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  • Meeting uunniiqquuee cchhaalllleennggeess to maximize recovery

    Carbonate Reservoirs

  • Proved oil reserves

    Oil in carbonates

  • 1

    Carbonate reservoirs: tthhee ffuuttuurree of world oil and gas production

    Carbonate reservoirs: hheetteerrooggeenneeiittyy at all scales

    Carbonate reservoirs: a ccoonnttiinnuuoouuss ffooccuuss




  • Although most energy experts agreethat the worlds energy resources are adequate to meet this projectedgrowth, more reserves will be needed to delaya peak in production. This means the petro-leum industry will have to increase recoveryfactors significantly from all types of reser-voirs. This increase in performance can beaccelerated at the same time as reducing thetechnical risk.

    From the statistics below it is clear that therelative importance of carbonate reservoirscompared with other types of reserves willincrease dramatically during the first half ofthe 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, morethan 85% of world energy consumption comes from fossil fuelsand the World Energy Outlook shows that energy demand couldrise by 53% between now and 2030.

    Carbonate reservoirs: tthhee ffuuttuurree of world oil and gas production



    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 reserveslie in carbonate reservoirs.

    World Energy Outlook 2006 Schlumberger Market Analysis, 2007 BP Statistical Review 2007

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    Proved oil reserves

    Oil in carbonates


    Techniques that were initially developed tocharacterize sandstone reservoirs are beingapplied to carbonate reservoirs, yet we knowthat these types of reservoirs have differentrequirements. The issues of addressing all of the uncertainties and variables incarbonates present great opportunities todevelop tailored technology.

    Schlumberger is committed to understandingand addressing the specific challenges andtechnical risks carbonates present. We are atthe forefront of technology development intothe challenges of carbonate reservoirs, andour continuous investment in R&D has led to a number of product and service introductionsthat are helping our customers optimize andimprove the characterization, the productionand the management of carbonate reservoirs.

    World Distribution of Carbonate Reserves

  • However, 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 challengesin reservoir characterization, production and management.


    Carbonates are sedimentary rocks deposited in marineenvironments with clear, shallow, warm waters and are mostly of biological origin. They are made up by frag-ments of marine organisms, skeletons, coral, algae andprecipitation, and consist mostly of calcium carbonate,which is chemically active compared to the sand whichmakes sandstones.

    Another key difference between clastic and carbonaterocks is the distance between the site where the sedimentwas created and where it was deposited. While sand andsilt may travel hundreds of miles down river systemsbefore deposition and lithification, the grains that comprisecarbonate sediments are usually deposited very close tothe place where they were created. This local depositioncontributes significantly to the heterogeneity of carbonategrains. Once carbonate rock is formed, a range of chem-ical and physical processes begins to alter the rock structure changing fundamental characteristics such


    Carbonate reservoirs: hheetteerrooggeenneeiittyy at all scales

    Carbonate rockstypically have acomplex textureand pore networkresulting fromtheir depositionalhistory and laterdiagenesis.Heterogeneitymay exist at allscalesin thepores, the grainsand 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 highporosities (35%75%) but this decreases sharply as thesediment is altered and buried to reservoir depths. As a result, carbonate reservoirs exhibit large and abruptvariations in rock type distribution.


    The porosity of carbonate rocks can be grouped into threetypes: connected porosity, existing between the carbonategrains; vugs, which are unconnected pores resulting fromthe dissolution of calcite by water during diagenesis; andfracture porosity which is caused by stresses followingdeposition. Diagenesis can create stylolite structureswhich form horizontal flow barriers, sometimes extendingover kilometers within the reservoir, having a dramaticeffect on field performance. Fractures can be responsible for water breakthrough, gas coning and drilling problemssuch 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 oflogging measurements and therefore on the determinationof oil in place.

    The complex world of carbonates:From top to bottom; connectedporosity, vugs, fracture porosity.



    In addition to the variations in porosity, wettability is a further heterogeneous characteristic in carbonates. The great majority of sandstone reservoirs are stronglywater-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 oilcan adhere to the surface of carbonate rock and it istherefore harder to produce. Most carbonate reservoirsare believed to have mixed wettability or to be oil-wet.

    Simulations show that in reservoirs under-going waterflooding only limited amounts of oil can be recovered fromoil-wet layers because the water tends to flow mainlythrough 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 produciblereserves and determining production strategies to maximize recovery.


    The water is injected in a vertical injector on theleft 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 waterpreferential path is throughthe water-wet layers. Theoil-wet layers are bypassed,significantly affectingrecovery.

    This scenario of hetero-geneous wettability distribution requires adapted production strat-egies combined with well-chosen enhanced oilrecovery (EOR) technologies.

    Montaron, B.: Increasing Oil Recovery Factors: A TechnicalChallenge Key to Future World Energy Supply, AFTPConference, Paris, October, 2005.

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    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

    Schlumberger recognizes the specific challenges of carbonatereservoirs and has a network of research laboratories andtechnology centers which is actively involved in carbonateprojects. In the Middle East, the Dhahran Carbonate ResearchCenter is entirely dedicated to carbonate research. Significantresearch activity in flow modeling, fracture detection and seismicservices is also taking place in research centers in Cambridge,Stavanger and Moscow, while our research center in Boston isengaged in geology and petrophysics, and our technology centers inClamart and Sugar Land are developing logging-while-drilling (LWD)and stimulation services targeting carbonate reservoirs.

    Our long-term commitment to research into carbonates is enablingus to develop and introduce solutions that are improving charac-terization, productivity and recovery in carbonate reservoirs. Ourregional technology centers support this commitment. The center in Abu Dhabi is focused on carbonates and enables customers towork alongside our experts to solve specific regional challenges.

    Carbonate reservoirs: a ccoonnttiinnuuoouuss ffooccuuss

    The DhahranCarbonate ResearchCenter (above) is entirely dedicated to understanding carbonates and developing global solutions.

    The worldwide net-work of Schlumbergerresearch laboratoriesand technology cen-ters (above right).

    Sustaining global oil and gas demandsrequires advanced and appropriate oilfield technology in carbonate reservoirs.


    Most carbonate reservoirs are naturally fractured. The fractures exist at all scales,from microscopic fissures to kilometer sizedstructures called fracture swarms or corri-dors, creating complex flow networks in thereservoir. As a consequence, the movement of hydrocarbons and other fluids is often notas expected or predicted. Just a few verylar