Page 137 - Primer on Enhanced Oil Recovery

11
Water altering gas injection

Abstract

Gas injection by itself can maintain reservoir pressure and assist in oil recovery.
Miscibility of injected gas with oil is very important and injection at high enough for mis-
cibility pressures lead to improved oil recovery. Instability of gas displacement front leads
to relatively fast gas breakthrough to the production wells and this is multiplied by low
sweep efficiency. Adding displacement agents with high viscosity leads to better oil
recovery. Just water can be injected in alternating with gas slugs and significantly
improve oil displacement. This is so named WAG technique Water Altering Gas. The
technique can be further modified to include various methods of water and gas mixing
and even by adding foam forming surfactants. Good understanding of interaction between
reservoir and injected fluids is needed for the maximum process efficiency.

Chapter Outline

11.1 WAG methods 128
11.2 WAG applicability criteria 131
11.3 Implemented projects 134
11.4 Technology of implementation 136
Further reading 138

The injection of natural or petroleum gas technology has been used to maintain res-
ervoir pressure and increase oil recovery in depleted oil fields. In particular, in the
United States it was used much earlier than waterflooding. At the initial stage, gas
was injected into the reservoirs at pressures below the pressure of its miscibility
with oil. In the period of time when the water flooding into oil reservoirs were not
used, applied gas injection technology used to contribute to an increase in oil pro-
duction and in general oil recovery from horizontal deposits from 5% to 10%. In
the inclined formations the technology contributed to between 15 and 25% extrac-
tion increase. It was an economically preferred choice as compared to the situations
when injection pressure was high enough to dissolve the injected gas.
Later, when the process of artificial flooding in hollow oil reservoirs was widely
introduced, it was found that water injection contributes to a greater displacement
of oil than the injection of gases that are not miscible with oil. The low efficiency
of gas injection as a displacing agent, due to its breakthrough in highly permeable
areas to production wells. This decrease in oil production from wells and the overall
Primer on Enhanced Oil Recovery. DOI: https://doi.org/10.1016/B978-0-12-817632-0.00022-0
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