Page 360 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Enhanced Oil Recovery Methods 327

mechanisms work as efficiently as desired. As a result, injected steam will tend
to break through prematurely into the offset producing wells without sweeping
the entire heated interval.
Technical Screening Guides

In some instances, only one type of enhanced recovery technique is applicable
for a specific field condition but, in many instances, more than one technique
is possible. The selection of the most appropriate process is facilitated by
matching reservoir and fluid properties to the requirements necessary for the
individual EOR techniques. A summary of the technical screening guides for
the more common EOR processes is given in Table 5-46. A distinction is made
between the oil properties and reservoir characteristics that are required for each
process. Generally, steamflooding is applicable for very viscous oils in relatively
shallow formations. On the other extreme, CO, and hydrocarbon miscible
flooding work best with very light oils at depths that are great enough for
miscibility to be achieved. Both steamflooding and in-situ combustion require
fairly high permeability reservoirs. Chemical flooding processes (polymer,
alkaline, or surfactant) are applicable in low to medium viscosity oils; depth is
not a major consideration except, at great depths, the higher temperature may
present problems in the degradation or consumption of some of the chemicals.
Screening guides or criteria are among the first items considered when a
petroleum engineer evaluates a candidate reservoir for enhanced oil recovery.
A source often quoted for screening criteria is the 1976 National Petroleum
Council (NPC) report on Enhanced Recovery [380], which was revised by the
NPC in 1984 [381]. Both reports list criteria for six enhanced recovery methods.
Some reservoir considerations apply to all enhanced recovery methods.
Because drilling costs increase markedly with depth, shallow reservoirs are
preferred, as long as all necessary criteria are met. For the most part, reservoirs
that have extensive fractures, gross heterogeneities, thief zones, or are highly
faulted should be avoided. Ideally, relatively uniform reservoirs with reasonable
oil saturations, minimum shale stringers, and good areal extent are desired.
Implementation of enhanced recovery projects is expensive, time-consuming,
and people-intensive. Substantial costs are often involved in the assessment of
reservoir quality, the amount of oil that is potentially recoverable, laboratory
work associated with the EOR process, computer simulations to predict recovery,
and the performance of the project. One of the first steps in deciding to
consider EOR is, of course, to select reservoirs with sufficient recoverable oil
and areal extent to make the venture profitable.
With any of the processes, the nature of the reservoir will play a dominant
role in the success or failure of the process. Many of the failures with EOR have
resulted because of unknown or unexpected reservoir problems. Thus, a thorough
geological study is usually warranted.
The technique of using cursory screening guides is convenient for gaining a
quick overview of all possible methods before selecting the best one for an
economic analysis. Common sense and caution must be exercised since the
technical guides are based on laboratory data and results of enhanced recovery
field trials, and are not rigid guides for applying certain processes to specific
reservoirs. Additionally, the technical merits of recent field projects are clouded
by various incentive programs that make it difficult to discern true technical
applications. Some projects may have been technical misapplications or failures,
but economic successes. Certainly, there have been enough technical successes,
but economic failures.