Page 227 - Hydrocarbon

214 Gas Reservoirs

request the producer to provide 1.4 times the DCQ. This means that the producer
has to be confident that there is sufficient well potential and transport capacity to
meet this demand, otherwise a penalty will be incurred. For most of the time this
means that the producer is providing a production potential (sometimes called
deliverability) which is not being realised. As compensation to the producer for
investing in additional capital to provide this level of redundancy, a higher gas price
would be expected.

9.3.3. Subsurface development of gas reservoirs
One of the major differences in fluid flow behaviour for gas fields compared to
oil fields is the mobility difference between gas and oil or water. Recall that
mobility is an indicator of how fast fluid will flow through the reservoir, and is
defined as
k
Mobility ¼
m

Permeability (k) is a rock property, whilst viscosity (m) is a fluid property. A
typical oil viscosity is 0.5 cP, whilst a typical gas viscosity is 0.01 cP, water being
around 0.3 cP. For a given reservoir, gas is therefore around two orders of magnitude
more mobile than oil or water. In a gas reservoir underlain by an aquifer, the gas is
highly mobile compared to the water and flows readily to the producers, provided
that the permeability in the reservoir is continuous. For this reason, production of
gas with zero water cut is common, at least in the early stages of development when
the perforations are distant from the gas–water contact.
The other main physical property of gas which distinguishes it from oil is its
compressibility; the fractional change in volume (V ) per unit of change in pressure
(P) at constant temperature (T ). Recall that

1 dV
Compressibility ðcÞ¼
V dP
T
1
The typical compressibility of gas is 500 10 6 psi , compared to oil at
1
10 10 6 psi 1 and water at 3 10 6 psi . When a volume of gas is produced
(dV ) from a gas-in-place volume (V ), the fractional change in pressure (dP)is
therefore small. Because of the high compressibility of gas it is therefore uncommon
to attempt to support the reservoir pressure by injection of water, and the reservoir
is simply depleted or ‘blown down’.

9.3.3.1. Location of wells
In a gas field development, producers are typically positioned at the crest of the
reservoir, in order to place the perforations as far away from the rising gas–water
contact as possible.