Squeezing more oil from existing fields

Anouk Creusen and Khalid Maamari believe that the ancestor of the common ghost shrimp could hold the key to unlocking billions of barrels of oil for an energy-hungry world. Creusen, a 32-year-old Dutch geologist, and her Omani petroleum engineer colleague Maamari, 29, have been analysing the network of channels created by shrimps burrowing in the sand over 100 million years ago in the Natih oil reservoir in northern Oman.

Over time, the sand has turned to rock and the burrows – known as nodules – have hardened to form spaghetti-like structures within the oil reservoir. Substantial volumes of oil can be found both in the surrounding rock and inside the nodules. However, gradually the nodules have hardened to form a natural barrier to the movement of oil, making it difficult to extract using conventional methods. The challenge facing Creusen and Maamari is to find new ways to get it out, while leaving as little as possible behind.

The potential rewards are enormous. The Natih reservoir in Oman’s largest oilfield, 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 barrels of oil in place can be produced. The application of enhanced recovery techniques could improve the recovery rate by an extra 10%. Reservoirs with similar structures to Fahud exist throughout the world and lessons learnt here could have implications for oilfields as far away as Alberta, Canada. 

Indeed, at a time of surging global energy demand, oil companies throughout the world are looking at ways to squeeze extra barrels from ageing oilfields like Fahud. In a typical oil field, conventional production methods pump out one third of the oil, on average. The rest remains in the ground because it is too costly or difficult to extract.

Several proven techniques, however, can enhance recovery, adding significantly to the amount of oil extracted from mature fields. For instance, if oil companies increased what they expect to recover from reservoirs globally by just 1%, it could perhaps yield 20-30 billion barrels of additional oil. That’s roughly equivalent to the proven oil reserves of the USA. Moreover, today’s elevated energy prices increasingly make these techniques economically viable.

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 oilfield. 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 oilfield, 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 oilfield to drive the oil out of the rock.

“Enhanced oil recovery (EOR) technologies are not cheap and $1 billion price-tags are normal when applied to whole oilfields,” says Colin Lothian, a senior Middle East analyst at oil industry consultants Wood Mackenzie. “But despite the high cost, oil companies and energy-rich countries believe that they are a sound investment at today’s high oil prices. They are also seen as a good way to ensure that finite resources have been tapped to their fullest.”

Oman in the vanguard

Oman is a good example of a country that has been quick to grab the opportunities offered by EOR technology. It is now in the vanguard of employing enhanced recovery techniques and its experience illustrates how such techniques are likely to become commonplace in the years and decades to come. 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,” says 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.”

The team working on the Fahud project believes 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,” explains 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 500 kilometres (310 miles) 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.

Generating enough steam to heat up whole reservoirs requires massive amounts of energy. A key element of all of PDO’s steam injection projects has been to maximise energy efficiency and limit carbon dioxide (CO2) emissions. The company, which supplies electricity to isolated communities near its operations, is locating most new electricity generating plants close to its steam projects. By using the steam that’s a by-product of power generation, energy used in oil recovery can be reduced by up to 65%.

Complex geology

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. Increasingly, geologists are opting to use enhanced recovery methods early in a field’s life in cases where conventional techniques might damage the reservoir.

One such case is the Harweel field in the far south-eastern corner of Oman. Here oil has been found in multiple reservoirs encased in salt more than 4 kilometres (2.5 miles) below the surface.

With an estimated age of 500 million years, this is some of the oldest oil ever discovered, dating back to the beginning of life on the planet. Petroleum engineers believe that the deposits originated as layers of dead algae on the ocean floor, which over millions of years were covered in tightly compacted salt. Unable to escape into the surrounding rocks, the oil and natural gas in the reservoir are mixed with a large proportion of highly toxic hydrogen sulphide 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 programme from 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 hydrogen sulphide 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 gas. The reservoir contains about 50,000 parts per million (ppm) of hydrogen sulphide. Breathing concentrations of just 600 ppm would be lethal within three minutes, which makes safety an important priority for the project. Hydrogen sulphide is also highly corrosive. Pipes and other production equipment must be built using anti-corrosion materials.

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. Again, the challenge is to increase production and extend the field’s life – in this case by injecting large amounts of treated water into the reservoir to flush out the oil. The company believes that the recovery rate will rise to over 22% and that the field will continue producing oil for another 20 years.

Despite the desert conditions above ground, water supply deep underground is not a problem. As in most oil fields in the world, the oil that flows from Marmul is mixed with water. For each barrel of oil currently extracted at Marmul, eight barrels of water are produced. It is highly saline and unsuitable for agriculture, let alone for human consumption.

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 centralised plant in the world, including treatment facilities to process 80,000 cubic metres (around 2.8 million cubic feet) a day 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 towards the producing wells.

Meanwhile, back at Fahud, now that the ghost shrimp research is winding down, Creusen is thinking about her new assignment. She has applied to join the field development team that will put her ideas into practice, drilling the steam injection wells at Fahud. For his part, Maamari is hoping to move to Shell’s technology centre in Westhollow, Texas, where his experience in understanding the fluid properties of oil would be applied to other projects.

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