Page 424 - Hydrocarbon Exploration and Production Second Edition

Managing Decline 411

a cyclic process in which the same well is used for injection and production, and the
steam is allowed to soak prior to back-production (sometimes known as ‘Huff and
Puff’). Alternatively, steam is injected to create a steam flood, sweeping oil from
injectors to producers much as in a conventional waterflood. In such cases, it is still
found beneficial to increase the residence (or relaxation) time of the steam to heat
treat a greater volume of reservoir.
Steam injection is run on a commercial basis in a number of countries (such as
the USA, Germany, Indonesia and Venezuela), though typically on land, in shallow
reservoirs where well density is high (well spacings in the order of 100–500 ft).
There is usually a trade-off between permeability and oil viscosity, that is higher
permeability reservoirs allow higher viscosity oils to be considered. Special
considerations associated with the process include the insulation of tubing to
prevent heat loss during injection, and high production temperatures if steam
residence times are too low. Safety precautions are also required to operate the
equipment for generating and injecting high-temperature steam.

17.3.2. In situ combustion

Like steam injection, in situ combustion is a thermal process designed to reduce oil
viscosity and hence improve flow performance. Combustion of the lighter fractions
of the oil in the reservoir is sustained by continuous air injection. Though there
have been some economic successes claimed using this method, it has not been
widely employed. Under the right conditions, combustion can be initiated
spontaneously by injecting air into an oil reservoir. However, a number of projects
have also experienced explosions in surface compressors and injection wells.

17.3.3. Miscible fluid displacement

Miscible fluid displacement is a process in which a fluid, which is miscible with oil
at reservoir temperature and pressure conditions, is injected into a reservoir
to displace oil. The miscible fluid (an oil-soluble gas or liquid) allows trapped oil to
dissolve in it, and the oil is therefore mobilised.
The most common solvent employed is carbon dioxide gas, which can be
injected between water spacers, a process known as ‘water alternating gas’ (WAG).
In most commercial schemes, the gas is recovered and re-injected, sometimes with
produced reservoir gas, after heavy hydrocarbons have been removed. Other
solvents include nitrogen and methane (Figure 17.7).

17.3.4. Polymer-augmented waterflood

The three previous methods tend to yield better economics when applied in
reservoirs containing heavy and viscous crudes, and are often applied either after or
in conjunction with secondary recovery techniques. However, polymer-augmented
waterflood is best considered at the beginning of a development project and is not
restricted to viscous crudes. In this process, polymers are used to thicken the
injected water to improve areal and vertical sweep efficiency by reducing the