The addition of LPG to the CO2 stream leads to minimum miscible pressure (MMP) reduction that causes more oil swelling and interfacial tension reduction compared to CO2 EOR, resulting in improved oil recovery. Numerical study based on compositional simulation has been performed to examine the injectivity efficiency and transport behavior of water-alternating CO2-LPG EOR. Based on oil, CO2, and LPG prices, optimum LPG concentration and composition were designed for different wettability conditions. Results from this study indicate how injected LPG mole fraction and butane content in LPG affect lowering of interfacial tension. Interfacial tension reduction by supplement of LPG components leads to miscible condition causing more enhanced oil recovery. The maximum enhancement of oil recovery for oil-wet reservoir is 50% which is greater than 22% for water-wet reservoir. According to the result of net present value (NPV) analysis at designated oil, CO2, propane, and butane prices, the optimal injected LPG mole fraction and composition exist for maximum NPV. At the case of maximum NPV for oil-wet reservoir, the LPG fraction is about 25% in which compositions of propane and butane are 37% and 63%, respectively. For water-wet reservoir, the LPG fraction is 20% and compositions of propane and butane are 0% and 100%.
CO2 injection has been found to be an efficient method for oil recovery worldwide through a miscible or an immiscible displacement process. Mechanism of CO2 enhanced oil recovery (EOR) is divided into two different processes, miscible flood and immiscible flood. Although miscible gas injection is a widely applied EOR process, it can be only applied when the reservoir pressure is higher than minimum miscible pressure (MMP). The main process of miscible gas injection is displacement efficiency improvement by oil viscosity reduction and swelling effect to reduce residual oil saturation. When reservoir pressure is higher than MMP, the injected CO2 and reservoir oil are completely miscible and the displacement efficiency can be enhanced by zero interfacial tension [
Injected CO2 and reservoir oil can be miscible by continuous contact. At the fore-end of injected fluid, CO2 is persistently contacted with fresh oil following flow direction, and they are eventually miscible by the vaporizing-gas drive process. In contrast, at the back-end of injected CO2, near injection well, reservoir oil is continuously contacted with fresh CO2 that causes the miscible state by the condensing-gas drive process [
Figure
Phase ternary diagram for reservoir oil and injection gas relation [
To improve sweep efficiency, WAG (water-alternating-gas) process is applied to CO2-LPG EOR method in this research. At the same WAG condition, injected LPG amount and composition are the variables considered in the study. Many experimental researches about the effects of LPG and impurities on MMP with oil have been actively developed [
It has been identified that CO2-LPG flood is an effective method for MMP reduction causing oil recovery enhancement through many experimental studies. Compositional model for CO2-LPG EOR is necessary to investigate how gas transport affects MMP reduction and oil recovery enhancement. In this research, compositional fluid and multiphase simulation models are developed and injected LPG mole fraction and composition are optimized based on recent oil, CO2, propane, and butane prices for maximum net present value (NPV).
Fluid data of Weyburn reservoir is referred for NPV based solvent injection simulation. Weyburn reservoir, located in southeast Saskatchewan and operated by PanCanadian Petroleum Ltd., has reached its economic limit of production by waterflooding. The reservoir is a target for CO2 miscible flooding to enhance oil recovery. The oil composition is shown in Table
Modelled fluid composition for Weyburn oil.
Components | Mole fraction |
---|---|
N2 | 0.0207 |
CO2 | 0.0074 |
H2S | 0.0012 |
CH4 | 0.0749 |
C2H6 | 0.0422 |
C3H8 | 0.0785 |
i-C4 to n-C4 | 0.0655 |
i-C5 to n-C5 | 0.0459 |
C6+ | 0.6637 |
|
|
Total | 1 |
Comparison between properties of fluid model and Weyburn data.
Parameters | Fluid model | Weyburn |
---|---|---|
Saturation pressure (psi) | 688 | 713 |
Oil gravity (°API) | 47 | 31 |
Formation volume factor (bbl/STB) | 1.11 | 1.12 |
Gas-oil ratio (SCF/STB) | 166 | 32 |
Minimum miscibility pressure (psi) | 1,996 | 2,059 |
Phase behavior of fluid model was determined by Peng-Robinson EOS [
The EOS constants for pure components are given by
Robinson and Peng [
Fugacity expressions are given by
Multiple mixing cell method [ Specify the reservoir temperature and an initial pressure. Calculate the tie-line length for each pressure step by using the equation below: where Draw a tie-line length graph as a function of pressures. Perform a multiple-parameter regression of the minimum tie-line lengths to determine the exponent Determine the MMP when the power-law extrapolation gives zero of minimum tie-line length.
After generating the fluid model which has approximate MMP to Weyburn fluid, MMPs were computed between oil and LPGs. The composition of LPG is propane 63% and butane 37%, and the calculated MMPs are indicated in Table
MMP estimates for injection gas according to LPG mole fraction.
LPG mole fraction (%) | MMP (psi) |
---|---|
0 | 1,996 |
5 | 1,995 |
10 | 1,825 |
15 | 1,820 |
20 | 1,412 |
25 | 1,354 |
30 | 1,046 |
The equation for calculating interfacial tension in multicomponent systems is as follows [
The reservoir model was assumed as 2D model which is discretized into 33 × 33 × 1 grid blocks. Each grid block has dimension as 10 ft × 10 ft × 20 ft as shown in Figure
3D view of simulation model.
Contact angle which is a determinant for wettability is defined by Young’s equation as follows:
Properties of reservoir rock and fluids.
Properties | Values |
---|---|
Depth (ft) | 4,000 |
Pressure (psi) | 2,000 |
Temperature (°F) | 145 |
Permeability (md) | 122 |
Porosity (%) | 24 |
Oil saturation ( |
0.64 |
Water saturation ( |
0.36 |
Relative permeability curves for different wettability conditions.
After waterflooding for three years, water-alternating CO2 EOR and CO2-LPG EOR were applied to water- and oil-wet reservoirs for ten years. WAG cycle of CO2 and CO2-LPG EOR is 1 : 1, and one cycle period is 6 months. Production pressure is 1,500 psi which is within a limitation of miscible condition by first or multiple contact miscibility process when added LPG concentration is larger than 20% (Figure
Operating conditions and injection design parameters.
Properties | Values |
---|---|
Producing pressure at bottom hole (psi) | 1,500 |
Total injection (PV) | 1.5 |
Period (years) | 10 |
WAG ratio | 1 : 1 |
Injected LPG mole fraction (%) | 0, 10, 15, 20, 25, and 30 |
Injected LPG composition (propane : butane) | 100 : 0, 63 : 37, 37 : 63, and 0 : 100 |
Ternary diagram for CO2/LPG/Oil system at reservoir pressure 1,500 psi.
The NPV of a time series of cash flows is defined as the sum of the present values. NPV considering prices of oil, CO2, propane, and butane and costs of water injection and produced water handling is calculated by the following equation [
Economic parameters for optimal design.
Parameters | Values |
---|---|
Oil ($/bbl) | 80 |
CO2 ($/ton) | 80 |
Propane ($/ton) | 800 |
Butane ($/ton) | 850 |
Water injection ($/bbl) | 0.25 |
Produced water handling ($/bbl) | 1.5 |
The aim of this study is to confirm the effectiveness of water-alternating CO2-LPG EOR process in oil recovery for different reservoirs. The performance of CO2-LPG injection process has been compared with that of CO2 WAG process. LPG is composed of 63% propane and 37% butane. Results of oil recovery with various LPG concentrations are indicated in Figure
Oil recovery factors with LPG mole fraction of injection gas for different wettability conditions (composition of LPG: propane 63%, butane 37%).
Oil-wet
Water-wet
Oil recovery factors with LPG composition for different wettability conditions (LPG mole fraction of injection gas: 15%).
Oil-wet
Water-wet
When reservoir oil and injected gas are miscible, gas saturation decreases further than immiscible condition (Figure
Gas saturation with LPG mole fraction of injection gas near production well for oil-wet condition.
Gas relative permeability with LPG mole fraction of injection gas near production well for oil-wet condition.
The addition of LPG to CO2 stream is more effective to lower interfacial tension between oil and gas phases. In particular, if the reservoir is in miscibility condition, interfacial tension reaches zero [
Interfacial tension between oil and gas phases with injected gas mole fraction after six months of gas injection (LPG composition: 63% propane and 37% butane).
CO2 100%
LPG 20%
If more butane content is injected than propane, more oil recovery is expected because of its higher molecular weight. It was proved that butane is much more effective in MMP reduction [
Oil saturation with LPG composition after six months of gas injection (LPG volume fraction of injection gas: 15%).
100% propane
100% butane
Tables
Maximum NPV improvements with LPG mole fraction and composition for oil-wet reservoir (base case: $2,652,887).
Maximum NPV improvements (%) | |||||
---|---|---|---|---|---|
LPG 10% | LPG 15% | LPG 20% | LPG 25% | LPG 30% | |
Propane 100% |
10.7 | 11.0 | 11.8 | 12.8 | 20.0 |
|
|||||
Propane 63% |
9.3 | 10.3 | 13.0 | 24.1 | 19.2 |
|
|||||
Propane 37% |
9.0 | 10.9 | 18.4 | 19.2 | 15.4 |
|
|||||
Propane 0% |
9.2 | 13.8 | 22.5 | 17.3 | 13.0 |
Maximum NPV improvements with LPG mole fraction and composition for water-wet reservoir (base case: $3,387,572).
Maximum NPV improvements (%) | |||||
---|---|---|---|---|---|
LPG 10% | LPG 15% | LPG 20% | LPG 25% | LPG 30% | |
Propane 100% |
12.2 | 12.0 | 12.3 | 12.6 | 15.5 |
|
|||||
Propane 63% |
11.8 | 12.2 | 13.1 | 16.0 | 16.7 |
|
|||||
Propane 37% |
11.8 | 12.5 | 15.6 | 16.7 | 14.2 |
|
|||||
Propane 0% |
11.7 | 13.6 | 17.0 | 15.4 | 12.4 |
In this study, water-alternating CO2-LPG EOR simulation model was developed. To examine the efficiency of CO2-LPG EOR considering oil, CO2, and LPG prices, extensive simulations have been performed for different wettability conditions and the following conclusions have been drawn. When LPG concentration is 30% and composition of butane is 100%, oil recovery increased by 46% and 25% for oil-wet reservoir. When LPG concentration is 30% and butane composition is 100%, the maximum increasing amounts are 22% and 15% in case of water-wet reservoir. As injected LPG concentration and butane composition increased, significantly enhanced oil recovery was observed from the reduction of MMP and interfacial tension. Oil recovery for different wettability by CO2-LPG EOR has become close to 100%. When LPG concentration is 25% and butane composition is 37%, maximum NPV improvement is 24.1% for oil-wet reservoir. When LPG concentration is 20% and butane composition is 100%, maximum NPV improvement is 17.0% for water-wet reservoir. For both oil- and water-wet reservoirs, when LPG concentrations are 10%, 15%, 20% (propane 100%), and 25% (propane 100%), the reservoir condition is immiscible and maximum NPV increment is lower than CO2 WAG process. When LPG concentration is higher than 20% (miscible condition), maximum NPV improved and optimum LPG concentration and composition exist for maximum NPV improvement. CO2-LPG EOR can be applicable in low pressure reservoirs that CO2 is not miscible. LPG addition to CO2 stream can appreciably improve oil recovery by zero interfacial tension bringing miscible condition. Moreover, the optimization of LPG concentration and composition is absolutely necessary for economic feasibility. The necessity of optimization is required more in oil-wet reservoir due to better performance of displacement efficiency.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (no. 20122010200060).