Keywords

1 Introduction

With the increasing global warming and climate deterioration, greenhouse gas emissions, especially CO2 emissions, have attracted more and more attention of countries. Achieving the geological storage of CO2 in reservoirs can not only greatly reduce the emission of CO2 in the atmosphere, but also greatly improve the oil recovery. Research on CO2 flooding and its storage has been carried out earlier, and a more complete theory of displacement and storage has been formed [1,2,3,4,5,6,7,8,9,10,11,12]. In 1997, Taber et al. summarized the screening criteria for CO2 flooding reservoirs based on the successful case of CO2 enhanced oil recovery [2]. In 2002, Shaw et al. screened the reservoir parameters suitable for CO2-EOR on the basis of summarizing the previous research results, and graded the target reservoir by setting the optimal value and parameter weight [5, 6]. The potential of CO2 flooding enhanced recovery was evaluated in the Alberta reservoir [8]. CO2 sequestration is widely evaluated using CO2 utilization coefficient in American and European countries, which is defined as total sequestration amount divided by cumulative oil production [9,10,11,12]. Shen Pingping et al. also proposed a similar approach in 2009 [13]. At present, China has a relatively late start in this field, especially for the potential of carbon dioxide storage in China’s major oil areas, it is necessary to further establish a reasonable evaluation system. There are a large number of low permeability reservoirs in Dagang oilfield. After decades of exploitation, the effect of water injection development has been worse year by year. Comparatively speaking, CO2 flooding has the advantages of low injection pressure and fast formation energy replenishment, and can obtain lower solvent/crude oil mobility ratio when miscible flooding is realized, which improves sweep efficiency and realizes geological storage of CO2 while increasing production and tapping potential. Based on the establishment of CO2 enhanced oil recovery and geological storage potential evaluation method, 272 oil layers in the low-permeability oil area of Dagang Oilfield are used as the evaluation objects, and the CO2 enhanced oil recovery and geological storage potential in this area are analyzed.

2 Method

2.1 Numerical Simulation Calculation Models

The current CO2 flooding enhanced recovery and geological storage numerical simulation calculation models mainly include four types: component model, black oil model, streamline model and split model. The advantage of the split model is that the three-dimensional problem is transformed into a one-dimensional flow problem, that the data volume needs to be reduced, and the operation speed is faster. The leakage characteristics of the split model are simplified. Considering that Dagang Oilfield CO2 flooding enhanced oil recovery and geological storage potential evaluation workload is large, detailed data is difficult to obtain, we adopted the split model.

According to the principle of conservation of mass, the following conservation equations are established:

$$ \frac{{\partial C_{ij} }}{{\partial t_{D} }} + \frac{{\partial F_{ij} }}{{\partial X_{D} }} = 0 $$
(1)
$$ C_{ij} = C_{i0} S_{0} + C_{i1} S_{1} + C_{i2} S_{2} $$
(2)
$$ F_{ij} = C_{i0} f_{0} + C_{i1} f_{1} + C_{i2} f_{2} $$
(3)

Here, \( C_{ij} \) is concentration of component i in j phase; \( S_{j} \) is j phase saturation; i is component of fluid; \( i = 0 \) is water component; \( i = 1 \) is crude oil component; \( i = 2 \) is injection gas component; t is time; X is length distance; f is split flow; D represents dimensionless.

Based on the traditional model, the calculation model considers the following main influencing factors in combination with the characteristics of Dagang oilfield reservoir: (1) viscous fingering and heterogeneity; (2) gas injection gravity differentiation; (3) introducing the minimum miscibility pressure calculation method to define the state of CO2 flooding phase; (4) the difference of area sweep coefficient between miscible flooding and immiscible flooding; (5) the influence of interfacial tension effect on relative permeability change.

From this model, the CO2 flooding enhanced oil recovery range can be determined, and the amount of CO2 storage in the reservoir can be calculated from the following formula.

$$ \begin{aligned} M_{{{\text{CO}}_{2} t}} & = \rho_{{{\text{CO}}_{2} r}} \times [R_{f} \times A \times h \times \varphi \times \left( {1 - S_{wi} } \right) \\ & \quad - V_{iw} + V_{pw} ] \\ \end{aligned} $$
(4)

Here, \( M_{{{\text{CO}}_{2} t}} \) is CO2 storage potential, t; \( \rho_{{{\text{CO}}_{2} r}} \) is CO2 density in the formation, t/m3; \( R_{f} \) is oil recovery; A is reservoir area, m2; h is reservoir thickness, m; \( \varphi \) is reservoir porosity; \( S_{wi} \) is reservoir irreducible water saturation; \( V_{iw} \) is injected water volume, m3; \( V_{pw} \) is producted water volume, m3.

2.2 Model Reliability Analysis

Based on the CO2 flooding enhanced recovery calculation model, a numerical simulation calculation program was developed. The program runs fast and the calculation results show that the reservoir engineering theory can be effectively reflected. Taking the typical reservoir of Dagang Oilfield as the research object, as shown in Fig. 1, the calculation results of the calculation software using this study are close to the calculation results of the commercial software Eclipse, and the difference between the two is within 10%, and the precision is high.

Fig. 1
figure 1

The calculation results of this study and the commercial software Eclipse

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2.3 Analysis of Influencing Factors

2.3.1 Influence of Reservoir Thickness

As shown in Fig. 2, as the thickness of the reservoir increases, the recovery of crude oil decreases. Mainly because the greater the thickness, the gravity differentiation causes the sweep volume coefficient to decrease.

Fig. 2
figure 2

Relationship between recovery factor and thickness

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2.3.2 Influence of Reservoir Heterogeneity

As shown in Fig. 3, as the heterogeneous coefficient increases, the gas injection process leads to the intrusion of gas, and the volume coefficient of the sweep decreases, which in turn reduces the recovery factor.

Fig. 3
figure 3

Relationship between recovery factor and heterogeneous coefficient

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2.3.3 Influence of Reservoir Crude Oil Viscosity

As shown in Fig. 4, as the viscosity of the crude oil decreases, the flow performance of the crude oil becomes better, the relative permeability of the oil phase increases, and the recovery factor increases exponentially.

Fig. 4
figure 4

Relationship between recovery factor and crude oil viscosity

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2.3.4 Influence of Reservoir Oil Saturation

As shown in Fig. 5, as the oil saturation increases, the recovery factor has an upward trend with a large increase. It indicates that the better the oil bearing of the reservoir, the better the development of gas flooding.

Fig. 5
figure 5

Relationship between recovery factor and oil saturation

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2.3.5 Influence of Reservoir Permeability

As shown in Fig. 6, as the reservoir permeability increases, the recovery rate increases, but when the permeability reaches a certain level, it is prone to gas enthalpy, resulting in a downward trend in oil recovery.

Fig. 6
figure 6

Relationship between recovery factor and permeability

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Based on the above analysis, the sensitivity rankings of factors affecting CO2 flooding enhanced oil recovery are: crude oil viscosity > permeability > oil saturation > heterogeneity > reservoir thickness.

3 CO2 EOR and Storage Potential Evaluation

3.1 Evaluation Process

We designed and developed the software, including four basic modules: basic data input, solution design, simulation calculation analysis, and report output. Based on the software, CO2 flooding and geological storage potential evaluation can be performed through the process shown in Fig. 7.

Fig. 7
figure 7

Potential evaluation process

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3.2 CO2 EOR and Storage Potential Evaluation

3.2.1 CO2 EOR Potential Evaluation

Through the evaluation of the potential of CO2 flooding enhanced recovery in the low-permeability reservoirs developed in Dagang Oilfield, the results show that the geological reserves of the miscible flooding are great potential, and the number of miscible flooding blocks is 16. These low-permeability reservoirs can enhance oil recovery by 8.6% by CO2 flooding, of which the average miscible flooding can be increased by 15.5%, and the average of immiscible flooding can be improved. 5.6%.

The evaluation results of the CO2 flooding enhanced oil recovery potential of the undeveloped low-permeability reservoirs in Dagang Oilfield indicate that 144 blocks can achieve miscible flooding, accounting for the total amount. 65%. These low-permeability reservoirs can enhance oil recovery by an average of 9.9% by CO2 flooding, of which the average recovery of miscible flooding is 12.3%, and the average of immiscible flooding increased oil recovery by 5.4%.

3.2.2 CO2 Storage Potential Evaluation

Figures 8 and 9 show the results of CO2 geological storage potential of 272 oil layers in the low-permeability oilfield of Dagang Oilfield. Figure 8 shows the CO2 geological storage potential of the miscible flooding area, and Fig. 9 shows the CO2 geological storage potential of the immiscible flooding area.

Fig. 8
figure 8

Storage potential of miscible flooding area

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Fig. 9
figure 9

Storage potential of immiscible flooding area

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Shen Pingping et al. [9] predict storage potentials as below:

$$ M_{{{\text{CO}}_{2} t}} = R_{{{\text{CO}}_{2} }} \cdot N $$
(5)

In which, \( M_{{{\text{CO}}_{2} t}} \) is CO2 storage potentials, t; \( R_{{\text{CO}}_{2}} \) is sequestration storage coefficient, dimensionless; N is geological reserves, t.

According to the formula (5), the sequestration storage coefficients of Figs. 8 and 9 can be obtained. Here, we divide the sequestration storage coefficients into three levels of the highest value, the median value and the lowest value, as shown in Table 1.

Table 1 Sequestration storage coefficients
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4 Conclusion

  1. (1)

    This paper establishes a method for evaluating the enhanced recovery and storage potential of CO2 flooding in actual reservoirs of Dagang Oilfield. This method can effectively evaluate CO2 flooding and geological sequestration potential of low permeability reservoirs in similar oilfields.

  2. (2)

    By evaluating the CO2 flooding potential of low-permeability reservoirs in Dagang Oilfield, the results show that Dagang Oilfield has a huge potential for CO2 flooding to enhance oil recovery.

  3. (3)

    By evaluating the geological storage potential of CO2 flooding in low-permeability reservoirs in Dagang Oilfield, the results show that Dagang Oilfield has a considerable amount of CO2 geological storage, and the highest sequestration storage coefficients can reach 0.5.