CO 2 miscible flooding for Aswad Oil Field

The current renewed interest on research and development of EOR processes and their oilfield implementation would allow targeting significant volumes of oil accumulations that have been left behind in mature reservoirs after primary and secondary oil recovery operations. During the life of a producing oil field, several production stages are encountered. Initially, when a field is brought into production, oil flows naturally to the surface due to existing reservoir pressure in the primary phase. As reservoir pressure drops, water is typically injected to boost the pressure to displace the oil in the secondary phase. Lastly, the remaining oil can be recovered by a variety of means such as CO2 injection, natural gas miscible injection, and steam recovery in the final tertiary or enhanced oil recovery (EOR) phase. The main parameter for determination of the possibilities to enhance oil recovery by e.g. CO2 injection into a specific oil field is the measurement of Minimum Miscibility Pressure (MMP). This pressure is the lowest pressure for which a gas can obtain miscibility through a multi contact process with a given oil reservoir at the reservoir temperature. The oil formation to which the process is applied must be operated at or above the MMP. And we obtained by using PVTi software. The objective of this paper is to check if the Aswad field is suited for the application of CO2 miscible flooding and to see how much the recovery factor will increase after applying this EOR technique and to obtain the best WAG ratio and injection rate.


Introduction
Enhanced oil recovery (EOR) processes include all methods that use external sources of energy and/or materials to recover oil that cannot be produced, economically by conventional means.
And CO2 flooding consists of injecting large quantities of CO2 (15% or more hydrocarbon pore volumes) in the reservoir to form a miscible flood.
The field that has been studied in this paper is Aswad field., the field is located in Sirte basin, south-east sirt basin, south of Zella field, operated by Zwitina oil company. The production from Aswad oil field started in October 1978. The field has gone through primary and secondary recovery mechanisms. The Aswad structure is a fault-bounded anticline that trends in a NW-SE direction. Table 1 below shows the fluid and rock properties of Aswad field.

Methodology
The technique that we used in this paper is divided into several steps. Firstly, we analysed the production performance of the field to understand the performance of reservoir, and remaining oil reserve and other important parameters such as WC and GOR, in the next step we evaluate the water flooding project by the calculation of voidage replacement ratio VRR Because we will use the same pattern that is used in the water flooding project to inject the CO2 because we didn't do a simulation to the reservoir (because we don't have the reservoir model) to select the best pattern that provide the highest production rate, in the third step we did a screening to the field by using (Rock and Fluid Properties) in order to check if the Aswad field is suited for the application of CO2 miscible flooding, by using EORgui software, and in the following step we Estimate the Minimum Miscibility Pressure for CO2 with oil by building EOS modelling via PVTi software, and finally we predict the future oil production after applying CO2 miscible flooding and we did a sensitivity analysis to see the best WAAG ratio and Injection rate that gives the highest recovery factor. The chart below gives summary of the steps   Figure 2 shows the production rate for oil, water and gas, the oil rate started high reach to 10,000 STB/day then declined to 6000 STB/day, but after water injection increased to 8000STB/day, finally declined to about 800 STB/day . To be noticed that the main potential of the reservoir is water injection.  Figure 3 shows the GOR performance with oil and gas rates, the GOR is mainly stabilize at first period of reservoir life and after decrease in the pressure the GOR increased but after injection water the pressure increase and GOR returned to stabilization at about 1500 SCF/STB. The reservoir is under-saturated oil reservoir.

Water flooding performance evaluation
As shown in production performance analysis the reservoir was flooded by water form early life, the performance of water flooding should be considered, to ensure that reservoir fractures that making water losing in the reservoir.
The main factor to evaluate the water flooding performance is called Voidage replacement ratio (VRR), calculated by: The performance of water flooding is shown in figure (4.7), its shows that VRR almost 1 for different period of flooding stages, with average VRR=1.165, that make indication for performance of water flooding was good.

EOR Screening Criteria
Screening criteria used to select the optimum EOR method to apply for Aswad field. This selection is done by using EORgui software. Figure 7 shows EOR methods screening for Aswad field. The input data that are required for the EOR selection are present in table 3.  As can be seen from figure 7, the left hand side column represents the properties (input data) that effect on the selection of EOR method. The top row in the same figure shows the different EOR processes. Red block means the property out of limitation of method, Green block means the property out of limitation of the method, light green block means slightly effect of property on method and whit block means no effect of property on method.
The above screens for EORgui screening shows high ability to applying CO2Miscibility flooding for this field about 67% so this is a good sign .

Minimum miscibility pressure (MMP) calculation
To ensure miscibility between injection gas and oil, the pressure should be higher than MMP that can be measured at laboratory by using slim tube test, but when test is missing, the correlation and EOS modelling can be used.
In our study, we build EOS model by using PVTi software to calculate MMP.After input the laboratory tests in PVTi, then select EOS (Soave-Redlich-Kwong (3-Parm) and Lohrenz-Bray-Clark Viscosity Correlation andsplitting the C7+to three fraction to tuning the modelling .

Sensitivity analysis
Enhanced oil recovery through the injection of CO2 as a tertiary recovery mechanism, with water flooding as Water altering gas (WAG), will be studied to predict the amount of oil that can be produced. The main factors that may be effect on this process are injection fluid rate and WAG ratio, so the sensitivity will be run to investigate the optimum injection rate and WAG ratio that gives high Recovery factor. We first used WAG ratio 1and change the injection rate to obtain the injection rate that provide the highest recovery factor. The table shows that the injection rate that gives highest recovery factor is 10000(bbl/day). So we going to change the WAG ratio and hold the injection rate at 10000(bbl/day).