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Carbonate reservoirs Meeting u
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Carbonate Reservoirs Proved oil reserves Oil in carbonates
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Carbonate reservoirs: tth
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of world oil and gas production
Carbonate reservoirs:
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eiittyy at all scales
Carbonate reservoirs:
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8 A
lthough most energy experts agree
that the worlds energy resources
are adequate to meet this projected
growth, more reserves will be needed to delay
a peak in production. This means the petro-
leum industry will have to increase recovery
factors significantly from all types of reser-
voirs. This increase in performance can be
accelerated at the same time as reducing the
technical risk.
From the statistics below it is clear that the
relative importance of carbonate reservoirs
compared with other types of reserves will
increase dramatically during the first half of
the 21st century. Therefore, so will the value
of this market for oilfield services companies.
However, there are significant challenges
in terms of recovery due to the highly

complex internal structure and specificity
of carbonate reservoirs.
In recent years, demand for energy has surged. Currently, more
than 85% of world energy consumption comes from fossil fuels
and the World Energy Outlook shows that energy demand could
rise by 53% between now and 2030 .
Carbonate reservoirs: tth
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of world oil and gas production
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More than 60% of the worlds oil and 40% of the worlds gas reserves are held in carbonates .
The Middle East has 62% of the worlds proved conventional oil reserves
§
; approximately
70% of these reserves are in carbonate reservoirs .
The Middle East also has 40% of the worlds proved gas reserves
§
; 90% of these gas reserves
lie in carbonate reservoirs . World Energy Outlook 2006 Schlumberger Market Analysis, 2007
§
BP Statistical Review 2007 3
Proved oil reserves
§
Oil in carbonates T
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Techniques that were initially developed to
characterize sandstone reservoirs are being
applied to carbonate reservoirs, yet we know
that these types of reservoirs have different
requirements. The issues of addressing
all of the uncertainties and variables in
carbonates present great opportunities to
develop tailored technology.
Schlumberger is committed to understanding
and addressing the specific challenges and
technical risks carbonates present. We are at
the forefront of technology development into
the challenges of carbonate reservoirs, and
our continuous investment in R&D has led to
a number of product and service introductions
that are helping our customers optimize and
improve the characterization, the production
and the management of carbonate reservoirs.
World Distribution of Carbonate Reserves H
owever, it is recognized that recovery factors
are higher for sandstone reservoirs than for
carbonates. Carbonate reservoirs present a num-
ber of specific characteristics posing complex challenges
in reservoir characterization, production and management.
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Carbonates are sedimentary rocks deposited in marine
environments with clear, shallow, warm waters and
are mostly of biological origin. They are made up by frag-
ments of marine organisms, skeletons, coral, algae and
precipitation, and consist mostly of calcium carbonate,
which is chemically active compared to the sand which
makes sandstones.
Another key difference between clastic and carbonate
rocks is the distance between the site where the sediment
was created and where it was deposited. While sand and
silt may travel hundreds of miles down river systems
before deposition and lithification, the grains that comprise
carbonate sediments are usually deposited very close to
the place where they were created. This local deposition
contributes significantly to the heterogeneity of carbonate
grains. Once carbonate rock is formed, a range of chem-
ical and physical processes begins to alter the rock
structure changing fundamental characteristics such
4
Carbonate reservoirs:
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he
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eiittyy at all scales
Carbonate rocks
typically have a
complex texture
and pore network
resulting from
their depositional
history and later
diagenesis.
Heterogeneity
may exist at all
scalesin the
pores, the grains
and the textures.
The average recovery factorthe ratio
of recoverable oil to the volume of oil
originally in placefor all reservoirs
is about 35%. as porosity and permeability. This is known as diagenesis.
At deposition, carbonate sediments often have very high
porosities (35%75%) but this decreases sharply as the
sediment is altered and buried to reservoir depths. As
a result, carbonate reservoirs exhibit large and abrupt
variations in rock type distribution.
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The porosity of carbonate rocks can be grouped into three
types: connected porosity, existing between the carbonate
grains; vugs, which are unconnected pores resulting from
the dissolution of calcite by water during diagenesis; and
fracture porosity which is caused by stresses following
deposition. Diagenesis can create stylolite structures
which form horizontal flow barriers, sometimes extending
over kilometers within the reservoir, having a dramatic
effect on field performance. Fractures can be responsible
for water breakthrough, gas coning and drilling problems
such as heavy mud losses and stuck pipe.
Together, these three forms of porosity create a very com-
plex path for fluids and directly affect well productivity.
This heterogeneity also has an impact on the response of
logging measurements and therefore on the determination
of oil in place.
The complex world of carbonates:
From top to bottom; connected
porosity, vugs, fracture porosity.
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In addition to the variations in porosity, wettability is
a further heterogeneous characteristic in carbonates.
The great majority of sandstone reservoirs are strongly
water-wet. However, the aging of carbonate rock

containing water and oil turns initially water-wet rocks
into mixed-wet or even oil-wet rocks. This means that oil
can adhere to the surface of carbonate rock and it is
therefore harder to produce. Most carbonate reservoirs
are believed to have mixed wettability or to be oil-wet.
Simulations show that in reservoirs under-going water
flooding only limited amounts of oil can be recovered from
oil-wet layers because the water tends to flow mainly
through the water-wet layers. In fact, recovery factors
can be less than 10% .
Characterizing the distribution of wettability and

understanding its effects of on fluid flow within a

complex reservoir is crucial in estimating the producible
reserves and determining production strategies to

maximize recovery.
6
The water is injected in
a vertical injector on the
left and produced through
a vertical producer on
the right.
Top view: this reservoir
is water-wet and the

injection front has the
usual uniform shape.
Bottom view: the water
preferential path is through
the water-wet layers. The
oil-wet layers are bypassed,
significantly affecting
recovery.
This scenario of hetero-
geneous wettability
distribution requires
adapted production strat-
egies combined with
well-chosen enhanced oil
recovery (EOR) technologies. Montaron, B.: Increasing Oil Recovery Factors: A Technical
Challenge Key to Future World Energy Supply, AFTP
Conference, Paris, October, 2005. 7
Complex Geology, Wettability and Reservoir Dynamics
Gravity-dominated flow in water-wet reservoir
Capillary-dominated flow in layered wettability
Simulations generated using ECLIPSE* software.
Water saturated zones
Oil saturated zones 8
S
chlumberger recognizes the specific challenges of carbonate
reservoirs and has a network of research laboratories and
technology centers which is actively involved in carbonate
projects. In the Middle East, the Dhahran Carbonate Research
Center is entirely dedicated to carbonate research. Significant
research activity in flow modeling, fracture detection and seismic
services is also taking place in research centers in Cambridge,
Stavanger and Moscow, while our research center in Boston is
engaged in geology and petrophysics, and our technology centers in
Clamart and Sugar Land are developing logging-while-drilling (LWD)
and stimulation services targeting carbonate reservoirs.
Our long-term commitment to research into carbonates is enabling
us to develop and introduce solutions that are improving charac-
terization, productivity and recovery in