Frac sand specs
Frac sand specifications are the responsibility in the USA of the American Petoleum Institute (API) and the current standard is API RP 56.
These specifications are very demanding and as a result suitable deposits are limited. The limited availability of natural reserves which are suitable for frac sand production coupled with growing demand ensures a high price for any producers able to meet the API RP 56 frac sand specifications.
Natural sands must be from high silica (quartz) sandstones or unconsolidated deposits. Other essential requirements are that particles are well rounded, relatively clean of other minerals and impurities and will facilitate the production of fine, medium and coarse grain sands.
Frac sand must be >99% quartz or silica. Most silica sand deposits are either already being exploited or are at least known of due to the use of this material in many other industrial applications including glass making and filtration media.
High purity quartz sands are common in the USA. These are made up of some deposits that are currently being exploited, some which have been abandoned and others that are so remotely located that costs of transporting material render them commercially unviable.
The tight specifications for frac sands – especially in relation to roundness and sphericity – make many deposits unsuitable for frac sand production.
From the work currently being done in the production of frac sands it seems that older quartzose sandstones have a better chance of producing a good frac sand. However, it is possible to upgrade other deposits if carefully controlled.
The sizes recommended by the API for frac sand are:
The 20/40 mesh size (0.42mm – 0.84mm) is the most widely used.
Sphericity & Roundness
The standards prepared by the API in this regard simply estimate how closely the quartz grain conforms to a spherical shape and its relative roundness.
The grain is assessed as follows:
"average radius of the corners / radius of the maximum inscribed circle"
Krumbein and Sloss devised a chart for the visual estimation of sphericity and roundness in 1955 as shown below. API recommends sphericity and roundness of 0.6 or larger.
API requires frac sand to be subjected to between 4000psi and 600psi pressure for two minutes in a uniaxial compression cylinder to determine its crush resistance.
The fines generated by this test are limited as shown below:
Size / Max fines by weight
6-12 mesh / 20%
16-30 mesh / 14%
20-40 mesh / 14%
30-50 mesh / 10%
40-70 mesh / 6%
This test measures the loss in weight of a sample that has been added to a 100ml solution made up of 12 parts Hydrochloric Acid (HCI) and 3 parts Hydrofluoric Acid (HCI) and subsequently heated at 150 degrees farenheit (approximately 65.5 degrees centigrade) in a water bath for 30 minutes.
The object of this test is to determine the amount of non-quartz minerals present.
API specifications require that losses by weight as a result of this test are restricted to <2% across all mesh sizes up to 40-70 mesh where the loss permitted rises to 3%.
Turbidity refers to the amoun t of silt of clay sized particles in the sand sample. This is generally not an issue in frac sand production as production requires a washing process to be introduced which effectively removes these particles.
There can also be an attrition process applied which also serves to remove unwanted fines as well as weaker grains.