Page 47 - Introduction to Petroleum Engineering

OIL AND GAS RESOURCES 31
2.3.2 Gas Hydrates

The entrapment of natural gas molecules in an ice‐like crystalline form of water at
very low temperatures forms an ice‐like solid called a gas hydrate. Gas hydrates are
also called clathrates, which is a chemical complex that is formed when one type of
molecule completely encloses another type of molecule in a lattice. In the case of gas
hydrates, hydrogen‐bonded water molecules form a cage‐like structure around low
molecular weight gas molecules such as methane, ethane, and carbon dioxide. For
more discussion of hydrate properties and technology, see Sloan (2006, 2007) and
references therein.
Gas hydrates have historically been a problem for oil and gas field operations. For
example, the existence of hydrates on the ocean floor can affect drilling operations in
deep water. The simultaneous flow of natural gas and water in tubing and pipelines
can result in the formation of gas hydrates that can impede or completely block the
flow of fluids through pipeline networks. The formation of hydrates can be inhibited
by heating the gas or treating the gas–water system with chemical inhibitors, but
these inhibition techniques increase operating costs.
Today, the energy industry recognizes that gas hydrates may have commercial
value as a clean energy resource or as a means of sequestering greenhouse gases. The
potential of gas hydrates as a source of methane or ethane is due to the relatively
large volume of gas contained in the gas hydrate complex. In particular, Makogon
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et al. (1997) reported that 1 m of gas hydrate contains 164.6 m of methane. This is
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equivalent to one barrel of gas hydrate containing 924 ft of methane and is approxi-
mately six times as much gas as the gas contained in an unimpeded gas‐filled pore
system (Selley, 1998, page 25). The gas in gas hydrates occupies approximately 20%
of the volume of the gas hydrate complex. Water occupies the remaining 80% of the
gas hydrate complex volume.
Gas hydrates are naturally present in arctic sands, marine sands, and nonsand-
stone marine reservoirs. They are common in marine sediments on continental mar-
gins and below about 600 ft in permafrost regions. Ruppel (2011) reported that
approximately 99% of gas hydrates occurs in the sediments of marine continental
margins. Methane hydrates form when both methane and water are present at appro-
priate pressure and temperature. The size of the hydrate resource is not well known.
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Boswell (2009) said that gas hydrates may contain approximately 680 000 trillion ft
of methane. Development of technology for commercially producing the hydrate
resource is ongoing.

2.3.3 Tight Gas Sands, Shale Gas, and Shale Oil

Low‐permeability hydrocarbon resources include tight gas sands (Holditch, 2007)
and shale (Kuuskraa and Bank, 2003; King, 2012). Both tight gas sands and shale are
characterized by very low permeability. The permeability of tight gas sand is on the
order of microdarcies (1 microdarcy is 1 thousandth of a millidarcy), while the per-
meability of shale is on the order of nanodarcies (1 nanodarcy is 1 millionth of a
millidarcy).