A Critical Review of Hydraulic-Fracturing Fluids for Moderate- to Ultralow-Permeability Formations Over the Last Decade

Abstract

Summary Hydraulic fracturing is a well-established process to enhance productivity of oil and gas wells. Fluids are used in fracture initiation and the subsequent proppant and/or sand transport. Several chemistries exist for these fluids. This paper summarizes the published literature over the last decade (more than 100 technical articles) and captures the advances in the design of water-based fracturing fluids for fracturing ultralow- to moderate-permeability reservoirs (nonfrac-pack applications). Guar-based polymers are still being used in fracturing operations for wells at temperatures of less than 300°F (148.9°C). To minimize the damage associated with this class of polymers, the industry has attempted several approaches. These included the use of a lower polymer concentration in formulating these fluids and alteration of the crosslinker chemistry so that one can generate higher viscosity values with lower polymer loadings. Moreover, the industry shifted toward the use of cleaner guar-based polymers because commercial guar contains a minimum of 5 wt% of residues, which can cause damage to proppant packs. The review has also revealed that shear and pressure effects on the rheological behavior of borate-crosslinked gels is significant. Although these fluids recover their viscosity after shear, they have been observed to lose a significant portion of their viscosity under high pressures. When fracturing deeper wells in hotter reservoirs, the need arose for a new class of thermally stable polymers. Thus, the industry shifted toward polyacrylamide- (PAM-) based polymers. These synthetic polymers offer sufficient viscosity at temperatures up to 450°F (232°C). Examples include 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and copolymers of partially hydrolyzed PAM-AMPS-vinyl phosphonate. To address the challenge of high-pressure pumping requirements on the surface, high-density brines have been used to increase the hydrostatic pressure by 30%. Breakers’ chemistry has seen introduction of new materials. These breakers decrosslink the gel by reacting with the crosslinker. They form ligands with the metallic crosslinkers and displace them from the crosslinking bonds with the guar-based polymers. Examples of these breakers include polysuccinimides and lignosulfonates. To minimize the environmental impact of using massive amounts of fresh water and to minimize costs associated with treating produced water, the use of produced water in hydraulic-fracturing treatments has been reported. In addition, the paper captures the advancements in the use of slickwater, which uses drag-reducing agents (PAM-based polymers) to minimize friction. The paper highlights the first use of breakers that were introduced to improve the cleanup of these drag reducers. For foamed fluids, new viscoelastic surfactants that are compatible with carbon dioxide are discussed. The paper also sheds light on the use of emerging technologies, such as nanotechnology, in the design of new, efficient hydraulic-fracturing fluids. For example, nanolatex silica was used to reduce the concentration of boron found in conventional crosslinkers. Another advancement in nanotechnology was the use of 20-nm silica particles suspended in guar gels. The paper provides a thorough review of all these advancements.