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PDO and Shell employ high-pressure steam and other aids to raise oil production and extend the life of reservoirs in Oman.
|The PDO Marmul polymer flood project will extract 10% more heavy oil from the 6-billion-bbl field. (Illustrations courtesy of Shell|
Anouk Creusen, a 32-year-old Dutch geologist, and her Omani petroleum engineer colleague Khalid Maamari, 29, have been analyzing the network of channels created by shrimps burrowing in the sand over 100 million years ago in the Natih oil reservoir in northern Oman.
The potential rewards are enormous. The Natih reservoir in Oman’s largest oil field, Fahud, has been producing oil for more than 37 years, but still holds several billion barrels of oil. Using existing production methods, just 30% of the field’s original 6 billion bbl of oil in place can be produced. The application of enhanced recovery techniques, however, could improve the recovery rate by an extra 10%.
If oil companies increased what they expect to recover from reservoirs globally by just 1%, it could perhaps yield 20 billion to 30 billion bbl of additional oil. That’s roughly equivalent to the proven oil reserves of the United States.
Techniques to enhance recovery change the properties of the oil in the reservoir to help it flow to the well. Three approaches have so far proven especially effective. The most widely used technique is to inject steam at high pressure into the oil field. The steam heats the oil to make it thinner and thus flow better, but it also helps push the oil to wells where it can be drawn to the surface.
Another common technique involves injecting a cocktail of gases into the oil field, which mix with oil and help it flow to the well. In the third well-tested method, chemicals are mixed with water to make it thicker before it is injected into the oil field to drive the oil out of the rock.
Oman in the vanguard
Oman is a good example of a country that has been quick to grab the opportunities offered by enhanced oil recovery (EOR) technology. Petroleum Development Oman (PDO), a joint venture between the Oman government and oil companies Shell, Total and Partex, is pursuing large-scale projects using all three of the most commonly used techniques.
“All the challenges PDO is facing today, other companies will face in the future,” says Malcolm Brinded, executive director exploration & production at Shell.
Back in Fahud, Creusen and Maamari have created computer models of the Natih structures and are using them to simulate different enhanced recovery techniques. “We decided to focus on small-scale models in order to better understand the big picture,” said Maamari. “We first studied the effects of injecting water under high pressure into the reservoir but found that much of the oil would remain locked into the rocks.”
Getting all steamed up
The team working on the Fahud project believe that injecting high-pressure steam could prove the most effective approach to producing the maximum amount of oil. “By using the steam to heat the nodules, the oil will both expand and become more liquid,” explained Creusen. “As a result, the oil will break out of the nodules and flow through the rock, draining into wider fractures and to a network of horizontal production wells at the bottom of the reservoir.” This technique will soon enter a pilot phase at the Fahud field. PDO is developing another major field at Qarn Alam using the same principle.
Steam works in different ways depending on each field’s geological characteristics. More than 310 miles (500 km) south of Fahud, high-pressure steam started flowing into the Amal West field in September 2007. Unlike Fahud, which has light oil, Amal’s crude is heavy and viscous, making it hard to extract from the surrounding rock. Here the steam starts by heating the oil to make it more liquid and providing the pressure to drive it to wells, from which it is pumped. In the neighbouring Amal East field, where the oil is so thick that it will never flow of its own accord, the company will test a variant known as “steam soak” in which steam is injected for a month and the heated oil then pumped out from the well. Both fields will use temperature sensors especially designed to withstand the intense heat and high pressure within the reservoirs.
Gas cocktail to the rescue
The conditions at Fahud and Amal are just two examples of Oman’s exceptionally complex geology. The country has multiple small oil deposits, each with unique geological structures and each requiring a new approach. One such case is the Harweel field in the far southeastern corner of Oman. Here oil has been found in multiple reservoirs encased in salt more than 2.5 miles (4 km) below the surface.
Unable to escape into the surrounding rocks, the oil and natural gas in the reservoir are mixed with a large proportion of highly toxic H2S and CO2 gases and have remained under huge pressure. But trying to produce the oil using conventional methods would lead to a rapid loss of pressure and reduce the total amount of oil that could be recovered from the field. So the company has decided to embark on an enhanced oil recovery program early on.
At Harweel, the principle has been to turn the field’s biggest problem — its toxic gases — into part of the solution by re-injecting the gas into the reservoir to force out more oil. This involves separating most of the natural gas from the H2S and CO2 before pumping them back into the reservoir. The gas cocktail blends into the oil, making it more fluid while maintaining the field’s pressure and making the oil easier to produce. Using this approach, the company projects that about 30% of the oil can be recovered, compared to just 10% using conventional methods. Harweel represents a massive technical challenge because of the high concentration of toxic and highly corrosive gas.
Nothing like a new liquid formula
Where injecting steam or gas isn’t likely to work, the petroleum engineer’s toolkit includes a third method known as polymer injection, a technique in which water is mixed with chemicals to help drive the oil out of the rocks. Not far from Harweel lies one of the first fields discovered in Oman. The Marmul field has been producing a thick, viscous crude oil for the last quarter of a century. Around 15% of the field’s oil has been produced, but using conventional methods the rate at which it comes out of the ground has started to decline. The company believes that the recovery rate will rise to over 22% and that the field will continue producing oil for another 20 years.
Oil companies routinely re-inject water they pump from reservoirs to help force out more oil. However, conventional water-injection methods do not work in Marmul. The reservoir’s crude is so thick that the water tends to pass right through the oil, rather than pushing against it. The solution is to add a chemical polymer to the water to make it more viscous and help drive more oil to the producing wells. Polymer flooding has been used extensively in China and elsewhere but never before in the Middle East, where it hasn’t been judged necessary until now.
For the process to work, the re-injected water has to be treated to remove impurities such as sand, which impede the flow of the water and polymer mixture in the reservoir. PDO’s plans at the Marmul field call for the biggest centralized plant in the world, including treatment facilities to process 2.8 MMcf/d (80 Mcm/d) of water as well as a polymer injection station. The blend of polymers and water is injected into the reservoir under high pressure, cracking the rocks. This increases the volume of water that can be injected and the amount of oil pushed toward the producing wells.