There are many types of fluids available for use in hydraulic fracturing. To select the proper fluid for a specific well, it is necessary to understand the properties of the fluids and how those properties may be modified to accomplish various desired effects.
The properties that a fracturing fluid should possess are how leakoff rate, the ability to carry a propping agent, and how pumping friction loss. The fluid also should be easy to remove from the formation; it should be compatible with natural formation fluids; it should cause a minimum of damage to the formation permeability. The degree to which these characteristics exist must be considered in the design and selection of a fracturing fluid.
Low leakoff rate is the property that permits the fluid to physically open the fracture and one that controls its areal extent. The rate of leakoff to the formation is dependent upon the viscosity and the wall-building properties of the fluid.
The ability of the fluid to carry the propping agent is important and also can be controlled by additives. Essentially, this property of a fluid is dependent upon the viscosity and density of the fluid and upon its velocity in the pipe for fracture. Density and velocity are not hard to describe; however, viscosity is difficult to measure and describe properly since many fracturing fluids are non-Newtonian. Two defferent fluids, such as an emulsion and gelled water, can appear ot have the same viscosity by one measurement, but may have widely varying abilities to carry propping agents in suspension. This aspect of propping agent suspension is frequently overlooked. The rate of movement is a major factor in the ability of fluid to carry the propping agent. Plain water with its low viscosity will carry proppants satisfactorily if it is pumped at a high rate.
Friction loss has become more important in recent years because it is controllable and because higher pumping rates have proved effective in fracturing treatments. The ability to reduce friction loss in pumping has been one of the governing factors in the present trend toward the use of water-base fracturing fluids.
To achieve the maximum benefits from fracturing, the fracturing fluid must be removed from the formation. This is particular true with the very viscous fracturing fluids such as viscous oils, gels, or emulsions. Most of the gelled-oil and water-base fracturing fluids have built-in breaker system that reduce the gels to low viscosity solutions upon exposure to the temperatures and pressures existing in the formations. When the voscosity is lowered, the fracturing fluid may be readily produced from the pay formation and no flow restrictions remain. Generally, viscous oils such as residual fuel oils are sensitive to heat and to crude oil diluted so that they flow easily from the well. Emulsions ususlly are diluted by formation fluids, have their emulsifying agents adsorbed on the rock durfaces, or are destroyed by contact with acid or heat. This allows the remaining liquids to flow from the well.