Hydraulic Fracturing Fluids
The types and use of fracturing fluids have evolved greatly over the past 60 years and continue to evolve. To select the fracturing fluid for a specific well, it is necessary to understand the properties of the fluid and how these properties may be modified to accomplish desired effects.
1. Low leak-off rate,
2. Ability to carry the propping agent,
3. Low pumping friction loss,
4. Easy to remove from the formation,
5. Compatible with the natural formation fluids,
6. Minimum damage to the formation permeability,
7. Break back to a low viscosity fluid for clean up after the treatment.
Fracturing fluid pumped during the process is generally in turbulent flow in the well bore and perforations and in laminar flow in the fracture. The accurate characterization of the rheological properties of the fluid is necessary for the successful application of the hydraulic fracturing process.
The turbulent flow frictional loss in the well bore and perforations is important to design and perform a fracturing treatment. The frictional losses are used to predict the surface treating pressure and injection rate.
The laminar flow behavior of the fluid in the fracture is critical to the design of proppant transport and fracture geometry. The fracture geometry and extension during the treatment depends to a high degree on the rheological properties of the clean as well as proppant-laden fluid. Fracturing fluids are generally classified into three types: aqueous-based, oil, and foam fluids. Aqueous-based fracturing fluids have been widely used in the oil and gas wells because of their low cost, high performance, greater suspending power, environmentally acceptable and ease of handling.
Aqueous-based fracturing fluids are classified based on the amount of polymer (gelling agent) per gallon of water. For example a 20-pound fracturing fluid mix consists of a ratio of 20 pounds of gelling polymer per 1000 gallons of base fluid. Various mixes of these gelling agents are used in hydraulic fracturing treatments, depending on the type of formation being stimulated.
Guar is the most popular polymer for preparing aqueous-based fracturing fluid. The guar polymer has a very high affinity for water. The guar polymer easily dissolves in water and readily establishes hydrogen bond with the water molecules and gets hydrated. The hydration of the polymer particle causes it to swell, exposing more sites on the guar to establish more hydrogen bonds with the water molecule. The hydration of the polymer continues till each guar molecule is well bonded with water molecule. The hydration rate is monitored with guar solution viscosity which varies as an exponentially with time for hydration. When no further increase in the viscosity is observed, the guar hydration is considered to be complete.
Oil-based fracturing fluids are primarily used for water sensitive formation. They normally employ gelled kerosene, diesel, distillates, and many crude oils. Aluminum salts of organic phosphoric acids are generally used to raise viscosity, proppant carrying capability, and improve temperature stability. Compared to aqueous-based fluids, they are more expensive and more difficult to handle. Oil-based fluids are more hazardous because of flammability and also possess environmental concerns.
Foam fracturing fluids are used in low pressure and fluid sensitive formations to aid in clean-up and reduce fluid contact. They are gas and liquid dispersions. Foams can use nitrogen and/or CO2 as the internal phase and water-methanol mixtures used as the external phase for the formation of foam fracturing fluids. The disadvantages associated with foam fluids are: they cannot be loaded with high proppant concentration, the cost of
foam fluid systems including field equipment is very high, and they are very uneconomical as compared to aqueous and oil-based fracturing fluids. Furthermore the rheological characterization of foams is not easy because of the many variables involved.