Page 40 - Drilling Technology in Nontechnical Language

Chapter 2 – OIL AND GAS RESERVOIR FORMATION 31

oil well. Water drive maintains the driving pressure much longer than gas
drive because the pressure comes from the surrounding area. Eventually,
as the oil is produced, water will move towards the well, and at a certain
point, it may break through the oil and reach the well. The water will then
be produced in preference to the oil, not only because it is less viscous but
also because the increase in the amount of water in the pore spaces blocks
the oil. This is called water blocking.

Problems Related to Fluids in the Reservoir

In some cases, the oil in a reservoir can become degraded by bacterial
action to produce hydrogen sulfide (H S). H S is extremely toxic; exposure

2
2
to over about 100 parts per million (ppm) of H S in the air can be fatal.
2
While that is bad enough, steel exposed to H S in a wet environment can
2
become brittle and fail (break) without warning. If a reservoir has H S
2
present, the rig must be equipped to deal with it. Crews must be trained
and equipped to work safely in the presence of H S. Expensive steel alloys
2
must be used in the well to prevent equipment failure. H S is also called
2
sour gas, and it has a distinctive odor of rotten eggs. H S quickly destroys
2
the sense of smell.
Carbon dioxide (CO ) can also be present in the reservoir. Exposure
2
of steel to CO in a wet environment can also cause serious corrosion
2
problems for steel tubulars in the well.
If both H S and CO are present in a reservoir, serious problems may
2
2
arise in producing the oil and gas and in treating them before they can
be sold.
Summary

1. For an exploitable petroleum reservoir to form, specific
conditions arising in the correct sequence are necessary.
2. Economically exploitable hydrocarbon reserves are generated
from about 0.01% of all organic matter deposited within the
earth’s crust.

_Devereux_Book.indb 31 1/16/12 2:06 PM