To understand the science and capabilities of an important technology, such as the polymer-free fracturing fluid, the product developers say a shift in mindset is required. This different approach is particularly true with recent emphasis on improving the performance of guar-based fracturing fluids. Good field results with this viscoelastic surfactant (VES) based fluid are being reported by operators offshore and onshore.
Until the damaging effects of in-situ polymer concentrations on proppant pack conductivity were reported, water-based fluids containing 45-50 lb/1,000 gal of polymer were the industry standard. Since then, to minimize polymer damage, fluids with polymer loadings as low as 15 lb/1000 gal have become commonplace.
Another method reported to lower formation damage and improve clean-up prior to production has been to use optimized breaker schedules that include live oxidizers, encapsulated breakers, enzymes, and breaker aids.
To overcome the damaging effects of polymer-base fracturing fluids and maximize fracture conductivity, Schlumberger Technology Corporation has introduced what it calls the "ClearFRAC system," a family of polymer-free fracturing fluids.
As an alternative to the polymer/breaker approach, the newly developed fluid system uses a viscoelastic surfactant (VES), similar to that used in shampoos or liquid detergents, to develop the viscosity necessary to create a fracture and transport proppant. Post-frac clean-up is as simple as putting the well on production, the developers say. Contact with oil or gas, or dilution by formation water, reduces the VES gel's viscosity, allowing the fracturing fluid to flow back to the surface with the produced fluids.
Other attributes
Reports from users in the field have identified other attributes of this fracturing fluid. The VES fluid:
- Reduces friction pressure, allowing fracturing through smaller pipe diameters, such as coiled tubing
- Reduces leak-off without forming a filter cake
- Improves fracture geometry (longer fractures with better height containment)
- Exhibits good proppant-carrying ability
- Reduces wellsite equipment and materials requirements (less proppant, less fluid, fewer additives, less hydraulic horsepower)
- Delivers an effective fracture length that closely matches the design
- Leaves no residue in the proppant pack, maximizing conductivity.
In its original formulation, operators often considered the VES fluid to be an expensive alternative for use only in special situations. Further research has uncovered a new generation of ClearFRAC fluid that brings the overall fracturing cost in line with conventional, polymer-base fluid approaches.
"While we still do a cost-comparison between the ClearFRAC fluid and other conventional-style fluids, the current design cost isn't that much of an issue," reports BP Amoco's stimulation engineer Dan Polson. "We can now concentrate on optimizing production enhancement and doing what's right for a given well."
As with any stimulation treatment, operator objectives and local conditions (geological, operational, and others) must be considered when designing the appropriate stimulation, including fracturing fluid selection. The developers say the VES approach can now be placed on the routine or preferred list of options for hydraulic fracturing, including frac packing.
The VES approach currently is suited to wells (including standard, slimhole, or coiled tubing completions) with a bottomhole temperature up to 250°F. Research is underway to develop a VES formulation suited to hotter wells. VES fluid may also be prepared with seawater, eliminating the need to transport fresh water to the offshore rig.
Fluid science
The surfactant used in the ClearFRAC fluid is a member of a family of compounds called viscoelastic surfactants. The surfactant molecule is 5,000 times smaller than a guar molecule. When mixed in brine, the molecules associate to form rod-shaped structures called micelles. If the surfactant is present in a sufficient concentration, the micelles associate with one another, resulting in a network structure that imparts viscoelastic properties to the surfactant. In this system, no biocides, polymer hydration or metal crosslinkers are required. The elastic nature of the VES fluid allows efficient transport of proppant into the fracture during a stimulation treatment.
Conventional polymer-based fluids may require several breakers. The amount and type of breakers used depend on the bottomhole temperature, fluid type, job pump time (fluid exposure time), and the polymer loading. Hence, optimizing a breaker schedule without jeopardizing the treatment requires time and experience.
The fluid can be broken without adding chemical breakers. When pumping is completed, the produced hydrocarbons (gas, oil or condensate) disrupt the micelle structure, resulting in a loss of fluid viscosity. The resulting fluid will flow back to the surface, leaving no residue in the proppant pack. Laboratory tests (results available in SPE paper 38622) have shown the following characteristics:
- VES fluid is compatible with commonly used clay stabilizers, such as potassium chloride, magnesium chloride, and tetramethyl ammonium chloride.
- Nitrates are the preferred brine in environmentally sensitive areas. No metals or chlorides are required.
- Viscosity can be controlled by changing the surfactant concentration, the brine concentration, and the brine type.
- The brine composition can be adjusted to impart optimum fluid performance at a given application temperature.
- The viscosity of VES fluids can be reduced by exposure to hydrocarbons (oil and condensate) or by dilution with aqueous formation fluids. In dry gas wells, clean-up is enhanced by pumping a preflush.
- The VES system prevents fluid leak-off rather than creating a filter cake, facilitating fracture clean-up and enhanced production.
- When VES fluids are used, the conductivity of proppant packs after clean-up is 100%.
The current ClearFRAC surfactant is cationic in nature. Some cationic surfactants have been shown to change the wettability of formation rock, which could impact production. A study of the fluid's effects on wettability of rock surfaces (SPE paper 17168) shows that, owing to the unique size, geometry, and charge distribution of the surfactant molecule, wettability is unaffected.
Certain heavy crudes may have a tendency to form emulsions with treatment fluids. For conventional fluids, a de-emulsifier is incorporated into the system to minimize this problem. For VES fluids, a de-emulsifier preflush has been proven effective. The preflush also helps to initiate a fast flowback of the fracturing fluid in wells with low bottomhole pressure.
Giovanna case study
Treatments with ClearFRAC recently exceeded 1,900, with a success rate greater than 99%, the developers say. The treatments have been performed onshore and offshore, in formations representing a wide range of permeability.
Eni-Agip's Dr. Enzo Pitoni, senior production engineer, says the fluid has been able to achieve highly conductive, economic fractures in the fracpack workovers conducted in the Giovanna field, located in the Adriatic Sea. At Giovanna, Eni-Agip encountered dirty, low permeability, productive dry gas sands that have an average permeability of 12 mD, 35 µm average grain size, and clay content up to 50% in some rock samples. In this case situation, Fracpack completions are being used to achieve the following:
- Bypass formation damage, if any, caused by mud invasion during drilling and cementing
- Create a conductive path to
economically produce wells in the field and stop clay fines movement
- Connect multiple layers with a single fracture.
To achieve these results, dimensionless fracture conductivity must be maximized, the fracture dimensions must be tightly controlled, high net pressures must be maintained, and tip screen-out (TSO) must be successful. Eni-Agip reported that conventional frac fluids were not able to reach these goals, so after a series of investigations, the ClearFRAC solution was selected.
Earlier this year, the Giovanna 6 well was worked over using 0.5% VES fluid and potassium chloride brine, creating a low viscosity (18-cp @ 170s-1) fluid. Three slurry batches were prepared with 2 ppa, 4 ppa, and 6 ppa 40/60 US mesh proppant. Pitoni said the fluid's simplicity resulted in improved operational reliability during treatment. "Only two additives were required for viscosity development, which could be adjusted any time at the wellsite with no polymer hydration time required. Since the fluid is polymer-free, there is no fear of bacterial growth or degradation. Crosslinkers or specific internal breakers also were not needed. The reduced complexity of the treatment not only simplified on-site mixing and metering, but increased its reliability, as well."
"Further," says Pitoni, "the VES fluid proved forgiving during fracturing. For example, when pumping was interrupted for several minutes, premature screen-out (landbridging inside the tubulars) did not occur, as is sometimes experienced with conventional fluids. The low friction pressure made the VES fluid ideal for fracturing through the existing, small-size production tubing in the Giovanna 6 well. The reduced friction pressure helped improve stimulation economics. Overall, the Giovanna 6 workover verified that this fluid's characteristics can produce TSO and thus high fracture conductivity to maximize productivity and well life."
BP Amoco also has tested the fluid in Wyoming. BP Amoco's Dan Polson said conventional frac fluids limit the ability to use new tubular or stimulation designs and handle friction poorly. The latter presents a problem, Polson said, "because we prefer slimhole completions in this area. We've also seen a lot of excess leak-off, which causes screen outs." He said the ClearFRAC fluid appears to be more effective in handling friction, reducing leak-off, and carrying proppant."
"With the cost of ClearFRAC fluid now competitive with crosslink solutions," he said, "we can now purposely design fracture stimulations with longer lengths and lower proppant concentrations. We can now design smaller jobs, using smaller pads."
Polson said area wells also have poor rock quality, and the VES fluid is preferred because it doesn't require a filter cake to control leak-off.
Comparative data showed the VES fluid saved over 2,200 bbl of fluid during proppant placement, and even with a much smaller pad (20%, compared to 45%, which is common in the area), the VES fluid placed 6 ppa proppant. A much larger conductive frac area resulted, which brought about 2.5 times more initial gas production. Because the VES fluid is able to do the job with less viscosity, less hydraulic hp, and reduced friction, more frac length is achieved without unnecessary frac height growth.
The fluid also provides a cleaner, simple operation, Polson said. "We supply fewer frac tanks, less water, and less labor for delivering raw materials. Schlumberger requires fewer pump trucks, no precision control mixing (PCM) truck for gel hydration, and considerably fewer additives."
"While we're still in the early stages with this product, as we learn more and more about its capabilities and Schlumberger continues to improve it, I expect to see its use spread to meet a fair share of our fracing needs."
Schlumberger is continuing to improve the performance of VES systems, including extending it to higher temperatures and expanding its environmental friendliness. Current system uses continue to expand into areas such as selectively fracturing through coiled tubing, acid fracturing, and using the surfactant with seawater in offshore settings. Other applications of VES technology are being found in scale control, water control, horizontal well clean-up, gravel packing, diversion, and reservoir drilling fluid clean-up.
References:
SPE papers: 38622 and 17168.
