Page 38 - Petroleum and Gas Field Processing

desired high level. Depending on the type of reservoir, pressure
maintenance may be achieved by either the injection of water through
wells drilled at the periphery of the reservoir or the injection of gas at the
top of the reservoir.

Water Flooding
In this method of improved recovery, injection wells are drilled between
the oil producing wells in a specific regular pattern. For example, for the
5-spot flooding pattern, one injection well is drilled between and at equal
distances from four producing wells. Water is injected into the injection
well to drive (push) the oil toward the four producers. Simulations of the
process are usually made before implementation to determine the optimum
well spacing, or pattern, and the injection rate and pressure to achieve
efficient displacement of oil by the injected water. Water-flooding
operations are always associated with a high water cut in the produced
fluid. The process continues until the oil production rate becomes too low
for economic operation.
Because the water viscosity is lower than the oil viscosity, water may
finger through the oil and reach the producing well prematurely, bypassing
a significant amount of oil that would be very difficult to recover.
Engineers have to be careful in determining the injection rate for the
specific reservoir and rock and fluid properties to avoid water fingering.
Water flooding could be improved if the water viscosity were altered to
become higher than the oil viscosity. This would create a more favourable
mobility conditions, make the oil–water interface more stable, and avoid
fingering. This is achieved by adding certain polymers to the injection
water. This is known as polymer flooding.

Chemical Recovery

Some liquids have a higher affinity than others to adhere to solid surfaces.
When two liquids are in contact with a common solid surface, one of the
two liquids (the one with the higher adhesion affinity) will spread over the
surface at the expense of the other. The solid surface is then identified as
wettable by that liquid. Most reservoir rocks are wetted by water than by
oil and are, therefore, identified as water-wet rocks. Such conditions are
favorable for displacing oil by water. There are situations, however, where
the reservoir rocks are oil-wet. In these cases, water would not be able to
displace the oil. Reservoir engineers and surface-chemistry scientists have
developed methods in which chemicals are used to change the wettability
of the rock for effective water-flooding operations. These methods are

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