Time Lapse Seismic to Assess Blowout EffectsS. Chakraborty*, Subhadeep Maji, P.K.Paul, Y.Ranjeet Singh and Dr. R. Dasgupta

Geophysics Department, Oil India Limited, Duliajansubhadeepchakraborty@oilindia.in

Keywords

Repeatability, Normalized Amplitude, Time Lapse

Summary

The following study utilizes 3D seismic data from 1995and 2008 to investigate blow-out related seismic amplitudechanges in the Eocene reservoirs of the field. Differences inreflection amplitudes and other attributes in these datavintages are used to interpret and constrain the spatialdistribution of reservoir properties and probe into insight ofthe depletion process due to blow-out. 4D seismic data isconsidered as a relative measure of change in the reservoirand it has to be suitably calibrated and normalized. NRMSdata is used to remove more anomalous measurements.NRMS amplitudes can be interpreted as a fractional changein amplitude relative to the original. Production profiles forseveral wells and seismic based attributes align towards theresults predicted through NRMS amplitude differences.This study normalizes the two seismic surveys acquired intwo vintages and interprets the amplitude differences, aswell as correlating the seismic volumes thereby identifyingthe regions which may be affected by the blowout. On theprocess of updating the reservoir by calibrating 4D data ofdifferent vintages has given a better understanding ofchanges to the reservoir and also reduces risk inforecasting, leading to better decisions about reservoirmaintenance and infill well targeting.

Introduction

In 1992, while drilling a well D15 on an elongatedanticline, in the southern bank of river Brahmaputra inupper Assam Basin structure, along up- thrown side ofa major normal fault, a blowout occurred.Accordingly, a high-resolution 3D survey around the saidwell was planned in the year 1995 with an aim to locate anddecipher the effect of blowout at various levels and toidentify the extent of such effect prior to drillingsubsequent wells in the vicinity, which was necessary tooptimally develop the profuse Eocene Reservoir at a depthof about 3600 m. In September 2005 there was anotherblowout in the vicinity which was producing oil inlakadong-therria formations of the Eocene age. The wellwas killed and shut in. General apprehension in context tofurther drilling in the vicinity of the distressed well,

however, demanded the delineation of the possible affectedzones at the reservoir level to assess the extent of damage.A time lapse seismic was planned in 2005 in accordance tostudy the changes in reservoir sands in the Eocene age afterthe simultaneous two blowouts which are also affected bycontinuous production from different wells in the field.Rock physics observations over the field on unconsolidatedsands show that Vp /Vs ratio increases for low effectivepressure and in most cases the horizontal stress is less thanvertical stress. Gassman fluid substitution is sufficient inmost cases. Fluid saturation changes in the reservoir affectsthe P wave while reservoir pressure changes impacts bothP wave and S wave velocities.

Since seismic signals related to reservoir geology arecommon to all surveys, they can be removed bysubtraction. In addition, differences in the signal acquiredin different vintages improve the ability of the seismic todetect subtle blow-out and production related changes inthe reservoir. Basemap of the study area with the blow-outwell is represented in Figure 1.

Figure 1: Basemap of the study area showing two Inlines,one passing along the Blowout well (represented with star)and another passing far away from the Blowout well

Figure 2 below, represents difference of RMS amplitudesover Inlines near the Blowout well and away from the

N

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Blowout well respectively. Differences in Reflectionstrength is observed in traces near the blowout well from2650ms-2850ms. Two 3D seismic surveys were carried outin 1995 (BASE survey) and in 2008 (MONITOREDsurvey) are analyzed in terms of acquisition repeatability.Repeatability (w.r.t shot and receiver position) in recordingthe seismic data in the monitored survey was made mostaccurately to the earlier survey.

Figure 2: RMS amplitude differences over Base andMonitored Survey. Notable changes in amplitude strengthare observed near the blowout well, whereas the amplitudechanges are relatively less over a region away from theBlowout field

Figure 2.1(a) and 2.1(b) below shows the acquisitionrepeatability in the Base and Monitored Surveysrespectively.

Both the field data was processed with the sameprocessing sequence and using the same parameters.

Figure 2.1 (a) and 2.1 (b): Source Layout for both BaseSurvey (Left) and Monitored Survey (Right) depictingrepeatability of sources and receivers is honored in dataacquisition

Methodology :

The objective of the study is to compare the 3D seismicdata acquired in 1995 (before the blow-out) and 2008 (afterthe blow-out) and find out the various geophysical attributedifferences and identify the areas affected due to blow out.As an effect of the blow- out, the acoustic/elastic propertiesof the reservoir will tend to change. Since seismic signalsrelated to reservoir geology are common to all surveys,they can be removed by subtraction. In addition,differences in the signal acquired in different vintagesimprove the ability of the seismic to detect subtle blow-outrelated changes in the reservoir.

The Figure 2 clearly demarcates that the changes inamplitude has occurred on seismic traces near the blowout

Blowout well respectively. Differences in Reflectionstrength is observed in traces near the blowout well from2650ms-2850ms. Two 3D seismic surveys were carried outin 1995 (BASE survey) and in 2008 (MONITOREDsurvey) are analyzed in terms of acquisition repeatability.Repeatability (w.r.t shot and receiver position) in recordingthe seismic data in the monitored survey was made mostaccurately to the earlier survey.

Figure 2: RMS amplitude differences over Base andMonitored Survey. Notable changes in amplitude strengthare observed near the blowout well, whereas the amplitudechanges are relatively less over a region away from theBlowout field

Figure 2.1(a) and 2.1(b) below shows the acquisitionrepeatability in the Base and Monitored Surveysrespectively.

Both the field data was processed with the sameprocessing sequence and using the same parameters.

Figure 2.1 (a) and 2.1 (b): Source Layout for both BaseSurvey (Left) and Monitored Survey (Right) depictingrepeatability of sources and receivers is honored in dataacquisition

Methodology :

The objective of the study is to compare the 3D seismicdata acquired in 1995 (before the blow-out) and 2008 (afterthe blow-out) and find out the various geophysical attributedifferences and identify the areas affected due to blow out.As an effect of the blow- out, the acoustic/elastic propertiesof the reservoir will tend to change. Since seismic signalsrelated to reservoir geology are common to all surveys,they can be removed by subtraction. In addition,differences in the signal acquired in different vintagesimprove the ability of the seismic to detect subtle blow-outrelated changes in the reservoir.

The Figure 2 clearly demarcates that the changes inamplitude has occurred on seismic traces near the blowout

Blowout well respectively. Differences in Reflectionstrength is observed in traces near the blowout well from2650ms-2850ms. Two 3D seismic surveys were carried outin 1995 (BASE survey) and in 2008 (MONITOREDsurvey) are analyzed in terms of acquisition repeatability.Repeatability (w.r.t shot and receiver position) in recordingthe seismic data in the monitored survey was made mostaccurately to the earlier survey.

Figure 2: RMS amplitude differences over Base andMonitored Survey. Notable changes in amplitude strengthare observed near the blowout well, whereas the amplitudechanges are relatively less over a region away from theBlowout field

Figure 2.1(a) and 2.1(b) below shows the acquisitionrepeatability in the Base and Monitored Surveysrespectively.

Both the field data was processed with the sameprocessing sequence and using the same parameters.

Figure 2.1 (a) and 2.1 (b): Source Layout for both BaseSurvey (Left) and Monitored Survey (Right) depictingrepeatability of sources and receivers is honored in dataacquisition

Methodology :

The objective of the study is to compare the 3D seismicdata acquired in 1995 (before the blow-out) and 2008 (afterthe blow-out) and find out the various geophysical attributedifferences and identify the areas affected due to blow out.As an effect of the blow- out, the acoustic/elastic propertiesof the reservoir will tend to change. Since seismic signalsrelated to reservoir geology are common to all surveys,they can be removed by subtraction. In addition,differences in the signal acquired in different vintagesimprove the ability of the seismic to detect subtle blow-outrelated changes in the reservoir.

The Figure 2 clearly demarcates that the changes inamplitude has occurred on seismic traces near the blowout

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well. Significant changes between the traces are alsoobserved over a window of 200ms (2650ms to 2850ms)which corresponds to producing formations of Lk+Th andLangpar sands, both of Eocene age in the study area. Across-correlation-based time shift is applied to the monitordata. Subsequently, phase / time shift has been applied to atime window between 2650ms and 2850ms which includesthe reservoir zone.

Phase shifting parameters are applied consistently to alltraces which ensure that trace -to -trace variations arepreserved. This is followed by a designing of a shapingfilter is applied to the output of the above monitor data thattries to match the wavelet of the Base & Monitor data setsas shown in Figure 3. The shaping filter attempts to matchthe frequency, phase and amplitude spectra of the waveletfrom one seismic volume to the other, where the cross-correlation of the two datasets is matched to theautocorrelation of the reference dataset. Amplitudedifferences caused by scaling are corrected using crossnormalization. Large amplitude differences below thereservoir may be related to induced time delays caused dueto the blowout and production changes, and time varianttime shifts are created whenever production /blowoutchanges the velocity in the reservoir. An attempt to removethese time delays is made through cross correlating thetraces above the reservoir and below the reservoir andapplying a threshold, because it is difficult to find a singletime shift that may align all the traces.

Figure 3 : Effect of shaping filter on traces having Highercross correlation (top) and Lower Cross Correlation (bottom). The shaping filter attempts to match the frequency,phase and amplitude spectra of the wavelet from oneseismic volume to the other, under same geologicalconditions. It is noted from the figure that the shaping filterattempts to significantly match the spectra of traces awayfrom the blowout region ( higher cross correlation values )without significantly affecting traces in the vicinity of theblowout well ( lower cross correlation value).

Consequently, repeatability of both base & calibratedmonitor is analyzed using the normalized RMS amplitude.NRMS amplitudes can be interpreted as a fractional changein amplitude relative to the original. The relatively higherNormalized Difference RMS amplitude as represented inFigure 4 & 5 shows Blowout effect or production inducedchanges or both, while the relatively lower NormalizedDifference RMS amplitude represents the areas which maynot have been affected due to Blow out as well asproduction changes.

Figure 3(a): Workflow adopted for the NormalizationProcess and to interpret differences.

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Figure 4: NRMS Difference Amplitude map generated overstudy area. Areas color coded red resembles low NRMSvalues and is possibly inferred as those regions not affectedby blowout/productions based on the scrutiny of theirproduction profiles.

Figure 5: Percentage variation of NRMS differenceamplitudes demarcated with a local outline where NRMSvalues are high enough and probable region affected bythe blowout effect.

Figure 6: Performance Plot represented above (left ) is ofa producing well where NRMS values are lesser, andinterpreted as a well not affected due to blowout process.The performance plot above ( top ) represents a wellwhich falls in a region with higher NRMS difference valueand shows the degradation of oil production rategraphically represented in red colour after the blowouttime highlighted with a black vertical line.

Seismic attribute maps as shown in Figure 7a and 7b alsoprovide some qualitative information of the geometry andthe physical parameters of the subsurface. The rate ofchange of any of these attributes with respect to time andspace helps to provide the stratigraphic and lithologicalparameters of the prospect. Figure-5 ( below ) shows theSeismic Amplitudes map close to Lk+Th formation withvolume window 20ms, and this deciphers a relativedecrease of Amplitudes in the monitored survey inaccordance with NRMS difference map.

Figure 4: NRMS Difference Amplitude map generated overstudy area. Areas color coded red resembles low NRMSvalues and is possibly inferred as those regions not affectedby blowout/productions based on the scrutiny of theirproduction profiles.

Figure 5: Percentage variation of NRMS differenceamplitudes demarcated with a local outline where NRMSvalues are high enough and probable region affected bythe blowout effect.

Figure 6: Performance Plot represented above (left ) is ofa producing well where NRMS values are lesser, andinterpreted as a well not affected due to blowout process.The performance plot above ( top ) represents a wellwhich falls in a region with higher NRMS difference valueand shows the degradation of oil production rategraphically represented in red colour after the blowouttime highlighted with a black vertical line.

Seismic attribute maps as shown in Figure 7a and 7b alsoprovide some qualitative information of the geometry andthe physical parameters of the subsurface. The rate ofchange of any of these attributes with respect to time andspace helps to provide the stratigraphic and lithologicalparameters of the prospect. Figure-5 ( below ) shows theSeismic Amplitudes map close to Lk+Th formation withvolume window 20ms, and this deciphers a relativedecrease of Amplitudes in the monitored survey inaccordance with NRMS difference map.

Figure 4: NRMS Difference Amplitude map generated overstudy area. Areas color coded red resembles low NRMSvalues and is possibly inferred as those regions not affectedby blowout/productions based on the scrutiny of theirproduction profiles.

Figure 5: Percentage variation of NRMS differenceamplitudes demarcated with a local outline where NRMSvalues are high enough and probable region affected bythe blowout effect.

Figure 6: Performance Plot represented above (left ) is ofa producing well where NRMS values are lesser, andinterpreted as a well not affected due to blowout process.The performance plot above ( top ) represents a wellwhich falls in a region with higher NRMS difference valueand shows the degradation of oil production rategraphically represented in red colour after the blowouttime highlighted with a black vertical line.

Seismic attribute maps as shown in Figure 7a and 7b alsoprovide some qualitative information of the geometry andthe physical parameters of the subsurface. The rate ofchange of any of these attributes with respect to time andspace helps to provide the stratigraphic and lithologicalparameters of the prospect. Figure-5 ( below ) shows theSeismic Amplitudes map close to Lk+Th formation withvolume window 20ms, and this deciphers a relativedecrease of Amplitudes in the monitored survey inaccordance with NRMS difference map.

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Figure 7a: Surface based seismic amplitude map overBase survey ( left ) and monitored survey ( right ) withinEocene formations showing decrease in seismic amplitudeattribute.

Figure 7b: Volume based seismic amplitude map overBase survey ( left ) and monitored survey ( right ) within amapped Eocene formation with a volume window of 20msshowing decrease in seismic amplitude attribute postblowout.

Conclusion:

The above study has facilitated in locating specific regionswhich seem to have been affected by Blowout. This isrepresented by relative higher differences in NRMSamplitudes. The change of seismic attributes generated overvolume windows specified to the target reservoir alsodemarcates and explains to some extent of the loss in theamplitudes over specified wells in the Monitored survey.Less differences in NRMS values are indicative of minorchanges in the reservoir production whereas largerdifference of NRMS amplitude values infer about the majorchanges in production. Production performance plots asshown in Figure 6 for more than a few wells incorresponding with the blowout period have shown atrend which supports of the analysis of NRMS amplitudedifferences as interpreted.

Acknowledgement

We thank the management of Oil India Limited for theauthorization to present this work

References :

Beasley, C. J., Chambers, R. E., Workman, R. L., Craft,K.L. and Meister, L. J. (1997) Repeatability of 3-D oceanbottom cable seismic surveys. The Leading Edge, v. 1 6,pp.1281–1285

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