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Environment UNCCD – 06

Liquefied petroleum Gas (LPG)

LPG is a predominant mixture of propane (C3H8) and Butane (C4H10) with a small percentage of unsaturated (propylene – C3H6 and butylenes- C4H8).  LPG may be defined as those hydrocarbons, which are gaseous at normal atmospheric pressure, but may be condensed to the liquid state at normal temperature, by the application of moderate pressures. Although they are normally used as gases, they are stored and transported as liquids under pressure for convenience and ease of handling. Liquid LPG evaporates to produce about 250 times volume of gas. LPG vapour is denser than air: Escape of even small quantities of the liquefied gas can give rise to large volumes of vapour / air mixture and thus cause considerable hazard. To aid in the detection of atmospheric leaks, all LPG’s are required to be odorized. There should be adequate ground level ventilation where LPG is stored. For this very reason LPG cylinders should not be stored in cellars or basements, which have no ventilation at ground level.

Natural Gas 

Natural gas is a source of relatively clean energy that can be obtained from such sources as conventional oil and gas fields, unconventional gas resources, landfill gas, and municipal solid waste gas (MSW gas).Methane is the main constituent of Natural gas and accounting for about 95% of the total volume. Other components are: Ethane, Propane, Butane, Pentane, Nitrogen, Carbon Dioxide, and traces of other gases. Very small amounts of sulphur compounds are also present.Natural gas is a high calorific value fuel requiring no storage facilities. It mixes with air readily and does not produce smoke or soot. It has no sulphur content. It is lighter than air and disperses into air easily in case of leak. 

Unconventional Gases 

Unconventional gas re-sources include gas hydrates, tight gas sands, coal gas, and shale gas.

Gas from landfills and municipal solid waste (MSW) gas are obtained from the decay of organic waste. Landfill gas and MSW gas are not fossil fuels; they are renewable energy sources. 

Gas Hydrates – are naturally occurring, crystalline complexes that are formed when one type of molecule completely encloses another type of molecule in a lattice. Methane hydrates contain a relatively large volume of methane in the hydrate complex. Methane hydrates are found around the world. They exist on land in sub-Arctic sediments and on sea beds where the water is near freezing. They can be found in Arctic sands, marine sands, fractured muds, and shale.

Shale gas 

 It is a low permeability source rock found throughout the world. It has enough porosity to hold large amounts of Gas production from shale has flow mechanisms that are similar to those required for gas production from coal. 

Shale gas refers to natural gas that is trapped within shale formations. Shale is fine-grained sedimentary rocks that can be rich sources of petroleum and natural gas. Over the past decade, the combination of horizontal drilling and hydraulic fracturing has allowed access to large volumes of shale gas that were previously uneconomical to produce. 

Horizontal drilling is used to provide greater access to the gas trapped deep in the producing formation. First, a vertical well is drilled to the targeted rock formation. At the desired depth, the drill bit is turned to bore a well that stretches through the reservoir horizontally, exposing the well to more of the producing shale.

 

Hydraulic fracturing

(commonly called “fracking” or “hydrofracking”) is a technique in which water, chemicals, and sand are pumped into the well to unlock the hydrocarbons trapped in shale formations by opening cracks (fractures) in the rock and allowing natural gas to flow from the shale into the well. When used in conjunction with horizontal drilling, hydraulic fracturing enables gas producers to extract shale gas at reasonable cost. Without these techniques, natural gas does not flow to the well rapidly, and commercial quantities cannot be produced from shale.

 

Conventional gas reservoirs are created when natural gas migrates toward the Earth’s surface from an organic-rich source formation into highly permeable reservoir rock, where it is trapped by an overlying layer of impermeable rock.

Shale Gas vs. Conventional Gas

Conventional gas reservoirs are created when natural gas migrates toward the Earth’s surface from an organic-rich source formation into highly permeable reservoir rock, where it is trapped by an overlying layer of impermeable rock. In contrast, shale gas resources form within the organic-rich shale source rock. The low permeability of the shale greatly inhibits the gas from migrating to more permeable reservoir rocks. Without horizontal drilling and hydraulic fracturing, shale gas production would not be economically feasible because the natural gas would not flow from the formation at high enough rates to justify the cost of drilling.

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