Page 48 - Handbook of Natural Gas Transmission and Processing Principles and Practices

FIGURE 1.4 Seismic survey used in exploration.

Once the intended depth is reached, a suite of electric logs is run from the bottom to the top of the
well, to determine the porosity, shale content, fluid saturations, and thickness of the rock layers
(Fig. 1.5). If a potentially gas-bearing reservoir is indicated, flow tests can be performed to estimate
the permeability of the reservoir. If no productive reservoirs are found, the well is abandoned. If
one or more productive layers are found, a steel pipe (casing) is placed in the wellbore and
cemented. A “wellhead,” which is an assembly of control valves, is placed on top of the well to
control the flow from the well.

1.8.1.3. Completion
The casing is perforated where the gas reservoir is known to exist. The perforations penetrate both
the steel casing and the surrounding cement sheath, thus creating a flow path, allowing the gas to
flow from the reservoir into the well (Fig. 1.6). When the reservoir rock has a low permeability, the
deliverability of the well can be improved significantly by acidizing or hydraulic fracturing.
Acidizing increases the permeability in the vicinity of the well. Fracturing increases the contact area
between the well and the reservoir. To fracture a well, a liquid (usually gelled water with a
proppant) is injected at high rates and pressures until the rock cracks. After the completion
treatment, flow and shut-in tests are conducted to determine the deliverability potential of the well
and to estimate the permeability of the reservoir and the effectiveness of the completion.

1.8.1.4. Production
The rate at which a gas well can produce depends, principally, on two fundamental equations,
namely Darcy's law and the Material Balance Equation (Dake, 1978). Darcy's law relates the flow
rate in a reservoir to the driving force (pressure difference) and the resistance (permeability).
Material Balance relates the decrease in the average reservoir pressure to the size of the reservoir
and production volumes (hence to time). The principal factors that determine the deliverability of a
well are therefore: reservoir pressure, flowing pressure, permeability, completion effectiveness,
original-gas-in-place, and wellbore configuration.

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