Enhanced recovery

Several factors have combined to force EOR to the front of the industry’s agenda worldwide. With cutting edge technology it is possible to develop heavy oil reservoirs that were not technologically feasible in the past.

High oil demands, prices, and the decline of conventional oil are all factors pushing the oil companies towards the heavy oil. The abundance of heavy oil resources is well documented, and in many cases, such as Oman, there are reduced exploration costs because a number of heavy oil reservoirs have already been discovered.

Petroleum Development Oman (PDO), a joint venture between the Government of Oman and Shell, has embarked on a major drive to reverse declining oil production.

As primary and secondary recovery techniques come to the end of their natural life, PDO is increasingly turning to Enhanced Oil Recovery (EOR) methods to extend Oman’s production plateau into the coming decades.

In 2006, the company spent in excess of US $3 billion in oil field development projects and is very likely to see continued high capital expenditure in the forth-coming years.

Shell Technology Oman (STO) was officially launched in November 2006 as Shell’s Enhanced Oil Recovery (EOR) Research and Development (R&D) Centre based in the Middle East.

This new centre is forming an integral part of Shell’s global Exploration & Production Technology organisation.

“The main change in PDO’s strategy towards EOR is due to the maturity of the company’s producing oil fields. All fields, anywhere in the world, have limited recovery from primary mechanisms and Oman is not an exception,” says Dr Xu Dong Jing, Manager of Shell Technology, Oman.

“We have now reached a point when we need to embark on tertiary recovery mechanisms which are known as enhanced oil recovery [EOR] methods.”

” However, in Oman there is an additional factor in oil production which is the existence of multiple fields with very heavy, viscous oil which needs thermal methods to heat the oil to improve recovery.”

There are three main methods of enhanced oil recovery being used by Shell, these are: Improved extraction is achieved by gas injection, thermal recovery, and chemical injection.

Gas injection is the most commonly used EOR technique. Here, gas such as carbon dioxide (CO2), natural gas, or nitrogen is injected into the reservoir whereupon it expands and thereby pushes additional oil to a production wellbore, and moreover dissolves in the oil to lower its viscosity and improves the flow rate of the oil.

Oil displacement by CO2 injection relies on the phase behaviour of CO2 and crude oil mixtures that are strongly dependent on reservoir temperature, pressure and crude oil composition.

These mechanisms range from oil swelling and viscosity reduction for injection of immiscible fluids (at low pressures) to completely miscible displacement in high-pressure applications.

In these applications, more than half and up to two-thirds of the injected CO2 returns with the produced oil and is can be re-injected into the reservoir to minimize operating costs. The remainder is trapped in the oil reservoir by interfacial tension with water, or in porous rock.

Other techniques include thermal recovery (which uses heat to improve flow rates), and, more rarely, chemical injection, where polymers are injected to increase the effectiveness of waterfloods, or the use of detergent-like surfactants to help lower the capillary pressure that often prevents oil droplets from moving through a reservoir.

“To tackle the issue we firmly believe we need capabilities in all three sectors covering miscible gas, chemical and thermal techniques,” says explains Val Brock, business development manager for EOR, Shell.

The key to productive EOR is understanding the reservoir, and bringing an integrated solution to it.

“Speaking broadly, the three main EOR techniques need to be paired with well technology analysis, smart wells and surface analysis come in to play in a much more significant way in EOR to bring a complete integrated package to the project,” adds Brock.

In Oman Shell are looking at energy efficient enhanced recovery. This is where the dependency on hydrocarbon gas is reduced. Field technology trials, research and development are critical to Shell’s presence in Oman.

The company hopes in time to be able to role out a much more comprehensive business plan for EOR field development there, and then put further groundbreaking projects into practice.

However, matching the right EOR technology to an individual field is a complex task. The available processes are not universally applicable as each field has its own unique characteristics.

Indeed, for some fields there might not exist at this moment in time an appropriate EOR method.

At the Marmul field in the south of Oman, a major polymer flooding project is currently being implemented by PDO.

The field, first discovered in 1956, has very heavy, viscous crude and it is hoped production will be substantially increased using polymer flooding as an EOR technique. It is the first polymer injection project in the region.

Polymer flooding works by adding polymer to water in order to make it thicker and more viscous before it is injected into the reservoir. Increasing the viscosity of water leads to a more effective oil sweep improving both production and the overall recovery factor.

At Harweel, in the southern corner of Oman, lies PDO’s most advanced EOR project. PDO is using a sour gas injection using gas from a nearby field. The Harweel reservoir poses several technical challenges as deep reservoirs are encased in salt.

The Harweel project currently revolves around the Zalzala oil field, which is one of a cluster of fields originally brought on stream in 2004.

The principle of the EOR technique being applied is miscible gas injection, which is based on the blending of natural gas with the crude oil in the reservoir rock to form essentially one fluid.

The single-fluid nature of the mixture makes it move much more easily through the reservoir and into the producing wells. In essence, the gas injected into the reservoir acts as an oil solvent that ‘cleans’ the reservoir of its oil.

Without the miscible-gas injection, only about 10% of the oil originally in place in the Harweel cluster reservoirs could be expected to be recovered. But with the application of miscible-gas injection, the quantity of oil recovered is may increase to nearer 40%.

The oil/gas mixture produced from the Zalzala field will be separated into oil and gas components at the processing facilities. The oil will then be transported by pipeline to the Mina al Fahal oil export terminal near Muscat.

Here, the gas will be re-compressed and re-injected again and again, and again, until as much oil as is economically feasible has been produced from the fields in the cluster.

“The gas injection projects are dictated by geography and demand, and ultimately which solution you pick becomes an economic question,” explains Brock.

“Where there is a market demand for that gas it may be preferable to exploit that, so usually hydrocarbon gas injection is done where there is a surplus of that gas. In the future the Middle East will see a move towards using CO2 so that the hydrocarbon products streams can be more efficiently used.”

The projects in Oman are actually follow-ups to initial work and analysis carried out in the 1980s. At that time the projects were put on hold because of the low price environment. In the current situation these projects have become attractive again.

In an energy constrained world optimising the use of hydrocarbon resources is vital, although the Middle East may be unique in its combination of surplus gas deposits, and maturing oil fields.

Tackling the green issue, carbon capture technology holds a great potential for enhanced recovery. Technology is being developed and coming on stream that can more efficiently trap the exhaust CO2 from heavy industry, and store that gas in the reservoir being tapped for EOR.

“Greenhouse gases, or man-made CO2 capture will play a huge part in our future. We are working on developing capturing techniques for CO2 that would otherwise be emitted, such as industrial flue exhausts,” says Brock.

The application of EOR is relatively new to the Middle East, however a combination of a sustained high oil price, maturing fields and global demand, means the technology is highly likely to become just as prevalent in this region as it has elsewhere.

The oil price is a component, but there are three hard truths all contributing to the acceleration of EOR. There is growing worldwide demand for energy.

“The reality is that there isn’t one solution to that first point and the truth is hydrocarbons are going to play a significant part in meeting energy demand for the foreseeable future, and there is also the environmental challenge, from greenhouse gases,” says Brock.

EOR techniques will transform where the next generation’s energy supply can be reached. On average worldwide, the industry only recovers about 34% of the barrels in a typical well.

With EOR trials and projects in California, Shell has seen results as high as 80%. With such factors in mind, plus an attractive market price to support implementation, it really is unacceptable not to embrace the technology that enhances our ability to extract oil.

However, it remains the case that a dominant factor in EOR roll-out is the sustained high oil price. Five years ago the long-term outlook for crude prices was still in the mid US$20 range. It’s only in the past three years that has surpass the US $50 mark.

“These projects typically have longer lead times, and what we’re seeing now is a huge amount of projects globally that are at the early stages of EOR development. I think its safe to say we’ll see a great rise in the number of EOR projects being announced in the coming years,” adds Brock.

“We’re positioning ourselves to have the best capability in all three major families in EOR technology. There isn’t one silver bullet when it comes to EOR, success will be achieved by a combination of techniques,” says Brock.

“When you consider field decline EOR is a necessity, but at the same time it takes courage and vision to push these projects through, and that is a combination we have seen in Oman,” concludes Xu Dong.