Silica gel as a potential viscosifier for frac fluids was introduced to the petroleum industry in the late 1970s.1 Early research identified this form of silica as providing a highly viscous, thixotropic fluid that can maintain viscosity at high temperatures.
A need for this particular set of performance attributes arose in geothermal wells being stimulated in Hokkaido, Japan.2,3 This provided an opportunity for the recently developed silica-based viscosifier to move quickly to field trials. Literature reports these early applications of silica gel-viscosified frac fluids were a technical and economic success. There was a significant increase in steam volume, and the cost of a silica-based system was described as being comparable to that of the modified guar.
Despite these promising early results, silica was not embraced as a viscosifier. Looking back some 30 years, it was likely a case of silica gel being a few years ahead of the curve.
Considering today’s needs for fracturing fluids, coupled with more openness to alternative versions of them, PQ Corporation and Well Completion Technology began development of a next-generation silica gel-based viscosifier. The newly developed silica gel has several attractive features compared to existing organic-based frac fluid viscosifiers, and significantly improves upon the previously developed silica gels, as well as other frac fluid viscosifiers.
Among these features, the silica gel ranks very high as an environmentally friendly chemical, and it has an ability to effectively carry larger, higher-strength and greater-density proppants. It also has a reduced affinity to rock and metal surfaces, and can be formulated with water that is very high in dissolved solids. Silica gel requires little or no biocide, and can be used in low-, normal-, high- and ultra-high-temperature applications. In addition, it can be produced on-site as a batch process or a continuous process, and also can be produced as a concentrate.
The cost structure of the silica gel is competitive with modified natural polymers, and less expensive than synthetic polymers or viscoelastic surfactants. As a product based on sand, silica is not subject to the wide price swings associated with natural polymers.
CHEMISTRY OF SILICA
Silica is a broad term that covers a wide range of forms of silicon dioxide (SiO2). In hydraulic fracturing, the form that first comes to mind is sand. Quartz or sand is a natural product that is hard, crystalline and with no porosity. These characteristics make this form of silica well-suited as a proppant, but it has no function as a viscosifier. This proposed silica gel can be considered at the other end of the silica spectrum. It is synthetic, amorphous and consists of a three-dimensional network of silica particles, Fig. 1. The silica gel is porous, with both internal and external surface areas.
Suspension and thixotropic properties are tied to the surface area and porosity, and surface chemistry of the silica gel. The silica gel developed by PQ for hydraulic fracturing produces a surface area in the range of ~600 m2/g of silica. Past work would have a silica gel closer to ~200 m2/g.
The surface of the silica gel also differs in the form and density of silanol groups (Si-OH), meaning the total concentration of silanol groups, as well as the relative concentration of isolated vs. associated silanols. The silica gel is produced so as to be closer to the isoelectric point. The silanol groups remain mostly protonated and in associated form (many SiOH groups near each other vs. having a negative charge
(SiO-) or in isolated acidic form).
The large concentration of associated silanols shows a high degree of hydration (hydrogen bonding), leading to a minimal charge on the silica and less effects from metals and salts. This creates several performance enhancements that are desirable in a frac fluid viscosifier. The inertness of silica enables the gel to incorporate various salinity brines, and recycled and produced water. Previous forms of silica require a much greater use of fresh water. The non-ionic nature also reduces its affinity for the metal and reservoir rock surfaces.
SILICA GEL VISCOSITY
The preparation of the silica gel differs from water-soluble biopolymers and synthetic polymers in that it is not a hydration process. Rather, the small molecules of sodium silicate are made to polymerize in an acid solution. The preparation of the silica gel does resemble the hydration of biopolymers, in that it can be prepared on-site as a batch process, or on-the-fly in a continuous process. Alternatively, the silica concentrate can be produced off-site as a concentrate, and then diluted on-site just prior to use. The batch process and use of a silica concentrate have the advantage of being operationally simple, and can be produced using existing field equipment. The continuous process allows for production of a large volume of viscosified frac fluid, but does require modification of existing fracture fluid equipment.
The viscosity build of the silica gel can be tailored to suit equipment, as well as reservoir environments. There are several factors affecting the gelation rate and final viscosity. These include controlling final pH, concentration of raw materials, the presence of salts, and formation temperature.
Silica gel builds viscosity via the smaller fragments that mechanically entangle and interlock to form a much larger, three-dimensional structure. Gel strength is further enhanced, as hydrogen bonds form between silica gels. The final viscosity build is dependent on several factors, with silica concentration being the most dominant. The method developed by PQ does lower the required concentration of silica gel to achieve desirable proppant suspension and transport.
The trend toward high-temperature and ultra-high temperature wells increases the need for viscosifiers with exceptional thermal stability. The prior use of silica gel as a geothermal frac fluid indicated a usage temperature of over 500°F.2 These data support the evaluation of silica gel for application in these environments. It is also supported by the basic chemistry of silica.
Silica gel does not lose viscosity through typical degradation methods of hydrolysis, oxidation or bacterial breakdown. There is, however, some loss of viscosity, due to the silica network structure undergoing syneresis. Syneresis is the process, whereby the silica gel shrinks and the entrained water/brine is extruded from the gel structure. The dehydration of the silica gel results in the loss of gel volume and surface area, which translates into a reduction in viscosity. The process of syneresis is dependent on time and temperature, and pH.
The viscosity to carry and transport most proppant materials and concentrations can be achieved, using a silica loading in the range of ~2 to 3% SiO2, by weight of water. The proppant suspension ability of the system is illustrated, using mono-mesh carbon steel shot with a diameter of 0.017 in. and a specific gravity of 7.6 (Fig. 2) in a 2.5% weight silica gel, Fig. 3a and 3b.
The viscosity of the three-dimensional silica gel can have a high tolerance to monovalent salts, as well as to multivalent metals, Table 1. Tolerance is dependent on the formation conditions of the silica gel, as well as the pH of silica gel-based frac fluid. The low affinity for metals and lack of a ligand structure means that metals, such as alkali metal borates and zirconium compounds, do not act as crosslinking agents.
The rheology of a silica gel-based frac fluid was modeled, using a variety of different rheological methods, including flow loop testing, different types of rheometers and settling rates of suspended material. The rheology can be characterized as following the familiar Power Law model.
Silica gel does not “break” or depolymerize in the same sense as natural polymers. The silica is removed through a combination of other factors. Viscosity loss can occur through an induced drop in the pH. Clean up is achieved through a combination of factors. The lack of charge results in the silica gel having a low affinity for formation rock and, therefore, has a low lift-off pressure from the fracture face, making it readily removable.
The other factor favoring the removal of silica is the previously described process of syneresis. The post-fracturing process favors accelerated syneresis by the closure pressure of the fractured reservoir. Closure forces induce a pressure difference between the silica gel and the hydraulic fracture fluid. The entrained water/brine is forced out at an accelerated rate. The rate of syneresis is accelerated further by temperature. As the gel undergoes a syneresis, there is a decrease in silica gel volume with a corresponding loss in viscosity. As the gel condenses or shrinks away from the water phase, it can be removed from the fracture via flowback of water.
Formation damage studies were done in the original patent (U.S. Patent 4,215,001), “Methods of treating subterranean well formations.”4 These regained formation damage studies showed that regained permeability in low-perm Bandara core (<10 md) was 80+%, compared to standard guar gum at 20–30%. Regained permeability in very low-perm Ohio core (<1 md) was 90%, compared to standard guar gum at 20–25%.
Fracture flow capacity tests with the silica gel, compared to a standard Guar gum gel (previously broken), indicated a 50% flow capacity with the silica gel, compared to 75% flow capacity for the standard guar gum gel. This loss of fracture flow capacity was offset easily by the high regained permeability in the fracture face. This test was conducted overnight in an air-conditioned lab. The temperature was well below 75°F. A higher temperature and longer time should result in a significant increase in flow capacity, as the silica gel becomes dehydrated.
The use of silica gel as a viscosifer helps to produce a frac fluid that is non-hazardous and environmentally benign. The method of on-site production of the silica gel results in a product with similar characteristics as food-grade silica. Depending on the jurisdiction, the use of silica as a food additive can be as high as 5% by weight, and in the U.S. has an upper limit of 2% for most food applications.5
The environmentally friendly nature of the silica gel is further enhanced by reduction, or elimination, of certain frac fluid additives, such as biocides and gel stabilizers. The starting raw materials are N grade sodium silicate and hydrochloric acid. Both materials have a minimal bacterial loading, due to their manufacturing process and pH. The resulting silica gel is not a food material for most microbes. This reduces or eliminates the need for biocides. Since the silica gel does not undergo oxidative degradation, there is no need for the use of oxygen scavengers.
Other commonly used frac fluid additives can be incorporated into the silica gel-based frac fluid. For example, friction-reducing polymers can be added to reduce friction losses in tubing during injection. The incorporation of other polymers can cause an increase in viscosity, as the polymers associate with the silica gel structure. The degree of viscosity increase is dependent on the type, and concentration, of polymer. Viscosity increases can be controlled by reducing the overall level of silica gel.
Made with readily available and cost-effective material of N grade sodium silicate and hydrochloric acid, the resulting silica gel is emerging as a unique, high-performance viscosifier for hydraulic facturing fluids. The gel can be produced in a batch or continuous-mix operation. The unique gel structure and chemistry provide superior proppant transportation and suspension of regular, as well as high-density proppants. Suspension properties can be maintained at normal-to-ultra-high temperatures, up to 500°F and perhaps beyond. Performance is achieved without sacrifice in environmental and health safety. Silica represents an overlooked technology that is starting to emerge as a high-performance viscosifer in aqueous systems. It is anticipated that silica gel will also find application in foam-based systems, as well as aqueous systems.
- Elphingstone, E., et al., “Methods of treating subterranean well formations,” U.S. patent 4,215,001, July 1980.
- Katagiri, K., et al., “Hydraulic fracturing aids geothermal field development,” World Oil, December 1980.
- Katagiri, K., et al., "Frac treatment boosts geothermal well production," World Oil, September 1983.
- Elphingstone, E., et al., “Methods of treating subterranean well formations,” U.S. patent 4,215,001, July 1980
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