Proppants are solid particles pumped into the fracture to keep it open after the pressure is released. Without proppants, fractures would close under in-situ stresses, and fluid flow would be severely restricted.
The ultimate goal of hydraulic fracturing is not just to create fractures, but to create conductive pathways that allow hydrocarbons to flow efficiently from the reservoir to the wellbore. Proppants are therefore essential because they directly control fracture conductivity, which determines production performance.
Fracture conductivity
Fracture conductivity represents the ability of a propped fracture to transmit fluids. It is defined as the product of the permeability of the proppant pack and the fracture width. In physical terms, it describes how easily fluid can move through the fracture once it has been created and propped.
To better understand fracture performance relative to the reservoir, the concept of dimensionless fracture conductivity is often used:
Where:
- \(\boldsymbol{C_{fD}}\): dimensionless fracture conductivity
- \(\boldsymbol{k_f}\): proppant permeability
- \(\boldsymbol{w}\): average propped fracture width
- \(\boldsymbol{k}\): formation permeability
- \(\boldsymbol{x_f}\): fracture half length
When the dimensionless fracture conductivity is too low (typically < 2), much of the created fracture becomes non-productive. Although fractures may extend thousands of feet, production often reflects effective half-lengths of only a few hundred feet.
This is commonly observed in slickwater fracturing, where low-viscosity fluids result in poor proppant transport. Proppant settles quickly, leaving only the lower portion of the fracture propped, while the rest remains unpropped and closes under stress.
These unpropped regions have very low conductivity (on the order of 0.1–5 mD·ft or less), which severely limits flow. As a result, a large portion of the fracture (sometimes up to 90%) may not contribute to production, leading to low recovery efficiencies.
Proppant Types
Proppants are commonly classified into three main categories based on their material and mechanical properties: natural sand, resin-coated proppants, and ceramic proppants.
Natural sand is the most widely used proppant due to its low cost and availability. High-quality sands, such as white (Ottawa) sand, are composed primarily of quartz and have good roundness and strength, resulting in relatively high conductivity. Lower-quality sands, such as brown sand, contain more impurities and are less spherical, which reduces conductivity and increases the likelihood of crushing under stress.
Resin-coated proppants are natural sands or ceramics that are coated with a resin layer. This coating helps bind grains together, reduces fines generation, and prevents proppant flowback. While the coating may slightly reduce conductivity at low stress, resin-coated proppants generally perform better at higher stresses by maintaining permeability and limiting damage.
Ceramic proppants are engineered materials designed for high-stress environments. They have higher strength and more uniform grain properties than natural sands, allowing them to better resist crushing and maintain conductivity. However, their higher cost limits their use to applications where improved performance justifies the expense.
