Carbonate evolution

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Bernard Montaron, carbonate theme director, Schlumberger.

Bernard Montaron, carbonate theme director at Schlumberger reveals the challenges and opportunities the world's carbonate reserves hold.

The significance of carbonate reserves cannot be overstated. The combined total of carbonate and sandstone reservoirs currently stands at around 3000 billion barrels of remaining oil in place, and more than 3000 trillion cubic feet of gas.

Analysis figures from Schlumberger estimate that 60% of the world's remaining oil, and 40% of its gas reserves are held in carbonate fields.

Indeed, its no exaggeration to say that these reserves present the industry with some of the greatest challenges and opportunities to develop new technologies and processes to tackle the need for ever more energy.

Last year's BP Statistical Review estimated the Middle East plays host to 62% of the world's proved conventional oil reserves, of which more than 70% of these are held in carbonate reservoirs.

To meet rising global energy demand and consumption, sustaining production from existing fields and increasing recovery factors will be critical. To do this, it will be necessary to improve our understanding of fluid flow mechanisms and reservoir systems.

Spearheading Schlumberger's efforts in this field is Dubai-based Bernard Montaron, theme director for carbonates and naturally fractured reservoirs.

"Several years ago Schlumberger decided to assign theme directors entirely dedicated to work on a small number of business themes. Heavy oil, deep water, and carbonates are example of such focused themes," explains Montaron.

"My main mission is to facilitate the integration of Schlumberger technologies and expertise from all business segments across the entire organisation to address specific challenges and market needs in my business themes."

Montaron's role is geared around understanding technical challenges that oil and gas companies need to address today and in the future regarding carbonate reservoirs and naturally fractured reservoirs. "The role is a world-wide position, but given that the Middle East is the centre of gravity for carbonates on the planet, it makes perfect sense to be here in Dubai."

Carbonates in the Middle East

The overwhelming majority of Middle East proven oil reserves are in carbonate reservoirs, as well as 90% of its gas. The statistics are slightly skewed by the sheer size of some of the fields found here.

"The largest gas reservoir on the planet is here, between Iran and Qatar (South Pars and North Field respectively). That single field holds close to 30% of all gas reserves known in the world today," says Montaron.

For oil, the reason the figure is slightly lower is because there are extremely large sandstone reservoirs on the Arabian Peninsula. The Burgan Field in Kuwait is among the largest sandstone reservoirs in the world, as is the Safaniya field in Saudi Arabia. Ghawar is the world's largest onshore field, covering a vast land area of around 280 x 26 kilometres, and it's a carbonate reservoir.

The concentration of hydrocarbons in and around the Gulf countries is often attributed to geographical serendipity, but the formation of carbonate hydrocarbon fields in the region can be credited to atmospheric conditions, which make the formation of carbonates a near certainty.

"In this region carbonate hydrocarbons have been typically formed by the precipitation of calcium carbonate from seawater. To reach a stage where the calcium carbonates form, the sea water has to be evaporated, so the high temperatures and coastal winds create the ideal elements needed," says Montaron.

As water is removed by the heat from the sun and by wind, the concentration of minerals increases. As the carbonates are formed, the remains of marine life are trapped, such as shrimps, fish and crabs. These layers of trapped carbon grow at a rate of around half a millimetre per year in regional conditions.

This doesn't sound like much, but over 2 million years that accumulates to 1 kilometre of carbonates," says Montaron. The weight of those carbonates force it deeper and deeper, to hotter parts of the earth's crust, where it gets cooked, and oil is formed.

"Basically, in this part of the world the atmospheric conditions are ideal for carbonate formation, the heat encourages a huge amount of evaporation, and there's a lot of marine life and corals that adds carbon matter."

Whilst the geological birth of the region's oil may seem an impossibly distant point in history, the creation of carbonate deposits is still being played out in the Gulf before our eyes today. "In Abu Dhabi you can visit geological sites where carbonates are being created right now. Carbonates are formed in shallow seawater by several natural mechanisms, and there are some excellent fields where you can see the initial deposits being created."

The other interesting thing about calcium carbonates is that it's nature's own way of removing carbon dioxide (CO₂) from the atmosphere. "The CO₂ gets dissolved in the seawater and is trapped in the carbonates, so the biggest CO₂ store on the planet is in the carbonates and carbonate reservoirs."

Carbonate structures tend to pose more complex problems for the oil industry because it forms a very brittle rock that tends to crack under tectonic pressure. This cracking creates fractures and corridors that pose significant challenges to drillers, seismologists and geophysicists, as pinpointing their exact location is exceptionally difficult.

The challenges

One of the major problems with carbonate reservoirs is the lack of accurate data on production available in the public domain. Most of the time oil and gas companies consider these statistics very sensitive figures. What we do know, from reputed sources, is that the overall average recovery factor is around 35% overall for sandstone and carbonate reservoirs. It's also fairly certain that the recovery factor, on average, is lower for carbonates.

"There are many reasons behind why recovery is trickier in carbonate reservoirs, but heterogeneity is the principal problem," explains Montaron. The rocks tend to be a lot more heterogeneous than in sandstone.

Sand that follows a 100km journey from weathered rock or mountain, and is then eroded along wadi's or river beds, is then rolled by the sea, has a very homogenous character. With carbonates, they form in a much more irregular pattern along the coast, and so as the formation gets deeper, there is much greater deviation."

These deviations can be caused by a myriad of factors. Crabs may dig holes in the carbonate in one place, whereas a couple of hundred metres away a mangrove might form which is laying down vegetative matter, but leaving traces of root networks in the rocks.

Over time these will form very different structures. Added to this, when you are three kilometres deep, some of this translates into vugs, which are small cavities in a rock or vein, often with a mineral lining of different composition from that of the surrounding rock, which complicates the picture further.

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