Salt-Tolerant High-Viscosity Friction Reducers Minimize Freshwater Consumption in Hydraulic Fracturing Operations

By Temi Yusuf, Schlumberger

Hydraulic fracturing is a well stimulation treatment involving the pumping of engineered fluids and proppant at high pressure (above fracturing pressure) into reservoir rock.

The intent is to create fractures propped open by special materials called proppant. This process creates high-conductivity communication with a larger reservoir area and bypasses any existing near-wellbore damage, thereby improving a well's productivity.

This stimulation method significantly improves the economic viability of most wells, especially in unconventional reservoirs, such as shale gas or other low-permeability reservoirs, which may not produce optimally without it.

Completions in unconventional reservoir development have continued to evolve over time—from vertical to horizontal wells, longer laterals and more stage counts per well. Stimulation jobs have followed this size trend, with more freshwater required to hydraulically fracture wells for viable production. In 2019, almost 5 billion barrels of water were required for fracturing operations in the U.S. only (IHS Markit 2021). This is equivalent to about 11 days of water consumption (domestic and industrial applications) for the city of Houston.

Likewise, significant volumes of water are produced from various well activities, including post-hydraulic fracturing flowback, water breakthrough and water naturally produced along with hydrocarbons. In 2019, the total flowback and produced water volume onshore in the lower 48 states was almost 20 billion barrels (IHS Markit 2021). This presents its own unique challenges for treatment, handling or disposal, with several billion dollars dedicated yearly by oil companies for water management.

Lower-permeability formations, as found in most unconventional plays, require long and narrow fractures to optimize production. This has primarily been achieved by using low-viscosity water-based fluids (known as slickwater) pumped at high rates into the formation. The main fracturing fluid additive is a friction reducer used to optimize pumping rates while minimizing pressure drop in the wellbore.

The major drawback of slickwater fluids, however, is their low proppant-carrying capacity. Consequently, higher-volume water is needed to place the designed amount of proppant within the fracture. This challenge is further amplified in regions where water scarcity is a growing concern.

Furthermore, slickwater operations require high hydraulic horsepower to somewhat compensate for the lower proppant transportation capacity of fluids through high fluid velocity. This results in higher energy consumption and associated emissions per well treatment.

Fracturing operation designs have explored the use of hybrid fluids—using slickwater for early, low-proppant-concentration stages followed by higher-viscosity guar-based fluids for later, higher-proppant-concentration stages. These hybrid operations introduced further operational complexity, requiring more equipment resources, mixing, and handling on location. Guar-based fluids are also a less sustainable option when compared with polyacrylamide-based friction reducers.

In recent years, major technological improvements in friction-reducer chemistry have enabled a drastic reduction of freshwater consumption in fracturing operations. Friction reducers were modified to provide higher viscosity. This supports flexibility in the design and enables pumping of much higher concentrations of proppant.

Consequently, water volume requirements are lower, which minimizes pump time and leads to a lower carbon emissions footprint. This improvement also further simplifies the process, reducing equipment requirements by using only one fluid system throughout the stimulation operation.

Historically, most friction reducers came in liquid form with a substantial part of the formulation being the solvent or liquid carrying agent. Effectively, several million pounds of product are handled and transported during fracturing operations worldwide with most of this being just the solvent.

The introduction of dry high-viscosity friction reducers (HVFRs) streamlines logistics and handling, offering a lower-weight, higher-efficiency (100% active material), and more sustainable alternative to liquid HVFRs. Chemical alterations to the particles assure dispersion and controlled hydration for higher-performance operations.

Most high-viscosity friction reducers are sensitive to salts naturally occurring in water and usually perform better in freshwater sources. Due to the ionic charge on the polymer, the presence of significant quantities of opposing charged ions such as calcium, magnesium or iron in water sources results in undesirable fluid performance and, in extreme cases, the formation of semisolid agglomerates of polymer complexes. This suggests that the quality of water and total dissolved solids (TDS) is critical to the fluid performance and well productivity.

The need for freshwater coupled with the sheer volume required in these operations puts pressure on water sources in the area—potentially competing with domestic and other industrial water needs.

Freshwater sourcing and post-treatment disposal of produced water significantly contributes to operators’ operating expenses today. These costs are higher still in arid areas around the world where freshwater is scarce. With unconventional gas developments expected to ramp up to align with the transition to more sustainable energy sources, a solution that resolves both water sourcing and disposal challenges is invaluable.

Dry HVFRs have now been modified so they become tolerant to higher salinities, up to 300,000 ppm TDS (30% salinity). The tolerance to high salinity offers attractive water flexibility. Operators can reuse produced and flowback water from neighboring wells in lieu of costly water treatment or disposal.

In summary, the growing demand for freshwater sources for stimulation and the need to dispose large volumes of produced or flowback water presents a unique opportunity to improve fracturing fluids used in unconventional well stimulation.

Slickwater fluid systems have evolved from low-viscosity, low-proppant-carrying, and freshwater-enabled liquids into dry high-viscosity fluid systems compatible with higher water salinities. This provides the industry with simpler, more sustainable, flexible, cost-effective and streamlined operations with lower water requirements and carbon emissions footprint.

Temi Yusuf is a Stimulation Chemistry Product Manager for Schlumberger. She has a chemical engineering background with 17 years’ experience in the oil and gas industry. Her experience and expertise span technical, operations, sales, and marketing in well stimulation (fracturing and acidizing), sand management, water and gas conformance, cementing, and coiled tubing domains.