Absolute Density – (also known as “Particle Density”) – The true measure of density of a discrete proppant particle.  Measurement is by a “pyncnometer” taking a fixed mass of proppant and passing argon through the mass in an enclosed chamber of specific volume.  Measures the density of the proppant only.

Acid Solubility – A mass loss (gravimetric) test method which determines the degree of solubility of natural sand and manufactured proppant in a 12:3 blend of Hydrochloric and Hydrofluoric acids, which is a measure of the resistance of proppants to acid attack, and an indication of the presence of contaminants which may negatively affect proppant performance.  

Apparent Density – (also known as “Specific Gravity”) – Determines the density of a particle by weighing a measured mass of proppant in a low-viscosity fluid such as kerosene or mineral spirits.  The fluid displaces the air that normally fills the interstitial space between particles.

Bulk Density – The unit mass of an untapped or unsettled proppant that will occupy a specific known volume; e.g., how many grams per cubic centimeter, or lbs per cubic foot.  Bulk Density includes both the mass of the proppant and the mass of air occupying the interstitial spaces between proppant particles.

Clusters – Undesirable agglomerated discrete proppant particles; typically occurs more with inefficiently processed natural sand proppants as opposed to manufactured ceramic proppants.  ISO 13503-2 and API RP19C limit the mass of clusters to less than 1%.  

Conductivity --- Conductivity is a calculated value based on the permeability and width of a proppant pack in a prepared API test cell, usually between sandstone cores or steel plates, depending on the objectives of the test.  Conductivity is typically expressed in units of millidarcy-feet (md-ft), the darcy being named after Henry Darcy, a pioneer researcher in hydraulics and fluid flow through porous media.  Standards are API RP-19D and its ISO 13503-5 equivalent.  The test is performed by sandwiching 2.0 lbs / sq ft of proppant between carefully-fitted sandstone cores or steel plates in a standard API test cell.  Degassed 2% potassium chloride solution is pumped through the proppant pack using precision, constant flow rate pumps while at the same time applying force to the pistons at the top and bottom of the API cell, forcing the sandstone cores or steel plates tightly against the proppant and simulating closure stress downhole.  Flow, pressure, temperature, and other data are recorded as the cell is stressed for 50 hrs +/- 2 hrs for each desired closure stress level.  Permeability and pack width are directly measured, and conductivity is then calculated and plotted.  Temperatures up to 350F and stresses up to 20,000 psi can be accommodated. 

Crush Resistance – The measure of the strength of a mass of screened, fines-free dry proppant to force applied over a fixed cross-sectional area, providing an equivalent stress to the proppant under test.  The mass of proppant introduced to the crush cylinder is a function of its Bulk Density and the specified loading of 4.0 lb/cu ft.  The load is applied in a controlled rate and held at the final test stress level for 2.0 minutes.  The mass is re-screened to determined the amount of fines generated by the applied stress, and the highest stress attained without producing more than 10.0% fines is the “K Number.”  For example, if Crush Resistance of a proppant yielded 9.78% fines at 10,000 psi and 10.44% fines at 11,000 psi, the K Number (K=1000) of that proppant would be “10K,” because the generated fines were below 10.0% at 10,000 psi (10K psi) and exceeded 10% at 11,000 psi.

 pH – A measure of the molar concentration of hydrogen ions in the solution and thus a measure of the solution’s acidity or basicity.  Acidity or basicity of a mass of proppant mixed into a mass of dionized water is an indication of the proppant’s chemical effects on completion fluid chemistry.  
 
Resin Coating Loss of Mass on Ignition (LOI) -- A mass loss (gravimetric) test method which determines the mass of resin coating applied to a natural sand or manufactured proppant by means of thorough combustion of the flammable resin from the non-flammable proppant.  Result reported as a % of original mass of coated proppant.  Applies to coated proppants only.

Sieve Analysis – Determines particle size distribution by means of passing a measured mass of proppant through a stack of progressively finer mesh sieves and a colletion pan for fines under shaking and “hammering” action provided by a Tyler Ro-Tap shaker device for 10 minutes.  ISO 13503-2 and API RP19C require that a minimum of 90% of the proppant mass fall between the designated sieve sizes (e.g., “20-40”).  Not more than 0.1% may be trapped by the first screen, and not more than 1.0% may fall into the pan.  The mass of proppant sample retained on each sieve is entered into software and statistical calculations determine the mean and median (D50) particle size.  

Sphericity and Roundness –  (also known as “Krumbein Shape Factors”) – Sphericity is the measure of how spherical a given proppant particle is.  Roundness is the measure of the lack of sharp edges or angularity.  For example, highly spherical and well rounded proppants resemble small potatoes under magnification, whereas particles possessing low sphericity and roundness resemble unfinished stone arrowheads.  Proppants must be highly spherical and well rounded in order to maximize  interstitial space between adjacent proppant particles to allow passage of oil, gas, condensate, etc., through the proppant pack in the frac width.

Turbidity – A method using transmittance or reflectance of light to measure the amount of fines which are <200 US Mesh in diameter, including clay, silt, proppant fines, etc.  A fixed mass of proppant is added to a fixed mass of deionized water, agitated, and the water is drawn off and measured in a tubidity meter.  The higher the fines content, the more “cloudy” or turbid the water will be.  ISO 13503-2 and API RP19C limit the Turbidity of proppants to 250 FTU / NTU; 1 FTU (Formazin Turbidity Unit) = 1 NTU (Nephelometric Turbidity Unit).