Abstract
Because the limitations of water-based fracturing fluids restrict their fracturing efficiency and scope of application, liquid CO₂ is regarded as a promising substitute, owing to its unique characteristics, including its greater environmental friendliness, shorter clean-up time, greater adsorption capacity than CH₄ and less formation damage. Conversely, the disadvantage of high leak-off rate of CO₂ fracturing due to its very low viscosity determines its applicability in gas shales with ultra-low permeability, accurate measurement of shale permeability to CO₂ is therefore crucial to evaluate the appropriate injection rate and total consumption of CO₂. The main purpose of this study is to accurately measure shale permeability to CO₂ flow during hydraulic fracturing, and to compare the leak-off of CO₂ and water fracturing. A series of permeability tests was conducted on cylindrical shale samples 38 mm in diameter and 19 mm long using water, CO₂ in different phases and N₂ considering multiple influencing factors. According to the experimental results, the apparent permeability of shale matrix to gaseous CO₂ or N₂ is greatly over-estimated compared with intrinsic permeability or that of liquid CO₂ due to the Klinkenberg effect. This phenomenon explains that the permeability values measured under steady-state conditions are much higher than those under transient conditions. Supercritical CO₂ with higher molecular kinetic energy has slightly higher permeability than liquid CO₂. The leak-off rate of CO₂ is an order of magnitude higher than that of water under the same injection conditions due to its lower viscosity. The significant decrease of shale permeability to gas after water flooding is due to the water block effect, and much longer clean-up time and deep water imbibition depth greatly impede the gas transport from the shale matrix to the created fractures. Therefore, it is necessary to substitute water-based fracturing fluids with liquid or super-critical CO₂ in clay-abundant shale formations.