The first hydraulic fracturing attempt in the late 1940s used gelled crude oil as a fracturing fluid and silica sand as a proppant. Since then, the technology has focused on the nature of the proppant (plastic and aluminum pellets, glass beads, ceramics, silica sand, or resin-coated sand [RCS]) and the composition/rheology of the fracturing fluids. For example, technological advancements in fracturing fluid rheology and chemical additives allowed the use of higher concentrations of proppants and resulted in increased fractured widths and lengths and productivity in terms of oil and gas recovery.
Today hydraulic fracturing incorporates a multidisciplinary approach that includes reservoir simulations, micro-seismic testing, fluid dynamics, etc. The combination of hydraulic fracturing and the use of sophisticated horizontal drilling techniques has been a game-changer, giving access to previously inaccessible and otherwise nonprofitable oil and natural gas reservoirs. During the last 15 years these technological achievements have resulted in a “fracing boom,” that has changed local economies and created thousands of jobs while providing an immediate viable solution to US energy needs in a growing world economy.
When the US foundry industry started its decline in the 1970s and 1980s, phenolic formaldehyde resins found a new application as a coating in silica sand proppants. These coatings reinforced the mechanical properties of the sand grain and provided an economical upgrade of silica sand. The robust thermal and mechanical properties of the phenolic resins coupled with short processing times widened the spectrum for use in different types of well applications.
In general, there are two types of RCS: the initial approach of reinforcing the strength of the sand particle with a highly converted phenolic resin coating (pre-cured), and the newer technology of partially cured or curable RCS that is engineered to fully cure under the temperature and closure stress in the fracture. Shallow oil and natural gas wells exhibit low temperature and closure stress but are still responsive to the hydraulic fracturing process; however, these conditions require a more sophisticated approach in engineering the material properties and performance of RCS. Two of the most significant objectives in hydraulic fracturing are the successful delivery of proppant into the fractures and the sufficient particle-to-particle bonding that allows the proppant to pack in the fracture and withstand oil and gas hydrodynamic drag forces during production (proppant flowback prevention). Interparticle bonding is crucial as it makes the proppant pack act as a consolidated piece that resists flowback while enhancing a better distribution of closure stress. This prevents embedment and packing rearrangement, which can reduce effective conductivity.
Since precured phenolic RCS failed to deliver flowback control and proppant embedment resistance, a curable coating became the new direction. However, a fully curable phenolic coating had substantial challenges such as dissolution of the coating within the water and negative interaction with fracturing fluids (reduction of the pH, interaction with oxidizing breakers). The designed thermoset chemistry of a lower onset temperature initially promised to deliver an approach to favor bonding in low-temperature and low-closure stress conditions. Unfortunately, the coating could not selectively perform at low temperatures while at the same time resisting agglomeration and caking during processing, storage, and handling of the RCS prior to its use or during the fracturing process. One of the biggest challenges of the use of fully curable resins was the uncontrolled setup in the wellbore, which required drilling for the removal of the agglomerated RCS mass. The above challenges not only required a more complicated fracture process design but also included a continuing high risk of operating errors during the fracture process.
The response was the introduction of a partially cured phenolic coating coupled with the use of an activator as a bonding promoter that enabled a wider range of the application. While activators sound as though they are a catalytic component of the bonding mechanism, they are not. In fact, these are binary mixtures of approximately equal amounts of an alcohol and a surfactant that soften/deform the coating and force an interconnection of the grains at their contact points. Then, after the removal of the fracturing fluids and the activator, the coating sets up. The amount of activator needed can be in the range of 0.5% to 2% weight of the fracturing fluids. While activators have allowed for some progress in the area of low-temperature, partially cured RCS (in temperatures less than 60°C [140°F]), they also have major disadvantages:
- They can facilitate a negative interaction with the rheology of the fracing fluid. Reduction in viscosity translates to a failure to deliver the proppant into the fractures;
- During the softening stage, partial dissolution of the coating can occur in the water;
- There is a high risk of flammability;
- In shallow wells (when productivity is expected to be lower than deep wells) activators add an additional cost to the service companies (transportation and proper storage); and
- Deformation of the coating may reduce porosity, which leads to a reduced conductivity.
New industry direction
A solution to the above challenges that addresses the industry’s needs is the engineering of a bondable coating that exhibits lower onset temperatures and performs under low temperatures and closure stresses without the negative effects of the use of activators. Recent developments have led to the launch of nonphenolic green chemistry as a coating for RCS. It is a solid response to the increasing regulations and environmental awareness about substances such as formaldehyde and hexamethylenetetramine that are associated with phenolic chemistry. The developed, bondable RCS performs in the fracture only under closure stress while resisting premature bonding or agglomeration due to temperature.
The coating has a proprietary topcoat that masks the highly bondable undercoat. Under closure stress the topcoat embeds within the coating and reveals the bonding potential of the lower onset temperature undercoat. Then, temperature and closure stress have a synergistic effect to the particle-to-particle bonding, forming a consolidated pack of RCS that resists flowback and embedment. The mechanism provides a wider window for the coating process, storage, and handling, and the product does not agglomerate until it is required to perform downhole and deliver its bonding characteristics. Conductivity tests have revealed that this topcoat mechanism bondable product yields higher conductivity (in the range of 50% to 65%) than a phenolic partially cured product that uses an activator. A finding from the same set of tests shows that the conductivity performance of the phenolic product is actually lowered with an activator. This finding reinforces the argument that the activator acts as a plasticizer deforming the coating, which may have a significant negative impact in porosity and therefore effective conductivity.
In summary, the use of low-temperature bondable RCS without the need of activators provides a technological solution that simplifies the fracturing design, prevents flowback and embedment, and forms a conductive resin-coated consolidated proppant pack.