Page 423 - Hydrocarbon Exploration and Production Second Edition

410 Enhanced Oil Recovery

the problem but is an expensive option. To repair cross flow behind casing normally
requires a full workover with a rig. Cement has to be either squeezed or circulated
behind the production casing and allowed to set, after which cement inside the
casing is drilled out, and the producing zones perforated and recompleted.
In very difficult situations the production interval is plugged back, a side-track
well is drilled adjacent to the old hole and the section completed as a new well.

17.3. Enhanced Oil Recovery

A considerable percentage (40–85%) of hydrocarbons are typically not
recovered through primary drive mechanisms, or by common supplementary
recovery methods such as waterflood and gas injection. This is particularly true of
oil fields. Part of the oil that remains after primary development is recoverable
through EOR methods and can potentially slow down the decline period.
Unfortunately, the cost per barrel of most EOR methods is considerably higher
than the cost of conventional recovery techniques, so the application of EOR is
generally much more sensitive to oil price.
Generally, EOR techniques have been most successfully applied in onshore,
shallow reservoirs containing viscous crudes, where recoveries under conventional
methods are very poor and operational costs are also low. The Society of Petroleum
Engineers (SPE) publishes a regular report on current EOR projects, including both
pilot and full commercial schemes (the majority of which are in the USA). EOR
methods can be divided into four basic types:

steam injection
in situ combustion
miscible fluid displacement
polymer flooding.
In the North Sea, which is more representative of large, offshore, capital-
intensive projects developing lighter hydrocarbon reservoirs, it has been estimated
that around 4 billion barrels are theoretically recoverable using known EOR
techniques, which is equivalent to 15% of the estimated recoverable oil from
existing North Sea fields. This represents a considerable target. Therefore, EOR
research also continues into methods more suited to this type of environment, such
as waterflooding with viscosified injection water (polymer-augmented waterflood).
The physical reasons for the benefits of EOR on recovery are discussed in
Section 9.8, Chapter 9, and the following gives a qualitative description of how the
techniques may be applied to manage the production decline period of a field.

17.3.1. Steam injection
Steam is injected into a reservoir to reduce oil viscosity and make it flow more
easily. This technique is used in reservoirs containing high-viscosity crudes, where
conventional methods only yield very low recoveries. Steam can be injected in