Home General Knowledge GK SPECIAL TOPIC : WHAT IS SHALE GAS?
GK SPECIAL TOPIC : WHAT IS SHALE GAS?
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Saturday, 17 May 2014 07:03


WHAT IS SHALE GAS?

Shale gas is natural gas formed from being trapped within shale formations. Shale gas was first extracted as a resource in Fredonia, NY in 1821 in shallow, low-pressure fractures.

Geological hydrocarbon formations are created under specific conditions from organic compounds of marine sediments.

Conventional oil and gas originate from the thermo-chemical cracking of organic material in sedimentary rocks, the so-called source rocks. With increasing burial below other rocks these formations were heated, on an average 30 °C every 1 km increment and the organic material decomposed into oil once a temperature of about 60 °C was attained and later gas. Depth, temperature and exposure time determined the grade of decomposition. The higher the temperature and the longer the exposure time, the more the complex organic molecules were cracked, finally being decomposed into its simplest constituent methane with one carbon and 4 hydrogen atoms.

Depending on the geological formation, the emerging liquid or gaseous hydrocarbons escaped from the source rock and migrated generally upwards into porous and permeable strata, which in turn had to be covered by impermeable rock, the so-called seal in order to create a hydrocarbon accumulation. These hydrocarbon accumulations form the conventional oil and gas fields.  The relatively high oil content, the position within a few kilometres from the surface, and easy access on land make them easy to extract by drilling wells.

Some hydrocarbon accumulations exist in reservoir rocks with very low porosity and permeability. These occurrences are called tight oil or tight gas. Typically the permeability is 10-100 times smaller than in conventional fields. Hydrocarbons can also be stored in large volumes in rocks which are in principle not reservoir rocks at all, but shales and other very fine grained rocks in which the volume necessary for storage is provided by small fractures and extremely small pore spaces. Such rocks possess extremely low permeability.

Natural gas from shale rock is providing reliable, affordable, cleaner and responsibly produced energy. Developing these natural gas resources can help enhance the country's energy security, strengthen local and state economies, and fuel job growth.

Shale gas is cleaner-burning than coal or oil. The combustion of gas emits significantly lower levels of key pollutants, including carbon dioxide (CO2), nitrogen oxides, and sulfur dioxide, than does the combustion of coal or oil. When used in efficient combined-cycle power plants, gas combustion can emit less than half as much CO2 as coal combustion, per unit of energy released.

Two major types of drilling are used to produce shale gas:

• 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 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.

Environmental Impact

• Unavoidable impacts are area consumption due to drilling pads, parking and manouvering areas for trucks, equipment, gas processing and transporting facilities as well as access roads.

• Major possible impacts are air emissions of pollutants, groundwater contamination due to uncontrolled gas or fluid flows due to blowouts or spills, leaking fracturing fluid, and uncontrolled waste water discharge.

• Fracturing fluids contain hazardous substances, and flow-back in addition contains heavy metals and radioactive materials from the deposit. Groundwater contamination by methane, in extreme cases leading to explosion of residential buildings, and potassium chloride leading to salinization of drinking water is reported in the vicinity of gas wells.

• The impacts add up as shale formations are developed with a high well density (up to six wells per km²).







 

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