Abstract: Gas and water are simultaneously produced from the reservoirs in the 5^th Member of the Upper Triassic Xujiahe Formation (T₃x⁵) in the Xinchang gas field in the Western Sichuan Depression,and there is no consensus on whether the gas is free or dissolved.For this purpose, methane solubility experiments were conducted under different temperatures and pressures on the T₃x⁵ formation water,and the results were compared with the methane solubilities in 1 mol/kg NaCl solution.The fluid state in T₃x⁵ was demonstrated in combination with the gas/water ratio of the produced fluid and ideal gas state equation.The results indicate that methane solubilities in T₃x⁵ formation water under present average depth and historical largest depth are 2.260 m³/m³ and 3.194 m³/m³,respectively.The water-soluble gas in T₃x⁵ formation water was mainly released by reduction of pressure and temperature during uplifting since the end of Late Cretaceous.The volumetric proportions of free gas and formation water in the T₃x⁵ pores were generally lower and higher than 50%,respectively.The volumetric proportion of free gas increased slightly due to the volumetric expansion caused by reduction of pressure and temperature since the end of Late Cretaceous,whereas that of formation water decreased as the water was displaced.Due to the partial degasification of water-soluble gas by reduction of pressure and temperature,the proportions of free gas in the total natural gas increased while that of water-soluble gas decreased.The T₃x⁵ natural gas was dominated by free gas,with water-soluble gas accounting for less than 5%.Moreover,about 29.3% of the T₃x⁵ water-soluble gas was released and turned into free gas by reduction of pressure and temperature during uplifting,and the total amount of gas released from the T₃x⁵ formation water reached 1.8×10⁹ m³.

Key words: solubility, fluid state, formation water, Xujiahe Formation, Xinchang gas field, Western Sichuan Depression, Sichuan Basin