What CO2-EOR Has Already Taught Us About Upscaling Permanent Carbon Sequestration
As the announcement of carbon sequestration projects becomes the norm, it’s time we look at what we know from a technical angle about how these projects need to be run based on the industry’s experience with enhanced oil recovery.
It is not often that one’s career coincides with the initiation and maturation of a technology. So it has been with my career and the use of carbon dioxide (CO2) to increase oil recovery, or CO2‑EOR (enhanced oil recovery).
I started in the E&P business in the early 1970s, about the same time as CO2-EOR became feasible (maybe economic). But in no way do I claim credit for this success; it is the result of hundreds of highly competent technologists working in labs, on computers, in the field, and planning, always planning. But I do claim a sense of perspective as the technology has matured over the years and now is about to morph to a similar one, though one with a very different objective—CO2 storage.
I am drawing comparisons between carbon capture and sequestration (CCS) and CO2-EOR that are largely based on analyzing field data, not numerical models nor laboratory-scale experiments, though both have played important supporting roles.
Many of the oilfield technologies are transferable from CO2-EOR to CCS. Technologies such as petrophysics, numerical simulation, and anything relating to injection wells should apply. This fact should make it easier for petroleum engineers to make the transition to CCS. Experience from fluid production provides valuable insight for fluid movement and displacement in the reservoir, but production is not directly transferable to conventional CCS, which does not envision the operation of extraction wells as of this writing.
There have been no detectable caprock breaches and few surface leaks in CO2-EOR. This observation has resulted in the need for minimal monitoring requirements. However, monitoring for CCS will be a major part of a project. Ascertaining the amount of CO2 retained in subsurface during CO2-EOR is difficult using data routinely available to the public. The importance of accurate measurements of rates and pressures at wellhead and surface facilities will be even more important for CCS and over a longer time.
The timescales of explicit concern between CCS and CO2-EOR differ. The length of time for injection for CCS and CO2-EOR (decades) are about the same; CCS is far more concerned with post-injection duration (centuries). After CO2-EOR there is a blowdown phase as a reservoir moves onto another recovery process or is abandoned altogether. In contrast, CCS expends much more time and more effort on the post-injection than on the injection period, with significant monitoring and verification activity required to evaluate the status of the stored CO2.
The importance of external boundaries differs between CCS and CO2‑EOR. In both technologies the presence of boundaries is important, and for the same reason: to contain the CO2 within the reservoir. (An oil reservoir must ipso facto have at least one boundary or there would be no accumulation—an argument for storing CO2 in depleted oil reservoirs.) However, the presence of multiple producers in CO2-EOR means that the average pressure in the reservoir will attain a steady state, or close to it. In CCS the presence of impermeable boundaries means that pressure will increase with time, ultimately reducing the ability to inject. This observation means that the storage capacity of a reservoir will be much less than that inferred by volumetric calculation. Avoiding this pressure buildup is an argument for placing a few producers in the reservoir to mitigate against it.
CO2-EOR is subject to small volumetric sweep efficiency because of large CO2 mobility and formation heterogeneity. Therefore, the CO2-EOR methods to improve volumetric sweep efficiency are valuable. It may be, however, because of the boundary effect mentioned earlier that injection that pressures up reservoirs would serve to lessen these effects. The technology of EOR, being largely based on steady-state flow, should be re-evaluated for compressible, bounded flow.
The source of the CO2 differs. Most of the CO2 for CO2-EOR is from naturally occurring CO2 sources that require little purification. A substantial quantity of injected CO2 is separated onsite from produced CO2. Although there is always room for improvement in this separation (e.g., there are a few CO2-EOR projects using human-made sources). It, like nearly everything associated with CO2-EOR, is mature; its expense is born by the cost of the project. The CO2 for CCS is largely the exhaust of industrial process, coal-fired power plants being preeminent. The CO2 concentration from these processes is relatively small, which means that the separation of it from these gas streams is quite expensive, usually the major expense of any project. There has been some interest in injecting these streams directly in a storage reservoir, but this would take up valuable pore space with a non-CO2 gas.
The most obvious difference between CCS and CO2-EOR lies in economics. CO2-EOR produces a product of commerce, something that can be sold; CCS does not, at least as of this writing. Moreover, there is no economic penalty in CO2‑EOR for not recycling CO2 other than the cost of makeup CO2, while CCS would be directly penalized were CO2 to escape from the storage reservoir. The solutions to this range from far out such as to find an end use for CO2, to possible but expensive, such as some sort of tax or tax credit, to financial products, or a barter and trade system. Basically, someone must pay for CCS, and it will probably be us through some public entity. My favorite analogy is how we pay our cities for garbage collection. Why not CO2?
Finally, an obvious product of commerce and storage is CO2-EOR itself. This can be a bit tricky because most of the crude oil produced is burned in one form or another which, by itself, contributes to greenhouse gases. Statistics show that CO2-EOR projects retain about 7 Mcf of CO2 (about 0.4 tonnes) for each barrel of oil produced, with (of course) large variations. Curiously this value is very close to being carbon neutral (i.e., equal amounts of CO2 are stored and released). In other words, instead of considering CCS storage and CO2-EOR as competitors, they should be viewed as complementary technologies. Most CO2-EOR projects are not designed with storage in mind; however, it may be that some minor adjustments to design would make this CCS/CO2‑EOR advantageous.
CO2-EOR has been a success in the US, spurred on by availability of good, reliable sources, and favorable economics. The business is likely to grow in the US as the technology moves into unconventional reservoirs and outside the US as other sources and separation technology become cheaper.
Most of the technology from CO2-EOR applies to CCS. The bottom line is we know how to separate and store CO2. Our remaining issues are political and economical.
Larry W. Lake is a professor in the Department of Petroleum and Geosystems Engineering at The University of Texas at Austin where he holds the Shahid and Sharon Ullah Endowed Chair. He holds BSE and PhD degrees in chemical engineering from Arizona State University and Rice University, respectively. He is the author or coauthor of more than 100 technical papers, four textbooks, and the editor of three bound volumes. Lake has served on the SPE International Board of Directors and received the 1996 SPE/AIME Anthony F. Lucas Gold Medal, the 2002 DeGolyer Distinguished Service Medal, and the 2000 SPE IOR Pioneer Award. He has been a member of the US National Academy of Engineers since 1997.