*1.1. Research Status*

In-situ combustion technique is also called the inner layer combustion or fire-flooding, which is one kind of enhanced oil recovery (EOR) technology developed in 1930s. There are mainly two ways for in-situ combustion: Dry combustion and wet combustion [1,2]. A lot of studies have been conducted to study the characteristics of these two combustion modes. Wilson and Root proposed relevant calculation formulas through dry forward combustion and wet forward combustion experiments and discussed the main influencing factors [3]. Alexander et al. studied the e ffect of original oil saturation on the combustion e fficiency [4]. Chleh and Gates proposed the methods for estimating the minimum required air flow to maintain oil combustion [5,6]. Thomas proposed a more mature energy conservation equation for the fired oil layers [7]. Parrish et al. conducted a forward wet combustion test, discussed the influencing factors of various parameters, and provided a design method for wet combustion [8]. Penberthy et al. have proposed a relationship between temperature and crude oil saturation distribution, material balance, air demand and oxygen concentration near the combustion front [9]. Garon et al. carried out inverse combustion experiments and discussed the related influencing factors [10]. Suat and Mustafa conducted in-situ combustion technique on Turkish heavy oil reservoirs [11]. In the dry combustion experiment, as the API (A measure of the density of petroleum and petroleum products developed by the American Petroleum Institute.) of the crude oil decreases, the fuel consumption rate becomes faster. In wet combustion, the higher the air-water ratio, the fuel consumption will decrease. Burger published a research report on the fired oil layer and proposed the oxygen demand calculation formula and the ignition time equation [12].

Besides the in-situ combustion technique, there are another two commonly used EOR technologies: Steam assisted gravity drainage (SAGD) and the polymer flooding. SAGD is a cutting-edge technology to improve the recovery of heavy oil, super heavy oil, and high condensate oil. SAGD technology can greatly increase oil recovery [13,14]. Polymer flooding technology is representative of the tertiary oil recovery stage. Through the application of polymer flooding, more supporting techniques can be studied to improve the sweep volume of the injection agen<sup>t</sup> and the recovery factor of the tertiary oil recovery stage [15,16]. In China, in-situ combustion experiments were carried out in Xinjiang, Yumen, Shengli and Fuyu oilfields from 1958 to 1976. Since 1993, Shengli oilfield has listed the burning reservoir as a key pilot experimental project and six field experiments have been carried out [17]. However, due to the limitation of technical conditions, only dry combustion experiments have been carried out, and no wet combustion experiments have been carried out. Since 1999, based on the experience of dry combustion experiments, laboratory research on wet combustion has been carried out, and some achievements have been achieved. In 2001, Cai et al. carried out wet combustion experiments on heavy oil in the Hekou oilfield by using physical simulation technology [18]. The e ffect of the parameters on the reservoir performance under wet spontaneous combustion was studied, such as fuel consumption, apparent hydrogen-carbon atom ratio, combustion front propulsion speed, and air requirement. The results show that wet combustion can recover heat more e ffectively than dry combustion. It reduces fuel consumption and air consumption, and also improves oil recovery. In 2005, Guan et al. provided a method to determine the reservoir ignition temperature through laboratory tests [19]. The method was adopted in the Zheng 408 block of the Shengli oilfield, and the ignition temperature was successfully determined to be about 370 ◦C. Jiang et al. used a combination of physical modeling and numerical simulation to systematically study the mechanism of oil displacement in low-permeability reservoirs [20].

Due to the complexity in the process of in-situ combustion, it is di fficult to summarize the general laws of thermal oil displacement in the fired oil layer by relying only on limited combustion experiments. In this case, the numerical simulation method is becoming more important. The numerical simulation of the in-situ combustion reservoir is more complex and di fficult than steam injection. Frequent changes of chemical reactions and phase states greatly increase the number of governing equations. Thus, the relatively perfect numerical simulation technique of in-situ combustion appears later than steam injection. The development of the numerical simulation of in-situ combustion reservoir is from one-dimensional to two-dimensional and three-dimensional. The phase number is generally three-phase (gas, oil, water) or four-phase (gas, oil, water, and solid). In addition the nature of reservoir rocks and fluids, the consideration of gravity and capillary force make the numerical simulation of in-situ combustion more complex. Currently, there are some commercial numerical simulators which are suitable for various thermal recovery methods. Through numerical simulation, Bottia et al. found that the delayed ignition indicates high probability to ge<sup>t</sup> a spontaneous ignition. Furthermore the distance at which at which ignition occurs can be modified by the air injection rate [21]. Rahnema et al. found that the oil displacement is mainly driven by gravity drainage through the experiment and numerical simulation. Vigorous combustion was observed at the early stages near the heel of the injection well [22]. Pei et al. studied the e ffect of nitrogen injection on the e ffectiveness of in-situ conversion process by numerical simulation [23]. Nesterov et al. found that the activation energy

of the light fraction in the oil is the most significant factor which affects the possibility of ignition through numerical simulation [24]. However, the effect of production and geological parameters on the heavy oil reservoir production with in-situ combustion has not been systematically studied in previous research.

For the Du 66 block, the fire-flooding is the main development method since 2011. However, with the expanding scale of fire-flooding pilot test in the Du 66 block and the influence of reservoir heterogeneity, there are some problems, such as the difference of the combustion state between the thin interbedded layers, the serious overlap of fire-flooding line in thick interbedded layers, the uneven spread of fire line and the unclear understanding of the combustion state, which affect the efficiency of in-situ combustion. In order to reveal the mechanisms of multi-layer in-situ combustion and understand the characteristics of multi-layer fire wave, and also determine the main factors affecting the oil recovery of the Du 66 block, it is necessary to carry out the numerical simulation study of in-situ combustion in the Du 66 block.

In this paper, the numerical simulation method was used to study the influence of production parameters and geological parameters on the fire-flooding efficiency based on the Du 66 block in the Shuguang oilfield. The rest of this paper is organized as follows: Section 2 presents the geological background of the studied field; Section 3 is the construction process of the geological model; Section 4 shows the results and analysis.
