Sulfur deposition in sour gas reservoirs:laboratory and simulation study

Sulfur deposition in the formation, induced by a reduction in the solubility of the sulfur in the gas phase, may significantly reduce the inflow performance of sour gas wells and some wells in sour gas reservoirs have even become completely plugged with deposited sulfur within several months. Accurate prediction and effective management of sulfur deposition are crucial to the economic viability of sour gas reservoirs. In this paper, a dynamic flow experiment was carried out to investigate formation damage resulting from sulfur deposition using an improved experimental method. The core sample was extracted from the producing interval of the LG2 well, LG gas field in the Sichuan Basin. The experimental temperature was 26 °C and the initial pressure was 19 MPa. The displacement pressure continuously decreased from 19 to 10 MPa, and the depletion process lasted 15 days. Then the core was removed and dried. The core mass and core permeability were measured before and after experiments. Experimental results indicated that the core mass increased from 48.372 g before experiment to 48.386 g afterwards, while the core permeability reduced from 0.726 to 0.608 md during the experiment. Then the core was analyzed with a scanning electron microscope (SEM) and energy-dispersive X-ray mapping. The deposition pattern and micro-distribution of elemental sulfur was observed and the deposited elemental sulfur distributed as a film around the pore surface.

A Multiphase Wellbore Flow Model for Sour Gas “Kicks”

This study presents a new multiphase flow model with transient heat transfer and pressure coupling to simulate HTHP(high temperature and high pressure)sour gas“kicks”phenomena.The model is intended to support the estimation of wellbore temperature and pressure when sour gas kicks occur during drilling operation.The model considers sour gas solubility,phase transition and effects of temperature and pressure on the physical parameters of drilling fluid.Experimental data for a large-diameter pipe flow are used to validate the model.The results indicate that with fluid circulation,the annulus temperature with H2S kicks is the highest,followed by CO_(2),and CH_(4) is the lowest.The phase transition point of H2S is closer to wellhead compared with CO_(2),resulting in a faster expansion rate,which is more imperceptible and dangerous.With fluid circulation,the drilling fluid density and plastic viscosity both first decrease and then increase with the increase in the well depth.The bottom hole pressure when H2S kicks is greater than that for CO_(2) with the same amount of sour gas,and the pressure difference gradually increases with the increase of H2S/CO_(2) content.In addition,a parametric sensitivity analysis has been conducted to evaluate qualitatively and rank the influential factors affecting the bottom hole temperature and pressure.

Evaluation of sour gas-low salinity waterflooding in carbonate reservoirs-A numerical simulation approach

Although significant amount of H_(2)S(sour gas)rich natural gas is estimated globally,but not much attention has been given to the application of H_(2)S in the oil recovery process.Recent studies on the use of H_(2)S in oil recovery processes showed that H_(2)S has the potential of improving the oil recovery,and it can be even more effective than using CO_(2) in some processes.H_(2)S can equally dissolve in the water,react with the reservoir rock to change its surface charge,porosity,and permeability.However,previous in-vestigations on H_(2)S oil recovery attributed the improved oil recoveries to the higher miscibility of H_(2)S in the oil,and the reduction in the oil viscosity.Therefore,there is limited understanding on the H_(2)S-oil-brine-rock geochemical interactions,and how they impact the oil recovery process.This study aims to investigate the interactions between H_(2)S,oil,and carbonate formations,and to assess how the combi-nation of H_(2)S and low salinity water can impact the wettability and porosity of the reservoirs.A triple layer surface complexation model was used to understand the influence of key parameters(e.g.,pressure,brine salinity,and composition)on the H_(2)S-brine-oil-rock interactions.Moreover,the effects of mineral content of the carbonate rock on H_(2)S interactions were studied.Thereafter,the results of the H_(2)S-oil-brine-rock interactions were compared with a study where CO_(2) was used as the injected gas.Results of the study showed that the seawater and its diluted forms yielded identicalζ-potential values of about 3.31 mV at a pH of 3.24.This indicates that at very low pH condition,pH controls the ζ-potential of the oil-brine interface regardless of the brine's ionic strength.The study further demonstrated that the presence of other minerals in the carbonate rock greatly reduced the calcite dissolution.For instance,the calcite dissolution was reduced by 4.5%when anhydrite mineral was present in the carbonate rock.Findings from the simulation also indicated that CO_(2) produced negative ζ-potential values for the car-bonate rocks,and these values were reduced by 18.4%-20% when H_(2)S was used as the gas phase.This implies that the H_(2)S shifted the carbonate rockζ-potentials towards positive.The outcomes of this study can be applied when designing CO_(2) flooding and CO_(2) storage where the gas stream contains H_(2)S gas since H_(2)S greatly influences the dissolution of the carbonate mineral.

Numerical simulation for erosion effects of three-phase flow containing sulfur particles on elbows in high sour gas fields

Sulfur particles carried by high-speed flow impact pipelines,which may cause equipment malfunctions and even failure.This paper investigates the scouring effect of mist gas containing sulfur particles on elbows in highly sour gas fields.The multiphase-flow hydrodynamic model of the 90
elbow was established by using the computational fluid dynamics(CFD)method.The scouring effects of the gasliquid mist fluid with the water-liquid fraction of 20%and particles with the diameter of 0.01 e0.05mm on elbows were explored within the flow velocity range of 0e20 m/s.In addition,the influences of secondary collision,mean curvature radius to diameter(R/D)ratio,inertial force,drag force,and Stokes number on trajectories of sulfur particles were studied.Moreover,the influences of hydrodynamic parameters of multiphase flow on corrosion inhibitor film were analyzed with the wall shear stress as the reference value.Serious erosion mainly occurred in the extrados of elbow as well as the junction between downstream pipeline and the intrados of elbow,the maximum erosion area occurs at 61.9
.When the incident position of the particle was far away from the top of the inlet plane,the probability of secondary collision became smaller.Furthermore,the erosion rate decreased with the rise in the R/D radio.The maximum erosion rate of elbow increased with the increase in the Stoke number.The maximum erosion rate reached 0.428 mm/a at 0.05mm particle diameter and 20 m/s fluid velocity.The wall shear stress increased with the increase in fluid velocity and mass flow rate of particle,the fitting function of the wall shear stress curve was the Fourier type.The results indicated that highvelocity particles had a serious erosion effect on elbows and affected the stability of the corrosion inhibitor film.The erosion effect could be retarded by controlling the velocity and diameter of particles.The results provided technical supports for the safe production in highly sour gas fields.

In-depth study on the solubility of elemental sulfur in sour gas mixtures based on the Chrastil's association model

Accurately predicting the solubility of elemental sulfur in sour gas mixtures is a primary task.As a current and widely-used model on the solubility of elemental sulfur in sour gas mixtures,Chrastil's association model has a big error in the process of predicting experimental data based on different fitting methods.This paper combined with experimental data reported by relevant scholars about elemental sulfur solubility in sour gases and selected density,temperature and pressure as three important influential factors.According to different fitting methods,we can calculate the correlation parameters in Chrastil's model.Then different solubility formulas can be used to predict the solubility of elemental sulfur in sour gas mixtures.Through in-depth research and analysis of Chrastil's solubility model from numerical aspects,it's easy to find the irrationality about Chrastil's solubility model and fitting methods.Especially in fitting methods,further improvement of the fitting method is proposed and used to predict the solubility of elemental sulfur in sour gas mixtures.The calculation results show that some improvements of the predicting precision have been achieved by using the improved fitting method in Chrastil's association model.

Hydrate capture CO_2 from shifted synthesis gas, flue gas and sour natural gas or biogas

CO2 capture by hydrate formation is a novel gas separation technology, by which CO2 is selectively engaged in the cages of hydrate and is separated with other gases, based on the differences of phase equilibrium for CO2 and other gases. However. rigorous temperature and pressure, high energy cost and industrialized hydration separator dragged the development of the hydrate based CO2 capture. In this paper, the key problems in CO2 capture from the different sources such as shifted synthesis gas, flue gas and sour natural gas or biogas were analyzed. For shifted synthesis gas and flue gas, its high energy consumption is the barrier, and for the sour natural gas or biogas （CO2/CH4 system）, the bottleneck is how to enhance the selectivity of CO2 hydration. For these gases, scale-up is the main difficulty. Also, this paper explored the possibility of separating different gases by selective hydrate formation and reviewed the progress of CO2 separation from shifted synthesis gas, flue gas and sour natural gas or biogas.

Rapidly Estimating Natural Gas Compressibility Factor

Natural gases containing sour components exhibit different gas compressibility factor （Z） behavior than do sweet gases. Therefore, a new accurate method should be developed to account for these differences. Several methods are available today for calculating the Z-factor from an equation of state. However, these equations are more complex than the foregoing correlations, involving a large number of parameters, which require more complicated and longer computations. The aim of this study is to develop a simplified calculation method for a rapid estimating Z-factor for sour natural gases containing as much as 90% total acid gas. In this article, two new correlations are first presented for calculating the pseudo- critical pressure and temperature of the gas mixture as a function of the gas specific gravity. Then, a simple correlation on the basis of the standard gas compressibility factor chart is introduced for a quick estimation of sweet gases＇ compressibility factor as a function of reduced pressure and temperature. Finally, a new corrective term related to the mole fractions of carbon dioxide and hydrogen sulfide is developed.

Effect of Plastic Deformation and H_2S on Dynamic Fracture Toughness of High Strength Casing Steel

The effects of plastic deformation and H2 S on fracture toughness of high strength casing steel（C110 steel） were investigated. The studied casing specimens are as follows： original casing, plastic deformation（PD） casing and PD casing after being immersed in NACE A solution saturated with H2S（PD＋H2S）. Instrumented impact method was employed to evaluate the impact behaviors of the specimens, meanwhile, dynamic fracture toughness（JId） was calculated by using Rice model and Schindler model. The experimental results show that dynamic fracture toughness of the casing decreases after plastic deformation. Compared with that of the original casing and PD casing, the dynamic fracture toughness decreases further when the PD casing immersed in H2 S, moreover, there are ridge-shaped feature and many secondary cracks present on the fracture surface of the specimens. Impact fracture mechanism of the casing is proposed as follows： the plastic deformation results in the increase of defect density of materials where the atomic hydrogen can accumulate in reversible or irreversible traps and even recombine to form molecular hydrogen, subsequently, the casing material toughness decreases greatly.

A review of enhanced oil recovery(EOR)methods applied in Kazakhstan

Application of enhanced oil recovery methods in Kazakhstan has been ongoing for decades alongside the continued discovery of new oil and gas fields in the Pre-Caspian Basin.The objective of this review is to provide an overview of the hydrocarbon reserves and production,and the status of the petroleum industry in Kazakhstan,with a focus on the EOR methods and projects being applied to recover those reserves.A summary of the specific EOR methods in use was prepared,and existing enhanced recovery projects in Kazakhstan were reviewed and their successes and challenges were investigated.The performance of these projects in the context of EOR performance indicators such as capillary number and mobility ratio,as well as operational and environmental issues,were examined.Recommendations for current and potential applications of EOR in Kazakhstan were also discussed.The widespread application of thermal EOR methods,in use for decades in Kazakhstan’s older fields,was found to be successful,with very favorable impacts on mobility ratio from the addition of thermal energy to the reservoirs.Miscible EOR methods in Kazakhstan have had more limited success,with some significant challenges due to high concentration of hydrogen sulfide in the injected gas.Polymer injection started in the late 1960s,achieving good results.A recent polymer injection pilot project has shown some promise,with a favorable impact on mobility ratio and oil production,although the project has not yet been expanded beyond two polymer injectors.These results indicate the huge potential of existing and future EOR projects.This review is the first compilation of Kazakhstan’s existing oil and gas reserves,production,and EOR project performance,and should be seen as a guide to the existing applications of EOR methods in Kazakhstan.

Stress corrosion cracking of L360NS pipeline steel in sulfur environment

Slow strain rate test(SSRT)and tensile fracture observation are applied to study the stress corrosion cracking(SCC)of L360NS pipeline steel.5.0 wt.%sodium chloride solution containing elemental sulfur and 0.5 wt.%glacial acetic acid solution containing elemental sulfur are used.The results show that brittle fracture mainly occurs on the samples in high sulfur environment.In the sulfur suspension solution,the SCC sensitivity of L360NS pipeline steel increases along elemental sulfur content.In sulfur melting cladding condition,obvious corrosion can be observed with a large amount of corrosion pits appearing on the gauge section of the sample.The corrosion products are Fe1txS,black easily detached,and H2.In sulfur suspension and sulfur melting cladding conditions,stress oriented hydrogen induced cracking(SOHIC)is the main cracking,which is caused by the joint effect of SCC and hydrogen induced cracking(HIC).