T

C⁰

Disp-lace-ment front

Ther-mal front

Injection temperature

Fig. 11.4. The scheme of oil displacement by steam.

Reference representations: а - steam; б -water; в - oil.

The zones of: 1 – saturated steam; 2 – oil displacement by hot water; 3 – oil displacement by water at the conditions of reservoir temperature; 4 –oil filtration under original conditions.

Thus, when wet combustion will be implemented the same mechanisms of oil displacement will be realized as during injection of steam into the reservoir, namely the mechanism of oil displacement by steam and hot water, the mechanism of miscible displacement by evaporated oil light fractions in the steam area. However, as the in-situ combustion is realized by means of air and water injection to the reservoir, the mechanism of oil displacement by water-alternated-gas mixtures is observed. In addition, the process of oil recovery is influenced by the products of combustion and oil oxidation in porous medium, and physical-chemical transformations of the rock in the reservoir. The combustion process produces a significant amount of carbon dioxide that creates the conditions for the manifestation of the mechanism of oil displacement by carbon dioxide.

This mechanism can be significantly increased if the process of in-situ combustion is realized in the reservoirs of carbonate type because of additional carbon dioxide appearance due to thermal and chemical decomposition of the carbonates. Carbon dioxide along with oil and water can form the foam, which has a positive influence on the process of oil displacement. In the process of combustion, there are also formed surface-active reagents (SAR), alcohols and other chemical compositions that may lead to the manifestation of the mechanism of oil displacement by emulsions.

Thus, during the in-situ combustion, there are manifested the majority of the known processes, increasing oil recovery: gas, physical-chemical, hydrodynamic. This explains, the observed during in-situ combustion in laboratory and field conditions, high oil production rates.

The size of the formation heating zone in front of the combustion front when the wet combustion is realizing is the same as in burned zones and in most cases can achieve the values of 100-150 m and more. That’s why, on the one hand, it is possible to apply the method of wet combustion at relatively rare grids of well patterns (0,16-0,20 km²/well). and more), but on the other hand - there is no necessity to bring the combustion front to the producing wells, resulting in reduced air consumption for oil production. Only the development of the formation heating zone in front of the combustion front can reduce air consumption on average in 1.5-2.0 times.

Additional economy in air consumption for oil production can be achieved by moving along the formation of not heated injected water, created by wet combustion of heat margin. In general, it is believed that during wet combustion air consumption on oil production is reduced in 2,5-3 times and more than in dry combustion. Significant reduction of the consumption impact on oil production during wet combustion is an important prerequisite to expand the scopes of thermal stimulation on deep-bedded formations.

The method of wet-situ combustion is implemented at the objects with a huge range of geological-physical conditions. You can develop oil fields of a middle and low viscosity, including the fields after the flooding with this method.

At high values of water-air factor there are observed the varieties of combined technology, based on a combination of flooding and in-situ oxidation reactions. In this case, both the combustion front and the adjacent areas of superheated steam, will cease to exist, and the injected air will enter the zone of saturated steam, where it enters the exothermic reaction with oil.

It should be noted that the rate of oxidation processes is quite high at the temperatures that are typical for the steam area (200⁰ C and higher). This process is called super wet combustion. During super wet combustion the cold water intrudes into the combustion zone before the full burn out the oil, remained as a fuel. In this case, the heating and evaporation of water, heat recovery and its generation in the result of oxidation reactions are concentrated in a single area. Water velocity is determined by the injection rate. It will be significantly higher than the velocity of the combustion front.

Thus, significant cost reduction of air on oil production can be achieved by means of super wet combustion. To maintain super wet combustion process it is necessary to use small concentration of fuel. So using the method of super wet combustion is associated with the significant prospects of oil recovery enhancement, containing oils of low viscosity, including the oils after the flooding.

The main disadvantages of oil displacement methods with the help of in-situ combustion include:

• The necessity of the environmental protection and utilization of combustion products measures implementation;

• The necessity to take measures to prevent corrosion of the equipment;

• The possibility of the gravitational effects influence and decrease, as a result, of the sweep coefficient by the thermal stimulation.