How to Use and Misuse Proppant Crush Tests -- Exposing the Top 10 Myths
Authors: Palisch Terrence T. , Duenckel Robert John , Chapman Mark Aaron , Woolfolk Scott , Vincent Michael C. 
Published: January 2009

When the American Petroleum Institute established standardized crush testing procedures in 1983 (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 misapply the results of crush testing as they select proppants for their fracture designs.

This paper will review the top ten 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?Does the loading procedure affect crush?What happens if proppant is not uniformly distributed 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 1) how crush testing is performed, 2) how crush results can be misapplied and 3) the correct use of crush test results. In addition, the authors will present an alternative methodology for evaluating proppant which 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.

Introduction

ISO 13503–2 (International Organization for Standardization, 2006), is a compilation of three API Recommended Practices (RP56, RP58 and RP60), developed and modified by industry committees, and ultimately approved worldwide by representative countries to evaluate proppant parameters. Contained in these standards are procedures to determine numerous proppant characteristics, including sieve, shape, acid solubility, turbidity, bulk density, apparent specific gravity and crush. While these tests have been "developed to improve the quality of proppants delivered to the well site" and to "enable users to compare the physical characteristics of various proppants", ultimately "qualified engineering analysis and judgment are required for their application to a specific situation" (ISO 13503–2). In other words, these standard procedures should be used to conduct the listed tests, but their results should not be blindly used to select the appropriate proppant.

One of the procedures described by the standard is that of "crush testing". The original purpose of API RP-56 was to provide a uniform set of procedures to qualify sand sources for potential use in fracturing. More detailed examination of sand sources, including conductivity testing, would follow. However, many engineers have come to rely on the reported proppant crush as a primary criterion in proppant selection. Crush data are reasonably inexpensive to obtain, and are readily available for most proppants. Crush testing remains a useful tool in some manufacturing and quality control applications. However, proppant crush results have the potential to be extremely misleading and are widely misunderstood. The following sections will address the primary myths associated with crush testing and address their significance to the design engineer.