While viscoelastic surfactant (VES) fluids have been widely used in the offshore oil and gas industry for several years for a variety of stimulation treatment applications, those applications have not extended to hydraulic fracturing of deep-shelf or deepwater wells because of: (1) the fluids’ tendency toward high fluid leak-off and low viscosities at elevated temperatures, and (2) lack of an internal breaker mechanism for dry gas applications. Recently, Baker Oil Tools introduced Diamond FRAQ fluid, a new VES fluid system with internal breaker technology that substantially improves viscoelastic surfactant performance and extends its applicability to deepwater and high-pressure hydraulic fracturing.

VES advantages

For many years, fracturing fluid technology centered on a wide array of polymers such as guar, hydroxypropyl guar (HPG) and hydroxyethylcellulose (HEC). Since high viscosities are

VES fluid without internal breaker had less than 2% regain permeability, while internally broken VES fluid showed 141% regain permeability. (All graphics courtesy of Baker Oil Tools)
needed to carry the required amount of proppant, high concentrations of polymer are used. At high polymer concentrations, several problems can occur, the most serious being formation and proppant pack damage due to unbroken gel in the pore throats.
As fracturing technology was applied to the offshore environment, different job design criteria were needed for frac packs and tip screen out (TSO) procedures. Fracture damage also became a more serious concern due to higher production rates. To address operator concerns of polymer damage to the fracture face and the proppant pack, the service company developed a polymer-free, non-ionic, viscoelastic surfactant fluid system.

Polymer-free fluids are made from surfactants that form an interconnected network of micelles. These micelles are highly polar in nature and are stabilized by high salt brines, seawater or completion brines. The network structure imitates the hydrated polymer fibers but with much higher efficiency and strength. The viscosity profiles of these fluids are non-Newtonian and viscoelastic in nature; hence, the “VES” designation. The dynamic range of the VES fluids is linear over a wide range without excessive shear thinning at high shear rates.

Since the viscosity of VES fluids is created by micelles instead of polymer fibers, there is no physical degradation with time, but they do experience thermal thinning. Constant rheological parameters offer job design advantages for both fracture generation mechanics and proppant transport control, resulting in a higher level of management of the properties of the fracture growth and propagation.

Another significant advantage of VES fluids is that, unlike polymer fluids, they are stabilized by high salt environments and thus able to viscosify high salt brines, seawater and
Rapid, controlled viscosity reduction can be achieved with higher breaker loadings.
high-density completion brines. While the viscosity can be controlled easily by the concentration of surfactant, the attained viscosities are usually not as high as those delivered by polymer-based fluids. This feature enables better fracture height control while maintaining high proppant-carrying capacity.

VES systems typically offer higher retained permeability and conductivity of the formation sand and proppant pack in sandstone reservoirs than do polymeric fluid systems because they do not have polymer residue. Additionally, while residues remaining after polymer fluid breakdown may impart 30% to 50% damage to the formation and proppant pack, VES fluids avoid this damage because they are non-wall-building and leave no filtercake.

Previous VES shortcomings

A select type and amount of cation in the mix water is typically required for VES elongated micelle structures to form and retain their stability. VES fluids used for frac pack completions have been shown to have good viscosity and proppant transport capability at temperatures approaching 300ºF (149°C).

During frac-pack treatments or regular frac jobs, VES fluids readily leak off into the formation matrix, setting up the potential for a viscosity wall — impairing production. Because leak-off
Fluid-loss agent significantly reduces leakoff in wells.
control depends on viscosity rather than wall-building, VES fluids can only be used when reservoir permeabilities are below about 800 to 1,200 md, and preferably below 400 md. Since these fluids do not form filtercakes, the invasion depth can be proportional to permeability. The fluid could possibly remain in its viscosified state and therefore plug the pores. The amount of leak-off is much higher than with polymeric systems and is fluid viscosity-dependent.

Until recently, no internal breaker existed that could be added to VES fluids to go where the fluid goes. In fact, some companies have contended that internal breakers were not necessary. Instead, the VES fluid was considered to break by reservoir conditions, primarily: 1) contact with reservoir hydrocarbons, and 2) contact and dilution with reservoir brine. However, many treatments have failed to clean up after a treatment, prompting industry experts to agree that internal breakers are preferred. Relying on external or reservoir conditions to break down the leaked-off fluid to achieve quick and complete treatment flowback has been a point of contention and is of questionable efficacy. Additionally, conventional cations that work within a low and narrow concentration range for VES viscosity yield and temperature stability have limited the range in salinity and density of VES fluids to only light brines.

Improved performance

In 2005 the service company introduced the first VES fluid to incorporate an internal breaker that goes wherever the VES goes during well treatment to ensure effective and complete removal of VES fluid that has leaked into the reservoir during treatment. Through laboratory core clean-up tests, company fluids scientists determined that internal breakers are much more effective in removing leaked-off fluid than VES fluids without internal breakers present.
The water-based, non-ionic VES fluid is designed for treating oil and gas sandstone reservoirs where minimizing formation damage and maximizing proppant and gravel pack gravel retained permeability are of primary importance. Unlike other VES systems that tend to oil-wet sandstones, this fluid retains the natural water-wet condition of either oil or gas sandstone reservoir rock. This capability is critical for maximizing hydrocarbon production. The fluid VES can be broken with either external breakers such as oil present in the reservoir or mutual solvents pumped ahead or behind the fluid, or with internal breakers that do not rely on hydrocarbons in the reservoir.

Both the VES concentrate and the internal breaker system were developed in-house. The VES concentrate is a single-additive concentrate that forms a non-ionic micelle viscous solution when added to aqueous fluids. The internal breaker system uses proprietary internal breaker agents that are pumped with the main frac fluid. The internal breaker works in dry gas, condensate and oil reservoirs.

The VES fluid will viscosify seawater and brines made with sodium chloride, potassium chloride, calcium chloride and most bromide brine blends to 14.5 ppg. The fluid system improves the thermal stability of VES micellular structures in brines up to about 300ºF. The improved salinity tolerance at elevated temperatures allows brines to 14.5 ppg (sg: 1.73) to be used to increase hydrostatic pressure when treating deep, high-pressure reservoirs. A proprietary friction pressure-reducing agent is added to enhance friction pressure reduction when treating wells at high rate down long workstrings. High salinity tolerance also allows low, true-crystallization-temperature (TCT) brines to be formulated for thermodynamic gas hydrate prevention. The formation of more thermally stable micelles requires 20% to 50% less VES product and operates at higher temperatures.

A new fluid-loss additive currently being developed will allow the fluid to treat reservoir permeability of up to 2,000 md and bottomhole static pressure of 250ºF (121.11ºC). The water-soluble material forms a pseudo-filtercake as well as a viscous layer of VES fluid on core faces to significantly reduce fluid loss. Typical usage concentration for the fluid loss agent is from 10 to 20 pptg. Use of this technology will decrease the VES fluid volume required for a given treatment by up to two-thirds.

The fluid does not form a filtercake and thus does not plug the formation face with polymer solids in the near-wellbore area and perforation tunnels. Retained permeability is an excellent 90% to 99%. As a result, drawdown is minimized and productivity is enhanced. The surfactant flows back with low differential pressure to recover treating fluid. It has demonstrated superior shale stabilizing characteristics for treating reservoirs with high shale content. And its wide temperature range (80°F/26°C to 300ºF) enables it to frac pack or gravel pack most formations.

The non-ionic chemistry permits the VES to viscosify weighted brines, which enables it to be used with calcium chloride and other brines with a very low freezing point in deepwater gas wells where the operator wants to flow test the well immediately after the frac pack and hydrate inhibition is required.

A patent is currently pending for using the recently developed fluid with weighted brines and a friction reducer for treating deep geo-pressured reservoirs. The weighted brine is used to control the hydrostatic head pressure as in drilling operations, and the friction reducer gives the fluid the same or better friction pressure than a borate cross-linked guar fluid system.

Field successes

The fluid was chosen by an offshore operator to treat a deepwater (>3,300 ft/1,000 m) dry gas well where the well was to be flow-tested as soon as possible after the frac pack. The operator chose a 10.8-ppg (sg: 1.29) calcium chloride brine for hydrate inhibition as the well was to be turned around for a production test before methanol injection was to be installed. The fluid was selected for its compatibility with the 10.8-ppg calcium chloride brine and to ensure the fluid was broken for the flow-back test. The flow test exceeded the operator’s expectations, and the high flow rate was partly attributed to the relative non-damaging effect of the frac fluid. After more than 2 years, the operator reports the well is still on production, sand-free, with higher-than-expected rate and total cumulative production.

Advancing options

Advancing VES fluid technology holds significant promise for enhanced performance and applicability. Improved thermal stability of VES micelles allows for lower VES loadings at higher temperatures. Internal breakers can ensure rapid and more complete clean-up of the VES fluid, even at lower differential pressures. Use of a proprietary fluid-loss-control agent during fracturing should allow more fluid to remain within the fracture to improve fracture size and geometry along with allowing a thicker interval with higher kH to be treated. These advancements are soon to be released to the market and should extend the fluid’s ability to treat high-permeability, high-pressure, and high-temperature reservoirs.

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