentially damaging particulate matter. It is not designed to remove large amounts of solids resulting from incomplete mud displacement or tank cleaning operations. For the best utilization of filtration equipment, and to minimize downtime, other strategies should be employed for the removal of bulk solids. The upper limit for an efficient plate and frame operation is a solid level of around 2% by volume. Fluids with solid content above 2% should be displaced with clean fluid.
Downtime may become significant if the filter is not correctly sized for both flow rate and solid content. As an example, a fluid with a solid content of 0.25% would have a filtration unit operating time of 2.6 hours using a 600 square foot filter unit. Filtration unit operating time would increase to 4.7 hours with an 1,100 square foot unit. Each precoat cycle may require up to 60 minutes. Downtime would be greatly reduced from 7.2 hr/day to 4.4 hr/day with the larger unit. There is a distinct advantage in using a larger unit.
Type of Completion. Some downhole activities like gravel packing or milling operations are likely to generate a larger volume of solids, which will influence the size of the filtration unit required. The rate at which solids accumulate in the filter will determine the active filtration time. This subject is discussed in more detail in Chapter 8. (See "Filtration.")
Available Tankage. Efficient filtration requires an available brine holding tank capacity of about 0.75 to 1.0 times the hole volume. The filter feed tank and clean brine tank should be about the same size. This allows larger particles to settle and increases the active filtration time for the filter unit. This is an advantage, because the filter press must be broken down, washed out, and precoated each time the chambers are filled.
Larger tanks provide a wide spot in the circulating path, allowing some settling and taking some of the load off the filter. If rig space is limited and tankage is small, more solids must be captured by the filter. If settling is limited, the filter chambers will fill more rapidly and a larger filter would be advisable. A smaller filter feed tank, for example 75 bbl, will reduce settling time. This also means a shorter interval for dumping and precoating, only 10 minutes at a circulating rate of 7.5 bbl/min, before rig operations would have to be suspended until filtration equipment could be brought back online.
Deck Space. Available floor space on the rig may dictate the size of the filter unit used. All TETRA filter units are specifically designed for completion fluid applications.
TABLE 8. Filtration Equipment
|TETRA Filtration Equipment
||L x W
||6 - 8
||22 x 15 feet
||8 - 10
||16 x 15 feet
||8 - 12
||24 x 15 feet
||12 - 14
||26 x 15 feet
||14 - 20
||26 x 15 feet
Some general specifications of TETRA’s filtration equipment are shown in Table 8 to assist you in making your selection. The Filtration section in Chapter 8, "Special Topics," explores the subject in greater detail.
Viscosity. Lightweight, single salt fluids typically have a viscosity of less than 10 centipoise (cp), which will generally result in lower head losses through the circulating system. Both the density and the viscosity of a completion fluid should be considered when determining the filter unit size required for a specific job.
Above 10.0 lb/gal, the viscosity of CBFs can range up to 30 cp. The effect of viscosity is most noticeable in the case of two salt summer blends which may be saturated with calcium chloride. The increased viscosity will also cause compression of the filter cake, reducing its permeability. This will lower flow rates and decrease operating times. In cases such as these, a slightly larger filtration unit is recommended.
- Plan and carefully execute a complete displacement of drilling mud to reduce CBF contamination.
- As a general guideline, plan a filtration unit that will provide 0.8 bbl/min to 1.0 bbl/min per 100 ft2 of filter area.
- Select filtration equipment that will minimize downtime by estimating solid loading and filtration unit operating time; where settling time is limited by pit volume, a larger filter unit should be selected.
- Provide adequate tankage; 100% of the hole volume is ideal, but at least 75% is recommended to maintain an uninterrupted supply of clean completion fluid at all times.
- Establish a baseline nephelometric turbidity unit (NTU) value for the fluid at the rig site as the reference turbidity value for filtered brine.