Intra-layer combustion

(Figure) INTRA-LAYER COMBUSTION
The temperature
The air
The water
The mixture of the steam and the air
The petroleum
The mixture of the steam and the gases of the combustion
The gases of the combusion

INTRA-LAYER COMBUSTION (EN: interbedding combustion; DE: in situ Verbrennung, Flozbrand; FR: combustion in situ; ES: combustion in situ, combustion en el interior de la capa; RU: внутрипластовое горение) is the method for the development of the petroleum fields, which is based on the exothermic oxidation reactions of the hydrocarbons, mainly of the petroleum within the layer, using the oxidant (usually using the oxygen of the air), which is injected into the layer; the hydrocarbon gas and water are sometimes also delivered into the zone of the generation of the heat. This method has been proposed for the first time within the USSR during the start of the 30-ies (A. B. Sheinman, and K. K. Dubrovai). This method is used within the USSR (for example, the Pavlova Mountain reservoir within the Krasnodar territory, Khorasany within the Azerbaijan SSR), Romania, USA, and other countries, on the experimental-industrial scales.

The essence of the intra-layer combustion is the creation of the zone of the exothermic reactions, which is moving through the layer, and which is allowing, during the process of the burning of the portion of the layer petroleum, to facilitate and increase the extraction of the remaining part of this petroleum. The changing of the technological characteristics of the petroleum assists the displacement of this petroleum from the layer.

The intra-layer combustion starts from the initiating of the combustion within the neighbourhood of the bottomhole of the borehole-igniter, using the injection of the air into this borehole, less often of the other gas (the dry intra-layer combustion). The ignition of the layer petroleum takes place spontaneously, or as the result of the additional heating of the bottomhole zone of the borehole with the help of the bottomhole electric heater, of the gas burner, of the incendiary chemical mixtures, and so on. The maintaining of the process of the combustion, and the movement of the zone (front) of the combustion through the layer, are ensured using the continuous injection of the air. The front of the combustion, and the flow of the air, which is injected, may move into the same direction, namely, from the heating borehole-igniter to the extracting borehole (the direct-flow intra-layer combustion), or one towards another (the opposite-flow intra-layer combustion). The latter method is practically not used.

In case of the direct-flow intra-layer combustion, there serves as the source of the combustion mainly the "petroleum coke" (the heat-creation ability is 29-42 megajoules per kilogram, the temperature of the combustion is 350-370 degrees Celsius and more). The "petroleum coke" is formed from the most heavy fractions of the petroleum, which are separated during the heating of this petroleum before the front of the combustion; the lightweight fractions are evaporated and displaced. The speed of the movement of the front of the combustion is determined according to the concentration of the "coke" (it increases with the increasing of the density and viscosity of the petroleum), and according to the tempos of the injection of the air. In case of the insufficient content of the coke, they inject into the layer, together with the air, the hydrocarbon gaseous fuel (for example, the methane gas). The effectiveness of the dry intra-layer combustion is relatively low. Because of the low heat capacity of the air, less than 20% of the heat, which has been generated, is transferred into the zone before the front of the combustion. For the improvement of the process of the transfer of the heat, simultaneously (alternately) with the air, there is injected the water into the borehole. This water, while evaporating within the scorched zone, arrives into the region before the front of the combustion, and forms there the zones of the saturated steam, and of the condensed hot water (see the Figure).

In case of the increasing of the volumes of the injected water, the process of the combustion is terminated. But the oxygen of the air, which is injected, within the zone of the saturated vapour, enters into the exothermic reactions with the petroleum (the intra-layer combustion with the partial extinguishing, or the super-humid intra-layer combustion). During this process, the speed of the movement of the zone of the generation of the heat (the temperature is mainly 200-300 degrees Celsius) is determined mostly by the tempos of the injection of the water, and is significantly greater, than the speed of the movement of the front of the combustion in case of the dry and humid intra-layer combustion. The processes of the intra-layer steam generation during the humid and super-humid intra-layer combustion assist the intensification of the thermal impact onto the layer, and lead to the decreasing of the expenditures of the compressed air for the extraction of the petroleum.

The mechanism of the thermal method for the development on the basis of the intra-layer combustion, besides the displacement of the petroleum with the water steam, hot gases of the combustion, water, water-gas mixtures, and so on, includes the action of the oxygen-containing components such as the surfactants, and of the evaporating lightweight fractions of the petroleum. There may influence the petroleum yield (50-70% on average) the physical-chemical transformations of the rock-collector itself. The favourable geological-physical conditions for the usage of the intra-layer combustion are: the viscosity of the petroleum is more than 10^-2 pascal-seconds, the thickness of the layer is more than 3 metres, the depth of the embedment is up to 2 kilometres, the permeability is more than 100 millidarcies, the porosity is more than 18%, the petroleum saturation is more than 30-35%. The systems for the placement of the injecting and extracting boreholes for the intra-layer combustion are area-based and row-based. The disadvantages of the intra-layer combustion are associated with the necessity of the undertaking of the activities for the protection of the environment, for the recycling of the products of the combustion, and for the prevention of the corrosion of the equipment. The development of the intra-layer combustion comprises the combination of this combustion with other types of the impact onto the layer, the enhancement of the effectiveness of the individual elements of the general mechanism for the displacement of the petroleum with the help of the thermal effect. About the development of the coal deposits with the usage of the intra-layer combustion, see within the "Gasification of the coals" article.