Page 47 - Handbook of Natural Gas Transmission and Processing Principles and Practices

1.8. Natural Gas Exploration and Production
There are four major activities involved when developing a natural gas resource. They are:
exploration, drilling, completion, and production. These activities vary, depending on whether the
natural gas is “conventional” or “unconventional,” and accordingly, the discussion will follow these
two broad categories. In the discussion of production, the emphasis will be on the flow
characteristics within the reservoir. However, as the gas needs to travel from the bottom of the well
to the wellhead, the wellbore's delivery capacity will also be reviewed.

1.8.1. Conventional Gas

1.8.1.1. Exploration
Gas is contained in porous rocks at different depths in different locations. So, the first step is to
locate where the gas is likely to be found. This involves the two disciplines of geophysics and
geology.
A seismic survey (geophysics) is conducted to determine the layering of the different rock strata.
This helps to determine where an accumulation of gas (or oil) may occur, and the potential extent of
this accumulation. A seismic survey consists of a wave generator (an explosive device or a vibrator)
and a series of geophones, all located at the surface. The geophones detect reflections of the wave
from the various layers. Geophysicists interpret the seismic data to generate a 2-D or 3-D picture of
the layers in the earth (see Fig. 1.4).
The seismic survey only defines the structure of the rock layers. It does not identify what is
contained within the rock. However, it indicates where the highs and lows of the structure are, or if
there are any faults. Because of buoyancy effects, the hydrocarbons are more likely to be found in
the highs of the structure. In combination with the geoscientist, who brings regional experience and
knowledge of the hydrocarbon bearing potential of these layers, a potential drilling location is
determined. The chances of success vary significantly, depending on the degree of oil and gas
development in the region. Where there is little knowledge (wildcat exploration) the chances of a
successful well are 15%–30%, but where there has already been a lot of successful drilling
(development wells), the chance of success improves to 75%–100%.

1.8.1.2. Drilling
The mechanics of drilling are complex, and are significantly more so for an off-shore well than for

one on land. The well depth can vary from 1000 ft to 20,000 feet. The well may encounter several
layers of gas, oil or water bearing rocks, but usually, there is one particular layer that is the primary
target of interest. While drilling, the wellbore is filled with drilling mud. The hydrostatic pressure of
the mud counterbalances the reservoir's pressure and avoids blowouts during drilling.

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