Jun. 10, 2024
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 -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 -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 ( 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 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 , API and ISO members formed a group to create ISO ; the current standard test methods for testing proppants used in hydrofracking. ISO -5 is the standard test method for measuring the long-term conductivity of proppants. ISO -2 is the standard test method for determining the hydraulic fracture proppant crush resistance. The test practices of ISO -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 -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 -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 -2, once the preload is achieved, control automatically moves to load rate control (e.g. psi/min).
When the American Petroleum Institute (API) established standardized crush-testing procedures (API RP-56 ), the committee indicated that the test results should "provide indications of the stress level where proppant crushing is excessive and the maximum stress to which the proppant material should be subjected." However, over time, many have forgotten not only how the test is conducted, but also its original intent. As such, many now unintentionally misapply the results of crush testing as they select proppants for their fracture designs.
This paper will review the top 10 myths associated with crush testing and its interpretation, addressing such common questions as
Do standard test conditions (high proppant concentration and low temperature) provide realistic predictions of proppant performance?
Should proppant be tested wet or dry?
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Does the loading procedure affect crush?
What happens if proppant is not distributed uniformly in a fracture?
Do all proppants fail in the same manner?
Are all proppant types equally damaged by 5% crush?
How can the industry misuse the test to report "superior" results?
Readers of this paper will be armed with a better understanding of how crush testing is performed, how crush results can be misapplied, and the correct use of crush-test results. In addition, the authors will present an alternative methodology for evaluating proppant that incorporates all of the benefits gained from crush testing, but avoids the common pitfalls. Armed with this information readers can improve the design of fracture treatments, thereby achieving increased production rates and superior economic returns.
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