Confronting Carbonates
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Bernard Montaron, Theme Director of Carbonates at Schlumberger writes on the challenges of carbonate reservoirs. 132 Oil Review Middle East Issue Five 2008 E E NHANCED OIL RECOVERY is a particularly significant factor for carbonate reservoirs, since traditionally recovery tends to be low from these fields. Bernard Montaron of Schlumberger explains. Carbonate reservoirs hold around 60 per cent of the world’s oil and 40 per cent of the world’s gas. However, the heterogeneous nature of the rock presents challenges for petroleum engineers. To help address exploration and production issues specific to carbonate fields, Schlumberger has set up a dedicated carbonate R&D centre in Dhahran. This is a region which holds 62 per cent of the world’s hydrocarbon reserves. In fact, the Middle East is dominated by carbonate fields, with around 70 per cent of oil and 90 per cent of gas reserves held within these reservoirs. Carbonate formations Carbonate reservoirs present a challenge, in part, due to the history of the rock formation. Carbonates rocks are formed locally by the deposition of minute marine organisms, coral and algae, together with the precipitation of calcium carbonate in shallow, warm water. By contrast, the sand and sediment that form sandstone reservoirs tends to be swept long distances by rivers and currents before settling in sites such as deltas and beaches. One important difference between these two reservoirs types is that sand (a silicon compound) is chemically inert when compared to calcium carbonate. Carbonate rock initially has a high porosity of 35 per cent - 75 per cent, but this decreases sharply as the rock ages. Over Abu Dhabi International Petroleum Exhibition & Conference ADIPEC 2008 The complex world of carbonates: vugs Confronting carbonates United Arab Emirates | Qatar | Kuwait | Saudi Arabia | Oman | Egypt | Sri Lanka Corporate Office +971 4 885 4449 www.amanabuildings.com Arab Emirates | Q United Corporate .am www Arabia | Oman | Egypt | Sri Lan Qatar | Kuwait | Saudi fice +971 4 885 4449 e Of ff ldings.com manabui ka at Ras Abu Fontas, Doha 9 m
Transcript of Confronting Carbonates
Bernard Montaron, Theme Director of Carbonates at Schlumberger writes on thechallenges of carbonate reservoirs.
132 Oil Review Middle East Issue Five 2008
EENHANCED OIL RECOVERY is aparticularly significant factor forcarbonate reservoirs, sincetraditionally recovery tends to be low
from these fields. Bernard Montaron ofSchlumberger explains.
Carbonate reservoirs hold around 60 per centof the world’s oil and 40 per cent of the world’sgas. However, the heterogeneous nature of therock presents challenges for petroleumengineers. To help address exploration andproduction issues specific to carbonate fields,Schlumberger has set up a dedicated carbonateR&D centre in Dhahran. This is a region whichholds 62 per cent of the world’s hydrocarbonreserves. In fact, the Middle East is dominatedby carbonate fields, with around 70 per cent ofoil and 90 per cent of gas reserves held withinthese reservoirs.
Carbonate formationsCarbonate reservoirs present a challenge, in part,due to the history of the rock formation.Carbonates rocks are formed locally by thedeposition of minute marine organisms, coraland algae, together with the precipitation ofcalcium carbonate in shallow, warm water. Bycontrast, the sand and sediment that formsandstone reservoirs tends to be swept longdistances by rivers and currents before settlingin sites such as deltas and beaches.
One important difference between these tworeservoirs types is that sand (a siliconcompound) is chemically inert when comparedto calcium carbonate. Carbonate rock initiallyhas a high porosity of 35 per cent - 75 per cent,but this decreases sharply as the rock ages. Over
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The complex world of carbonates: vugs
Confronting
carbonates
United Arab Emirates | Qatar | Kuwait | Saudi Arabia | Oman | Egypt | Sri Lanka
Corporate Office +971 4 885 4449
www.amanabuildings.com
Arab Emirates | QUnited
Corporate
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Arabia | Oman | Egypt | Sri LanQatar | Kuwait | Saudi
fice +971 4 885 4449e Offf
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eons of time, as the rock iscompressed deep underground, itscharacter is changed due to achemical process called diagenesis.During this process, the poreschange in size and geometry.Understanding the complexheterogeneous rock matrix can bevery problematic.
Diagenesis can create styloliteflow barriers, which extend overlarge areas, creating a dramaticeffect on field performance. Therock may also be fractured bygeological processes caused bypressure within the earth‘s crustarising from continental drift. Alarge fracture corridor maymeasure 10 m wide, 100 m highand a kilometre long. The historyand topography of a carbonateformation provides some idea of the likelyposition, orientation and direction of theselarge corridors. Some may run along the crestof a fold where the formation has been subjectto bending. But shear stress will tend to resultin a crush zone because of the slippage of therocks on either side of a failure.
This is a very important consideration forreservoir management because major fracturecorridors can have a permeability a thousandtimes greater or more than the surrounding rockmatrix. On the positive side, fracture corridorscan provide highways for draining the reservoirs.However, the fracture corridors are also an easy
path for water, and injected gas, tobreak through to production wells.Fractures can also lead to problemsduring drilling, such as mud lossesand stuck pipe.
Fracture cluster mapping workflowLast year the Fracture ClusterMapping (FCM) workflow waslaunched to improve fracturecharacterisation and themodelling of carbonate reservoirs.The workflow integrates Q-Technology services, boreholemeasurements and Petrel seismic-to-simulation software with expertinterpretation and flexible workprocesses, resulting in improvedproduction performance.
Modelling fracture systems hasalways been and still is a challenge forgeoscientists in the oil industry. The FCMworkflow helps to make better decisions for thelocation of injectors and producers, plan welltrajectories, improve production predictions andform a comprehensive Discrete Fracture Network(DFN) model. It makes a clear distinction
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The topography of the field suggested thatthere would be extensive cracking along a foldrunning in a NNW to SSW direction. This thesiswas supported by an initial run of discontinuityextraction software (DES) when it was used toprocess high resolution Q-Technology seismicdata. However, this was not the full story.Borehole data from one well also showed more
than 400 open fractures runningperpendicular to the main fracturecluster. Sequential steps of theworkflow were then used, with filterwindows, to produce more finelydetailed fracture images at a seriesof azimuths (angles) and inclinations(dips) windows.
The constrained computer runsrevealed additional fracture clusters.The filtered images were finallymerged together to produce anoverall 3D picture of the fracturecorridors. The results were correlatedwith data from the four existingwells, and two new ones. Theavailability of an accurate fracturecorridor map has great benefits forreservoir management, both toenhance production by extending
the area that can be drained by each productionwell, and to avoid a water breakthrough surprise.
Understanding reservoir producibilityFCM addresses production concerns at areservoir scale; however it is equallyimportant to understand reservoir producibilityfrom a rock-textural perspective at the pore
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between diffuse fractures that can bemodelled using geo-statisticaltechniques, and fracture corridor'highways' that must be detectedand placed in the reservoir modelat their exact field location.
In practice, the frkacture clustermaps can be used in conjunctionwith technology that allows drillersto steer wells to the most favourabletargets within the reservoir.PeriScope directional, deep imagingwhile drilling, is particularly useful inthis respect as it enables petroleumengineers to geosteer horizontaldrains using measurement-while-drilling (MWD) and logging-while-drilling (LWD) technology.
Applying the workflowIn one example, the FCM workflow was appliedto five carbonate fields located in northernKuwait. One of these fields provided aparticularly good test site for validating theworkflow, because further drilling was takingplace in the complicated structure. Four wellshad been drilled at the start of the study, withtwo further wells drilled as the study progressed.
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scale. The Schlumberger Carbonate Advisorpetrophysics and productivity analysis,launched this year, centers on characterisationand size partitioning of the pore geometry byusing texture-sensitive borehole logs, such asnuclear magnetic resonance and capturespectroscopy. It is a comprehensive formationevaluation system that leverages poregeometry analysis to confirm theidentification of petrophysical rock types(mineralogy and pore system class), determinefluid saturations and estimate permeabilitiesand relative permeabilities.
Many different measurements are needed tobuild-up a picture of the petrophysical propertiesand producibility of a carbonate formation. Somemeasurements such as photoelectric effect,gamma ray and nuclear magnetic resonanceprovide information about the nature of the rockand its pores, while others, such as resistivity,provide information about its fluid content andmobility. It is the integration of all the availabledata that is key to determining the potentialproductivity of a formation. Carbonate Advisor isan interpretive methodology that integratespetrophysical data in three sequential steps. Thefirst step determines the lithology and porosityof the rock. This evaluation is then used in the
second step to estimate the pore type andpermeability. In the third step, relativepermeability and fluid saturations are derivedfrom a detailed analysis of the radial variation ofresistivity measurements.
Traditionally, petrophysical properties areoften determined from core samples, which areshipped to laboratory for assessment. However,in many cases, key petrophysical characteristics,such as porosity, pore type and permeabilityfrom core measurements may not be availablefor days, weeks or even months. Carbonate
Advisor, log measurements and analysistechniques, goes a long way to providing criticalpetrophysical information in a fraction of thetime. Results, presented continuously as afunction of well depth, are available just hoursafter data has been acquired from logging tools,enabling critical decisions to be made in atimely manner.
The futureToday, carbonate reservoirs continue to posea number of challenges. After all, therecovery factor from these reservoirs is belowthe 35 per cent average for fields of all typesand some fractured carbonate reservoirs haverecovery factors below 10 per cent. Thechallenge in coming years will be tointegrate data even more effectively toproduce ever-more accurate reservoir models,with an even better definition of features.With continued collaboration to refineexisting techniques and the commitment todevelop specialist new technologies, thefuture for carbonate reservoirs is lookingmore productive. ■
Further information is available fromSchlumberger at ADIPEC 2008.
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