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A Novel Model of Brittleness Evaluation for Unconventional Reservoirs Based on Energy Consumption

Abstract

Brittleness evaluation plays an important role in unconventional oil and gas. However, the definition of brittleness is controversial and the models for brittleness evaluation have no reliable theory to support them, which makes brittleness evaluation unreliable. Rock failure is a process of energy dissipation and release. The energy dissipation leads to plastic deformation and damage of rock, while the releasable strain energy results in abrupt structural failure of rock. In this study, brittleness evaluation has been done in terms of energy. By defining it in terms of energy, rock brittleness from different areas can be compared. The influence factors ignored by other models of brittleness evaluation, such as confining pressure, temperature and rock texture, can be addressed. Cyclic loading-unloading tests under different confining pressures have been done in order to investigate the effect of energy dissipation and release on rock failure. Most of the input energy is converted into the elastic strain energy during this process and the rest is dissipated. The energy dissipation cannot be released after unloading, while the elastic strain energy does. Therefore, the unloading curve normally falls below the loading curve to form a hysteresis loop. Then, the elastic strain energy and the energy dissipation of rock failure can be calculated from stress-strain curves or corresponding formulas in this paper. Our study shows that energy release leads to rock failure abruptly and energy dissipation determines the degree of rock fragmentation. Usually, the larger the energy dissipation during the process, the smaller the fragments after rock failure. Also, the energy dissipation of brittle rock stays low before failure and increases sharply when the failure happens, between which the ratio is usually less than 1:5. For ductile rock, there is relatively less difference in energy dissipation before and during the failure. This is because more energy is converted into plastic energy instead of the energy that increases the degree of rock fragmentation. The result is that even though the rock is failure, it doesn't broke into pieces. Therefore, by comparing the value of energy dissipation and release not only could we evaluate rock brittleness, but also predict the degree of rock fragmentation after rock failure.