| In the developing process of the ultra-deep and high-sulfur gas reservoirs under the condition of high temperature and pressure,with the decline of reservoir pressure and the pressure drop caused by the continuous movement of the gas in the formation,the solubility of elemental sulfur in sour gas decreases as well.Once the reservoir fluid has reached a sulfur-saturated state,further reduction in pressure and temperature will cause sulfur to deposit in the pore and throat of the formation,and accordingly the porosity and permeability of reservoir will also reduce.If the temperature is higher than the freezing point of the elemental sulfur under the pressure,the precipitated sulfur is liquid.At present,there are few related theories and experimental studies on the high sulfur gas reservoirs under these conditions,especially when the elemental sulfur precipitates in the liquid state.The studies of its adsorption and aggregation in the formation on the dynamic development of gas reservoirs are also rare.And the domestic research on this aspect is still in the initial stage.Through the liquid sulfur adsorption experiment,this paper theoretically established the relevant model and got calculation analysis through the case data.The main results of this paper are as follows:(1)On the basis of fully investigating the mechanism of elemental sulfur dissolution in domestic and foreign studies,it is found that in the development of high sulfur gas reservoirs,the liquid sulfur in the formation is mainly precipitated in physical form.There may be chemical precipitation.After the reduction of temperature and pressure decomposition of sulfur compounds form sulfur and hydrogen sulfide.The existence of liquid sulfur in the formation is discussed,including suspension,sulfur lock and adsorption.(2)The prediction model of sulfur solubility in high sulfur-containing gas is improved.On the basis of Chrastil's model,a new model of sulfur solubility prediction with higher accuracy is obtained by taking the form of function.This provides an important basis for the precise prediction of the amount of elemental sulfur under different pressures.(3)Through the experimental design,implementation and analysis and summary of the experimental results,when liquid sulfur appeared in the core outlet section,the decrease of the gas permeability from the initial permeability to damage after the permeability is at least 70%.The smaller the initial porosity,the greater is the effect of liquid sulfur adsorption on it.(4)Based on the study of sulfur deposition mechanism,a mathematic model of liquid sulfur saturation is established considering the physical properties of gas reservoir and the influence of fluid parameters on pressure,non-Darcy flow and stress sensitivity.At the same time,the liquid sulfur adsorption model under the condition of one-dimensional and two-dimensional steady seepage is established,and the influence of the adsorption on the porosity and permeability of the formation is studied.(5)The effect of liquid sulfur adsorption on the porosity and permeability of the formation was studied by using the model established above.The calculated results show that the impact of liquid sulfur on the formation is very small in the unit production time,but it cannot be ignored,because the accumulation of liquid sulfur in the formation under a long time production will cause some damage to the formation.According to the calculation of radial model,it can be seen that the damage of the liquid sulfur adsorption to the formation is very large for the range within 5m of the wellbore,and the gas-liquid two-phase flow will occur.With the increase of distance,the effect of liquid sulfur on the formation is getting smaller and smaller,and the effect of liquid sulfur is almost negligible at 10m.Once the liquid sulfur precipitates and adsorbs in the formation,especially in the near-well zone,it will result in a rapid decrease in production after a period of production and a gentle reduction in the subsequent length of time. |
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