Natural Gas Production and Processing Operations

There are two types of wells producing natural gas. Wet gas wells produce gas which contains dissolved liquids, and dry gas wells produce gas which cannot be easily liquefied

After natural gas is withdrawn from producing wells, it is sent to gas plants for processing. Gas processing requires a knowledge of how temperature and pressure interact and affect the properties of both fluids and gases. Almost all gas-processing plants handle gases that are mixtures of various hydrocarbon molecules. The purpose of gas processing is to separate these gases into components of similar composition by various processes such as absorption, fractionation and cycling, so they can be transported and used by consumers.

Absorption processes

Absorption involves three processing steps: recovery, removal and separation.

• Recovery.

Removes undesirable residue gases and some methane by absorption from the natural gas. Absorption takes place in a counterflow vessel, where the well gas enters the bottom of the vessel and flows upward through absorption oil, which is flowing downward. The absorption oil is “lean” as it enters the top of the vessel, and “rich” as it leaves the bottom as it has absorbed the desirable hydrocarbons from the gas. The gas leaving the top of the unit is called “residue gas.”

Absorption may also be accomplished by refrigeration. The residue gas is used to pre-cool the inlet gas, which then passes through a gas chiller unit at temperatures from 0 to –40 °C. Lean absorber oil is pumped through an oil chiller, before contacting the cool gas in the absorber unit. Most plants use propane as the refrigerant in the cooler units. Glycol is injected directly into the inlet gas stream to mix with any water in the gas in order to prevent freezing and formation of hydrates. The glycol-water mixture is separated from the hydrocarbon vapour and liquid in the glycol separator, and then reconcentrated by evaporating the water in a regenerator unit.

• Removal

The next step in the absorption process is removal, or demethanization. The remaining methane is removed from the rich oil in ethane recovery plants. This is usually a two-phase process, which first rejects at least one-half of the methane from the rich oil by reducing pressure and increasing temperature. The remaining rich oil usually contains enough ethane and propane to make reabsorption desirable. If not sold, the overhead gas is used as plant fuel or as a pre-saturator, or is recycled to the inlet gas in the main absorber.

• Separation.

The final step in the absorption process, distillation, uses vapours as a medium to strip the desirable hydrocarbons from the rich absorption oil. Wet stills use steam vapours as the stripping medium. In dry stills, hydrocarbon vapours, obtained from partial vaporization of the hot oil pumped through the still reboiler, are used as the stripping medium. The still controls the final boiling point and molecular weight of the lean oil, and the boiling point of the final hydrocarbon product mix.

Other Processes

• Fractionation.

Is the separation of the desirable hydrocarbon mixture from absorption plants, into specific, individual, relatively pure products. Fractionation is possible when the two liquids, called top product and bottom product, have different boiling points. The fractionation process has three parts: a tower to separate products, a reboiler to heat the input and a condenser to remove heat. The tower has an abundance of trays so that a lot of vapour and liquid contact occurs. The reboiler temperature determines the composition of the bottom product.

• Sulphur recovery.

Hydrogen sulphide must be removed from gas before it is shipped for sale. This is accomplished in sulphur recovery plants.

• Gas cycling.

Gas cycling is neither a means of pressure maintenance nor a secondary method of recovery, but is an enhanced recovery method used to increase production of natural gas liquids from “wet gas” reservoirs. After liquids are removed from the “wet gas” in cycling plants, the remaining “dry gas” is returned to the reservoir through injection wells. As the “dry gas” recirculates through the reservoir it absorbs more liquids. The production, processing and re circulation cycles are repeated until all of the recoverable liquids have been removed from the reservoir and only “dry gas” remains.

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Compressed Natural Gas and Liquefied Hydrocarbon Gases

LNG

The composition of naturally occurring hydrocarbon gases is similar to crude oils in that they contain a mixture of different hydrocarbon molecules depending on their source. They can be extracted as natural gas (almost free of liquids) from gas fields; petroleum-associated gas which is extracted with oil from gas and oil fields; and gas from gas condensate fields, where some of the liquid components of oil convert into the gaseous state when pressure is high (10 to 70 mPa). When the pressure is decreased (to 4 to 8 mPa) condensate containing heavier hydrocarbons separates from the gas by condensation. Gas is extracted from wells reaching up to 4 miles (6.4 km) or more in depth, with seam pressures varying from 3 mPa up to as high as 70 mPa.

Natural gas contains 90 to 99% hydrocarbons, which consist predominately of methane (the simplest hydrocarbon) together with smaller amounts of ethane, propane and butane. Natural gas also contains traces of nitrogen, water vapour, carbon dioxide, hydrogen sulphide and occasional inert gases such as argon or helium. Natural gases containing more than 50 g/m3 of hydrocarbons with molecules of three or more carbon atoms (C3 or higher) are classified as “lean” gases.

Depending how it is used as a fuel, natural gas is either compressed or liquefied. Natural gas from gas and gas condensate fields is processed in the field to meet specific transportation criteria before being compressed and fed into gas pipelines. This preparation includes removal of water with driers (dehydrators, separators and heaters), oil removal using coalescing filters, and the removal of solids by filtration. Hydrogen sulphide and carbon dioxide are also removed from natural gas, so that they do not corrode pipelines and transportation and compression equipment. Propane, butane and pentane, present in natural gas, are also removed before transmission so they will not condense and form liquids in the system. (See the section “Natural gas production and processing operations.”)

Natural gas is transported by pipeline from gas fields to liquefication plants, where it is compressed and cooled to approximately –162 °C to produce liquefied natural gas (LNG). The composition of LNG is different from natural gas due to the removal of some impurities and components during the liquefaction process. LNG is primarily used to augment natural gas supplies during peak demand periods and to supply gas in remote areas away from major pipelines. It is regasified by adding nitrogen and air to make it comparable to natural gas before being fed into gas supply lines. LNG is also used as a motor-vehicle fuel as an alternative to gasoline.

Petroleum-associated gases and condensate gases are classified as “rich” gases, because they contain significant amounts of ethane, propane, butane and other saturated hydrocarbons. Petroleum-associated and condensate gases are separated and liquefied to produce liquefied petroleum gas (LPG) by compression, adsorption, absorption and cooling at oil and gas process plants. These gas plants also produce natural gasoline and other hydrocarbon fractions.

Unlike natural gas, petroleum-associated gas and condensate gas, oil processing gases (produced as by-products of refinery processing) contain considerable amounts of hydrogen and unsaturated hydrocarbons (ethylene, propylene and so on). The composition of oil processing gases depends upon each specific process and the crude oils used. For example, gases obtained as a result of thermal cracking usually contain significant amounts of olefins, while those obtained from catalytic cracking contain more isobutanes. Pyrolysis gases contain ethylene and hydrogen.

Combustible natural gas, with a calorific value of 35.7 to 41.9 MJ/m3 (8,500 to 10,000 kcal/m3), is primarily used as a fuel to produce heat in domestic, agricultural, commercial and industrial applications. The natural gas hydrocarbon also is used as feedstock for petrochemical and chemical processes. Synthesis gas (CO + H2) is processed from methane by oxygenation or water vapour conversion, and used to produce ammonia, alcohol and other organic chemicals. Compressed natural gas (CNG) and liquefied natural gas (LNG) are both used as fuel for internal combustion engines. Oil processing liquefied petroleum gases (LPG) have higher calorific values of 93.7 MJ/m3 (propane) (22,400 kcal/m3) and 122.9 MJ/m3 (butane) (29,900 kcal/m3) and are used as fuel in homes, businesses and industry as well as in motor vehicles (NFPA 1991). The unsaturated hydrocarbons (ethylene, propylene and so on) derived from oil processing gases may be converted into high-octane gasoline or used as raw materials in the petrochemical and chemical-processing industries.

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How to make LNG - Liquefied Natural Gas

In liquefaction plants, NATURAL GAS (mainly METAN ) is brought to the state of saturated liquid at a temperature of about-161 ° C and ambient pressure, with a volume reduction of more than 600 times; In international trade, LNG ( LPG NATURAL LIQUID) is loaded and transported by sea into the double hull tanks of special vessels, called methane, with a loading capacity of up to 150,000 m3.

The regasification process takes place in special terminals and consists in generating liquid pumping from the vessel tank to the terminal tank, in a subsequent COMPRESSION and heating up to the inlet temperature in the pipeline (Figure - Scheme of the regasification and layout process of a type plant ).

LNG drainage from the ship to the terminal takes place through the submersible pumps in the ship's tanks; the liquid is then sent from the STOCCAGGIO storage tank to the vaporizers by means of delivery pumps (multistage centrifuges), which have the task of ensuring the pressure required by the regasification operations.

The operating pressure of the terminal varies significantly according to the intended use for NATURAL GAS and, in the case of a DISTRIBUTION network , they are normally higher than the critical one (46 BAR for the CH4).

In vaporizers, the LNG passes over heated to a temperature dependent on the heat source.

The two most popular types are: open ranks, using sea water, and submerged flames, which use as a source of heat water heated by an internal burner.

If the required heat is supplied by seawater, this, after being pumped into vaporizers, is discharged into the sea at a temperature below 7 ° C compared to the inlet.

The NATURAL GAS is sucked and pressurized by compressors to then be injected into the network or sent to the user; one part is often used for self-consumption of the plant: in fact, the terminal is normally equipped with an electric power generation system consisting of Diesel or turbocharged engines.

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Nigeria, The First Crude Oil producer in The African Continent

Nigeria, or rather the region of the Niger Delta, is notorious for the continued tensions between local multinationals and guerrillas (and the consequent repercussions on the country's oil activity and crude oil prices) is one of the richest areas of hydrocarbons.

The quantity and quality of these resources have attracted the interests of the major Western companies that have been operating in the oil and, most recently, in the gas sector for decades.

The first crude oil producer in the African continent, member of OPEC, the country oscillates between the sixth and the eighth position as a world exporter and is the fifth supplier of the United States, while the recent results obtained under the NATURAL GAS liquid prelude to a protagonist future also on this market.

Nonetheless, over 60% of Nigeria's 150 million people live in an endemic poverty stash, with less than a dollar a day.

A situation of marginalization and exploitation to which the institutions could not answer - complicit also the corruption of a political class more attentive to their own personal interests than to the needs of the population - and who is degenerated into rebellion and violence perpetrated against the foreign oil installations and Western technicians, by local militias fighting in the name of the emancipation of their land and direct control over their resources.

In a descriptive and accessible way to everyone, the book by Agata Gugliotta, "Nigeria, whose resources? Oil and gas in the Niger Delta "reconstructs the economic and political life of Nigeria seen through black gold, the resource that still hinders the way of being a state enslaved to the needs of private capital; contextualizes the motives and developments of a revolt that, overwhelmingly overwhelmingly over time, has just recently swung to the backdrop of the media; analyzes what might prove to be a ransom for the country, or, conversely, an accelerator of the crisis: the exploitation of gas resources.

Burned in torch for decades, gas - considering the magnitude of RESERVES on site and its growing role in the international energy landscape - could offer the country a new stage of development and create opportunities to get out of the economic crisis and the climate of violence which attracts him.

But regardless of the time and the uncertainties related to the development of the gas sector, the economic and social degradation, the ' pollution of air, water and land every day that the Nigerian population is forced to suffer, they require urgent attention.

On the other hand, as the pages of this page show - written in a delicate but acute civil passion - the conflict that has bloomed Nigeria for a long time is likely to get stuck further, leading to a collapse of an economy already on the brink and making it increasingly difficult to see the presence and the " activities of Western multinationals.

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What is Lifting Gas ?

Gas Elevator is a method to increase the natural reach of an oil well by reducing the weight of liquid in the column by means of high pressure gas injection. The weight of oil in the column well, with the resistance caused by the viscous crude oil flow through the system well, the natural pressure of the reservoir must be exceeded to provide flow. Gas injection near the bottom of the column and reduces the density of the oil, and the total weight of liquid within the column well. Gas lift plants are generally more compact and require less energy than other methods of increasing flow rates, and are a popular solution for offshore drilling projects.

Most oil reserves are under adequate natural pressure to provide an economic rate of flow at the time of the first exploited. As oil is removed from the tank, however, the pressure decreases and the flow rate slows or stops completely. Since this usually occurs before the bulk of the oil has been removed from the tank, the rest of the oil can be utilized by reducing the downward pressure of the column and reservoir. This can be done by pumping the oil directly through the column, replacing the missing oil in the tank with water or other liquids, or by reducing the weight of the liquid in the column.

The gas is injected into the column and either through the well of the well or directly through the production tube. If the gas is injected through the well coating, the gas inlet valve is usually placed in a spindle, a kind of niche built on the side of the production tube. This allows the oil to flow through the pipe without being obstructed by the gas injection equipment, and is generally favored in low volume wells. In larger wells, the gas lifting system can be lowered into the production tube directly without significantly affecting the oil flow.

In the case of most land-based wells, other streamlining methods are simpler and cheaper than gas lifting. It is mainly used on offshore drilling rigs, where space is a premium and the compact nature of the injection mechanisms is an advantage. It is also used in petroleum fields that produce a high volume of natural gas. The gas can be passed through a washing plant to purify and dry the gas on site, where it can be injected immediately into oil wells with marginal production. Once the gas is injected into a well, the majority is recovered to the surface and can be compressed and re-injected without a large amount of waste.

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How Much Methane Hydrates in the Earth

One can not fail to make a nod in the described picture, to a perspective that in the long run could potentially mitigate concerns, at least on the physical consistency of METAN's resources .

We're talking about HYDRATES of NATURAL GAS : solid compounds formed by water and gas (mostly CNG ), similar in appearance to dry ice.

It is estimated that in the oceans there are about 60 billion of billions of cubic meters of HYDRATES gas, from which, potentially you could get METHANE far exceed 100 times RESERVES estimated to CNG .

The HYDRATES of NATURAL GAS (or gas hydrates) are widespread in large areas of the planet.

They are predominantly formed under low temperature conditions and high pressures typical of oceanic seabed; but are also present in polar and sub-polar continental areas.

They are the result of the decomposition of organic material by the microorganisms present in the sediments, a process that determines the formation of METHANE .

In particularly low temperatures and high pressures - parameters that occur precisely in the seabed or in the areas covered by icy soil - METHANE molecules remain trapped in the ice resulting in hydrated gases.

The main sources of HYDROGEN gas are located along the margins of virtually all ocean platforms, at depths of between 500 and 4000 meters, with thicknesses of even hundreds of meters.

Any commercial exploitation of this hidden treasure at the bottom of the sea is anything but simple.

The problems are due not only to the marine environment and to the depth of the deposits, but above all to the difficulty of managing the present METHANE to bring it to the surface.

Hydrated gases are of metastable nature: if they change the ambient temperature and pressure conditions they pass quickly from solid to gaseous state, dissociating violently into the two water and methane components .

The problem is currently being investigated in many countries, with particular focus on Japan, Canada, the USA and Norway.

Italy is also doing research, thanks in particular to the activity of the National Oceanographic and Experimental Geophysical Institute (OGS), which has been involved in the development of geophysical methods for the purpose of identifying and quantifying the presence of hydrated gases for about ten years.

Research is carried out in numerous ocean areas, including Antarctica, where OCT researchers have recently discovered the first continental WATER gas field .

As for the concrete possibility of recovering this METAN and considering it as an energy reserve for the future, the first steps are moving now and there are still no short-term solutions that can be expected, even if the technological issues to solve do not seem to be prohibitive.

Apart from the great POTENTIAL of available energy, the greatest incentive for research into possible exploitable GIRATI gas fields is represented by their already widespread geographic location, which makes them particularly attractive at a time when, in the international energy market growing geopolitical variable.

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Cracking and reforming

Cracking is to break the long hydrocarbon molecules into smaller molecules. This can be done by thermal process under high pressure, or catalytic (under high temperatures and in the presence of a compound that facilitates the chemical reaction). When hydrogen is involved, it is called hydrocracking, is when water is called steam cracking. 

The reforming to convert naphtha to produce gasoline or premium. 

There are other processes refining as isomerization, alkylation, etc. ... We can thus influence the characteristics of the products obtained (octane, color, odor, volatility ...).

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Exploration and prospecting for new oil fields

To find oil, we look for sedimentary basins where oil and gas have formed. Gas and oil then had the opportunity to migrate through porous rock capable of holding large quantities. 

In spite of modern tracking techniques (satellite imagery, geophysical, three-dimensional seismic surveys), the discovery of a new deposit is uncertain until the introduction of drilling. Only the latter can confirm the presence of the precious liquid.

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