--> Effect of Temperature on Brittleness Evaluation of Unconventional Reservoirs

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Effect of Temperature on Brittleness Evaluation of Unconventional Reservoirs

Abstract

The influence of temperature on brittleness evaluation has been underestimated. It is generally recognized that temperature is conducive to the improvement of rock ductility. However, the effect of temperature on brittleness evaluation is more complicated than that. In this study, temperature influence on brittleness evaluation of unconventional reservoirs has been analyzed from two aspects: rock texture and pore pressure. According to the analysis of uniaxial compression test, SEM and X-ray diffraction, mechanical property, texture and crack characteristics of rocks has been obtained. Stress-strain curves of shale and sandstone samples at temperature from 25°C to 800°C have been used to analyze temperature dependencies of the elastic modulus, peak stress and strain. The result shows that the degree of rock fragmentation increases after an initial decrease at elevated temperatures in drained conditions. From 25°C to 200°C, thermally induced microcracking is unconspicuous. Rock porosity decreases accompanying with a more compact texture. From 200°C to 800°C, rock porosity keeps decreasing while the number and width of the microcrack inside rock increases, which increases the brittleness of rocks. In undrained conditions, the strength decreases at elevated temperatures, but the ductile behaviour may turn into the brittle one because of pore pressure. If a sufficient amount of fluid or gas exists in the pores or it is being released by other compounds, pore pressure will increase with an increase in temperature. The effective stresses in the reservoir decrease at elevated pore pressure, which is equivalent to reducing the influence of confining pressure and thus increases the brittleness of rocks. Since the thermally induced microcracking is unconspicuous under 200°C, thermal cause of abnormally high pore pressure is the primary factor need to be considered in brittleness evaluation of unconventional reservoirs. Finally, a relationship between temperature and pore pressure has been given in this paper, which can be used to predict the effect of temperature on brittleness evaluation.