Introduction
Sour gas reservoirs are characterized by high content of hydrogen sulfide (H2S) and or elemental sulfur (ES). Such natural gas reservoirs with high sulfur content are one of the significant clean energy resources and are widely distributed around the world. In this case, sulfur is present in trace amounts in deep sour gas reservoirs and is dissolved within the hydrocarbon gas phase at reservoir pressure and temperature conditions. One of the important topics in the study of sour gas reservoirs is deposition of sulfur which causes major problems such as the formation of damage by reducing permeability and porosity, blocking tubing string, and production facilities. Sulfur deposition resulting from a change in the temperature and pressure of high sulfur gas reservoirs (e.g., ZareNezhad and Ziaee, 2013; Roberts, 2017; Bemani et al., 2020; Shao et al., 2021; Zou et al., 2021). Sulfur deposition is a powerful resistance factor for ensuring safety and efficiency during development of sour gas reservoirs that resulting in the low well productivity. In this regard, it is worth developing an accurate production prediction simulation considering sulfur deposition for fractured reservoirs.
Sulfur is the tenth most abundant element in the universe. The chemistry of sulfur in fossil fuels is of both practical and academic interest. The behavior of sulfur during processing high-sulfur
crude oils, coals and natural gas to make environmentally acceptable fuels poses special engineering problems and economic penalties. Sulfur is also known to exist as multiple allotropes. Indeed, sulfur is second only to carbon in terms of the number of known allotropic forms. These allotropes largely take the form of chains and rings, with the most common being the puckered ring structure of S8. In addition to the known allotropes, several polymorphs are also known to exist. The most common structures are the α- (rhombic) and β- (monoclinic) polymorphs of S8 (Greenwood and Earnshaw, 1997). Chemical allotropes such as S2 to S20 are exists particularly in lean sour gas reservoirs. These allotropes (especially S6 to S8 – called elemental sulfur, ES) can be crystalized and deposited in the porous media under specific physical conditions and this is a well-known issue in most of the sour dry gas reservoirs. General formulation of S8 deposition can be defined as:
(1)
Natural gases with high H2S contents often contain dissolved ES and polysulfides. Bojes et al. (2010) also showed that several mechanisms can be used to explain how ES is formed and transported in sour gas reservoirs including poly-condensation of H2S at high pressure and high temperature reservoirs, the generation of H2S in the reservoir, thermochemical sulfate reduction, the formation of ionic polysulfide by the dissociation of sulfides (H2Sx), as well as the air oxidation of H2S, oxidation of aqueous solution and dissolution of ES in hydrocarbon liquid. Many virgin crude hydrocarbon reservoirs contain small but significant amounts of H2S that is easily oxidized to ES by exposure to air. However, the presence of H2S, a corrosive and toxic gas, has adverse effects on production and safety. The ES is widely distributed on the Earth’s surface and occurs primarily in sedimentary basins containing both hydrocarbons and evaporites. Sulfur deposition in the reservoir formation and its impact on well productivity and ultimate recovery has been investigated for close to 50 years. With the expanding energy contradiction between the supply and demand in the world, the development of high sulfur content gas reservoirs has caught more and more attention in public. Experimental and numerical modelling researches have focused on the phase behavior of the sulfur gas systems and the flow of sulfur and natural gas through the formation. However, the dissolution and deposition of ES will take place during the developing process of sour gas reservoirs, compared with sweet gas reservoirs.
The ES solubility in sour gas decreases with the descent of formation pressure, the ES will precipitate once the sulfur content exceeds its solubility. The precipitated sulfur will block the formation pore and reduce its permeability, which leads to lower the capacity of gas well seriously and even brings great detriment to the normal production and management of gas well. The ES deposition in sour gas reservoirs depends on the geological formation characteristics, reservoir depth, changes in temperature and pressure during production operations, liquid and gas phases pressures, and sour gas composition (Orr and Sinninghe Damste, 1990; Hands et al., 2002; Shedid and Zekri, 2006).
The vast majority of economic ES deposits (~98%) are found in shallow basins that include salt domes and or evaporites series. The ES that is encountered in sour reservoirs has been described as occurring in three distinct forms, including: 1) a free phase either as a liquid or as a solid; 2) physically dissolved sulfur mostly in the gas phase, but with small amounts in the aqueous phase; and 3) chemically dissolved sulfur as polysulfides with occurs in the gas phase as well as in liquid sulfur (Hyne, 1991; Nöth, 1997; Steudel, 2003; Ellis et al., 2019). One of main reasons to the ES deposition is the pressure draw-down due to gas production which results in sulfur solubility decrement. The ES may precipitate in the reservoir, wellbore and surface facilities. Deposition of the ES in the vicinity of the wellbore may alter the porosity, absolute and relative permeabilities around the wellbore and consequently may significantly reduce the inflow performance of sour gas well. Considering the effect of ES deposition on production performance of dry gas reservoirs, modelling of the ES deposition is the most important in dry gas reservoirs with chance of existence of ES.
Many analytical and numerical models were developed to predict and evaluate the effect of sulfur deposition on the inflow performance of gas wells (e.g., Roberts, 1997; Xiao et al., 2006). However, some of these models assumed that the gas properties are constant and they are not changing with the reservoir pressure and temperature. Moreover, some of them rely only on a single sulfur solubility correlation from the literature. In this study, we have developed a novel model which takes into consideration the effects of ES deposition and ES deposition in porous media and near wellbore applied on the study gas field as a typical example of high pressure, high temperature reservoirs (HPHT)
dry gas field that is of interest for chemical and petroleum engineers. This study contains pressure-volume-temperature (PVT) analysis, which is Schlumberger equation of state-based software for generating PVT data from laboratory analysis of oil and gas samples, and equation of state (EoS) tuning for compositional run, calculation of S8 deposition and finding an accurate correlation between S8 deposition and permeability reduction, and finally permeability reduction with both ECLIPSE (ECL Implicit Program for Simulation Engineers, Standard Schlumberger Reservoir Simulation Software) Black Oil and Compositional simulations.
Summary
Accurate prediction and simulation of the elemental sulfur (ES) deposition in sour gas mixture systems has been recognized as a key issue in the development of high sulfur content gas reservoirs. Many experimental measurements and simulated models have shown the complicated relationships between sulfur deposition and sour gas properties. However, the efficient simulation that can be used to analyze sulfur deposition in sour gas over a wide range of temperature and pressure is rare. Therefore, the main objective of the present work is to build an efficient simulation approach to evaluate the ES deposition in the developing process of the sour gas reservoir accurately, a novel simulation procedure was devised to establish measurement method for determining ES deposition by purchasing corresponding simulation accessories. The modelling of the ES deposition is not a straightforward task and there is no specific workflow for such studies. This is due to limited number of reservoirs in the world with ES deposition issue and for most of them, the issue occurs on surface facility devices, instead of porous media. On the other hand, well-known available simulators in the market are not able to directly simulate these phenomena. ECLIPSE was utilized to modelling of the ES deposition in porous media and consider the impacts of ES deposition on production performance of sour dry gas reservoirs. Moreover, the Chemical Reaction option was used commingled with Solid Option and mobility multiplier as a function of solid adsorbed saturation by the rock formation in order to compositional simulation. Equation of State (EoS) tuning on a typical sour dry gas sample was performed, considering the effect of existence of S8 component (with Zero mole percent) in the initial reservoir fluid composition. Results of simulation indicate that ES deposition in porous media can considerably alter the well productivity performance as the most of the deposition occurs around the wellbore, due to pressure drop as a consequence of gas production. The simulation output can be in form of modified gas production rate, due to ES deposition and rate of deposition in Solid and Liquid phases. Furthermore, it provides insights to the evaluation of sulfur deposition in sour gas reservoirs and helps operators to develop the sour gas reservoir better.
Conclusions
1. A novel simulation approach has been developed for calculating and predicting sulfur deposition associated with sour gas production. The novel approach has been used to successfully match and predict sulfur deposition and gas production rate of the typical example of typical dry sour gas field. The simulating results have been used as a design basis for downhole sulfur treatments and clean-out operations, the optimization of well completions and off-take rates to minimize the impact of sulfur deposition, and the development of new well design and operating strategies for sulfur producers.
2. Although there is no direct and straightforward method for modelling of elemental sulfur (ES) deposition in ECLIPSE Black Oil and Compositional, however alternative approaches such as permeability reduction in Black Oil and chemical reactions in Compositional simulation can be employed for this purpose.
3. Parameters such as “reaction rate constant” with “activation molar energy in chemical reaction rate” play key role in chemical reaction characterization and consequently, performance of simulation. In absence of these parameters, out puts of ECLIPSE Black Oil model can be used for characterization of chemical reaction parameters in Compositional simulation.
4. Considering the effect of ES deposition on production performance of gas fields with high amount of H2S, modelling of such phenomena is inevitable for these kinds of reservoirs.
Acknowledgements
Authors would like to express their sincere gratitude to Mr. Mohsen Shafi’ee who willingly shared his experience with regard to EOS tuning and simulation during the work.
References
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