New trends in Earth Sciences- Strategies in Geophysics for hydrocarbon exploration
IDEAZNew Trends in Earth Sciences -Exploration of Energy ResourcesAkhil Prabhakar (Int. Mtech 4th yr GPT)Swapnil Pal (Int. Mtech 4th yr GT)Ojaswita Singh (Mtech 1st yr GPT)IIT Roorkee
STRATEGIES IN GEOPHYSICS FORESTIMATION OF CONVENTIONALANDUNCONVENTIONAL RESOURCES.Why this topic?With every passing day, energy security and availability is becoming the mostimportant concern for people. In this day and age, can we imagine a dignifiedlife and growth without access to energy?We are fully aware of the facts that…• Fossil fuels reserves are depleting fast. (Are they?)• Renewables are intermittent and still expensive.• Nuclear future seems uncertain.• Any other breakthrough technologies don’t seem to be on the horizon yet.Demand for energy Source of energy ?
Either let the tourist go to the otherhotel as they do not have thatsophisticated technique to exploitthose immature lemons and thusbecome bankruptInnovate in their existingtechniques to exploit thoseimmature lemons and meet thepresent demand, thus securing theirjobs and sustaining their business.The choice made by chef will decide thefuture of Hotel A…
Analogy between Energy Industry and this “Nimboo Pani” story.Hotel ATouristGeneralPopulationOil and GasIndustryGeoscientistChefFreshlemonsConventionalhydrocarbon
Analogy between Energy Industry and this “Nimboo Pani” story.ImmaturelemonsUnconventionalhydrocarbonEnergyJuiceHotel BRenewableEnergy
Innovate in their existingtechniques to exploit thoseunconventional hydrocarbons andmeet the present demand, thussecuring our jobs and credibility ofbeing a geoscientist.Either let the policy makers go tothe renewable energy as we do nothave that sophisticated technique toexploit those unconventionalhydrocarbons and thus becomebankruptTherefore the choice made by a geoscientist will bedecisive to decide the future of Oil and Gas Industry.
PART IThe role of geophysics in petroleum resources estimationand classification.— New industry guidance and best practicesPublished in “The Leading Edge, November 2012” issue.
Key areas where seismic technologies impact “modern”resource estimation:-• Faulted Reservoirs• Prediction of rock and fluid properties.• Reservoir surveillanceEASY OIL is an OLD STORY… !
For faulted reservoirsSeismic attributes: Used as part of integrated analysis to assess the likelihood of economicallyproducible reservoir in an undrilled fault block.Seismic amplitude anomalies may be used to support reservoir and fluid continuity across faultedreservoir provided that the following conditions are met:• Within the drilled fault block, well logs, pressure, fluid data and test data demonstrate a strongtie between the hydrocarbon-bearing reservoir and the seismic anomaly.• Fault throw is less than reservoir thickness over (part of ) the hydrocarbon bearing sectionacross the fault and the fault is NOT considered to be a major, potentially sealing, fault.• The seismic flat-spot or the seismic anomaly is spatially continuous and at the same depthacross the fault.Faulted hydrocarbon-bearing reservoirWell logs, pressure, fluid dataIntegration of data sets
Faulted Reservoirs…A seismic amplitude anomaly has been calibrated to a hydrocarbon-bearing reservoir by well D. Thedrilled area is separated by faults from three other fault blocks to be targeted by development wells A,B, and C.Type of resourcesassociated with undrilledblocksHydrauliccommunicationacross the faultIntegratedanalysis
If Complex Geology…??• Increased level of uncertainty inherent in the data• Appropriately classify the volumes BASED ON the uncertainty components.• The levels of risk and uncertainty should be commensurate the quality of the data,velocity uncertainty, repeatability, and quality of supporting data.
Faulted Reservoirs…Before any seismic attribute analysis, mapping the faults in 3D is a must.This impacts the likelihoodassessment of economicallyproducible reservoir inundrilled fault block, andhence the resourceclassification.The detailed mapping of the faults PROVIDES evidencefor reservoir continuity across the fault relay-ramp,
Fluid Properties• Seismic technology can be used to predict the rock and pore-fluid properties of thereservoir and sometimes its pressure regime.• In PRMS-AG section 3.2.2, the following guidance is given:“The reservoir properties that 3D seismic can potentially predict under suitableconditions are porosity, lithology, presence of gas/oil saturation as well aspressure.”
An overview on seismic data quality &requirements for such an analysisThe oil accumulation is trapped against a fault to the northeast dipping to an oil-water contact (OWC) to the southwest. The seismic amplitude maps are from anear-offset (left) and far-offset (right) volume. The oil-water contact appears as anamplitude increase on the near offsets and an amplitude decrease on the far offsets.Both run along a structural contour. The seismic amplitude response is consistentwith the trap geometry, the depositional model, and the seismic rock propertiesfrom the well data.
• In this example, the key uncertainty for estimation of in-place volumes is thedistribution of net sand thickness. The low, mid, and high net sand maps are theoutput of a probabilistic seismic inversion. Each map fits the well data used toconstrain the model. The three net sand maps reflect the uncertainty in the net sanddistribution and can be used to constrain three different “oil-in-place” scenarios inlow, mid and high case static models that can be carried through to reservoirsimulation and are thus key input to the resource volume assessment andclassification.Fluid Properties…
Reservoir Surveillance• Time-lapse seismology (4D seismic) impacts estimation of resources and reservesin various ways• Bypassed oil reserves can be spotted on time-lapse seismic when a compartmentfault block or other discrete component of the trap) is identified by time-lapseseismic as an isolated pool that previously was believed to be part of the field’sconnected pool or pools.
…Updated reservoir estimates• Time-lapse seismic results revealed an area in the west of the F block without 4D sweep thatdiffered from what was expected.• Spectrally boosted 3D seismic evidence for a (newly mapped) normal fault cutting the Fblock into two separate blocks.• The new fault was incorporated in the model update, allowing an improved history match byadjusting the fault seal properties.• Now, the initially bypassed volumes block F will have to be reclassified from “developedreserves” to “contingent resources”.BUT…Until furtherdevelopment activitiesmature!
…Updated reservoir estimates• Changes of saturation in the interval swept by the water can noticeably alter theacoustic/elastic impedance of the reservoir. This impedance change can bedetected by time-lapse seismic comparisons.• These OWC changes, as derived from time-lapse seismic results, can subsequentlybe mapped out laterally and be used to UPDATE the static and dynamic reservoirmodels hat underpin the resources and reserves volumes estimate.
…Updated reservoir estimates• Steam-assisted gravity drainage (SAGD) (in-situ thermal recovery method) is being used todevelop some 20 billion barrels of bitumen below the surface.• The acoustic properties of bitumen sands strong response to temperature changes.a significant velocity decrease through zones in thereservoir which have been thermally altered by theSAGD process.monitoring of the thermalevolution over time.
Uncertainty in seismic predictionsPredictions from 3D seismic data aimed at definingInherent UncertaintyFluid flowRock/FluidpropertiesTrapgeometryThe accuracy of a given seismicbased prediction is dependent on:1. The quality of the seismic data(bandwidth, frequency content,signal-to-noise ratio, acquisitionand processing parameters,overburden effects, etc.).2. The uncertainty in the rockand fluid properties and thequality of the reservoir modelused to tie subsurface control to the3D seismic volume.
ANSWER lies inIntegrationof data…• It is important to realize that thisuncertainty assessment will needto be kept evergreen and needsto be revisited when new databecome available.This integrated workflow is the key to:•Near future exploration worksin complex geologies.•Efficient extraction ofhydrocarbons from wells.• Exploration ofunconventional resources.
Unconventional resourcesConventional resourcesConventional resources exist in discretepetroleum accumulations related to alocalized geological structural featureand/or stratigraphic condition(typically with each accumulationbounded by a down-dip contact with anaquifer) that is significantly affected byhydrodynamic influences such as thebuoyancy of petroleum in water.Unconventional resourcesUnconventional resources exist inhydrocarbon accumulations that arepervasive throughout a large area andthat are generally not significantlyaffected by hydrodynamic influences(also called “continuous-type deposits”).
PART IITransition from Conventional to UnconventionalStrategies in geophysics for estimation ofunconventional resources.Published in “The Leading Edge, November 2012” issue.
Overview• The rise of unconventional resource plays to prominence in the oil and gas industryhas presented geophysics with a set of unprecedented challenges, chief amongwhich is problem of resources and reserve estimation.TraditionalconcernUnconventionalresourceReservoir qualityTrap mapping
Parameters for reserve estimationConventional PetroleumSystemChance Factors Critical ChanceSourceChargeTrapReservoirContainmentUnconventional PetroleumSystem
Important areas which need to beaddressedThe quantification of“deliverability system” is theprincipal area whichgeophysical methods mustaddress.Reliance on seismicinversion, attributes such ascurvature and coherence, andmicro-seismic data.The low porosity of manyunconventional reservoirdemands greater trace to tracefidelity and low noise in thegeophysical data.Premium acquisition programare needed, together withstrict quality assurancestandards.
Defining the containerReservoir estimation begins bydefining the boundaries of thereservoir unit.Non-interpretable coal bedsare marked using isopachs andisochrons from usablehorizons.Shale gas reservoir can exhibitsubtle reflection characterand may not be map able fortraditional horizon picking. Insuch cases, inversion volumesmust be used to definereservoir boundaries.It is already knownwhere the hc’s are!Additional Geologicalconstraints are the key!
Establish reservoir continuityUnconventional reservoirs can be surprisinglyheterogeneous.Oil sands reservoirs often contain channelsystem whose fill can now be non-reservoir.Gas shales can contain non-reservoir faciesthat also act as frac barriers.These volume cannot contribute to estimatesof original gas in place (OGIP) or oil in place(OOIP), also they can render reservoir zonesinaccessible to drainage.All these reservoir must be identified andmapped and their volumes subtracted from thegross reservoir volume before a developmentplan with project economics can be prepared.Source: The leading edge, the strategies in geophysics forestimation of unconventional resources.Subtle nature of features being mappedmakes consistent interpretation difficult!
Establish reservoir continuityThe problem arises in connection withthe often low contrast nature of thesefeatures in unconventional reservoirsand with the subtle character expressionof lithology and mechanicalstratigraphy. Statistical approachescan be of value.What are the tools of geophysicswhich can be used? Inversion productssuch as Poisson’s ratio and young’smodulus may be needed to characterisegeobodies; coherence volume,character-based facies classification,and such proprietary techniques as Anttracking are useful, but they requireregional calibration.Source: The leading edge, the strategies in geophysics forestimation of unconventional resources.
Predict reservoir propertiesThe resource play geophysics differs the most from conventional-play-geophysics. Inshale gas projects, geophysics is called upon to characterize reservoir propertiesbefore and after the frac program.The engineers want to know the in-place resource, the fracability, faults to avoid,variations in principal stress direction, the location and orientation of natural fracturesystems and the location of barriers. After the frac program, the engineers want to knowthe SRV, the overlap between fracture patterns, and the location or distribution ofproppant emplacement, all which are critical input for geophysical recovery factorcalculation.Source: The leading edge, the strategies in geophysics for estimation ofunconventional resources.Principal stressdirections andpore pressuresare same…
Resource-play unconventional resource geophysics has 2principal tasks:Characterise thestate of thereservoir beforecompletesimulationEvaluate themodification ofreservoir into astate thatpermitseconomicproduction.Resource playunconventionalresourcegeophysics
Resource-play unconventional resource geophysics has 2principal tasks:To characterize the state of thereservoir before completion simulation.• In both shale and tight oil projects,hydrocarbon recovery is dependenton the existence and connectivity ofnatural fractures. Often the fairwayof interest for the exploitation of suchresources is found by mapping thesenatural fracture systems. In additionto define fracture related producibilityare velocity and amplitude anisotropycombined with shear wavebirefringence.To evaluate the modification reservoirinto a state that permits economicproduction.• This requires us to monitor thestimulated rock, identify bypassedresource pay, verify the resourceconfinement after stimulation, andpredict or forecast the hydrocarbondelivery success from stimulation-induced changes in observedgeophysical characteristics. In oilsands work, SAGD projects requiregeophysics to track the growth ofsteam chambers with 4D seismicsurveys.
Integrate with other data analyses• This stage in the workflow is critical for unconventional resource geophysics.• Coherence techniques, curvature, horizontal anisotropy, and edge-detectionalgorithms can predict fracture zones, but they must be compared with core andimage-log data from horizontal wells to calibrate them.• In unconventional plays, the geophysicists at present is perceived more as asupporting player than in traditional exploration and development, and geophysicalinterpretation products and predictions sometimes encounter scepticism.• The standards for reliability, repeatability, and accuracy are high.• This should clearly be the future direction of R&D efforts in order forgeophysics to contribute seriously to resource and reserve estimation.
Data requirements, processing and analysis standards, andemerging technologies3D seismic acquisition, processing, and elasticinversionJoint P- and S-wave 3D surveys can offeradvantagesVSP surveys for unconventional reservoirsMicro seismic data for unconventional resourceestimationHere, theANSWERlies in…!
Innovate in their existingtechniques to exploit thoseunconventional hydrocarbons andmeet the present demand, thussecuring our jobs and credibility ofbeing a geoscientist.Either let the policy makers go tothe renewable energy as we do nothave that sophisticated technique toexploit those unconventionalhydrocarbons and thus becomebankrupt
“The MORE we get to know, andthe MORE we try to quantify (nature’s complexities),the MORE we realize,We know Less!”