Expert Viewpoint - CO2 Injection and Applicability to Heavy Oil
Some people believe that CO2 injection—or as it is often described, “CO2 flooding”—can be used to enhance oil recovery from sub-surface heavy oil reservoirs. In a wide-ranging interview, Schlumberger’s scientific adviser on CO2, T.S. (Rama) Ramakrishnan, explains the complex issues surrounding this subject while separating the myths from reality.
Q. There is a view amongst the general public, as well as some scientists, that CO2 injection or flooding can also be used to enhance heavy oil recovery. How advanced is this technology?
A. Amongst the technical oilfield community, there is no such misconception. Most respectable reservoir engineers conversant with the practices of reservoir flooding realise that CO2 injection for heavy oil reservoirs is never likely to produce any significant result unless a breakthrough of some significance is achieved. However, as you say this has not stopped many members of the public prematurely holding onto the belief that CO2 injection, still holds the key to the enhanced oil recovery (EOR) of heavy oil reserves.
Q. So why is simple CO2 injection not the answer to enhancing heavy oil recovery?
A. This is probably best explained by reference to how water acts as a “driver” to assist and enhance oil production from conventional black oil reservoirs, and then comparing this to how CO2 acts.
As you know, conventional primary oil production is achieved by the de-pressurisation of oil (i.e. the expansion of oil as it races towards the surface). This de-pressurisation can, in certain circumstances, be due to natural water drive, where pressure from a large aquifer pushes the oil out of the reservoir and up the production well to the surface. Although this mechanism is the best and most efficient primary mechanism to recover oil, it is not perfect for two major reasons. Firstly, water does not sweep or contact with the oil completely, and areas of the reservoirs are simply by-passed. Secondly, water by itself is not capable of “washing” oil between particles away unless assisted by soaps or surfactants which then allows for easy displacement or for an oil-in-water microemulsion to be created; this is an exceptionally efficient production method.
However, CO2 works by employing completely different mechanisms. CO2 is partly miscible with conventional light crude oil. I say partly, because it dissolves slightly in light crude oil (which results in a partial change in the composition of the crude oil). In addition, some of the lighter components of the crude oil evaporate into the CO2 rich phase. As time progresses the compositional difference between the crude oil and the CO2 leads to a state of equilibrium where they will have completely mixed with each other, and there is no longer an interface between the CO2 and the crude oil.
Q. Is this a positive or a negative situation in terms of oil recovery?
A. It is positive….this “developed miscibility” state is desirable. You definitely want it to happen, but—and here is the crux of the matter—for it to occur at all, you need light elements in the oil that can evaporate into the CO2, and as heavy oil does not possess any light elements, it will not develop any miscibility at all.
The second drawback (and one which can never be totally satisfied) has to do with density. The general rule is that with an API (density) of over 25…and bear in mind that the higher the API value the lighter the crude….it becomes a strong candidate for CO2 enhanced recovery. Crude oils with an API value below 25 are not suitable for CO2 enhanced recovery; heavy oils have an API of 22 or below.
Q. Does this mean that CO2 injection as an aid to enhance heavy oil recovery is a non-starter?
A. No, not necessarily. As a scientist, I am reluctant to rule out anything altogether, but if CO2 injection is going to contribute to heavy oil production then a great deal more research needs to be undertaken. At the moment steam injection (or flooding) to enhance heavy oil recovery is certainly seen as the way forward.
Q. Are there any fundamental differences between heavy oil limestone and sandstone reservoirs in terms of their potential for either CO2 injection or CO2 sequestration?
A. There is no doubt that the majority of CO2 sequestration (and indeed CO2 injection) projects have focused on sandstone reservoirs. CO2 dissolves limestone (i.e. carbonate) formations and although we are still examining the long term effects that this has, initial indications are that sandstone formations are relatively inert and therefore more likely to prove to be the most suitable for CO2 injection or storage. There are enough open questions regarding limestone reservoirs, so I would not expect them to be targeted in the shorter term.
Q. Despite its contribution to CO2 emissions, does steam flooding still have a future?
A. As far as heavy oil recovery is concerned, the most effective method available is still steam assisted recovery. What the industry is working on now is how to produce this steam as efficiently as possible. One avenue of research is to look at other forms of steam generation which do not necessitate the burning of fossil fuels or at least do not have the adverse effects of burning fossil fuels.
Q. Are there any CO2 injection operations involving heavy oil reservoirs at the moment?
A. I am not aware of any occasion where a heavy oil reservoir has been commercially enhanced by CO2 injection. However, we need to bear in mind that this is all new territory, and industrial scientists are still feeling their way in what is still a very exciting and potentially dramatic development. In theory, it is a great idea; at the moment we are concentrating on how to make it work. Indeed, one possibility being explored is to capture the CO2 produced for heavy oil steam generation and pipe this to conventional light oil fields where it will be used for CO2 flooding.
Q. Who is providing the research momentum to examine the relationship between CO2 and enhancing recovery from heavy oil reservoirs? Is it governments, the oil industry, equipment manufacturers, research establishments, the general public or environmentalists?
A. The pressure is coming from all of these groups. The research for efficient heavy oil recovery is still wide open. Steam flooding is the tried and trusted method, but we need to move forward. Having said that, I do not think advances will come about by refining current practices or expanding an existing research pilot—we need a step-change vis-à-vis enhancing heavy oil recovery. Oil at $60/bbl should be enough to provide the impetus.
