Investigation of Thermal Fracturing and Permeability Enhancement in Tight and Shale Rocks During In-Situ Heating

Abstract Thermal fracturing could lead to a remarkable enhancement in rock permeability, which is quite crucial for unconventional reservoirs. This study focuses on assessing the capacity of thermal fracturing and permeability improvement in tight and shale reservoirs, and mathematically characterizing the essential relationship between them during the in-situ heating (ISH) process. For this purpose, ISH treatment, pulse-decay permeability (PDP) testing, and computerized tomography (CT) scanning for core samples from tight sandstone and shale reservoirs was carried out in real-time on a self-designed apparatus to explore the dynamic evolutions in permeability and thermal crack during the ISH process. Experimental studies reveal that only the temperature exceeding the critical value of 500 °C, some thermal cracks propagate quickly on a small scale and form a complicated crack system in situ, therefore causing a permeability increment to 11.23 and 29.82 times. Synergizing the mechanism analysis of thermal activity and geochemical data, the large difference in mineral thermal expansion coefficient (DTEC) arising from α/β phase transition in quartz (α/β-QPT) at 573 °C is demonstrated to be the most critical mechanism for thermal fracturing in two rocks. On the basis of crack geometry, thermo elasticity, fracture mechanics, and percolation theory, a permeability model is established. α/β- QPT is fully proven as a dominant reaction through incorporating it into our model and getting preferable permeability matching. Practical guidance and prospect for the actual variations in physical properties of ultra-low permeability reservoirs in situ stimulated by thermal recovery could be provided by this work.