Behavior of Proppants Under Cyclic Stress
W.T. Stephens, Saint Gobain Proppants; S.K. Schubarth, Schubarth Inc.; and K.R. Dickson, E.M. Snyder, K.J. Doles, and D.C. Herndon, Saint Gobain Proppants
SPE Hydraulic Fracturing Technology Conference, 29-31 January 2007, College Station, Texas, U.S.A.
2007. Society of Petroleum Engineers
Abstract
Size distribution of proppants is one factor that determines fracture conductivity. Sufficient closure stress can cause mechanical failure of proppants changing the size distribution and altering the bulk properties of the pack. Crush resistance is one characteristic that describes how the size distribution changes under a static load. In many cases, proppants within a fracture experience a dynamic stress state that permits proppant motion and redistribution, altering the stress concentration across the proppant pack.
This paper documents the behavior of proppant size distribution under the influence of cyclic stress. While standard sieve analysis provides a good quantitative value for crush resistance, optical measurement of the size distribution illustrates proppant failure and the alteration of size distribution under load. Advanced load frames permit monitoring and measurement of the compaction process. This paper discusses the sensitivity of the size distribution with respect to closure stress and different types of stress cycles as well as the influence of alternative size distributions and proppant materials.
The results of this paper address expanding the technical evaluation of proppants using new tools and test methods to provide a greater understanding of the evolution of the proppant size distribution. Operators will be able to select proppants based upon data measured under more practical application conditions.
Introduction
The American Petroleum Institute (API) defines the crush resistance test for proppants and specifies one test cycle per specimen.1 Schubarth and Milton-Tayler2 state that proppants typically experience multiple stress cycles in the production environment and that crush resistance tests using multiple stress cycles sufficiently simulates this situation. These stress cycles often occur early in the life of the proppant and significantly change the size distribution with each cycle. Typically, the greatest change to the proppant size distribution occurs during the first five stress cycles. The data from the cyclic crush resistance tests finds use in the correlation developed by Berg2-3 to describe permeability based upon properties of the proppant pack.
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