Mathematical Model For Overall Formation Damage Of Fractured Gas Reservoirs With High H₂S Content

Flow of fluid, mixture of natural gas. H2S and elemental sulfur, in porous media in fractured gas reservoirs with high H2S content is a complex process. During the course of production of fractured gas reservoirs with high H2S content, formation pressure falls continually and temperature drops near the wellbore due to Joule-Thomson effect to a certain extent. On the one hand, change of the thermodynamic conditions leads to decline of solubility of sulfur particles in gas phase, and sulfur particles will precipitate from gas phase after running up to saturation state and transport and deposit at pore space and throat, 'sequentially resulting in formation porosity and permeability reduction. In addition, the size of sulfur particle deposited in the pores has effect on the permeability. On the other hand, fracture will close continually due to decrease of formation pressure, which will also result in descending of porosity and permeability. Productivity and economic benefits are severely impaired by combined effects of both sulfur deposition and fracture close and gas well production may halts when sulfur deposition and fracture closure become severe. Classical percolation theory cannot meet the fractured gas reservoirs with high H2S content.This thesis'comes from the National High Technology Research and Development Program (863 Program) project "key technology for safe exploitation of sour gas reservoirs (No.2007AA06Z209)", and the main content includes establishment and application of mathematical model of formation damage.According to characteristics of complex flow through porous media in fractured gas reservoir with high H2S content, breaking through classical filtration theory, based on aerodynamics theory used to build a mathematical model for describing transportation and precipitation of sulfur particles in fractured gas reservoir with high H2S content, a new comprehensive formation damage model in fractured gas reservoir with high H2S content, accounting for sulfur deposition, fracture closure, temperature drop near the wellbore, has been proposed.The main contents of the works are as follows:(1) On the basis of analyzing H2S content change law in the process of developing the gas in the gas reservoirs with H2S content, the chemical reaction equilibrium theory of inorganic chemistry is introduced to establish formation sulfur saturation calculation method, and X gas field was taken as a case of application. Results indicate that, in the process of the development of the reservoirs with high H2S content, under the condition of minor sulfide content change, the sulfur content precipitated from polysulfide takes very low proportion of total precipitated amount. Therefore, physical deposition is the main way of sulfur deposition, so as the deposition theory of element sulfur is further recognized. (2) based on Joule-Thomson effect theory and formation pressure distribution formula, a mathematical model of temperature change in vicinity of wellbore was proposed for gas well with high H2S.content, which can forecast temperature change in vicinity of wellbore and make predicting of solubility of sulfur in sour gas more precise.(3) On basis of analyzing fracture closure due to reservoir stress sensibility for gas reservoirs with high H2S content, porosity and permeability change mathematical model which can comprehensively consider sulfur deposition resulting from drop of formation pressure, temperature change near wellbore and reservoir stress sensibility was proposed.(4) A new mathematical model of formation damage for fractured gas reservoirs with high H2S content, considering fracture closure resulting from reservoir stress sensibility and temperature change near wellbore and reservoir, was proposed. Some well with high H2S content was taken as a case of analyses of production mechanism. Results indicate this mathematical model of formation damage can describe the process of development of fractured gas reservoirs with high H2S content, and have been preliminarily applied in reasonable allocation of production rate and construction of productive capacity in PG gas field.