The benefits and advantages of waterflood are well-known with many decades of application in a wide range of reservoirs with different crude oil and rock types. However, the average global recover factor for waterflood is only around 30%. There is, therefore, great interest in developing methods that can augment waterflood and improve its recovery factor from the current low values. It has been shown that enriching water with CO2 and injecting it in the form of carbonated water can improve the performance of water flood significantly 1-15 . However, a complete understanding of the pore-scale interactions and events taking place during carbonated water injection (CWI) in an oil reservoir and the actual mechanisms by which additional oil may be recovered are still missing and therefore the true potential of CWI is not yet well known. This is further complicated by the fact that the current commercial reservoir simulators are not able to adequately simulate the complex and multi-physics processes that take place during CWI which include both fluid/fluid and rock/fluid interactions.

The objective of the Carbonated Water Injection (CWI) JIP at Heriot-Watt University is to perform a thorough investigation of the performance of CWI under reservoir conditions and systematically study the parameters that impact the amount of oil recovery by CWI and its underlying mechanisms. Here we present the results of a series of CWI experiments performed under reservoir conditions at pore-scale and core-scale. Direct flow visualisation results of our high-pressure micromodel experiments reveal very vividly the pore-scale events that take place as CO2 gradually leaves the injected carbonated water and dissolves in the oil. The results show that the pore-scale interactions of carbonated water with crude oil are quite different from the well-known mechanisms observed in conventional CO2 flood. Apart from the usual CO2-related mechanisms such as oil swelling and viscosity reduction, in CWI, formation of a new fluid phase within the oil is observed. As we will show, this is a major mechanism that significantly improves the performance of CWI and the amount of additional oil recovery achieved by CWI. Our coreflood experiments confirm our pore-scale flow visualization results and clearly show that, compared to conventional waterflood, CWI can lead to substantial additional oil recovery under both secondary mode (injected instead of conventional water flood) and tertiary mode (injected after conventional water flood). The performance of CWI is significantly affected by the composition of the oil including the amount of light and intermediate hydrocarbons dissolved (solution gas) in crude oil.

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