In-situ Generation of CO2 to Eliminate the Problem of Gravity Override in EOR of Carbonate Reservoirs

Carbon dioxide (CO2) injection into hydrocarbon reservoir is reported by many researchers to be one of the best enhanced oil recovery techniques. CO2 for enhanced oil recovery has favorable characteristics such as miscibility between CO2 and oil under most reservoir conditions, intermediate component extraction and heavy oil viscosity reduction which is named CO2 flooding process. As a result of viscous fingering and gravity override, sweep efficiency decreases and significant amounts of oil are left behind during CO2 flooding. Foaming was used to increase the injected CO2 viscosity but a lot of problems were encountered with the foaming agents such as stability and thermal stability, etc. In this paper we will introduce a new method of generating CO2 in-situ in carbonate reservoirs during enhanced oil recovery processes. The generation process includes the injection of low pH HEDTA or EDTA chelating agents followed by sea water, high pH chelating agents, or low salinity water. First the low pH chelating agent will react with the carbonate rock and produce CO2 that will diffuse to the oil and increases the oil mobility and in turn more oil will be produced. The chelating agents used are H2Na2EDTA (pH = 4.5), H3HEDTA (pH = 2.5), and H2NaHEDTA (pH = 4). The acid part which contains the hydrogen ions will attack the carbonate rock and produce CO2 that will increase the oil recovery, and then the high pH chelating agent or even sea water can be used to displace the low pH chelating agent and CO2. The experimental results showed that about 90% of the oil in place was recovered from the carbonate cores without using of surfactants or any other additives. The new method will eliminate the problem of gravity override which is the main problem of CO2 EOR. The chelating agent can be placed in the whole reservoir by introducing a low reactive form of these chemicals or by encapsulating these chemicals to start reacting with the reservoir after the placement is complete. The reaction of the encapsulated chemicals can be triggered by temperature or any other triggering mechanism.