10.3. Water-alternated-gas cyclic injection.

The technology of water-alternated-gas cyclic injection is the alternate bank or simultaneous mixture injection of gas and water in the same or separate injection wells.

The physical mechanism of oil displacement is the following. Water fills the small pores and narrow pore channels; thereby it increases the sweep coefficient of the reservoir. The gas, injected into the reservoir, due to higher mobility, occupies large pores and the upper part of the reservoir; it is partly dissolved in oil and increases its mobility and displacement factor. Thus, the gas increases one of the factors of oil recovery coefficient, and the water - the other.

These features of water and gas have led to the conclusion about expediency of combining their advantages with the aim of disadvantages reducing and to the use of periodic, cyclic injection of water-gas mixture. Optimal ratio of the volumes of water and gas injection under such kind of stimulation should be proportional to the ratio of volumes of small pores (lower then medium size) and large pores (above average size) in the reservoir (controversial). In this case, you can count on achievement of maximum effect from combined water and gas injection, i.e. water-alternate-gas mixture displacement.

Phase permeability depends on wetting phase (water), free gas provides oil displacement. Alternate displacement of oil by gas and water increases oil displacement and sweep coefficients due to the reduction of the relative permeability of high-permeability interlayers, filled with water-gas mixture. Combined oil displacement from heterogeneous reservoirs by water and gas is more efficient for the ultimate oil recovery in comparison with the separate oil displacement only by water or gas. When the drive is chosen in a correct way it can increase oil recovery in 7-15% compared with regular flooding. The main condition of optimal water-alternate-gas injection process is the even distribution of the injected gas through the flooding volume of the deposit when the simultaneous gas and water breakthrough happens to producing wells. The cycles’ duration of every injected agent lasts for 10-30 days.

The disadvantages of water-alternated-gas cyclic injection: the intake capacity of the injection well for each operating agent reduces after the first cycle: for gas in 8-10 times, water 4-5 times due to the decrease of the relative phase permeability of the reservoir bottom-hole zone.

Depending on the structure and heterogeneity of the formation gravitational separation of water and oil may reduce the effectiveness of the use of the technology in 10-20%.

The conducted laboratory studies have shown that the effect from the change of proportions of the injected water and gas is insignificant. The size of the sample is small, homogenous, and therefore, in the laboratory to create an environment that is close to the real formation conditions, is almost impossible. There are two ways-out: computer simulation, or field- experimental works with reliable knowledge about the geological and physical structure of the reservoir.

CHAPTER 11. THERMAL, THERMIC ENHANCED OIL RECOVERY METHODS.

One of the interesting methods of enhanced oil recovery are thermal methods. In the literature to describe such kind of impact on the layer there are two terms: thermal or thermic methods. Further, we will use both terms.

Thermal methods are divided into thermophysical: injection of hot water, steam, injection of hot water containing chemical reagents, cyclic steam wells treatment; and thermochemical: in-situ combustion. Hot water, steam are called heat-transfer agent. Thermal methods are applied to the deposits: containing high-viscosity oil; when the reservoir temperature is close to the temperature of the oil saturation by paraffins; for bituminous deposits of clays. A Brief classification of oils is contained in Annex 3.