Sulfur Solubility in Sour Gas

Abstract

Summary. It is well known that sulfur is soluble in sour gas and often precipitates during production if the temperature and pressure decrease. precipitates during production if the temperature and pressure decrease. Of more significance is the possibility of sulfur precipitation in the reservoir as the pressure is reduced. This applies particularly to high- temperature reservoirs, where sulfur is believed to be more viscous, or to moderate-temperature, low-permeability reservoirs. In either case, sulfur precipitation cam impair well productivity and thus the economics precipitation cam impair well productivity and thus the economics of reserve depletion. In the U.S., large sour-gas reserves with reservoir temperatures between 390 and 500 K [242 and 440 degrees F] are being exploited. The purpose of the work was to obtain sufficient sulfur-solubility data in sour gas to develop a widely applicable predictive model of sulfur solubility vs. sour-gas conditions. The data include the effects of temperature, pressure, and gas composition, including variations in the amount and composition of gas-condensate components. Pressures and temperatures ranged from 6.7 to 155 MPa [970 to 22,500 psi] and 394 to 486 K [250 to 415 degrees F], respectively. The influence of H2S on the melting curve was also reported. The value of the experiments was two-fold: to provide information that would allow production and reservoir engineers to predict more reliably the reservoir pressure at which sulfur will precipitate and to yield design data for production systems in which sulfur plugging problems in tubing, flowlines, and production facilities could be avoided. Introduction Many natural gas reservoirs with high concentrations of H2S have recently been discovered. In several of these gas fields, production is severely hampered by elemental sulfur, which is production is severely hampered by elemental sulfur, which is dissolved in the sour gas. The source of this sulfur is not always known. Elemental sulfur sometimes is present in the reservoir as a separate phase. In these cases, the gas is saturated with elemental sulfur. Often, however, elemental sulfur is not present in the reservoir as a detectable separate phase. In these instances, although it is theoretically possible phase. In these instances, although it is theoretically possible for the sour gas to be saturated with elemental sulfur, the gas is usually partially saturated with elemental sulfur. The sulfur solubility is strongly dependent on the pressure, temperature, and composition of the gas. As a result of the geothermal gradient, the gas stream cools as it rises in the tubing, and pressure drops as a result of both the hydrostatic pressure gradient and friction. Consequently, the solubility decreases, and sulfur is deposited whether solubility limit is exceeded. Sulfur may also precipitate in the geological formation and thus impede production because the reservoir pressure usually drops as the reserves are depleted. The presence of sour gas lowers the sulfur freezing point. The freezing presence of sour gas lowers the sulfur freezing point. The freezing point of sulfur may vary from 363 to 393 K [194 to 248 degrees F]. point of sulfur may vary from 363 to 393 K [194 to 248 degrees F]. The actual value depends on crystalline structure, the sour-gas pressure, and the sour-gas composition. Having more data on the pressure, and the sour-gas composition. Having more data on the solubility of sulfur in compressed sour gases of various compositions over a wide range of temperatures and pressures would be of great value in solving the problems associated with production of sulfur- containing sour gas. Some literature is available on the solubility of sulfur in different natural gases and in supercritical H2S. Kennedy and Wieland reported results on (methane+H2S+CO2+sulfur) at pressures of up to 41 MPa [6,000 psi] and at temperatures of 338.7, 366.5, and 394.3 K [150, 200, and 250 degrees F]. Roof measured the solubility of sulfur in H2S up to 31.2 MPa [4,520 psi] and within the 316.5-to-383.2-K [110-to-230 degrees F] temperature range. His results differ considerably from those of Kennedy and Wieland. Swift et al published data on the solubility of sulfur in H2S up to 137.9 MPa and 449.8 K [20,000 psi and 350 degrees F]. Brunner and Woll measured the solubility in sulfur in H2S and in compressed sour gases at pressures of 10.3 to 60.0 MPa [1,500 to 8,700 psi] and at temperatures of 387.6 to 447.6 K [238 to 346 degrees F]. Woll reported data on the influence of sour gases on the melting curve of sulfur. The data presented here are an extension of and are consistent with previous Brunner and Woll data. By means of high-pressure optical cells, we have measured the solubility of sulfur in seven synthetic sour-gas mixtures containing methane, H2S, CO2, nitrogen, and C2 to C6 alkanes. Experimental Techniques The measurements were performed in three high-pressure optical cells of simple design and large capacity: 30, 110, and 550 cm3. The entire contents of the 30- and 110-cm cells could be observed. In conjunction with a simple metering, stirring, and heating system, the cells permitted precise measurement of the solubility of sulfur in compressed sour gases. Fig. 1 shows how the apparatus functions. A known mass of sulfur is first introduced, and the sour gas is then metered stepwise into the thermostatted cell. The mixture is kept thoroughly stirred until the last droplet of sulfur dissolves in the compressed supercritical sour gas. Two different methods were used for metering the mixtures into the high-pressure optical cell. The choice depended on the phase equilibrium of the synthetically mixed sour gases. The first method applies to high H2S concentrations, an example of which is Mixture 1, which is liquid at room temperature. In this case, the mixture is prepared in a thermostatted 1000-cm3 high- pressure autoclave. It is subsequently metered into the pressure autoclave. It is subsequently metered into the optical cell through a thermostatted hand-operated, high-pressure screw press. The second method derives from the fact that at low H2S concentrations, the sour gases are already in the supercritical state at room temperature or at moderately elevated temperatures. The mixtures were prepared in a heated gas cylinder by weighing out the individual gases at pressures of up to 4.8 MPa [696 psi]. They were metered directly into the high-pressure optical cell by means of a diaphragm gas compressor with a heated head. Fig. 2 shows the 30-cm3 cell with two synthetic sapphire windows and a heating jacket. The maximum operating pressure was 200 MPa [29,000 psi]; the maximum operating temperature was 620 K [656 degrees F]. Fig 3 shows the 550-cm cell. Its maximum operating pressure is 100 MPa at 620 K [14,500 psi at 656 degrees F]. Preference is given to this cell for measurements of low sulfur solubilities and sulfur concentrations. The contents of the three cells were stirred by a simple magnetic laboratory stirrer and a PTFE-jacketed bar magnet. The mass of sour gas in equilibrium at the dewpoint pressure and temperature was calculated from the volume of the cell and from separate PVT measurements and density calculations with the Lee-Kesler modification of the Benedict-Webb-Rubin equation of state (EOS) considering the estimated sulfur partial volume in the cell. JPT p. 1587