Frac Gel

Frac gel is a viscous fluid used as the primary carrier fluid in a hydraulic fracturing treatment. It is made by dissolving a polymer (most commonly guar gum or a derivative such as hydroxypropyl guar, HPG) in water and then activating a crosslinker (borate ions at high pH, or metal ions such as zirconium or titanium at lower pH) that links the polymer chains into a three-dimensional network. The resulting gel is far more viscous than the base water, thick enough to carry proppant (sand or ceramic beads) in suspension at concentrations of 0.5 to 4 kilograms per litre of fluid without the proppant settling to the bottom of the fracture before the gel breaks. After a set time at downhole temperature, a breaker chemical degrades the polymer chains and the gel reverts to a thin liquid that can flow back to surface.

Key Takeaways

Guar gum is the dominant frac gel polymer globally. It is derived from the guar plant (Cyamopsis tetragonoloba), which is grown primarily in Rajasthan, India, and parts of Pakistan. Guar is concentrated in the guar bean's endosperm and processed into a powder. Roughly 85 percent of the world's guar supply goes to the oil and gas industry for hydraulic fracturing, making guar price a significant cost input for completion programs.
Crosslinked gels (borate-crosslinked at pH above 9, or metal-crosslinked at lower pH) can achieve viscosities 10 to 100 times higher than the same polymer at the same concentration without crosslinking. This high viscosity keeps proppant suspended during pumping and creates a wider fracture width that accommodates the proppant pack.
Gel breakers are chemicals (persulfate salts, enzymes, or oxidizers) that degrade the polymer chains after the proppant has been placed and the fracture has closed onto it. Incomplete breaking leaves residual polymer gel coating the proppant grains, reducing the permeability of the proppant pack and cutting the well's post-frac productivity.
Linear gel (polymer dissolved in water but not crosslinked) is less viscous than crosslinked gel but still significantly more viscous than water. It is used for lower-rate, lower-temperature treatments or as a pre-flush ahead of the crosslinked stages.
Slickwater, which uses water with only a friction reducer (not a gel polymer) as the fracturing fluid, has largely replaced crosslinked gel in unconventional shale and tight gas completions. Slickwater creates more complex fracture networks in naturally fractured rock but cannot carry high proppant concentrations, so it uses smaller proppant mesh sizes at lower concentrations. Frac gel remains the preferred fluid for conventional tight reservoirs that require high proppant loading to achieve an effective conductive fracture.

What Is Frac Gel and How Does It Work?

Pour water into a glass. It is thin and flows easily. Now dissolve a small amount of guar powder (about 0.5 percent by weight) and stir. The mixture is slightly thicker, like thin soup. Add a few drops of borate solution at high pH and stir again. Within 30 seconds, the mixture transforms from a thin fluid to something resembling a firm jello that holds its shape when you tilt the glass. That is a crosslinked guar gel.

The chemistry behind this transformation is elegant. Guar gum molecules are long polymer chains with hydroxyl groups (-OH) spaced along them. Borate ions at high pH (above 9) react with pairs of these hydroxyl groups on adjacent chains, forming a boron-oxygen bridge that links the two chains together. Thousands of these linkages form simultaneously throughout the mixture, creating a three-dimensional elastic network rather than isolated polymer chains in solution. The network is what gives the gel its remarkable viscosity.

When this gel is pumped at high pressure down the wellbore and into the formation, it forces open a crack in the rock (the hydraulic fracture) and carries proppant grains forward into the fracture. The high viscosity keeps the proppant suspended (without it, proppant would settle to the bottom of the fracture within seconds). At downhole temperature, the breaker chemical that was added to the gel mixture before pumping gradually attacks and cleaves the polymer chains. Over 4 to 24 hours after pumping, the gel thins back to water and flows back out of the fracture, ideally leaving clean proppant packing the fracture open.

Fast Facts

Guar prices became a major cost issue for North American completions during the 2011 to 2014 unconventional drilling boom. In 2011 and 2012, guar prices rose from approximately USD 1.00 per kilogram to over USD 5.00 per kilogram as demand from the booming hydraulic fracturing market overwhelmed global supply. A single large completion job using crosslinked guar gel at 5 kilograms per cubic metre of fluid, with 2,000 cubic metres of gel pumped, required 10,000 kilograms of guar, costing USD 50,000 at peak prices compared to USD 10,000 at pre-boom prices. The guar price spike accelerated the industry's shift toward slickwater completions that use only friction reducer (a different, cheaper polymer) rather than guar-based gel, permanently changing completion design in unconventional plays.

Frac Gel in Conventional Tight Gas Completions

Crosslinked guar gel remains the workhorse for conventional tight gas completions in formations where proppant concentration in the fracture is critical to achieving an effective conductivity. Formations like the Cardium, Glauconitic, and Viking sandstones in Alberta, the Falher in the Deep Basin of west-central Alberta, and the Niobrara Chalk in the Denver-Julesburg Basin in Colorado require proppant-laden treatments to prop fractures open against the closure stress of the formation.

In the Falher tight gas play, where wellbore temperatures at 3,000 to 3,500 metres reach 80 to 110°C, high-temperature crosslinked gels (typically titanium or zirconium crosslinked HPG or CMHPG) are needed because borate gels lose their viscosity above about 65°C. The metal-crosslinked systems maintain viscosity at high temperature and provide the proppant transport needed to achieve deep fracture placement before the gel breaks.

Foam gel (nitrogen or CO₂ dispersed in a guar gel base) is used in water-sensitive formations and in coalbed methane wells in the Horseshoe Canyon and Mannville formations of Alberta, where the foam's low water content reduces formation damage and the gas phase helps the fluid flow back to surface quickly after the treatment.

Gel Damage and Breaking

Incomplete gel break is one of the most common causes of poor post-frac productivity. If the breaker chemistry does not fully degrade the polymer chains at downhole temperature and pH, residual gel coats the proppant grains in the fracture and fills the pore throats between them. A theoretical proppant pack permeability of 100,000 millidarcies (common for 20/40 mesh white sand) can be reduced to 10,000 to 20,000 mD by unbroken polymer residue. This 5 to 10-fold reduction in conductivity directly cuts the post-frac well deliverability.

Breaker loading (how much breaker is added to the gel) must be carefully matched to the downhole temperature. Too little breaker and the gel does not fully break. Too much and the gel breaks prematurely during pumping, dropping proppant before it reaches the designed fracture length. Temperature-activated encapsulated breakers (which release only at downhole temperature) allow higher breaker loadings without early breaking at surface conditions.

Frac gel is also called fracturing gel, crosslinked gel, or simply gel in completion engineering. Related terms include guar (a natural polysaccharide polymer derived from the guar bean, grown predominantly in India; the primary raw material for crosslinked frac gels used in hydraulic fracturing; also used in food and industrial applications), crosslinker (a chemical that forms bridges between polymer chains in a frac gel, dramatically increasing viscosity; borate is used at high pH for lower-temperature applications; titanium and zirconium crosslinkers are used at higher temperatures), gel breaker (a chemical (persulfate, enzyme, or oxidizer) added to frac gel to degrade the polymer chains after the proppant is placed, allowing the gel to thin and flow back out of the fracture), proppant (sand, coated sand, or ceramic beads pumped into a hydraulic fracture to hold it open against formation closure stress after pumping stops; the carrier fluid (frac gel or slickwater) transports proppant into the fracture), and slickwater (a hydraulic fracturing fluid composed primarily of water with a small amount of friction reducer polymer; much lower viscosity than crosslinked gel; used in unconventional shale completions where fracture complexity is preferred over high proppant concentration).

When Premature Gel Break Destroyed a Deep Basin Completion in West-Central Alberta

A completion team was fracturing a Falher-C tight gas well in the Deep Basin of west-central Alberta. The well's bottomhole static temperature was 108°C. The treatment design called for 1,800 cubic metres of CMHPG crosslinked with zirconium acetate, pumped at 4 cubic metres per minute carrying proppant at concentrations ramping from 0.25 to 1.5 kilograms per litre. The breaker was a persulfate encapsulated in a membrane designed to release at temperatures above 90°C.

During the pre-job review, the laboratory analysis of the gel samples showed acceptable viscosity at 108°C. What was not adequately characterized was the gel stability at the wellbore temperature of 115°C encountered in the first 300 metres of perforated interval. At 115°C, the zirconium crosslinks were reversible and the gel was significantly thinner than at the design temperature.

During pumping, the first 600 cubic metres of gel and proppant screened out (blocked) in the perforations because the gel carrying capacity was insufficient and proppant bridged across the perforations. The job had to be shut down. Post-job analysis showed that 90 percent of the proppant placed was within 15 metres of the perforations rather than distributed along the intended 150-metre fracture half-length. A restimulation job was required at a cost of CAD 420,000 compared to the original treatment cost of CAD 280,000. The failure was traced to inadequate gel rheology testing at the actual downhole temperature. Designing at the nominal bottomhole static temperature rather than the maximum perforation interval temperature was the root error.