The term “hydraulic fracturing” refers to the practice, used in drilling, of injecting a mix containing water under high pressure with “proppants”, as well as chemicals in small amounts, into the shale petroleum-bearing layer of the rock, or first down and then along the plane of the deposit in case of more superficial shale gas deposits. Proppants, also called fracking or frac sands, consist of small particles which prop the drilled fissure open so that gas or fluid can flow without obstruction to the surface, as shown in Figure 1.
Proppants are of three types – sand, ceramics and either coated with resin, as depicted in Figure 2. Table 1 shows their properties.
The quality control parameters set up by the American Petroleum Institute according to API RP 19C Standard, “Measurement of Properties of Proppants Used in Hydraulic Fracturing and Gravel-Packing Operations”, make proppant use mandatory and also specify the distribution of proppant size from 106 microns to 3.35 mm, as in Table 2.
The appropriate particle size distribution is ensured by using Laser Diffraction (LD) technology in place of sieves, because LD produces measurements which are rapid, with a low margin of operator error and increased precision.
The standard also sets the values for two different types of shapes which must also be met. The ideal proppants have high Krumbein Sphericity and Roundness values, which are measured on a scale of 0 to 1.0, with higher values indicating rounder edges and sphericity. When such proppants are used, the flow or conductivity of the gas or liquid petroleum is optimized through the propped fissures for the best possible extraction rates.
Krumbein parameters for roundness and sphericity have been in use for decades, from the 1960s onwards, and are measured by sieving of the proppant sample. This separates it into different fractions based on the size. This is followed by size analysis and manual selection of 20 particles from each of the fractions for microscopic examination.
At this time, the particle shape is visually compared with a chart of shape silhouettes called the Krumbein-Sloss chart (shown below), each silhouette having a specific value. Each particle is matched as closely as possible to the silhouette and the corresponding value is assigned to it.
According to the standard, this chart is for visual estimation of sphericity and roundness of proppant particles, but there are some evident shortcomings, as described below:
- The sample is so small as to preclude it being representative of the whole proppant batch at each size fraction
- Each analyst has a different way of deciding whether the particle conforms to the specification, making the method highly subjective
- The measurement takes much time because of the various steps required, including picking out the samples physically, sieving, manual examination under the microscope, deciding on each particle individually, and reporting
- Manual processes such as sieve size measurement and shape estimation tend to increase the rate of errors
For all these reasons, Automated Digital Image Analysis (DIA) has come into widespread use because of the many benefits:
- Automated process
- Measures 30 distinct shapes and sizes including those on the Krumbein Roundness and Sphericity parameters
- Measures samples containing many hundreds of thousands of particles, but takes minutes instead of hours to complete the process
- Each particle is individually measured and given a value for each separate parameter from the 30 used, rather than measuring only 20 particles in an entire size fraction
- Each size fraction, as well as the whole sample, is reported for shape parameter values, using all the particles in each of the fractions
- The calculations for shape parameters use the same formulas as the Krumbein-Sloss chart to arrive at its values
Microtrac has brought out a powerful new instrument which puts LD together with DIA, one being the most widely used, and the other, the new and highly powerful technology for size analysis, in one instrument. This measures the same sample at the same time within the same sample cell, using two different technologies.
Microtrac Sync – combining LD and DIA
The Microtrac Sync is set up for wet analysis performance but a dry feed module is also available. Both are shown in the figure below.
The following is the inner configuration of the Sync.
LD size report for proppant
LD size analysis reports come in the following format, as shown in Figure 6, including the graphical and tabular size distribution trends, ten percentile sizes, summary data and SOP parameters.
DIA reports on size and shape of proppants
Figure 8 and Table 3 show the corresponding DIA results, giving the Krumbein Roundness and Sphericity values for each size fraction in the form of the area equivalent diameter.
The standard prescribes a value of 0.6 or more for all fractions (accurate to the nearest tenth). The report above shows that all roundness values meet this specification but two of the sphericity values are just outside it, at 0.57 and 0.58, while remaining within one tenth.
These fractions contain only 2.4% of the total particle count of the whole sample. The report thus gives more detailed data without room for analyst subjectivity or operator error, which means that the specifications can be adjusted for tight quality control based upon automated measurement of particle size and shape.
Figure 9 shows the data in the form of a scatter diagram, with the blue graph displaying particle location within the sample with respect to the Krumbein roundness and sphericity values on the x-axis and y-axis, respectively, showing how these values are distributed for the whole proppant sample. Again, this indicates the superior detail and increased yield of data from DIA.
A glance shows how much of the proppant sample falls above the 0.6 specification (within a tenth), as well as the D50 values which show the 86th and 95th percentile for Krumbein Roundness and Sphericity respectively.
Hydraulic fracturing is an important technique in the extraction of oil and natural gas from shale petroleum deposits in many parts of the world.
The shale fissures created by drilling must be propped open to allow smooth extraction, using proppants.
The proppant particles must be round and spherical to a very high degree to allow free conductivity so that extraction rates can be optimized.
The American Petroleum Institute specifies size distribution parameters, as well as two particle shape values based on Krumbein Roundness and Sphericity, as per the API RP 19C Standard for proppant test and reporting.
These parameters have been traditionally tested using manual methods which are slow and cumbersome, with a high possibility of operator error as well as statistical inaccuracy with respect to particle shape, because of small sample size.
The use of automated technologies, specifically laser diffraction and digital image analysis, provide a powerful, rapid and convenient way of measuring the quality of proppant particles within minutes using a very high sample size, and based on 30 different size and shape parameters applied to each particle in the whole sample.
The Microtrac Sync is a cutting-edge instrument which combines the best of both technologies to offer rapid and error-free comprehensive quality control measurements using two different techniques on the same sample in the same sample cell at the same time.
This information has been sourced, reviewed and adapted from materials provided by Microtrac, Inc.
For more information on this source, please visit Microtrac, Inc.