Page 224 - Hydrocarbon

Reservoir Dynamic Behaviour 211

long sustainable plateau (typically 10 years) to attain a good sales price for the gas;
the customer usually requires a reliable supply of gas at an agreed rate over many
years. The RF for gas reservoirs depends on how low the abandonment pressure can
be reduced, which is why compression facilities are often provided on surface.
Typical RFs are in the range 50–80%.

9.3.1. Major differences between oil and gas field development
The main differences between oil and gas field development are associated with
the economics of transporting gas
the market for gas
product specifications
the efficiency of turning gas into energy.
Per unit of energy generated, the transportation of gas is significantly more
expensive than transporting oil, due to the volumes required to yield the same
energy. In other words, the energy density of gas is low compared to oil. On a calorific
basis approximately 6000 scf of gas is equivalent to one barrel (5.6 scf ) of oil. The
compression costs of transporting gas at sufficient pressure to make transportation
more economic are also high. This means that unless there are sufficiently large
quantities of gas in the reservoir to take advantage of economies of scale,
development may be uneconomic.
For an offshore field requiring significant infrastructure for development,
recoverable volumes of less than 0.5 trillion scf (Tcf ) are typically uneconomic to
develop. This would equate to an oil field with recoverable reserves of
approximately 80 MMstb. If close to existing offshore infrastructure, this threshold
is closer to 50 Bcf.
For the above reasons, gas is typically economic to develop only if it can be used
locally, that is if a local demand exists. The exception to this is where a sufficient
quantity of gas exists to provide the economy of scale to make transportation of gas
or liquefied gas attractive. As a guide, approximately 5 Tcf of recoverable gas would
be required to justify building a liquefied natural gas (LNG) plant. Globally there are
around 30 such plants, but an example would be the LNG plant in Malaysia which
liquefies gas and transports it by refrigerated tanker to Japan. The investment capital
required for an LNG plant is very large; typically in the order of $5 billion.
Whereas a ‘spot market’ has always existed for oil, gas sales traditionally require
a contract to be agreed between the producer and a customer. This forms an
important part of gas field development planning, since the price agreed between
producer and customer will vary, and will depend on the quantity supplied, the
plateau length and the flexibility of supply. Whereas oil price is approximately the
same across the globe, gas prices can vary very significantly (by a factor of two or
more) from region to region.
When a customer agrees to purchase gas, product quality is specified in terms
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of the calorific value of the gas, measured by the Wobbe Index (WI) (MJ/m or
Btu/scf ), the hydrocarbon dew point and the water dew point, and the fraction
of other gases such as N 2 ,CO 2 ,H 2 S. The WI specification ensures that the gas the