Study on Numerical Simulation of Fire Flooding Reservoir

. Fire flooding, also known as in-situ combustion technology, is a mining technology that can greatly improve the recovery of heavy oil reservoir. The composition of the output of the fire flooding oil well is very different from that of other heavy oil thermal recovery wells, and shows regular changes with different stages of fire flooding. These changes have a great impact on the efficient lifting of the fire flooding oil well. Numerical simulation can be used to simulate the combustion process of fire flooding and the heat and mass transfer process in the reservoir, so as to simulate the changes of the components of the output with different stages of fire flooding, and provide guidance for the design of fire flooding lifting. The results show that it is feasible to use reservoir numerical simulation software to simulate the production performance and the change of production composition of in-situ combustion oil wells.


Introduction
In-situ combustion is the continuous injection of combustion-supporting gas such as air or oxygen into the oil layer through the injection well, and the continuous combustion of crude oil in the oil layer by ignition or spontaneous combustion.The heavy components in the heavy oil are cracked into light oil at high temperature, and the injected gas, light oil, combustion-generated gas and water vapor drive the crude oil to flow to the production well.The composition of the output of the fire flooding oil well is very different from that of other heavy oil thermal recovery wells, and shows regular changes with different stages of fire flooding.These changes have a great impact on the efficient lifting of the fire flooding oil well.
Kuo C H proposed a mathematical model, introducing two temperature fronts, heat flow front and combustion front [1].The two-dimensional numerical simulation proposed by Smith J W et al. can be used to predict the effect of fire flooding in closed well pattern [2].A fully implicit and universal mathematical model for thermal recovery was proposed by Grabowski J W [3]. The model can be used for both fire flooding and steam flooding, and can deal with one-dimensional, two-dimensional and three-dimensional problems.University of Calgary scholar R. G. Moore defines crude oil as light oil, medium oil, and heavy oil based on the number of carbon atoms.His model has seven reaction equations [4].Stanford University scholar A. R. Kvoscek obtained the correlation equation of heavy oil combustion reaction by numerical simulation through RTO results combined with DSC heat release curve.In this method, the fuel is assumed to be two coke substances [5].
Through the investigation of the numerical simulation of fire flooding, it can be seen that the numerical simulation can simulate the combustion process of fire flooding and the heat and mass transfer process in the reservoir, so as to simulate the change of the composition of the output with different stages of fire flooding, and provide guidance for the design of fire flooding lifting.Based on these studies, it is believed that it is feasible to simulate the oil well production performance and the change of production composition by reservoir numerical simulation software.

Numerical Simulation of Fire Flooding Reservoir Development Based on Mechanism Model
The reservoir in the block is a reservoir after long-term fracturing and thermal recovery.The basic parameters of the model are set with reference to the field data of the oilfield, which makes the simulation closer to the field reality and more intuitive and accurate.Describe how the fire flooding production performance changes.

Establishment of Mechanism Model
The reservoir in the block is a reservoir after long-term fracturing and thermal recovery.The basic parameters of the model are set with reference to the field data of the oilfield, which makes the simulation closer to the field reality and more intuitive and accurate.Describe how the fire flooding production performance changes.In order to accurately characterize the fire flooding process, it is necessary to consider both the accuracy and reliability of the model and the operation speed of the model.It is necessary to reasonably plan the grid and unit selection of the model.Select a similar proportion to establish a conceptual model corresponding to the actual.The length and width of the model are set to 300 m, the grid blocks are 50 in the horizontal direction, the grid step is 60 m, and there are 7 layers of grids in the vertical direction, with a total of 17500 grid nodes (50 × 50 × 7).The top depth is set to 700 m and the total thickness of oil layer is 7 m.The three-dimensional structure of the model is shown in Figure 1.

Establishment of Component Model
The model mainly considers 4 phases and 7 components, in which 4 phases are mainly gas phase, water phase, oil phase and solid phase.The 7 components are mainly heavy oil, oxygen, methane, water, carbon dioxide, inert gas (carbon monoxide and nitrogen) and coke.
There are three main chemical reactions involved in the formation: heavy oil is decomposed into light components (methane) and coke, heavy oil combustion, coke combustion.

Establishment of Fluid Model
The variation curve of reservoir crude oil viscosity with temperature is shown in Figure 2. It belongs to heavy oil under the condition of reservoir temperature, and the viscosity of crude oil decreases exponentially with the increase of temperature, which indicates that the viscosity of crude oil is sensitive to temperature, so it can be explained that it is feasible and inevitable to exploit by fire flooding.Relative permeability is one of the main factors affecting the relative flow of underground fluid.The oilwater relative permeability curve, oil-gas relative permeability curve and oil-gas-water three-phase relative permeability used in the model are shown in Figure 3 to Figure 5. Through relative permeability curve and absolute permeability, the effective permeability of each phase fluid can be calculated when oil, gas and water coexist.Effective permeability is an important parameter in fire flooding simulation.

Establishment of Production Dynamic Model
In the model, a well group is used for fire flooding development, the well layout scheme is selected by the inverse five-point method, and the well spacing is 160 m.The perforation scheme adopts the scheme of perforating the upper two layers.Table 1 is the operating parameters of injection-production wells.
Table 1.Operating parameters of injection-production wells.
Property Parameter Bottom hole flowing pressure, MPa 1 Gas injection pressure, MPa 16 Gas injection speed, MPa 48000

Gas injection type Air
The operating parameters of production wells and gas injection wells are shown in the table.Commissioning began on January 1, 2020, with simulated production lasting eight years until December 1, 2027.

Production change of fire flooding reservoir 3.1 Change Law of Fire Flooding Productivity
In the process of oil layer combustion, it is necessary to grasp the position of fire line at any time.According to the radial distance of the fire line, the gas injection intensity at different stages can be reasonably adjusted and corresponding control measures can be taken to make the combustion zone advance uniformly and stably.In the numerical simulation results can be seen in the model at different times each section of the temperature distribution, according to the characteristics of the fire drive front, the highest temperature part can be seen as the combustion front, so you can intuitively find the combustion front in the model, so that the production process of the fire drive stage line division.According to the production characteristics of each stage, the production stage of in-situ combustion can be divided into the following four stages: combustion establishment stage, combustion effect stage, thermal efficiency oil displacement stage and fire stage.At the same time, there is usually an oil wall in the process of fire flooding.The oil wall is the area where the oil saturation increases within a certain range in the porous medium in a certain period of time.The area has a large oil saturation, and the seepage resistance is large.It can play a certain plugging role and can block the high permeability channel in the process of fire flooding, thereby improving the sweep efficiency.After passing through this area, the pressure will decrease greatly, and this area is the main area of pressure drop.
The fire flooding after steam injection is different from the primary development of fire flooding in the original reservoir.Its characteristic is that the secondary water body caused by steam injection provides a horizontal channel and also improves the speed of fire line advance.The secondary water body leads to a large amount of drainage in the early stage of production, and several stages of water content fluctuation appear in the middle stage.Under the drive of exhaust gas, the secondary water body is continuously expelled.When the fire line advances beyond a certain range, only bound water is left near the production well.At this time, the water content decreases and the daily oil production increases.

Fire Drive Exhaust Composition
According to the survey results and numerical simulation results, the main components of the fire flooding exhaust gas are CO2, N 2 , and CH 4 from more to less, and there is almost no generation of O 2 , H 2 S, and CO.By drawing the gas production rate of these four components, the exhaust gas composition characteristics of the four stages of fire flooding are determined.In the first three stages, the CH 4 production is high.At this time, the combustion front of the fire flooding is normally advanced, and the combustion reaction is fully carried out.The methane production increases rapidly after ignition, and begins to decrease after ignition is completed.This is due to the initial stage of ignition.The combustion is intense, the heat generated is large, the temperature rises rapidly, and more crude oil is cracked.When the fire line continues to advance to the production well, reaching the high temperature effective stage and the thermal effect oil displacement stage, the spread range of the fire flooding becomes larger, the high temperature area becomes larger, and the reaction speed is proportional to the area of the reaction.The combustion reaction is faster, so the yield of small molecule saturated hydrocarbons such as CH4 is also greater.The gas production rate of CO 2 and N 2 components was found to be basically consistent with the oil production rate, indicating that the output of nitrogen and carbon dioxide changed with the change of reaction rate.

Conclusion
In this paper, a three-dimensional injection-production geological model is established using reservoir numerical simulation software.
(1) According to the simulation results of the fire flooding mechanism model, the fire flooding production in this block is divided into the following four stages: combustion establishment stage, combustion effective stage, thermal flooding stage and fire stage.Among them, the reservoir pressure increases first and then decreases with the progress of fire flooding, and the reservoir pressure and daily oil production are the highest in the thermal oil displacement stage.
(2) The gas-liquid ratio remained in the range of 1000-2000 m 3 /m 3 after the establishment of fire flooding.In the combustion effect stage, the composition of tail gas was N 2 (47.33 %), CO 2 (30.06 %) and CH 4 (27.97 %).In the thermal effect oil displacement stage, the composition of tail gas was N 2 (43.51 %), CO 2 (33.59 %) and CH 4 (22.90 %).There was almost no formation of O 2 , H 2 S and CO in the two stages.

Figure 2 .
Figure 2. Variation curve of crude oil viscosity with temperature.

Figure 3 .
Figure 3. Oil and water relative permeability curve.

Figure 4 .
Figure 4. Oil and gas relative permeability curve.

Figure 5 .
Figure 5. Three-phase relative permeability of oil, gas and water.

Figure 6 .
Figure 6.Composition of fire drive exhaust gas.