Proppant is the term given to any material that is added to a fracturing fluid with the purpose of holding open the fracture and creating a medium with greater permeability than the reservoir, for hydrocarbons to flow through to the wellbore. Proppant can be sand, or ceramic, and in some cases either can be resin coated for reducing proppant flow-back, as the resin cures to bond the proppant together. There are several key qualities that determine the quality of a proppant:
- Size – the size of proppant for each job should be appropriate for the reservoir permeability, that is, large enough to create insitu permeability larger than reservoir permeability, but not large enough to create unnecessary risk or screen-out through particle bridging across pore throats and perforations. Sizes are given as ranges in mesh size, where each corresponds to a specific value in microns. For example, size 20/40 proppant will have grain sizes between 420 to 850 microns, where the larger mesh sizes refer to the smaller proppant diameters. QAQC is carried by testing proppant in a series of sieves equal to the smallest and largest mesh sizes, and proppant quality is acceptable if 90% of proppant sample is between the mesh sizes specified.
- Strength and density – the proppant pack should be able to maintain permeability at fracture closure stress. The density of the proppant is also related to the stress that the proppant pack may withstand; sand, with a specific gravity of 2.65, may withstand pressures up to 5,000 psi, medium strength ceramic proppant, a denser material, may withstand pressures up to 10,000 psi, and high strength proppants such as sintered bauxite which is denser still, up to 15,000 psi.
- Sphericity and roundness – measured on a scale of 0 – 1, the values will indicate the uniformity of the proppant, which will have an impact on the conductivity of the proppant pack when in place in the formation; low values will imply perhaps flatter shaped proppant with irregular edges, and higher values a more sphere-like shape. The more spherical and uniform a proppant, the higher the conductivity will be. Values are normally determined visually, following guidelines.
- Fines – crush tests can be completed in accordance with the American Petroleum Institute (API) guidelines, on proppants to show the tendencies of a proppant to produce fines under pressure. High percentage production of fines is undesirable, as they may can plug the proppant pack and reduce flow, rather than enhance flow.