An Experimental Study for the Impact of Heating and Cooling on Strength of Unconventional Reservoirs

Abstract

Abstract Unconventional reservoirs (shale and tight sandstone) have gained significant attention in meeting increasing energy demand despite unconventional ways to exploit these resources. Numerous studies have been carried out to evaluate different aspects of unconventional reservoirs including geomechanical characteristics at high pressure and temperature. However, less attention has been given to understand the impact of temperature change on the strength of the rock. This study focuses on two unconventional rocks namely Kentucky Sandstone (0.98mD) and Eagle Ford Shale (21.39μD), aiming to investigate the effect of heating and cooling on strength. XRD analysis showed that the main mineral in the tight sandstone is quartz (61%), whereas in the shale, it is calcite (88.6%). A central hole was drilled in a 2.5-inch core to accommodate a temperature logger and heated to 250 °C. The rate of reduction in rock temperature was then recorded using a temperature probe till 30 °C. Micro CT-Scan imaging was performed before and after heating the samples to 250 °C to observe any microstructural changes. Finally, the strength of the rocks was determined through a scratch test. The obtained results demonstrated that the sandstone core exhibited a heating rate of 2.3 °C/min initially to reach up to 68 °C, then reduced to 0.8 °C/min to reach the final temperature of 85°C. In contrast, the Eagle Ford Shale showed a consistent single heating rate of approximately 1.7 °C/min to reach 85 °C. The final temperature was set to maximum value of 85 °C during heating due to limitation of temperature logger inside the oven, however for rate of cooling the samples were heated in the oven at 250 °C and cooled at room temperature, measuring the rate of cooling. Comparatively, sandstone was more conducive to heat than shale. During the cooling phase, the temperature of sandstone initially reduced at a rate of 6.5 °C/min to reach 110 °C, 2.7 °C/min till 60 °C, and eventually reached 30 °C at a rate of 0.7 °C/min. The Eagle Ford Shale exhibited a temperature reduction trend, ranging from 5.7 °C/min up to 110 °C, 2.0 °C/min till 60 °C, and reached 30 °C at a rate of 0.6 °C/min. The heating and cooling cycles resulted in reduced strength for both samples. The average strength of Kentucky Sandstone decreased from 5896 to 5415 psi (8.1% ↓), while the strength of Eagle Ford Shale decreased from 18240 to 16840 psi (7.7% ↓). These findings contribute to a better understanding of induced thermal impact on mechanical properties of unconventional rocks.