Effects of Mineral Displacement on Geothermal Reservoir Properties at High Temperatures Identified using Micro-CT and Digital Volume Correlation

Abstract

Characterization of reservoir rock samples under in situ conditions is crucial for evaluating the quantity and exploitable potential of geothermal energy. However, reservoir characterization is impeded by the lack of precise assessments of rock properties at in situ temperatures. To address this, high-temperature micro-computed tomography was deployed, integrating digital volume correlation (DVC) technology to ascertain the strain exhibited by pores and minerals. The findings reveal the neglect of the effects of mineral displacement at high temperatures previously. The strain within the sandstone is heterogeneous and primarily concentrated at the edges of large grains of brittle minerals and the fillings among them. The weak interfaces among diverse large-grain brittle minerals and their fillings cause strain in sandstone. At 105 °C, the average equivalent strain in sandstone was 0.03275 determined by DVC, significantly surpassing the strain of mineral thermal expansion, which remained below 0.001. Most of the strain was caused by mineral displacement, not mineral thermal expansion. The porosity of the sandstone decreased from 5.02 to 4.84% as the temperature increased from 30 to 105 °C, and some of the connected pores were transformed into independent pores at high temperatures. The tortuosity of the sample increased from 3.88 to 3.97 from 30 to 105 °C, respectively, and the temperature increase caused permeability reduction from 67.9 to 58.2 mD (1 mD = 9.869233 × 10⁻¹⁶ m²). The thermal treatment experiments demonstrated that mineral displacement in sandstones is a universal phenomenon at high temperatures and it leads to changes in sandstone pore structure and permeability. This study advances a new path to investigate geothermal reservoir properties at high temperatures and offers novel understanding.