5. Fluid Systems
5.2 Water-Based Crosslinked Systems
The majority of hydraulic fracturing treatments are carried out using water based crosslinked gels. These systems offer the best combination of low cost, ease of use, high viscosity and ease of fluid recovery. Generally, water based crosslinked gels will be used unless there is a specific reason not to use them – they are the default option.
The starting point for a crosslinked system is a linear gel, as described above in Section 5.1.
When used for crosslinked systems, linear gels are often referred to as base gels. The most commonly used linear gels are guar and its derivatives; HPG, CMG and CMHPG.
5. Fluid Systems
A crosslinked gel, as illustrated in Figure 5.2a, consists of a number of hydrated polymer molecules, which have been joined together by the crosslinking chemical. This series of chemical bonds between the polymer molecules greatly increases the viscosity of the system, sometimes by as much as 100 times.
In order for an efficient crosslink to occur, two separate things need to happen. First, the base gel needs to be buffered to a pH which will allow the crosslinking chemical to work. Usually, this is at a different pH to that required for polymer hydration, so a different pH buffer has to be used. Secondly, the crosslinking radical needs to be present at sufficient concentration. If both these conditions occur, the gel will experience a dramatic increase in viscosity.
Figure 5.2a – A crosslinked polymer. ‘A’ shows the hydrated polymer prior to addition of the crosslinker. ‘B’ shows the crosslink chemical bonds between the polymer molecules.
Obviously, a fully crosslinked polymer is extremely viscous, and can result - under the wrong conditions - in a high level of fluid friction as it is pumped downhole. To counter this, it is quite common to use a delayed crosslinker. A delayed crosslinker can take anything up to 10 minutes before the gel is fully hydrated, depending upon the temperature, initial pH and shear that the fluid experiences. The ideal crosslink delay system would delay the onset of crosslink as long as possible, but would still have the fluid fully crosslinked by the time it reaches the perforations.
The most commonly used crosslinking systems are as follows:-Borates
“Exotic” Borates Zirconates Aluminates Titanates
Figure 5.2b – pH ranges for crosslinkers (after SPE 37359)
A B
Zirconates
Aluminates
Organic Titanates
Borates
0 1 2 3 4 5 6 7 8 9 10 11 12
Of these, the borates and “exotic” borates are by far the most commonly used, followed by the zirconates. Figure 5.2b illustrates the pH ranges of these crosslinkers, whilst Figure 5.2c shows their temperature
ranges:-Figure 5.2c – Temperature range for crosslinkers (after SPE 37359)
All crosslinked gels tend to be shear thinning, which means that the apparent viscosity of the fluid decreases with shear rate. This is because the shear acts to break the crosslink bonds between the hydrated polymer molecules. Borate crosslink bonds will reconnect and produce a good quality gel after the shearing has taken place. However, zirconate bonds are much more shear sensitive and may not reconnect. Therefore, it is essential to consider the level of shear that a fluid will experience when selecting a crosslinker.
Like most fracturing companies, BJ Services tends to classify its crosslinked fluids systems by the type of crosslinker
used:-Viking™ is a guar-based system that uses conventional borates for the crosslink. It is a cheap, easy to use fluid intended for low temperature applications. There is no crosslink delay. Crosslinkers used are XLW-4, XLW-32 or XLW-10.
Viking D is the delayed crosslink version of Viking, and uses the crosslinkers XLW-30 or XLW-30A.
Spectra Frac G® is probably the most commonly used of all BJ’s borate frac fluid systems. It is guar based, and uses an organo-borate crosslinker for a much greater temperature range than the Viking systems. The system is a premium system at lower temperatures, typically providing more viscosity. The crosslinker can be delayed, and the length of time for the delay can be varied over a significant range. The crosslinker for the system is XLW-24.
Spectra Frac G® HT is the high temperature version of Spectra Frac G®. It is guar based, and uses an organo-borate crosslinker for a much greater temperature range than the Viking systems. The crosslinker also employs a self-breaking mechanism, which helps to reduce the viscosity over a period of time above +/- 230°F. The crosslinker can be delayed, and the length of time for the delay can be varied over a significant range. The crosslinker for the system is XLW-56.
Lightning™ is a new fracturing fluid system that uses a newly developed low-residue guar polymer, GW-3. The system uses the same borate crosslinkers as Viking™.
Medallion Frac® is a CMHPG based system that uses a zirconate crosslinker. Unlike the borate systems, which operate at a pH above +/- 9.0, Medallion Frac® operates at a pH below neutral, usually around 4.5 to 5.5. Because of its low pH, it is the fluid of choice for CO2 foam fracs, pads for acid fracs, and for formations which are sensitive to high pH’s.
100 150 200 250 300 350 400
Zirconates Aluminates
Titanates
High Temperature Borates Conventional Borates
Temperature, oF
5. Fluid Systems
Crosslinkers for the system are XLW-41, XLW-53 or XLW-60. XLW-60 is a delayed crosslink, whilst XLW-41 and XLW-53 are designed for a rapid crosslink. The crosslinkers can be used together in varying proportions to adjust the crosslink time as desired.
Medallion Frac HT® is a high pH version of Medallion Frac®. It uses a different buffer to achieve the required pH (usually around 8.0 to 9.0), but otherwise is the same as Medallion Frac®. The high pH zirconate system is more temperature stable than the low pH. Generally, the high pH system uses XLW-60 as the crosslinker.
Vistar™ is a low or high pH, zirconate crosslinked system, designed so that only very low polymer loading is needed, as compared to other fluid systems. The base gel is a guar-derivative (GW-45). Crosslinkers for the system are 63 (lower temperatures) and XLW-14 (high temperatures).
Crosslinked systems are also characterized by the quantity of polymer used in the base gel.
For instance, a “35 lb” system has the base gel mixed with 35 lbs of polymer in every 1000 gals of water. If this base gel were to be used in Spectra Frac G®, the fluid system would be known as Spectra Frac G® HT 3500.
LFC, XLFC, VSP and GLFC
LFC (which stands for Liquid Frac Concentrate) and XLFC are slurried polymer systems, usually designed to carry 4 lbs of polymer in every gallon of slurry. Simply add the slurry to water and the base gel will form. Slurrying the polymer in an oil-based system helps disperse the polymer in the water (preventing fish-eyes) and is much easier to meter when hydrating gel on-the-fly. The liquid base for the slurry is usually diesel or a low toxicity diesel-derivative.
However, LFC and XLFC systems have been developed that use vegetable or fish oil as the base liquid, although these hold reduced amounts of polymer per gallon. In addition to the base oil and polymer, LFC and XLFC also contain suspending agents to prevent settling during storage, dispersants to help mix the slurry and wetting agents to help the polymer hydrate quickly once the LFC or XLFC is added to water. A pH buffer can also be incorporated to help the polymer hydrate more quickly, especially at low temperatures.
LFC-1, GLFC-1 and XLFC-1 contain guar (GW-27) LFC-2, GLFC-2 and XLFC-2 contain HPG (GW-32) LFC-3, GLFC-3 and XLFC-3 contain CMHPG (GW-38) XLFC-5 contains GW-3
XLFC is the updated version of LFC. VSP (or Vistar Slurried Polymer) is a version of XLFC developed for the Vistar™ system and contains CMG (GW-45).
GLFC systems, which can be mixed with guar, HPG or CMHPG, use an organically-derived base oil in order to meet increasingly tight environmental regulations in many areas of the world.