Exergy investment in producing hydrocarbons is a relatively small fraction of the energy of the oil produced; yet it can reduce energy consumption in the order of percentages. In areas of high insolation or high wind speed, it can be considered that part of the exergy required for these purposes can be retrieved from sustainable energy sources. This idea is expected to be more important when applying enhanced oil recovery. As an example we use solvent (Dimethyl Ether - DME) enhanced water drive recovery. DME is a chemical solvent that has proven to be an efficient oil recovery agent. The recovered DME and oil are both considered products. The main invested exergy considered are the circulation costs of the fluids, separation/retrieval costs and the manufacturing costs of DME – it is assumed that DME is manufactured from natural gas using the single step direct method.
To improve the insight in the production process we develop a simple model of DME enriched brine injection in a 1-D reservoir. The model shows that about 92% of the oil in place is recovered using DME, which includes about 30% incremental production after water flooding. Moreover, 100% of the DME injected is recovered.
For the production /retrieval costs, we use a data set from the literature. The data set gives us the amount of DME /water injected and the amount of DME /oil/water produced. Moreover it gives the pressure drop, which allows us to calculate the power required for circulation of the fluids. Using these data, the exergy recovery factor (ExRF), which is defined as the exergy of the resources minus the exergy invested divided by the exergy of the resources produced (oil and DME) is calculated. It is observed that the ExRF initially increases with time before it declines and becomes negative. The time at which the ExRF becomes zero is called the exergy zero time. The result shows a negative exergy at the beginning of the DME enhanced water flood (DEW) process. As the incremental oil produced increases due to the presence of DME, and as more DME is back produced, which leads to less manufacturing of DME, the ExRF becomes positive. For DME enhanced recovery the initial area below exergy zero time plus the area above the exergy zero time is positive. Cumulatively, the result shows that at the end of the project, about 71% of the exergy is recovered.
The exergy analysis helps us to identify the various components that contribute the most to the exergy loss (~29%). DME manufacturing is found to be the most important contributor to the exergy loss, contributing ~80% (cumulative) to the total invested exergy. It shows that reducing the exergy of manufacturing DME increases the ExRF. The amount of DME lost in the reservoir is shown to also have an effect on the ExRF (not as much as the exergy of manufacturing DME), as it affects the utilization factor of DME. The utilization factor is the ratio of the oil produced (bbls) and the mass of DME injected. If DME is lost more DME must be injected without any increase in oil recovery and thus, DME loss reduces the ExRF.
CO2 hydrogenation is chosen as one of the innovative ways of producing DME from renewable sources. The method utilizes CO2 captured from burning the oil produced from the field in power plants and uses solar PV (photovoltaic) as the source of energy to produce H2 from water electrolysis
The results show that the CO2 captured from the power plant can be used to produce more DME than what is needed in the field. The excess DME can be reinjected or used for other purposes such as electricity generation, methanol production or for other uses e.g. as transportation fuel. It is also found that using CO2 hydrogenation has the potential to reduce greenhouse gas emissions by about 82% compared to using natural gas for DME production, which means the method is cleaner and more sustainable.