Effect Of Liquid Sulfur Adsorption On Mining Dynamics Of High Sulfur Gas Reservoirs

During the development of high sulfur gas reservoirs,the solubility of elemental sulfur in acidic gas will decrease with the decrease of reservoir pressure,so that the saturated sulfur in acidic gas will gradually precipitate in porous media and be adsorbed and deposited on the porous surface of porous media.Because of the high temperature environment of the formation,the sulfur adsorbed and deposited in the pore of the rock takes the form of liquid state.The liquid sulfur precipitated changed the original pore structure of the formation,resulting in changes in formation permeability and ultimately affecting the productivity of gas wells.At present,there are few theoretical and experimental studies on the influence of liquid sulfur in high temperature and high pressure sulfur-bearing gas reservoirs,especially considering the change of formation porosity and sulfur saturation caused by the adsorption of liquid sulfur in formation.Based on the adsorption effect of liquid sulfur and the stress sensitivity effect of rock,the prediction model of sulfur saturation is established,and the stress sensitivity experiments of sulfur-bearing cores and gas-liquid sulfur phase permeability experiments are carried out to analyze the effect of liquid sulfur on formation porosity and permeability.Finally,an example is given to analyze and calculate the effect of liquid sulfur on formation porosity and permeability.The research results of this paper are as follows:(1)To investigate the dissolution mechanism of elemental sulfur,elemental sulfur is precipitated in the form of liquid sulfur in high temperature and high pressure formation environment,and it will exist in formation pore by suspension,sulfur lock and adsorption after precipitation.The effects of temperature,pressure and gas composition on the solubility of elemental sulfur were analyzed.The solubility of sulfur in natural gas was positively correlated with temperature,pressure and H2S content in gas.(2)A new sulfur solubility parameter is obtained by power function fitting,and the original sulfur solubility model is improved.Compared with the common sulfur solubility model,the improved model has higher accuracy in predicting the solubility of liquid sulfur.A sulfur saturation model considering the effects of liquid sulfur adsorption and stress sensitivity is established.It is concluded that both the effects of liquid sulfur adsorption and stress sensitivity will increase the sulfur saturation,and the effect of sulfur saturation is more obvious when considering the comprehensive effects of both.(3)Designing and conducting stress sensitivity experiments,analyzing the damage of stress sensitivity to reservoir and its effect on gas-liquid sulfur phase permeability.By comparing different cores,the permeability of fractured cores is more significantly affected by stress sensitivity.However,the permeability damage rate of fractured cores under strong effective stress is also great.The permeability damage rate of fractured cores corresponding to 60 MPa effective stress is about 35%higher than that of matrix cores.The stress sensitivity of cores,the variation of gas-liquid sulfur dial on permeability under different confining pressures and the gas-liquid sulfur phase permeability under different confining pressures are analyzed from two aspects.(4)The pore volume of adsorbed liquid sulfur and the effect of adsorption on permeability were analyzed by the model of liquid sulfur adsorption.The results show that the effect of liquid sulfur adsorption is more serious within 5 meters from the wellbore.By analyzing the influencing factors of gas well productivity,considering the comprehensive effects of non-Darcy percolation,liquid sulfur adsorption and stress sensitivity,gas well productivity declines more obviously.