carbonate evolution

7/27/2019 Carbonate Evolution

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CARBONATE EVOLUTION

26 Oil&Gas Middle East August 2008

Bernard Montaron,

carbonate theme director,

Schlumberger.


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CARBONATE EVOLUTION

August 2008 Oil&Gas Middle East 27www.arabianbusiness.com/energy

The significance of carbonate

reserves cannot be over-

stated. The combined total

of carbonate and sandstone

reservoirs currently stands at around3000 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 worlds 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 great-

est challenges and opportunities to

develop new technologies and proc-

esses to tackle the need for ever

more energy.

Last years BP Statistical Review

estimated the Middle East plays host

to 62% of the worlds proved conven-

tional oil reserves, of which more

than 70% of these are held in carbon-

ate reservoirs.

To meet rising global energy

demand and consumption, sustain-

ing production from existing fields

and increasing recovery factors will

be critical. To do this, it will be nec-

essary to improve our understanding

of fluid flow mechanisms and reser-

voir systems.

Spearheading Schlumbergers

efforts in this field is Dubai-based

Bernard Montaron, theme director

for carbonates and naturally frac-

tured reservoirs.

Several years ago Schlumberger

decided to assign theme directorsentirely dedicated to work on a small

Carbonate Evolutionnumber of business themes. Heavy

oil, deep water, and carbonates are

example of such focused themes,

explains Montaron.

My main mission is to facilitatethe integration of Schlumberger

technologies and expertise from all

business segments across the entire

organisation to address specific chal-

lenges and market needs in my busi-

ness themes.

Montarons 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 cen-

tre 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 Mid-

dle 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 reser-voirs on the Arabian Peninsula. The

Bernard Montaron, carbonate theme director at Schlumberger revealsthe challenges and opportunities the worlds carbonate reserves hold.

In this part of the world the atmospheric

conditions are ideal for carbonates. The

heat encourages a huge amount of

evaporation, and theres a lot of marine

life and corals that adds carbon matter.

Burgan Field in Kuwait is among the

largest sandstone reservoirs in the

world, as is the Safaniya field in Saudi

Arabia. Ghawar is the worlds largest

onshore field, covering a vast landarea of around 280 x 26 kilometres,

and its a carbonate reservoir.

The concentration of hydrocar-

bons in and around the Gulf coun-

tries is often attributed to geographi-

cal serendipity, but the formation of

carbonate hydrocarbon fields in the

region can be credited to atmos-

pheric conditions, which make the

formation of carbonates a near cer-

tainty.

In this region carbonate hydro-

carbons have been typically formed

by the precipitation of calcium car-

bonate from seawater. To reach a

stage where the calcium carbonates

form, the sea water has to be evapo-

rated, so the high temperatures and

coastal winds create the ideal ele-

ments needed, says Montaron.

As water is removed by the heat

from the sun and by wind, the concen-

tration of minerals increases. As the

carbonates are formed, the remains

of marine life are trapped, such as

shrimps, fish and crabs. These lay-

ers of trapped carbon grow at a rate

of around half a millimetre per year

in regional conditions. This doesnt

sound like much, but over 2 millionyears that accumulates to 1 kilome-

tre of carbonates, says Montaron.

The weight of those carbonates force

it deeper and deeper, to hotter parts

of the earths 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 evapo-

ration, and theres a lot of marine life

and corals that adds carbon matter.

Whilst the geological birth of the

regions 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. Carbon-

ates are formed in shallow seawa-

ter by several natural mechanisms,

and there are some excellent fields

where you can see the initial deposits

being created.


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CARBONATE EVOLUTION

28 Oil&Gas Middle East August 2008

The other interesting thing about

calcium carbonates is that its natures

own way of removing carbon dioxide

(CO2) from the atmosphere. The

CO2 gets dissolved in the seawaterand is trapped in the carbonates, so

the biggest CO2store on the planet is

in the carbonates and carbonate res-

ervoirs.

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 cre-

ates fractures and corridors that pose

significant challenges to drillers, seis-

mologists and geophysicists, as pin-pointing their exact location is excep-

tionally difficult.

THE CHALLENGES

One of the major problems with car-

bonate reservoirs is the lack of accu-

rate 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 sand-

stone and carbonate reservoirs. Its

also fairly certain that the recovery

factor, on average, is lower for car-

bonates.

There are many reasons behind

why recovery is trickier in carbon-

ate reservoirs, but heterogeneity

is the principal problem, explains

Montaron. The rocks tend to be a lot

more heterogeneous than in sand-

stone. Sand that follows a 100km

journey from weathered rock or

mountain, and is then eroded along

wadis or river beds, is then rolled

by the sea, has a very homogenous

character. With carbonates, they

form in a much more irregular pat-

tern 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 digholes 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 verydifferent 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 compli-

cates the picture further.

On top of all these factors impact-

ing the carbonate structures is the

issue of tectonic activity. When the

plates beneath us move they tendto buckle, bend or crack under the

huge pressure. This forms fractures

and these structures can have a

huge impact on the recovery factors.

When seawater is injected to push

the oil towards the production well,

under natural pressure it may go into

these cracks and get directed away

from the matrix or body of oil you are

targeting.

Heterogeneity is the number one

challenge when dealing with carbon-

ate reservoirs. The secret to maxim-

ising production in these fields lies

with accurately mastering the reser-

voir description. Being able to locate

where those big fractures are, know-

ing how the different rock types are

distributing the reservoir, and from

that being able to create a complete

description of the permeability of

the reservoir is making a step in the

right direction.

Ultimately, if this can be achieved,

then placing the wells and pinpoint-

ing where to inject water or gas will

have a potentially massive impact on

the production yield.

FIELD EVALUATION

A reservoir may be 30 km East

West and 50 km North South, and

maybe 1 kilometre deep. When you

are looking at a field on this scale its

vital to understand how the fluids aregoing to move within it.

A good analogy is to imagine the

reservoir as a roadmap of a country,

and understanding the vehicular traf-

fic through it, says Montaron. For

example, throughout a country therewill be lots of small roads running all

over, but thats not an efficient means

of getting around. If any significant

distance is going to be covered, then

highways are the quicker routes. Just

like a road map, within the matrix of

the carbonate reservoir are fractures,

which act as highways. Most of the

movement happens along these frac-

ture corridors formed by tectonic

activity. Some of these major arteries

could be 10 metres wide, three kilo-metres long, and if fracture corridors

can be found and tapped, then you

hit these highway for fluids.

When drilling, if by sheer luck,

the vertical well hits the fracture cor-

ridor then you have an area where

permeability and the flow of oil

from all the surrounding cracks will

migrate - essentially taking advan-

tage of a giant natural feature ofthe reservoir to drain all of the oil

towards the well.

However, if you drill just a hun-

dred metres away you could be in

a very tight matrix of the reservoir,

and the productivity of the well could

be 50 times less that the one that

struck a fracture corridor. Of course,

it could still produce 2000 bpd, but

if you found the natural highway for

the oil, it would be more like 20 000

bpd.In carbonate reservoirs it can be

quite common that out of 25 wells

drilled, just one of those may pro-

duce 60% of the total yield, and this is

an illustration of the difference strik-

Qatars vast North Field and Irans South Pars hold 30% of the worlds known natural gas reserves.


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CARBONATE EVOLUTION

ing a corridor could make.

Of course, on the other hand, it

could also happen that the fracture

corridor is connected to water, in

which case your well will produce

and recycle a lot of water, which is far

from ideal, so understanding which

fluids these structures will bring to

your wells is essential.

Knowing that the fracture corri-

dors act as conduits for oil or water

is just the first step. The major prob-

lem is that in the scheme of an oilfield

seismic report, these fractures are

very difficult, almost invisible to see

on seismic images.

Understanding the big structures

and taking advantage of high-resolu-

tion seismic imaging, and new work-

flows to characterise the reservoir is

a fundamental step towards improv-ing recovery yields.

Its only very recently with seis-

mic weve been able to see these

fracture corridors. The seismic sig-

nature is very small, and they are

easily confused with noise. To get a

picture of these structures you cant

filter anything out of the pictures; you

want all the noise and a very smart

way of pinpointing these hard to see

structures.

Fracture corridors are impos-sible to see with the naked eye and

even the best seismologists will only

be able to see the folds above, but the

corridors remain near invisible.

Smart processing of high resolu-

tion seismic a new workflow called

FCM for fracture cluster mapping -

will show you these structures, but

you need to understand each rock

layer within the reservoir. Compre-

hensive, detailed reservoir charac-

terisation is the answer. Having a

complete picture of what youre deal-

ing with is absolutely crucial.

This is achieved through full

data integration. Seismic will give

you an image of the big and interest-

ing structural features, but then you

have several other tools such as wire-

line data logging.

Precision monitoring of the

downhole conditions is the best way

of understanding the rock types andthe drilling environment.

An example of this data inte-

gration is available through West-

ernGecos Q-Technology surveys.

Each survey is a unique combina-

tion of acquisition, processing, and

inversion technologies required to

produce solutions to reservoir prob-

lems, whether defining reservoir

geometry, characterising reservoir

properties, or monitoring fluid move-

ments. Acquiring the right surveythe first time increases its value as a

reservoir-management tool at every

subsequent stage in the fields life.

These technologies are rather

more advanced than standard seis-

mic surveillance methods because

they record the data from individual

seismic detectors without summa-

tion. The seismic wavelet is control-

led through source signature moni-

toring. Together, this results in the

highest quality seismic fidelity and

the maximum suppression of noise.

Hence, subsequent Well-Driven

Seismic processes such as well cali-

bration, inversion, and classification

techniques all produce more accu-

rate results.

Using this sort of technology

enables incredibly powerful data

analysis which in turn generates res-

olution that could not be generated

by more traditional bundled seismicsystems, says Montaron.

A new workflow has been devel-

oped by Schlumberger to improve

fracture characterization and to effec-

tively model carbonate reservoirs.

The FCM Fracture Cluster Mapping

workflow integrates Q-Technology

services, borehole measurements

and Petrel seismic-to-simulation soft-

ware with expert interpretation and

flexible work processes, resulting in

improved production performanceThe workflow helps production

engineers make better decisions for

the location of injectors and produc-

ers, plan well trajectories, improve

production predictions and form a

comprehensive Discrete Fracture

Network (DFN) model. It makes

a clear distinction between diffuse

fractures that can be modeled using

geo-statistical techniques, and frac-

ture corridor highways that must be

detected and placed in the reservoir

model at their exact field location.

This year Schlumberger intro-

duced Carbonate Advisor petrophys-

ics and productivity analysis serv-

ice. Carbonate Advisor offers a sys-

tematic analytical framework to effi-

Bernard Montaron is based in Dubai, UAE.

Global Experts, Local Solutions

Setting Performance Standards by

Flow lines, Tanks, Bunds, Structures, On shore and offshore Applications

Guaranteed and Insured solutions in: Anti Corrosion WaterproofingEnvironmental Protection Pipe CoatingsAsset Longevity Insulated CompositesBlast Mitigation and Protection Primary & Secondary Containment

Tel: 971 4 320 4342 Fax: 971 4 320 4401 P.O.Box 126274 Dubai United Arab Emirates email: [email protected] website: www.ncs-me.net


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CARBONATE EVOLUTION

ciently deliver a timely, comprehen-

sive petrophysical evaluation of car-

bonate rocks. This integrates infor-

mation from magnetic resonance

and elemental capture spectroscopy,

as well as other logs and core data, to

produce a single, complete formation

evaluation of carbonate reservoirs.

CARBONATES AND

ENHANCED OIL RECOVERY

Most of the data available from pilot

tests worldwide, stretching back

over the last three decades reveals

very few EOR pilot studies have

been in carbonates. The established

methods of EOR, such as polymer or

surfactant injection work very well

with sandstone reservoirs because

these chemicals are employed veryefficiently when pushing the oil out

because the oil mobility is higher.

Porosity and permeability are higher,

but in carbonates, surfactant chemi-

cals are needed because the oil sticks

to the rock so by definition you will

have to inject thousands of tonnes of

these chemicals which will stick on

the rock surface, which raises the

problem of profitability and economi-

cal production.

However, Montaron says in the

next five years the industry will

undertake a great many more pilots

for EOR in carbonate fields.

Already we are seeing some

interesting results from CO2injection

in carbonates. People are increas-

ingly experimental with surfactants,

CO2, and some very clever tech-

niques to find the best, which may bea combination of several methods.

Pilots in carbonate reservoirs can

be expensive. Each can last a course

of several years. In that time you

have to inject thousdands of tonnes

of chemicals, at a cost of hundredsof millions of dollars, which is affect-

ing your bottom line. The outcome is

uncertain, so theres a financial risk

element.

What we are aiming to do is to

bring down the cost of running pilots.

Imagine if the cost could be capped

and some tangible answers obtained

in less than six months, then the

whole venture looks a lot more

appealing. Within a timeframe of just

one year, three different processescould be investigated and a realistic

picture of where you want to invest in

the future can be established.

These mini-pilot schemes aim to

have enough scope to capture the

heterogeneity of the reservoir, and

enable enough data capture to ana-

lyse everything that is happening in

the downhole environment, so that

recovery factors can be accurately

gauged.

The possibilities for pilot stud-

ies are exciting, says Montaron. If

ultimately CO2

works best this could

really change the landscape of car-

bon capture in the Gulf countries.

LOOKING AHEAD

A casual analysis of worldwide oil

reserves and yearly production

reveals some quite staggering fig-

ures. Currently we are producing

roughly 87 million bpd 32 billion

barrels per year. This means, every

year the industry has to find twice

the remaining volume of oil in the

North Sea just to meet reserve-

replace targets.

Out of this 32 billion barrels pro-

duced each year almost 22 billion

barrels are coming out of sandstone

reservoirs. We are emptying the

sandstone reserves much faster than

the carbonate fields. The reserves

and production ration in sandstonefields have around 20 years produc-

tion time left. The proven and proba-

ble reserves in carbonate fields have

around 80 years production left, so

around four times more.

This means that as time goesby the market share of carbonate

reserves is increasing.

Because we are emptying the

sandstone reservoirs so fast it is pos-

sible that in 30 years we could be in

a situation where 75% of the global

reserves are in carbonates.

Traditionally we had been replac-

ing reserves with more discoveries

in sandstone than in carbonates. But

something happened in 2007 that

changed all of that, says Montaron.An oil and gas company discov-

ered a field 300 kilometres offshore

of Rio de Janeiro in the Santos Basin,

which was a truly huge carbonate

field below a thick salt layer.

The hydrocarbons in the Tupi

and Carioca Field are an ultra-deep

water environment buried beneath

2000 metres of water, 1000 metres of

rock, and 2000 metres of salt.

We dont know exactly how much

is there, but its a huge amount of oil.

This field is particularly interesting

because it is below a huge deposit of

salt, which is a perfect seal.

That means the entire hydrocar-

bon reserve that was there before

is still held in place. There are

no leaks.

There are plenty of areas world-

wide where there are large salt

deposits that fit a similar profile.

Offshore Angola and the Mediter-

ranean spring to mind. Six million

years ago the Mediterranean region

was almost isolated from the Atlan-

tic, and it evaporated leaving a mas-

sive layer of salt. Its not impossible

that we could discover a vast amount

of oil underneath these subsea

salt deposits. This sort of well was

not economical five years ago, but

with the price where it is now, they

are very viable. These sub-salt car-

bonate territories may well be thenext exploration boom.

It is possible that in just 30 years we

could be in a situation where 75% of

the global reserves are in carbonates.

SaudiAramco

KSA has mix of sandstone and carbonate reserves, but the sandstone fields are being depleted faster.

carbonate evolution

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