Calculation Plate Used to value CCUS-EOR Potential in the Oil-Water Transition Zone

2. Simulation method

In the oil-water transition zone, the formation mechanism of the oil saturation distribution can be described as follows: due to the capillary force of the reservoir, the area between the pure oil zone of the reservoir and the free water interface is an interval of oil layers, with oil saturations decreased gradually, rather than an abrupt interface. According to the principle of capillary force, after simplifying the reservoir into a capillary model, the smaller the pore radius of the rock, the higher the water column would be formed, leading to the results that the crude oil in the oil-water transition zone mainly exists in larger pore spaces [7,8,9,10].

In this study, the core of the experiment is the simulation of the state of unsaturated crude oil in the core. When using the crude oil saturation method of conventional flooding, the injected crude oil is often concentrated at the injection end. In order to be more consistent with the occurrence of crude oil in the oil-water transition zone, the entire core should be uniformly unsaturated with crude oil, and the crude oil should mainly be distributed in larger pore spaces. In this study, with the principle of capillary force, through repeated experimental verification, a core centrifugal saturation method has been proposed. The initial oil saturation of the core is controlled by the size of the centrifugal force.

The results of CT scanning (Figure 1) have verified that with the centrifugal saturation method, the saturation state of crude oil with different initial oil saturations in the core can be obtained more uniformly, and the crude oil is mainly contained in larger pore spaces, which is consistent with the real state of the reservoirs in the oil-water transition zone.

3. Experimental results

Due to the increase of heavy components in the TZ crude oil, which makes it difficult to mix with CO₂, CO₂ immiscible flooding is chosen as the displacement method. By comparing the results of a series of experiments, it has been found from Figure 2 that the unsaturated crude oil has a larger movable oil interval during CO₂ flooding. Under the same permeability condition, the residual oil saturation decreases with the decrease of the initial oil saturation, rather than a constant value. Moreover, the lower the permeability, the stronger the influence of initial oil saturation on residual oil saturation. This may be related to the proportion of dominant seepage pore space in the overall pore space. Crude oil will preferentially saturate dominant seepage pores in the process of saturation, and the CO₂ drive efficiency of dominant seepage pores is also relatively higher. Therefore, in contrast to the fully saturated crude oil state, the unsaturated crude oil state has a greater proportion of crude oil saturated in the dominant seepage pores and the oil displacement efficiency is correspondingly higher. When the initial oil saturation is lower than 40%, the viscosity of crude oil has little influence on the residual oil saturation, and the residual oil saturation basically does not change with the initial oil saturation. When the initial oil saturation is greater than 40%, under the same crude oil viscosity conditions, the higher the initial oil saturation, the higher the residual oil saturation.

Based on the above experimental results, it can be inferred that during the development process by CO₂ injection, with the decreasing in initial oil saturation, the residual oil saturation in the oil-water transition zone will decrease accordingly. When calculating the production potential, the recovery factor should be dynamic variables, determined by the initial oil saturation and reservoir conditions. If constant residual oil saturation is used to calculate the recoverable reserves in the transition zone, the exploitation potential will be underestimated. Compared with the conventional oil reservoirs saturated with crude oil at the top, the reservoirs in the oil-water transition zone have relatively higher crude oil viscosity. Many scholars believe that the residual oil saturation is higher in the transition zone than that of conventional oil-saturated reservoirs as the viscosity is higher. In this paper, through a series of scientific comparison experiments, it is proved that when the viscosity of crude oil is between 1 and 100 cp and the initial oil saturation is lower than 40%, the viscosity has little influence on the residual oil saturation. Under the conditions of low permeability and high viscosity conditions, the control degree of initial oil saturation over residual oil saturation is relatively strong. Therefore, the influence of initial oil saturation on CO₂ flooding efficiency should be fully considered when calculating the recovery factor.

4. Calculation chart of residual oil saturation in the oil-water transition zone

Based on the results of the above 28 sets of CO₂ flooding experiments, the initial oil saturation & residual oil saturation calculation chart in the oil-water transition zone has been drawn (see Figure 4) by considering the three main controlling factors of permeability, viscosity, and initial oil saturation. This chart can be used to calculate the key development parameters, including the recovery factor, initial water cut, and final recovery factors of various reservoirs with low initial oil saturation. According to the situation of literature searching, this is the first time that this chart is proposed at home and abroad, which is of great significance for evaluation of the production potential of such reserves. By using the core and oil test data of Daqingzi Reservoir in Jilin to verify the accuracy of this chart, the prediction accuracy of core residual oil saturation is above 90%

The residual oil saturation is divided into two parts controlled by permeability and viscosity respectively. By determining the permeability, viscosity and initial oil saturation, data are fitted with different calculation methods, such as using difference and empirical parameters according to the shape of the experimental data curve. The fitting formula established is as follows:

$$S_{or}=\alpha \left(e^{\theta {\mu S}_{oi}}-1\right)+\frac{\beta S_{oi}}{K}$$

In the formula, S_or refers to the residual oil saturation, %; θ refers to the fitting coefficient of viscosity difference, mPa•s^-1; α refers to the viscosity control coefficient, which is dimensionless; β refers to the permeability control coefficient, mD; S_oi refers to the initial oil saturation, %; k refers to permeability, mD; μ refers to crude oil viscosity, mPa•s.

With the oil-water transition zone of the Daqingzijing oil reservoir in Jilin as an example, two natural cores in the target reservoir were obtained, with the permeability of 53.3mD and 52mD, and the crude oil viscosity under formation conditions was 5cp. CO2 flooding experiments were carried out under reservoir conditions. The initial oil saturation was 41.2% and 65.2%, and the residual oil saturation after CO2 flooding was 14.6% and 29.9%. The calculated residual oil saturation was 15.6% and 32.6%, and the fitting accuracy was 92%. In the traditional calculation method, unified residual oil saturation was used to calculate the recoverable reserves in the oil-water transition zone. By using the chart to conduct the dynamic correlation between the initial oil saturation and the residual oil saturation, the recoverable reserves in the oil-water transition zone can be calculated by clarifying the relationship between the initial oil saturation and the residual oil saturation. In this way, the exploitation potential of such reserves can be re-understood, which is worthy of promotion.

Figure 5. Verification of the prediction accuracy of the influence factors of Jilin core (K=53.3mD and 52mD).

Figure 5. Verification of the prediction accuracy of the influence factors of Jilin core (K=53.3mD and 52mD).

5. Conclusion

• With the centrifugal saturation method, the saturation state of crude oil with different initial oil saturation in the oil-water transition zone can be restored to the greatest extent through the relationship between centrifugal force and crude oil saturation, and the crude oil is mainly contained in larger pore spaces.
• In unsaturated oil reservoirs, the residual oil saturation is not a constant value. With the decreasing in initial oil saturation, the residual oil saturation will become lower. The lower the permeability, the greater influence the initial oil saturation has on the residual oil saturation. When the initial oil saturation is lower than 40%, the viscosity of crude oil has little influence on the residual oil saturation. When the initial oil saturation is greater than 40%, under the same crude oil viscosity, the higher the initial oil saturation, the greater the residual oil saturation.
• Based on the capillary force-phase permeability fractal model, a key variable, initial oil saturation, was added to establish the chart evaluation method of residual oil saturation, forming a three-dimensional control model based on the core permeability, oil viscosity, and the initial oil saturation. Based on 36 sets of CO2 flooding experiment results, the chart has been drawn, which can be used to calculate the residual oil saturation and evaluate the production potential of typical reservoirs.