Induced hydraulic fracturing or sand fracking is a recent technique used to release petroleum and natural gas for extraction. Fracking creates fractures from a wellbore drilled into reservoir rock formations through the use of proppant materials. Proppant is an important granular material transported in the hydraulic fluid to fill the fracture, propping it open as high-pressure pumping stops. Common proppants used in hydraulic fracking include silica sand, resin-coated sand, and man-made ceramics. A proppant-filled fracture then creates a permeable channel through which hydrocarbons flow, increasing production rate and the amount of oil or gas recovered. Increased hydraulic pressure loads to increased hydraulic fracture in oil-rich formations, however, there is a limit to the pressures the proppant can take before is crushed into a cement that does not allow hydrocarbons to flow through.
Mechanical compression tests are needed to determine the maximum stress level where proppant crushing slows down the production and porosity drops. Proppant crush resistance tests aim to determine the proppant porosity under high pressures. Mechanical properties such as proppant crush resistance, specific gravity and bulk density are determined in hydraulic fracture tests. ISO 13503-2 Sec 11.3 Proppant Crush Resistance Test defines the test method currently used to test proppants used in hydrofracking. The 316SP compact electromechanical test frame is a special compact design capable of generating pressures of 21 psi (146 MPa) when used with a test cell piston diameter of 50 mm (2 in). The special load frame is configured for compression tests with an upper compression platen attached to the load cell mounted on the upper crosshead. The lower platen is coupled to the baseplate and includes hard stops to enable quick centered placement of the cell. It also includes circular rings. The proppant crush cell (ISO 13503-2 section 11.3.2) uses a piston diameter of 50,8 mm. The test speed is controlled at a constant rate of 13,8 MPa (2000 psi) per minute. The crosshead loading is based on PID closed loop servo control, so system response is capable of very high quality speed control using proportional-integral-derivative gain settings. The controller measures the load status from the load cell at a rate of 1000 times per second and adjusts the crosshead motion to achieve the desired loading rate. Arrival time (ref 11.5.10) is user adjustable with different gain settings. Hydraulic Fracture Crush Test for Proppant Porosity The American Petroleum Institute (API) was the original organization for standard testing hydrofracking. Specifically, API RP-56, API RP-60, and API RP-61 testing standards were used to determine properties of proppants used in hydrofracking.
In 2001, API and ISO members formed a group to create ISO 13503-#; the current standard test methods for testing proppants used in hydrofracking. ISO 13503-5 is the standard test method for measuring the long-term conductivity of proppants. ISO 13503-2 is the standard test method for determining the hydraulic fracture proppant crush resistance. The test practices of ISO 13503-2 for determining the crush resistance of proppants call for a test machine that has a frame capable of applying loads of 15 ksi. ISO 13503-2 specifies that the test machine used to determine the crush resistance of proppants must be capable of constant rate of extension (CRE) and capable of load rate control. The ideal machine for testing hydrofracking proppant crush resistance is TestResources 316 Series Electromechanical Test System. The 316 electromechanical test system features unique improvements over hydraulic test machines, including precision control and automation to enable the user to load samples with a consistent process, improving test consistency and accuracy. For proppant compression testing the 316 electromechanical test system is programmed to automatically follow the test procedure of ISO 13503-2. This includes the constant rate of extension (at 0.05 mm/min) to an initial load (of 50 to 100 psi). According to ISO 13503-2, once the preload is achieved, control automatically moves to load rate control (e.g. 2000 psi/min).
Applicable Testing Standards
- API RP-56/58/60 Proppant Crush Test for Frac Sand, Resin Coated Sand, Ceramic Proppant
- ISO 13503-2 Proppant Crush Test for Hydraulic Fracturing and Gravel-Packing Operations
Recommended Test Machine
Force range of 5 kN to 600 kN (1,125 lbf to 135,000 lbf)
Adjustable test space
The most popular choice for static tension and compression tests
These dual column testers are available in both tabletop and floor standing models
Recommended Testing Accessories
Standard load ratings up to 300 kN (67,500 lbf) *higher ratings available upon request
Platen diameters from 56 mm to 346 mm (2.2 in to 13.6 in)
Available in steel, stainless steel, aluminum, or delrin construction
Machined surface for flat interface with test specimen
Tests in accordance to ISO 13503-2
Minimum Rockwell Hardness of 43C
