Using Acoustic Velocities and Microimaging to Probe Microstructural Changes caused by Thermal Shocking of Tight Rocks

Large scale, Earth processes and bulk rock properties are influenced by underpinning, dynamic, microstructures within rocks and geomaterials. Traditionally, porosity has been considered the primary control on important bulk rock properties like seismic wave velocities (Vp and Vs) and permeability (k). However, in tight rocks, velocity and permeability can still change substantially despite slight porosity changes. Therefore, other microstructural features must be controlling these bulk rock properties. Understanding which microstructural features control Vp, Vs, and permeability in tight rocks is particularly useful in applications like enhanced geothermal systems, where methods like thermal shocking are used to increase the k of the subsurface. Thermal shocking involves quickly injecting surface water in the subsurface to cool minerals, induce contraction of minerals crystals, and cause thermal cracking. We tested three tight lithologies with unique microstructures; Sierra White Granodiorite (SWG), Porphyritic Trachybasalt (PTB), and Monte San Angelo (MSA) carbonate. We simulated the effects of thermal shocking by slowly heating (<2C min-1) samples to 350C and then quenching them. Using time-lapse microimaging, we assessed whether and how thermal cracking occurs in each lithology. We also explored how thermal cracks influence bulk rock properties like porosity, Vp, Vs, along with k. Vp, Vs, and k were measured in the presence of confining pressure.
We found thermal shocking is most effective in SWG, then MSA, and the least in PTB. Our time-lapse microimaging shows extensive cracking in the SWG and MSA lithologies, and little to no cracking in PTB. Vp and Vs for all lithologies became more pressure sensitive, and elastic moduli of each lithology decreased with thermal shocking. These results show that the presence of thermal cracks reduces stiffness between mineral crystal boundaries. Additionally, the presence of minerals with high thermal conductivity and/or thermal expansion coefficients leads to increased thermal cracking in SWG and MSA. This is especially true when these minerals are juxtaposed with other minerals that are less sensitive to temperature changes. In MSA, vuggy pores and calcite, a mineral with high thermal anisotropy, both contributed to thermal cracking. SWG and MSA k increased but remained the same for PTB. Therefore, in thermally shocked SWG and MSA, Vp and Vs are good indicators of thermal cracking and k increases. Vp and Vs are not as useful for PTB, specifically when Vp and Vs are measured at high pore pressures. Additionally, lithologies like PTB may require multiple thermal shock stimulations to increase k. Our results highlight which microstructures are altered with thermal shocking, how micro-scale changes alter bulk rock properties, and when we can monitor k increases and microscale thermal cracking with measurements of bulk rock properties.