Reducing H2S in Khurmala Oil Field by Using Different Kind of Chemicals - Paper Example

Hydrogen sulfide (H2S) concentration in several Petroleum reservoirs has been noted to increase surprisingly in oil, gaseous, and aqueous states of fluids produced. The concentration in the fluids is usually determined in parts-per-million by volume in the gaseous phase in relation to a portion of crude oil, and an aqueous state having a pH less than or equal to 5 and a temperature of 20 degrees. If the hydrogen sulfide (H2S) concentration is greater than 10 parts per million volume (ppmv) in its gaseous phase, then the oil in that field is considered sour. The souring of Petroleum reservoirs in the oil fields is mainly caused by sulfate-reducing bacteria known to increase hydrogen sulfide concentration in the fluids produced.

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The H2S is highly toxic and very corrosive on the plugs of the reservoir formations, which leads to increased sulfur content in the produced oil. Essential precautions must be put in place in the design and production of the transport and storage equipment, so as to minimize the corrosion and the resultant operational risk. Among the measures employed in the reduction of hydrogen sulfide is the injection of relatively expensive chemical scavengers in the production pipelines. The main objective of this project is to explore the various chemicals used in reducing (H2S) in Khurmala oilfield in Iraqi Kurdistan region. The reduction of (H2S) in this particular oilfield is critical as it improves the efficiency of refinery operations, minimizes the potential for hydrogen sulfide exposure risk to the refinery personnel, and eliminates sulfur dioxide production when refinery off-gas is used to fire the heaters and boilers in the reservoirs.

Literature Review

Hydrogen Sulphide Gas (H2S)

Hydrogen sulfide is a colorless gas, with an offensive odor and a sweetish taste. It is soluble in water, alcohol, oils, and many other solvents. It has a specific gravity of 1.1895 with reference to air. It is considered a weak acid; it is toxic to humans and corrosive to metals. Hydrogen sulfide can be dangerous to personnel on the surface as it is extremely toxic to human and even animal life, and is extremely corrosive to most metals as it can cause cracking of drill pipe and tubular goods, and destruction of testing tools and wire lines.

During oil and gas operations, hydrogen sulfide can be found as a component of formation gasses, dissolved in water, hydrocarbons, or even liquid sulfur.Thermal degradation of organic materials and sulfate-reducing bacteria (SRB) can create hydrogen sulfide along with other gasses.

The hydrogen sulfide content of fluids in the permeable formations of oil wells has an important impact on the economic value of the produced hydrocarbons and production operations. Typically, the sulfur content of crude oils is in the range 0.3-0.8 weight percent and the hydrogen sulfide content of natural gas is in the range 0.01-0.4 weight percent, although concentrations of hydrogen sulfide in natural gas of up to 30 weight percent have been reported. Several recent reports have claimed a systematic increase in the sulfur content of crude oils over the past 10-20 years and anticipate further significant increases in hydrogen sulfide concentration in both oil and natural gas.

Hydrogen Sulphide Scavenger Technology

Like many oilfields, Khurmala Oilfield employs hydrogen sulfide (H2S) scavengers in the hydrogen sulfide removal from their crude oil. According to Garrett et al, the term sulfide scavenger refers to any chemical (usually a commercial additive) that can react with one or more sulfide species and can convert them to a more inert form. Effective scavenging is based on attaining an irreversible and complete chemical reaction between the scavenger and one or more sulfide species. Because a mutual equilibrium exists between the three species in solution, irreversible end complete removal of one species serves to remove all three. The incomplete chemical reaction between a species and the scavenger cannot remove all soluble sulfides present. Most H2S scavengers function on a surface adsorption manner or through ionic precipitation. If the scavenger being used is based upon the surface adsorption technique, the mud must be in constant flow to assure that the additive and the sulfides actually collide with one another in order for the necessary reactions to take place. A turbulent flow type of situation would be ideal for this and would assure many random collisions of the two particles. When the scavenger is based on an ionic reaction, properties of the scavenger must be understood to assure that variables such as pH and salinity are conducive to the usage of the additive.

Theoretical Background

The application of sulfide scavengers is a widely adopted practice in production and processing operations in the Oil and Gas Industry.Crude oil is among the most important and commonly used fossil fuel in the modern times and arguably in the near future as well. The crude oil, however, exists in deep underground reservoirs and is susceptible to various non-hydrocarbon components, such as hydrogen sulfide (H2S) and carbon (CO2) dioxide. These impurities are undesirable compounds known to cause several problems, such as corrosion which may cause a breakdown in the refinery equipment thereby incurring huge losses, environmental pollution among other problems. Table 1and 2 below shows some of the health effects of exposure to hydrogen sulfide by humans, both from short-term exposure to longer inhalation.

There have therefore been several approaches used in the reduction of hydrogen sulfide in crude oil which is particularly aimed at safeguarding the health of personnel in the refineries and maintaining good protection of materials under various conditions and being environmentally acceptable at the same time.

By October 2013, The Eagle Ford Shale Play in Southern Texas was the second most productive oil field in the United States with a daily production 1.07 million barrels of crude oil. After extraction, however, the crude oil had high and hazardous levels of hydrogen sulfide(H2S). The (H2S) is removed with chemical scavengers, such as triazine-based scavengers. These triazine-based chemical additives are mostly present in excess of the required levels for the removal of the H2S. The excess triazine, combined with the by-products of the scavenging reaction with hydrogen sulfide, may cause corrosion and breakdown in downstream equipment and machinery.

The Khurmala oilfield has great potential and can be exploited either to feedstock the Kurdistan region or trade with the global market.

Methodology

Most H2S scavengers function on a surface adsorption manner or through ionic precipitation. If the scavenger being used is based upon the surface adsorption technique, the mud must be in constant flow to assure that the additive and the sulfides actually collide with one another in order for the necessary reactions to take place. When the scavenger is based on an ionic reaction, properties of the scavenger must be understood to assure that variables such as pH and salinity are conducive to the usage of the additive. Before a particular scavenger may be selected, a determination of the form of sulfides that exist in a particular mud system must be reached at.

The following are some of the chemical scavengers used in the reduction of hydrogen sulfide in crude oil:

Copper compounds, especially copper carbonate have been used in the drilling operations to remove H2S. If added to water-based muds, the H2S precipitates out as insoluble copper sulfide according to the following equation:

CuCO3 + H2S _ CuS_ + H2O + CO2

Although Copper compound scavengers have shown through tests that the reaction with sulfide is very fast and efficient, it is impractical to use it in hydrogen sulfide removal as copper plates out on ferritic material setting up corrosion cells. These scavengers cause lots of corrosion problems in the production fields and are only recommended as hydrogen sulfide removers where there is no contact with ferritic materials.

Hydrogen peroxide is also used as an additive to mud at the flowline to convert hydrogen sulfide to free sulfur according to the reaction:

H2O2 + H2S _ S0 + H2O

The practical application of the chemical is however very limited as the hydrogen peroxide is too reactive with other components of the system. Due to this, it is practically impossible to have a satisfactory reduction of hydrogen sulfide.

Zinc-containing chemicals and zinc oxide (ZnO), zinc carbonate (ZnCO3) and basic zinc carbonate (Zn5(OH)6(CO3)2) are also used as scavengers. The solubility of ZnO and ZnCO3 rapidly increase at either high pH or low pH because of the amphoteric nature of zinc compounds but basic zinc carbonate is soluble at both ends of the pH scale.11 If mud pH is higher than about 11 (as is frequently the practice in drilling), zincate ions form, which greatly increases the solubility of the basic zinc carbonate.

Zincate ions form because of the abundant OH- combine with the zinc ions.

Zn2+ + 3OH- _ Zn(OH)3- (6)

Zn(OH)3- + OH- _ Zn(OH)4- (7)

Due to this phenomenon, a zinc-base scavenger can dissolve completely in high pH muds giving a high concentration of zinc or zincate ions for fast and complete precipitation of the sulfides. This makes the zinc-base scavengers efficient.

Iron compounds- a synthetic magnetic iron oxide with the trademark name ironic sponge (magnetite Fe3O4) is the best among the iron oxides for sulfide scavenging tasks. But, they observed in various tests that low pH speed the reaction of ironic sponge with hydrogen sulfide. Laboratory tests of sulfide scavengers have used easily handled sodium sulfide crystals instead of highly toxic H2S gas.

Data Processing and Results

The removal of hydrogen sulfide in Khurmala Oilfield employs a chemical system referred as subsea flow-line and practically connects a large subsea oil well to a smaller off the shore production plant, as illustrated in the figure below:

Figure 1: Flow path and flow regimes of the chemical scavenger system

Reservoir fluids such as crude oil, water, and existent gas, flow into production lines at reservoir point 13 in the above illustration. Triazine has been popularly used as a benchmark scavenger in several oil fields. The lift gas having scavenger mist is incrementally directed into the production line through a mandrel. The hydrogen sulfide scavenger droplets scatter through the fluid produced. A gaseous state bearing a portion of the H2S is formed as the pressure drops below the level of the bubble point. The triazine droplets added to the gas phase decompose hydrogen sulfide through nucleophilic replacement of sulfur to the triazine ring thereby hydrolyzing water droplets. These reactions occur in multiphase flow along the flowline. Reactant products disintegrate in the liquid state. As noted earlier, the H2S removal from the crude oil in the Kurlmala oil field is a flowline connecting the oil well to the production facility as illustrated in figure 2 below.

Fig 2. EPRI H2S scavenger metering and injection system

As indicated in the flow diagram above, crude oil among other reservoir fluids namely formation water and existent gasses flow into the production lines and the crude oil put above bubble point pressure. The H2S scavenger is injected into the production line through oil, water, and gas.

The scavenger droplets disperse through the produced fluid substantially homogeneously because of the natural turbulence of the fluid flow. As the pressure drops below the bubble point, a gas phase containing part of the H2S is formed. The droplets of H2S scavenger injected into the gas phase decompose H2S by the substitution of sulfur into H2S scavenger ring, and the droplets...