An experimental investigation into the elastic and thermal properties of oil shales: Yanchang formation, Ordos basin

Abstract

The relationship between oil shale's elastic behavior and geothermal parameters is vital in sweet spot identification and in-situ oil conversion techniques. To qualitatively study the elastic P- and S-wave velocities and thermal properties of oil shales, we simultaneously make ultrasonic measurement s and investigate geothermal properties in the Triassic Yanchang Formation. The results demonstrate that ultrasonic P-wave velocities are sensitive to the pressure of <5 MPa mainly due to the closure of the soft pores/cracks and kerogen content and distribution. Thermal experiments reveal that kerogen content has a significant influence on shale's thermal expansion coefficients (TEC) and heat capacity. Specifically, organic-rich shales show the highest thermal expansion coefficients in both directions in core plugs (0° and 90°), reaching a maximum of 5 × 10−5 °C−1 and the highest heat capacity compared to the other samples. On the contrary, mudstone has a relatively low thermal expansion coefficient of 1 × 10−5 °C−1. The variation trend of the TEC of typical shale samples with the change of TOC (total organic carbon) content indicates that existing four phases of TEC correspond to different dominant rock evolution. These observations imply that low ultrasonic velocities accompanied by high thermal properties (heat capacity, TEC) can be ‘sweet spot’ indicators for high TOC reservoir zones. The experiments prove that the quartz content has a significant influence on the ultrasonic P- and S-wave velocities and TEC of shale rocks. Both ultrasonic velocities and thermal parameters of shales exhibit a distinct change at ∼40% quartz volume, responding to a change in the rock skeleton, and a change in TOC content. These observations will thoroughly assist us in understanding the elastic signature and thermal properties of shales during in-situ conversion process of shale oil reservoirs in Ch7 Member of the Yanchang Formation, in the Ordos Basin, Northwest China.