Page 25 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

folded and raised as a result of tectonic movements, the hydrocarbons
migrate out of the deposits and upward in porous rock and collect in crests
under the non-permeable rock, with gas at the top, then oil and fossil water
at the bottom. Salt is a thick fluid and if deposited under the reservoir will
flow up in heavier rock over millions of years. This creates salt domes with a
similar reservoir forming effect, and are common in the Middle East for
example.

This extraordinary process is still continuing. However, an oil reservoir
matures in the sense that an immature formation may not yet have allowed
the hydrocarbons to form and collect. A young reservoir generally has heavy
crude, less than 20 API, and is often Cretaceous in origin (65-145 million
years ago). Most light crude reservoirs tend to be Jurassic or Triassic (145-
205/205-250 million years ago) and gas reservoirs where the organic
molecules are further broken down are often Permian or Carboniferous in
origin (250-290/290-350 million years ago).

In some areas, strong uplift, erosion and cracking of rock above have
allowed the hydrocarbons to leak out, leaving heavy oil reservoirs or tar
pools. Some of the world's largest oil deposits are tar sands, where the
volatile compounds have evaporated from shallow sandy formations leaving
huge volumes of bitumen-soaked sands. These are often exposed at the
surface and can be strip-mined, but must be separated from the sand with
hot water, steam and diluents and further processed with cracking and
reforming in a refinery to improve fuel yield.

The oil and gas is pressurized in the pores of
the absorbent formation rock. When a well is 101 kPa
10 °C
drilled into the reservoir structure, the
hydrostatic formation pressure drives the
hydrocarbons out of the rock and up into the
well. When the well flows, gas, oil and water
is extracted, and the levels will shift as the
reservoir is depleted. The challenge is to plan
drilling so that reservoir utilization can be
maximized. Gas expands
20 MPa and pushes oil
100 °C
Seismic data and advanced 3D visualization downwards
models are used to plan extraction. Even so,
the average recovery rate is only 40%,
leaving 60% of the hydrocarbons trapped in
the reservoir. The best reservoirs with 40 MPa
200 °C
advanced Enhanced Oil Recovery (EOR)

23 Reservoir hydrostatic pressure

pushes oil and gas upwards.