In most low permeability reservoirs, fracture fluid loss and efficiency is controlled by the formation permeability. In high permeability formations, a fluid-loss additive must be added to the fracture fluid to reduce leak-off and improve fluid efficiency.
Typical additives for a water based fluid are :
Polymers – used to viscosify the fluid
Crosslinkers – used to change the viscous fluid to a pseudo-plastic fluid
Biocides – used to kill bacteria in the mix water
Buffers – used to control the pH of the fracture fluid
Surfactants – used to lower the surface tension
Fluid loss additives – used to minimize fluid leak-off into the formation
Stabilizers – used to keep the fluid viscous at high temperature
Breakers – used to break the polymers and crosslink sites at low temperature
Formation temperature is one of the main factors concerning the type of additives required to mix the optimum fracturing fluid. In deep, hot reservoirs (>250°F), more additives are required than in shallow, low temperature reservoirs. The owner of the oil or gas well normally does not own the equipment or the additives required to pump a fracture treatment. The operator will hire a service company to pump the fracture treatment. Each service company has their own research department for developing fracture fluids and additives. Each service company obtains their additives from various suppliers. As such, there is no set of rules one can use to select the proper additives for a fracture fluid, without first consulting with the service company that will mix and pump the fluid into the well. Pilot tests of the fracture fluids must be conducted to be certain all the additives will work properly at the temperature in the reservoir and for the duration of the treatment.
Selection of the fracture fluid is a critical decision for the design engineer of fracture treatment. Fracture fluid's selection is based on factors such as reservoir temperature and pressure, the expected value of fracture half-length, and a determination if the reservoir is water sensitive.