Issues Relating to Optimising WAG Ratio in CO2 Flooding
CO2 injection into UKCS reservoirs is a current topic of interest due to the complementary needs of sequestering CO2 for environmental reasons and mitigating the projected decline in UKCS oil production. In general, there is a trade-off between the efficiency of Improved Oil Recovery (IOR) and the effectiveness of CO2 sequestration. With the WAG technique water is injected alternately with gas in order to improve sweep efficiency for IOR. However, such injected water occupies reservoir pore space which might otherwise be filled by sequestrated CO2. Does maximising CO2 sequestrated necessarily optimise project value? Eugene Balbinski (Eugene.Balbinski@ecltechnology.com), together with co-authors Jose Gil Cidoncha and Roy Wikramaratna (all of ECL Technology Ltd), have investigated. This work was undertaken as part of DTI's OG-MRP (Maximising Recovery Programme).
Relative Value of WAG and Continuous Gas Injection
Figure 1 illustrates the relative value of 1:3 optimised WAG and continuous CO2 injection for a field model example using crude economics based only on the value of incremental oil and sequestrated CO2.
Only if the oil price is low and the value of CO2 sequestration credits is high is continuous gas injection more valuable. Generally the total project value is maximised by maximising the IOR, rather than the CO2 sequestrated. Of course, this is only one example and this balance can be affected by many different factors and so will be case sensitive. For example, it is important to consider an optimised WAG flood. The costs of re-injecting produced CO2 may also have a differential effect on the total economics. However, this example does demonstrate that continuous CO2 injection isn't necessarily more valuable just because of its greater CO2 sequestration potential. These results were obtained using a reservoir simulation model - further details below.
Figure 1: 3:1 WAG has Greater Value than Continuous CO2 injection Unless Oil Price is Low and CO2 Sequestration Credit Value is High (Field Model)
Model Description
The simultaneous optimisation of IOR and CO2 sequestration was investigated using various 3D models with properties typical of UKCS reservoirs various:
- A generic model with homogeneous reservoir properties.
- A generic layered model with variable reservoir properties.
- A field type model which included realistic levels of heterogeneity on both large and small scales.
Miscibility was represented using the Todd-Longstaff model and all cases were run with realistic well controls and constraints.
The generic layered model shown in Figure 2 contains two relatively thin high connectivity pathways between the injector and producer. Typical UKCS reservoirs may also encounter such pathways, but they will be more complex.
Figure 2: High Connectivity Pathways in Generic Layered Model
Flood Optimisation
Apart from varying the relative volumes of injected CO2 and water, the most successful means of optimising the flood was found to be by varying the completion policy. Water was best injected into the upper part of the formation and CO2 into the lower part. This helped to counter the tendency for water to slump downwards and CO2 to rise to the top of the formation. The generic layered model example (Figure 3) clearly illustrates this, but the same principle is also evident in the more realistically heterogeneous field model.
Figure 3: WAG Gives Better Sweep (Optimised Generic Model)
Figure 3 shows that it is harder to mitigate poor sweep from channelling through higher connectivity pathways or override with Continuous Gas Injection (CGI) than with WAG. In the CGI case, low oil saturations from miscible CO2 injection are limited to regions around the higher connectivity pathways. In the WAG case, a much more widespread attainment of low oil saturations is possible from injecting water in the upper part of the formation and CO2 in the lower part. Similar results were also derived from the field model.
Sensitivity to Proportions of CO2 and Water Injected
To illustrate the dependence of IOR and CO2 sequestration on the proportion of water and CO2 injected, these quantities have been plotted against the gas injection percentage by reservoir volume for the generic layered and field models. Both these models exhibit qualitatively similar behaviour in that in each case the IOR reaches a maximum at a particular WAG ratio, while the CO2 sequestration increases monotonically with WAG ratio. The quantitative differences are due to the different heterogeneities modelled. For the field case the heterogeneity is more distributed and complex and the IOR starts to decline if more than three-quarters of the injected fluid is CO2, but for the generic layered case this occurs if more than a fifth of the injected fluid is CO2. In the field case, CO2 sequestration continues to increase rapidly if more than three-quarters of the injected fluid is CO2, but there is little increase after one quarter in the generic layered case.
Figure 4: Injecting Too Much CO2 Reduces IOR
For all the field model WAG cases, IOR is greater than for CGI, but CO2 sequestration is less. The total project value depends on the relative value of these two benefits. Table 1 gives figures for the optimised field model case.
| Gas injection case | Incremental recovery efficiency (% STOIIP) | CO2 sequestered (% HCPV) |
| 3:1 WAG | 14 | 15 |
| 1:1 WAG | 20 | 21 |
| 1:3 WAG | 20 | 24 |
| Continuous | 13 | 45 |
Table 1: Summary of Results for Field Model, Optimising Injector Completions