New Technology for direct hydrocarbon reservoir detection using seismic informationXueping Hu*, Yajun Chen, Xiuwen Liang, KerangLang, BGP, CNPC

Summary

Hydrocarbon reservoir, as the main research target in oil industry, is made up of multi-phase medium, namely theporous rock matrix and the fluids such as oil, water and gas in rocks pore. In this case, it is hard to get objective andaccurate description on the transmission characteristics of real seismic wavefield using conventional single-phasemodel.Biot theory presumes that fluid may move relative to solid,so when seismic wave transmits through double-phasemedium, between solid phase and fluid phase, thereproduces displacement and interaction, making first P wave (quick P wave) and second P wave (slow P wave), the two kinds of wave with reverse polarities.According to the results of numerical simulation, physicalmodel test and petrological analysis, It was found that oiland gas reservoir shows the characteristics of LFR & HFA(low frequency resonating and high frequencyattenuating), this discovery therefore provides the evidenceto directly detect oil and gas using seismic information, and become one of the technical highlights of the softwareKLInvesion (Oil & Gas Reservoir Detection and SyntheticInversion) developed by BGP. The experimental resultsfrom over 90 wells in 20 work areas proved that it is effective to detect oil and gas reservoir with over 90%coincidence to drilling data.

Theoretical basisDouble-phase modela. Generally, oil and gas reservoir is made up of multi-phase medium, the porous rock matrix and fluids includingoil, gas and water and so on.b. The research on the wavefield of double-phase medium is based on Biot theory, which presumes fluid can move relative to solid and the two phases interact with each other.The presumption unveiled the second P wave (slow P wave). Since the presumption is reasonable and accords toactual situation, Biot model is regarded as the emphasis in our research.

The elastic wave equation of double-phase mediumReal reservoir is not only double-phased, but alsoanisotropic. For the sake of simpleness, we only discuss

double-phase isotropic medium in this paper. According to Generalized Hooks Law and Generalized Darcys Law, if considering the energy consumption resulted from therelative displacement between fluid and solid, we can getthe elastic wave equation(1), shown as the following vectorexpression:

Using higher accuracy difference technology, we get thefinite difference numerical solutions of the elastic waveequation of double-phase isotropic medium. Subsequently,we can conduct forward and inverse calculation on various multi-phase models, research their seismic response features.

Test foundation and theoryThe redistribution of the seismic wave energy in double-phase mediumThe relative displacement between solid and fluid, on theother hand, results in the redistribution of the seismic wave energy in double-phase medium. In this case, the seismic wave with different frequency in double-phase medium will change obviously in proportion, showing as that the wave energy moves to low frequency direction, which hasbeen proved by real data. For example, Wang Yujing et al(2000) discovered the strong attenuation characteristics of the high-frequency seismic wave in oil& gas zone. SomeRussian geophysicists found the resonance characteristicsof the low-frequency seismic wave. The phenomenon has been accepted publicly; however the explanations to it are very different. Someone explained it as particles inertialmotion, another thought it as fluids viscosity, even one attributed it as the turbulence of eddy current. All theseconclusions explain the phenomenon only from single aspect. According to our research, we think the key factor to produce the phenomenon is the displacement betweenfluid and solid. So the conclusions particles inertialmotion, fluid viscosity or the turbulence of eddycurrent, whichever is only one of the behaviors of the displacement between fluid and solid.

SI P1.4

SEG/Houston 2005 Annual Meeting 1735

Main Menu

Direct reservoir detection using seismic information

The characteristics of LFR & HFA in double-phasemediumLFR & HFA is the most distinct expression of the waveenergy redistribution in double-phase medium. a and b in figure 1 show two seismic records from forward modeling. They are direct waves with same transmitting path. Theonly difference lies in the media through which they pass,b passed a 10m thick double-phase medium, while asmedium, 10m thick too, but its P wave velocity is same asthat of the first P wave of double-phase medium. c and d are the normalized frequency spectrums of a and b respectively. e is the result after minus c from d. From thediagrams we can see , compared with a, b has richer low-frequency wave but rather weaker high-frequency wave, clearly explaining the phenomenon of the wave energyredistributed after passing through double-phase medium,namely the LFR & HFA.

The LFR & HFA in double-phase medium provides the evidence for us to directly detect oil/gas reservoir using seismic information. Now the problem focuses on how topick up the useful data. As we know, in real data, thewave path from different CDPs are not completely same,complex wave paths will result in frequency changing,whats more, strong noise also covers the frequencyspectrum changes after seismic wave transmitted through double-phase medium. As a result, it is hard to catch theuseful change simply using spectrum analysis.The seismic wave with various frequency can be easilypicked up using resonant filter. Next it is time to choose suitable algorithm to process the selected wave to extractunderground oil and gas information. In this paper, we used energy normal probability logarithmic summation algorithm to extract oil and gas information.

Model testWe selected a representative geological model to run numerical simulation, and verified the correctness andeffectiveness of the above arithmetic result usingforwarding result. During inversion test, the low -frequency sensitive band is 1Hz-10Hz, and the high-frequency band is 35Hz-45Hz. Figure 1 shows the seismic response of a transmitted wave model.We analyzed the reflection from the bottom interface. Themaximum energy cumulative result is shown in figure 2, where the dash line is the original record, and the solid lineis the smoothed record. In the figure, we can see theoscillation of the dash line which is caused by diffraction.Clear LFR & HFA also appears in oil & gas zone. Allthe abnormities of double phase and single phase have

their own typical expression in the figure. The former shows the characteristics of LFR & HFA, while the latershows strengthened energy both in low frequency and high frequency bands. This clearly tells us that LFR & HFA is the behavior of double-phase medium. According to aboveconclusion, it is feasible to use the two methods we proposed above to differentiate double-phase and single-phase medium, therefore avoiding the multi-solution of inversion result. Meanwhile, we found, during processingof real data, there didnt appear LFR & HFA when themedium contains water. So water interference can avoid bythe methods.

Double-phase medium Low velocity medium

Figure 1 The stack record of the model (gather)

a. low frequency

b. high frequency

Figure 2 The max energy cumulative result

Example analysis

Using the finished KLInversion, we analyzed 2D field data(totaled 16 lines, 450km) and 3D field data (covering 19blocks, 2200 km2) in 17 working zones. The result provedthat 108 out of 120 wells are successful, only 12 wells fails. The coincidence rate is up to 90%. The incoincidence ofthe 12 wells, located in a 3D zone, may be caused by thecomplicated geological conditions, incorrectly interpreted

SI P1.4

SEG/Houston 2005 Annual Meeting 1736

Main Menu

Direct reservoir detection using seismic information

layers or hole deviation). Since we got no other effectivelyassistant data, we couldnt discover the prime criminal.

The detection result based on 3D data in west ChinaFigure 3 shows the local T0 diagram of the top interface ofJurassic oil formation in some 3D exploration zone in westChina. The warm colors mean high structure. Among 5wells in the zone, only the well Pb6 produced commercialoil and gas flow. The well Pb6-1 is oil-water homo-layered.Others are dry wells, located at the relatively high place ofthe structure (even Pb7 was drilled at the independentanticline). Pb601, the latest well at the highest place, is another dry one. Figure 4 shows the RMS amplitude (overlay with the structure contour map) including oil formation (60ms). From this map, we can understandneither the relationship between amplitude and structure,nor that between amplitude and oil -gas indication sincethe amplitude is low no matter where there is oil well. So it is not suitable by using amplitude attribute to detecthydrocarbon reservoir. Figure 5 shows the detection result of some oil-bearing formation limited in 60ms timewindow. The red zone is the potential oil and gas zone, theblue is non-oil and gas zone or poorest zone. Our detection result is completely coincident to the drilling outcome.The oil production well Pb6 is located at the very narrow

oil-bearing belt nearly along south-north direction, Pb6-1near to the boundary of the oil-bearing zone, and others areat non-oil-bearing zone, the blue zone in the figure 4. We

also found that a large potential reservoir, 0.8km2,is1300m away from the well Pb6 nearly along its southeastdirection. This may be a structurally lithologic trap, its left

updip direction top is fault-screened, and the up and downtops are lithologic-screened. Recently, an exploratory wellwas determined, and we are waiting for the furtherresearch on the detection result to decide the next plan.

Figure 4 The RMS amplitude map including oil formation (60ms).

Figure 5 the result of oil and gas detection.

Discussion of Result/Conclusions

ConclusionsFigure 3 The T0 diagraph of the top interfaceof oil formation. According to the results above, we can conclude that the

software is effective to detect hydrocarbon reservoir inwide scope since all experimental data used in our example is common (the experimental data hasnt been purposelyprocessed). The most advantage of the detection techniqueof the software is the freedom from wells limitation, so it can be used to define the lateral boundary of any reservoirwith higher precision. Used before drilling, KLInversion

SI P1.4

SEG/Houston 2005 Annual Meeting 1737

Main Menu

Direct reservoir detection using seismic information

can predict oil and gas reservoir, this will avoid dry holesat extremely extent. Used after exploring, KLInversion can accurately calculate reserve. Used during producing, it willbe a good proposer for optimized development plan.

Pending problems1. Bad seismic data or over proceeded data (i.e. the data

was over subdued to the low-frequency componentwhen high resolution processing);

2. Insufficient migration precision;3. Well deviation;4. Error of demarcating or interpreting layers and

incorrect time window unable to cover oil and gaslayer completely;

5. Failing to interpret low-resistance hydrocarbonreservoir when logging interpretation;

6. Long produced or water flooded oil wells (contrastingthe detection on the current seismic data to theoriginal years-old oil-test result.)

Acknowledgements

The authors would like to thank the Sichuan Oil &Gas Company Limited Branch, the Xinjiang Oil & GasCompany Limited Branch for providing the seismic andgeological data. Meanwhile, special thanks go to all of thesoftware developers for their support to this project.

References:1. Robert, 1989: Reservoir Forward and Inverse

Modeling: Oversea Oil & Gas Exploration.2. Mu Shen, 1992: Seismic Data Structural Sequence

Interpretation, Global Russian Geophysical ResearchInstitute.

3. Robert. Sherif, 2000: Seismic Exploration Summary4. Huixing Zhang and Xiuwen Liang, SEG, 2004:

Double-phase Seismic Wavefield Simulation 5. Yongguang Mu, 2000, Oil Seismic Physical Model and

Application Research6. New Development Series of Oversea Oil & Gas

Exploration7. Wenqu Zeng, et al, Form-Dividing Theory and Form-

Dividing Computer Simulation, Northeast CollegePublishing Company.

8. Wenchong Zeng and Jian Ouyang, et al, Logging Seismic Analysis and Oil Reservoir Evaluation.

SI P1.4

SEG/Houston 2005 Annual Meeting 1738

Main Menu

EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2005 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. New Technology for direct hydrocarbon reservoir detection using seismic information References Huixing, Zhang, and Xiuwen Liang, 2004, Double-phase seismic wavefield simulation:

Presented at the 74th Annual International Meeting. Shen, Mu, 1992: Seismic data structural sequence interpretation: Global Russian

Geophysical Research Institute. Sheriff, Robert, 1989, Reservoir forward and inverse modeling: Oversea Oil & Gas

Exploration. Sheriff, Robert, 2000, Seismic exploration summary: SEG. Wenchong, Zeng, and Jian Ouyang, Logging seismic analysis and oil reservoir

evaluation. Wenqu, Zeng, Form-dividing theory and form-dividing computer simulation: Northeast

College Publishing Company. Yongguang Mu, 2000, Oil seismic physical model and application research new

development series: Oversea Oil & Gas Exploration.

Main Menu