TESTING STANDARDS FOR PROPPANTS
It is common for industries to develop and implement testing standards. While their use by manufacturers of products in an industry is usually not mandatory unless required by regulatory bodies of the government, such standards provide many important benefits. These benefits include evaluating the suitability of new products, defining performance envelopes, comparing different products to determine which product to use, guiding research and development (R&D) activities towards the attainment of specific and well-quantified targets, and building credibility by demonstrating that a product will work as advertised.
This post summarizes the use of industry standards for the proppants used during hydraulic fracturing operations in the oil and natural gas exploration and production industry. In such operations, (1) a fluid is injected into a wellbore in a subterranean formation at sufficiently high rates and pressures to cause the formation to fail, (2) the resulting fracture is kept open (propped up) by the placement of a packing of load-bearing “proppant” particles which have been carried by the fluid into the fracture, and (3) hydrocarbons (liquids and/or gases) flow towards the wellbore through the packing of proppant particles and are extracted.
The most important properties of proppant particles include their shapes, size distribution, density, resistance to acids, and crush resistance. Shape is generally described in terms of the “sphericity” and “roundness” parameters. Shape and size distribution are important because they affect how the particles pack within the fracture. The density is important because, for any given particle shape and size, a particle of lower density is easier to transport deeper into a fracture without settling. Acid resistance is important because greater acid resistance means greater robustness under chemical exposures that the proppant particles may encounter during a hydraulic fracturing operation. Crush resistance is important because fewer proppant particles with greater crush resistance will be crushed under the same closure stress of the fracture to generate tiny particles referred to as “fines” blocking the open pathways for hydrocarbons to travel through the packing
The most important property of a proppant pack is rate at which hydrocarbons can be extracted through it. This property is quantified in terms of a parameter named “conductivity” which, in this context, does not refer to electrical conductivity bur instead to the liquid conductivity of the packing. In addition to depending on the factors summarized in the preceding paragraph, the conductivity of a proppant pack depends on the mechanical properties of the subterranean formation, the compression resistance of the proppant particles at the temperature of the area where the fracture is located, and the density of the surface coverage (expressed in mass/area, such as lb/ft2 or g/cm2) by proppant particles.
Testing standards that are very similar or identical to each other have been issued by the American Petroleum Institute (API) and the International Organization for Standardization (ISO) for proppants. The following are the relevant API standards:
- API STD 19C, Measurement of and Specifications for Proppants Used in Hydraulic Fracturing and Gravel-packing Operations, 2nd Edition, August 2018. This standard provides standard testing procedures for evaluating proppants used in hydraulic fracturing and gravel-packing operations. The objective of this standard is to provide a consistent methodology for testing performed on hydraulic fracturing and/or gravel-packing proppants. These procedures have been developed to improve the quality of proppants delivered to the well site. They are for use in evaluating certain physical properties used in hydraulic fracturing and gravel-packing operations. These tests should enable users to compare the physical characteristics of various proppants tested under the described conditions and to select materials useful for hydraulic fracturing and gravel-packing operations.
- API RP 19D, Measuring the Long-term Conductivity of Proppants, July 2008, reaffirmed May 2015. Provides standard testing procedures for evaluating proppants used in hydraulic fracturing and gravel-packing operations. The "proppants" mentioned in this publication refer to sand, ceramic media, resin coated proppants, gravel packing media, and other materials used for hydraulic fracturing and gravel-packing operations. The objective of RP 19D is to provide consistent methodology for testing performed on hydraulic-fracturing and/or gravel-packing proppants. It is not intended for use in obtaining absolute values of proppant pack conductivities under downhole reservoir conditions. The tests and test apparatus herein have been developed to establish standard procedures and conditions for use in evaluating the long-term conductivity of various hydraulic fracture proppant materials under laboratory conditions. This procedure enables users to compare the conductivity characteristics under the specifically described test conditions. The test results can aid users in comparing proppant materials for use in hydraulic fracturing operations.
The image below, which summarizes some data for CARBOLITE low-density ceramic proppant manufactured by CARBO Ceramics, shows an example of how such data are often presented in product literature.
The following image, from the product literature of Fairmount Santrol which manufactures many sand proppant products, shows how such data can be used to define performance envelopes and to compare different products to determine which product to use in different subterranean environments.
The development and commercialization of ultralightweight (ULW) thermoset polymer nanocomposite proppants with high load-bearing ability by Sun Specialty Products during the last 15 years is the most significant recent breakthrough in proppant technology. Such ULW proppants are nearly neutrally buoyant in water, so that they can be transported with much greater ease than sand and ceramic proppants deep into a fracture. Data and photos reproduced from the product literature for the two current commercial product grades (OmniProp® and FracBlack HT®) are shown in the image below.