11.5. Thermal-polymer reservoir treatment (tpt)

The TPT technology is based on the injection of heated up to the temperature of 90-95⁰ C of the PAA (polyacrylamide) solution with concentration of 0.05-0.1% [25]. The viscosity of heated water solution of polyacrylamide is 1.5-2 mPas. The viscosity of oil in the system of cracks decreases, a part of hot solution, mainly hot water permeates the blocks, improves the hydrophilicity of the rock, increases oil mobility, and thus it leads to oil displacement. The same happens in the stratified reservoirs (Kazemi or Serra model). During this technology there is a complex or simultaneous physical treatment of three methods: hydrodynamic, physical - chemical and thermal. When water polymer solution moves through the reservoir it gets cold; its viscosity increases and becomes comparable with viscosity of the displaced oil. The displacement coefficient is increased.

The modification of the considered technology is cyclic in-situ polymer - thermal treatment. The heat-transfer agent (hot water, steam) is injected to the formation, then cold water solution of PAA. There are made several cycles of successive heat-transfer agent and PAA injection. As well as in TPT technology there is occurred the simultaneous physical influence of three methods: hydrodynamic, physical - chemical and thermal. It should be noted that the above-mentioned technologies are applicable for the fractured-porous reservoirs, and also, for the reservoirs, consisting of hydrodynamically related interlayers of different permeability.

11.6. Cyclic steam treatment of producing wells

Cyclic steam treatment of producing wells refers to the methods of production stimulation (PS). When cyclic steam treatments there is injected steam to the producing well by the volume of 100-300tones per 1m of formation thickness during15-20 days[3]. Then the well is terminated for 10-15 days for the heat redistribution, capillary counterflow oil displacement of low-permeability interlayers (LP) in high-permeability interlayers (HP). Further the well is being operated to achieve the maximum cost-effective production rate for 2-3 months.

The physical essence of the process consists in the following: steam dilutes high-viscosity oil, increases the oil mobility factor. Depending on the change of temperature and pressure the steam transits first of all into two-phase state of steam - water, then after condensation, into hot water, intruded into the low-permeability interlayers, reducing the oil viscosity there. After well termination as well as with the cyclic flooding, the water begins to displace oil from the LP to the HP. At the third stage of the well operation – the bottom-hole pressure falls, oil withdrawal increases due to its greater mobility. Thus, the cycle of the technology consists of three stages. A full cycle lasts for 3-5 months. There are usually made 5-8 cycles for 3-4 years with the increasing duration of each cycle. If the layer lies superficially, the grid density should be not more than 1-2ha/well. There are recovered in average1,5-2 tons of oil for all the cycles per 1 ton of the injected steam (decreasing from 10-15t to 0.5-1 t).

The used equipment includes a steam generator, pipelines, compensators of temperature deformations, wellhead and downhole equipment.

When the heat-transfer agent is injected the complications can occur in the well operation: sand production, emulsions formation, premature steam breakthrough, the casing string and production equipment heating. To prevent the complications there should be made the bottom-hole zone stabilization, the withdrawals processing limitation up to the wells termination.

11.7. In-situ combustion.

In-situ combustion (ISC) is based on the ability of hydrocarbons (in this case, oil) to enter into a chemical reaction with oxygen. In the result of combustion in the reservoir a large amount of heat is emitted, the physical properties of the reservoir fluids and rocks are changed. Unlike the other thermal methods of enhanced oil recovery ISC allows to remove technical problems and heat loss, which arise when generating it on the surface and delivering to the formation of the heat-transfer agent by injection [7].

The process of in-situ combustion is carried out at the well bottom that is called a fireflood well. The oxidizer (usually air) is pumped to the injection well with simultaneous heating of the bottom-hole formation zone using downhole electric heater, gas burner, incendiary chemical mixtures, etc. Due to this the exothermic oil oxidation reactions are accelerated, which eventually lead to the creation of thecombustion process in the bottomhole formation zone.

After initiation of combustion the continuous air injection provides both the maintenance of the in-situ combustion process and the movement the combustion zone through the formation. Due to the small size of the combustion zone compared with the distance between the wells the combustion zone is called a combustion front. When the air for combustion maintenance is occurred to be in the fireflood well then, under pressure, the combustion front moves in the direction from the injection well to the producing, i.e. in the direction of the injected air. This combustion process is called direct-flow unlike counterflow, when the combustion front is moves in the direction from the producing well (fireflood well) to the injection one, i.e. against the motion of the injected air. Counterflow combustion has not received a significant use yet, and the further only a direct-flow combustion is observed.

In the process of combustion the heavier fractions of oil, called coke (gas carbon), are burnt. The rest part of the oil is heated to reduce viscosity, density, increase oil mobility. The lighter fractions transform into the vapor phase and participate in the displacement of the liquid heated oil. For different geologic-field conditions the coke’s concentration can be 10-40 kg per 1 m³ of the reservoir. This important parameter of the combustion process is recommended to determine experimentally in laboratory conditions. It is established that with the increase of density and viscosity of oil the coke’s concentration increases, and at high values of permeability of rocks it decreases. It is believed that during the combustion of coke the heat in the quantity of 29-42 MJ/kg is emitted.

There are three main types of in-situ combustion: dry, wet and super wet.

11.7.1. Dry-situ combustion

At a dry-situ combustion to maintain the combustion process it is necessary to pump only air. The main share of the generated heat in the formation (80 % or more) remains in the area behind the combustion front and gradually dissipates in a surrounding rocks of the formation. This heat has some positive impact on the process of displacement of not covered by the burning process adjacent parts of the reservoir [3,7].

It is established, that in case of maintenance of in-situ combustion process by the injection of only gaseous oxidizer (air) into the reservoir, the loss of heat from the heated rock by means of the burning process happens slower than the rock is heated by the moving combustion front. When the combustion front moves as a fuel, a part of oil is spent that remains in the reservoir after its displacement by combustion gas, water steam, evaporated light fractions of oil ahead of the combustion front.

Air consumption for oil production in the dry-situ combustion, according to the results of field tests, varies in the range between 1000 to 3000 m³ (under normal conditions) per 1 m³ of oil.

The transfer of heat to the area ahead of the combustion front will approach the generated heat in the formation to the areas where the oil is displaced from the reservoir. This heat transfer is connected with the acceleration of heat transfer in the reservoir due to the addition of water to the injected air.

11.7.2. Wet-situ combustion

The combination of in-situ combustion and flooding is called wet-situ combustion.

The essence wet combustion is that the injected along with the air in certain proportions water, evaporating in the surrounding zones of the combustion front, transfers the generated heat to the area ahead of it; as a result, extensive zones of warming-up, formed by the areas of saturated steam and condensed hot water in this area are developing (Fig. 11.1). The process of the in-situ steam generation is one of the most important distinguishing features of the wet combustion, determining the mechanism of oil displacement from the reservoir [3,7,22] .

The ratio value of the injected volumes of water and air into the reservoir are within the range of 1-5 m³ of water per 1000 m³ of air (under normal conditions), i.e. water-air factor should be (1-5)*10^-3 m³/m³. Specific values of water-air factor are defined by different geological-field conditions of the process realization. However, with the increase of density and viscosity of the oil (more precise, with the increasing of coke’s concentration) the values of water-air factor are decreased. If the values of water-air factor are less than the indicated ones, the transfer of heat to the area ahead of the combustion front decreases. When water is injected in more volume the wet-situ combustion method passes into other modifications of the combined stimulation by means of burning and flooding.

It is important to emphasize that the increased values of water-air factor do not lead to the termination of exothermic oxidation processes in the reservoir even if high-temperature combustion zone is terminated. At the same time low values result in a decrease of the efficiency of thermal influence on the formation and oil recovery process. Therefore, the wet-situ combustion process is advisable to do with the maximum possible values of the water-air factor. Temperature distribution in the reservoir during the process of wet combustion is schematically depicted in fig. 11.3 [7] .

The highest temperature is in the zone of combustion front - here it reaches 370⁰ C and even higher. As the combustion front moves there are formed several specific, distinctly allocated temperature zones in the reservoir. In the burned area behind the combustion front there are two temperature zones. In the transition zone the temperature changes from the temperature of the injected agents (water and air) to the evaporation temperature of the injected water. The zone of the superheated steam, formed as a result of evaporation of the injected water and air to the rock, heated to a high temperature by the moving ahead combustion front, joints the combustion front.