Mechanical-Damage Characterization in Proppant Packs by Use of Acoustic Measurements

Abstract

Summary The strength and conductivity of proppant packs are key parameters for assessing proppant-pack performance. Mechanical damage in the propping agents, which leads to compaction and crushing, significantly reduces the conductivity of the proppant pack. Mechanical damage of proppants is usually analyzed by use of crush tests. However, measurements from these tests remain questionable because of discrepancies in procedures and test results. Therefore, a need emerges to develop techniques for characterizing the properties and mechanical damage in proppant packs. In this paper, we introduced a new technique that is based on interpretation of acoustic measurements from a granular effective-media model to quantify mechanical damage in propping agents. We performed uniaxial compression tests in the laboratory and measured the compressional- and shear-wave velocities in proppant packs loaded at axial stresses ranging from 10 to 110 MPa. After unloading the tests in which maximum axial stresses of 28, 55, 69, 97, and 110 MPa were applied, we conducted sieve analysis on the proppant packs. We applied an effective-medium theory modeled after the Hertz-Mindlin granular contact model to approximate the effective elastic properties. We then calibrated the model by use of the elastic properties estimated from the experimental measurements to characterize the mechanical damage of the proppant packs. We observed that the increase in grain-to-grain contact as the axial stress increases results in compaction and crushing in the proppant pack. We showed that the compaction effects and elastic and plastic behaviors in the stress–strain profile of the proppant pack were in agreement with the analysis of fines generated at different stress levels. The combined effect of compaction and crushing resulted in a reduction of porosity and, consequently, decreased the compressional- and shear-wave velocities of the proppant pack. The Hertz-Mindlin model showed a good approximation of the effective elastic properties estimated from the acoustic-wave velocities when calibrated with the pressure-dependent grain contact and the fraction of nonslipping grains as parameters. We demonstrated that the calibrated parameters can be correlated with the mechanical damage in the proppant pack. The characterization of mechanical damage in proppant packs can improve the design of the propping agents and quantification of proppant performance. Furthermore, the laboratory procedure can be extended to the use of borehole acoustic measurements in providing a real-time in-situ assessment of proppant performance.