Optimization of Associated Gas Sweetening Process in a Steam Injection Project Using Fluid Simulation

Abstract

Abstract Hydrogen sulfide (H2S) is present in the natural gas that is produced from most steam injection thermal enhanced recovery projects. Unlike the natural gas that is produced from non-thermal oil recovery processes, the gas contains high amounts of carbon dioxide (CO2), water vapor, and condensable C7+ hydrocarbons. It has a low BTU content (e.g. 400 – 700 Btu/scf) and is not suitable for selling. Instead, it is usually incinerated through flaring or commingled with the steam generator fuel gas stream. In either case, all of the H2S that is present must be removed so that sulfur products of combustion are not released to the atmosphere with the flare or steam generator exhaust gas. A term used for H2S removal is natural gas "sweetening." Among the various effective sweetening methods is the flow of natural gas through a bed of wood chips impregnated with iron oxide within a reactor called an iron sponge. The H2S in the produced natural gas reacts with the iron oxide to produce insoluble iron sulfide and water. The reaction renders the natural gas free of H2S. But, the reaction is impeded when the bed is contacted by hydrocarbon liquids. This is because a thin film of the liquid forms over the iron oxide and blocks the reaction from occurring. Hydrocarbon liquids form when the feed gas pressure and temperature go beyond the gas mixture's dew point and cross into the PVT phase equilibrium envelope. The capacity for H2S removal by the iron sponge is reduced as hydrocarbon liquids precipitate from the flowing gas. In this study, fluid simulation software is used to calculate the effects of different fluid compositions on the produced gas dew point. Understanding this behavior enables one to pick suitable temperature and pressure-operating points that promote high H2S removal by avoiding hydrocarbon liquid precipitation. Phase envelopes, from which the dew point is obtained, change depending on gas composition. Relative amounts of light and heavy hydrocarbons were found to have the most significant effects. The gas compositions used in this study correspond to that measured at Santa Maria Energy's (SME) cyclic steam stimulation (CSS) project located in the Orcutt oil field in northern Santa Barbara County, California. Corresponding phase equilibrium envelopes were calculated using the Peng-Robinson equation of state. This paper provides a range of practical operating pressures and temperatures for improving the performance of SME's iron sponge H2S removal system. This discussion follows an overview of the iron sponge sweetening process and the phase envelope calculation results for the project's produced gas.