The idea of gasifying (reforming) a fraction of a light crude oil in the reservoir, was conceived originally by Davidson and Yule. This involved ‘stringing’ a series of downhole gasification (DHG) units, along either a horizontal producer well or vertical producer well, or other suitable well arrangement.
We present results obtained using a small, ‘pilot-scale’ DHG unit, or reforming reactor, operated at up to 100 bar pressure. The feed to this unit was a light naphtha fraction, cut from Statfjord crude oil. The effect of pressure, catalyst loading, steam to hydrocarbon ratio and gasifier temperature were investigated.
Basically, the conversion to inert gases, principally hydrogen and carbon dioxide, at high pressure, was sufficiently high to make the process technically feasible in depleted light oil reservoirs. Furthermore, the economics of this novel enhanced oil recovery process, which also produces (and stores) hydrogen, appears to be very favourable. However, the experiments, which were conducted under pilotscale conditions, using a single-tube reactor unit, were not of sufficient duration time to test the long term effects on catalyst activity, due to carbon fouling and sulphur poisoning. These factors can be controlled (at least carbon deposition), as demonstrated in the experiments, by adjusting the steam to hydrocarbon ratio and the depth of naphtha-cut taken from the crude oil. These aspects of the process are to be further investigated in a Phase 2 project.
Davidson and Yule1 originally conceived the idea of a selfcontained, downhole gasification (DHG) unit, as a safer, ‘greener’ method for transporting oil out of environmentally sensitive areas, such as the Everglades – as gas. Subsequently, the focus was directed towards applying DHG in depleted light oil reservoirs. The inert gases generated by a DHG unit(s), are directed into a gas cap, or used to create a gas cap (Fig. 1). This would then allow gravity stabilised displacement (GSGI), WAG, or other assisted gas injection technique, to be implemented for the purpose of improved oil recovery. This is still a high priority, especially in such provinces as the North Sea, and the many 100s of depleted light oil fields in the US, where output continues to fall. Furthermore, the increasing awareness of anthropogenic emissions and their potential effect on global warming, lends emphasis to the other intrinsic benefit of DHG – generation of hydrogen in the reservoir. Thus, two important goals are achievable: improved oil recovery and hydrogen storage. Because all of the inert gases generated are stored-up in the reservoir, except for the very small dissolved amounts, the ‘carbon footprint’ of the process is considerably reduced. Secondly, the stored gas can provide a substantial source of hydrogen for the future ‘hydrogen economy’. In addition, if the hydrogen can be separated from the carbon dioxide in situ, it can then be simultaneously produced with incremental oil. Other potential benefits of the DHG process are shown in Table 1.
