Carbon Dioxide Enhanced Oil Recovery (CO2-EOR) is gaining more traction around the world, not only because of its favourable economics but also as part of sustainable practices, as well as preserving natural gas currently used for gas injection, for power generation purposes. Until recently, in any CO2 EOR project naturally occurring CO2 was used. However, with modern carbon-capturing and storage (CCS) technologies, whereby CO2 generated by the power industry and other energy-intensive manufacturing, such as steel and aluminium, is captured, stored and can be used.
The principle of CO2 EOR is common to many other ways of secondary and primary recovery methods. CO2 is injected into a reservoir to space between the rocks to push the oil out. One of the technical reasons why injecting CO2 is beneficial is due to its ability to mix with oil, also known as miscible with oil. Why it is required? Oil and water cannot be mixed into a homogeneous liquid, which in turn makes it less effective when water is used to push the oil. However, if the injection material (CO2) is miscible with the oil, it will become a homogenous mixture that will allow the forces of injected CO2 to be used effectively in "detaching" the oil from the rock and moving it more easily towards a producing well. The miscibility is achieved when high density (compressed) CO2 is used and with light crude oil.
The system works as a closed-loop, whereby CO2 coming to the surface with oil, is separated and re-injected back to the formation. The pictures below show the complete process.
The utilization of CO2 EOR requires is a significant amount of planning and engineering, not only due to down-hole conditions and reservoir modelling but as a result of the extremely corrosive nature of CO2. Well design, handling facilities and alike, all must be able to handle pure CO2. This includes utilization of corrosion-resistant materials, such as stainless steel and various grades of superalloy steel, elastomers (rubber) materials for packers, internally coated and fibreglass lining for tubing and many more. This, in turn, hugely affects the costs and lead times.
While onshore, the logistics make it easier to conduct a CO2 EOR project, in the offshore environment it is more complex, challenging and costly, due to the fact that existing infrastructure was not designed to accommodate it, in terms of weight, space, power, and materials used in the equipment installed.
Depending on the size of the project, a specialized platform might be required to embrace offshore CO2 EOR projects. There are a number of constraints that should be considered and planned in advance, such as tanks and storage capacity, manning capacity to accommodate the required crew, pumping and supply capacity. In general, best practices and facilities for onshore CO2 EOR may not apply offshore.
Enhanced Oil Recovery (EOR) is a set of activities and techniques designed to reduce oil saturation and increase the oil recovery rate. In general, oil production is divided into 3 stages: primary, secondary and tertiary (improved). The tertiary method is also called Enhanced Oil Recovery (EOR).
EOR is used when the primary (natural flow) and secondary (water and gas injection) methods are no longer effective and normally leave around 60-80% of oil unrecovered. Using EOR can help to recover more oil, and help to achieve higher recovery rates. For gas wells, the primary production method is normally enough to produce around 80% of the reserves. Hence, EOR is used in crude oil production.
EOR techniques are very costly and the price of oil is a key factor for an operator when decisions are made on what EOR technique to implement. What could be technically feasible, might be expensive and not economical. In addition, there is always a dilemma between exploration (to find more oil) and EOR, and sometimes exploration risks are too high, thus companies choose the EOR approach instead. In addition, the pace of new oil discoveries and their size has been declining; hence producing more oil from existing fields is dominant thinking for a few years to come.
Screening criteria on which EOR method to deploy require a detailed analysis of a particular field and its location, reasons of lower recovery rate following primary and secondary methods, is conducted to understand the strategy and what EOR approach to applying. Due to a number of reasons, both technical and commercial, EOR makes economic sense in larger fields.
Offshore EOR makes it even more challenging and costly, due to the fact that existing infrastructure was not designed to accommodate equipment packages associated with it in terms of weight, space, treatment facilities and power generation. In addition, due to the well-placement offshore and the distance between the wells, any EOR campaign will require more time to have a noticeable effect. On average an EOR project takes 1-7 years till it is fully deployed. Laboratory testing and pilot projects are prerequisites of any full-scale EOR development and can take up to 4 years to complete.
EOR is divided into 4 distinctive categories as shown below.
Each EOR technique has its own use in and it is hard to be definitive with any particular EOR technique, as every method unique enough to work better in some cases than the others. Below are a guide and major characteristics of available EOR methods.
Worldwide, EOR contributes to less than 2% of global crude production. CO2- EOR is the largest and most widely used EOR method, followed by Thermal injection and Gas Injection.
After long years of production, the recovery rate in most of the fields in the GCC is coming to a point, whereby focusing on EOR considerations become more and more important - most oil fields in the region have been under primary and secondary production for decades. Although the fields do not require any EOR yet, some of the EOR projects in the GCC are one of the world's biggest and challenging, whereby the most advanced technology is used. Those include steam injection and polymer flooding in Oman, gas injection in UAE and CO2 pilot projects in UAE, Saudi Arabia, and Qatar, with more steam injection projects on the way in Kuwait. Oman is the regional leader in EOR projects and has implemented thermal, chemical and gas injections projects. It is estimated that by 2020, EOR production in Oman will be around 22% (PDO).
As a result of the growing population and efforts to maintain and increase oil production in the GCC, it is becoming more challenging to meet the demand for natural gas to cater to power generation, gas re-injection projects and steam generation. Hence, the focus on other EOR methods, such as chemical and CO2, will be evident in the years to come.
CO2 EOR projects are gaining more traction, not only because of their favourable economics but also as part of sustainable practices, as well as preserving natural gas used for gas injection, for power generation purposes and alike. There have been a number of research centres established in UAE, Qatar and Saudi Arabia to make progress in applying CO2 EOR in the region. NOCs in the region are taking proactive steps to establish a knowledge base and expertise ahead of time.
EOR is challenging and time-consuming and ineffective screening may result in lower-than-expected recovery rates. It is a common practice to conduct small trial projects to understand which EOR technique delivers the best project economics. In addition, the environmental impact and effects of chemicals could be best understood in smaller projects.
Some of the giant fields in GCC will not require EOR for the next 20-30 years. However, certain fields may need one in the next 3-5 years. The chart below represents the projected growth in EOR production worldwide. Growth in EOR production in GCC is evident and represents a notable proportion of GCC oil production.