Genesis of high-salinity formation water and salinity sensitivity of deep coal-rock gas reservoirs in the Ordos Basin, China

Deep coal-rock gas reservoirs in the Ordos Basin are characterized by high-salinity formation water, low water saturation, and high gas saturation. During hydraulic fracturing, injected fluid can easily permeate the coalbed, which restricts the development of coal-rock gas to further increase production. The No.8 deep coal of Benxi Formation in the Ordos Basin was selected, and the salinity sensitivity experiment was done via the pressure decay method, soluble substance immersion experiment, and thermal evolution-hydrogeological analysis. We analyzed the genesis of high-salinity CaCl₂ type formation water and quantitatively evaluated the permeability damage of different salt fractions in the coal rock. The study shows that: the high-salinity formation water in deep coal rock of the Ordos Basin mainly originates from the synergistic effect of the thermally evolved hydrocarbon drainage-driven primary water and the deep formation water extrusion from the karst layer. The proportion of Ca²⁺ and Mg²⁺ in the cationic fraction of formation water is as high as 16%–66%. The coal rock salinity sensitivity damage is significantly enhanced with the increase in salinity (up to 61.93%). The damage rate of divalent calcium and magnesium was much higher than that of monovalent sodium and potassium, which were 72.15%–85.92% and 36.82%–45.40%, respectively. The brine with salinity lower than 20000 mg/L can enhance permeability, but the intrusion of high-salinity fluid can easily trigger irreversible salinity sensitivity damage. Deionized water can dissolve a small amount of soluble salts and trace organic matter in coal rock. Based on this study, the countermeasures of using clear water fracturing fluid and flowback fluid softening are proposed to provide theoretical basis for reservoir protection and efficient development of deep coal rock gas reservoirs.