Pre-stack Inversion in Hampson-Russell Software

Transcription

1 Pre-stack Inversion in Hampson-Russell Software HRS9 Houston, Texas 2011 Brad Hickenlooper

2 What is Simultaneous Inversion? Simultaneous Inversion is the process of inverting pre-stack CDP gathers for P-Impedance (Zp),S-impedance (Zs), and (data dependant) Density (ρ) at the same time. The relationship between the back ground trends of the Zp, Zs and Density logs are used to guide the inversion process and couple the seismic based impedance volumes. 2

3 Why is Simultaneous Inversion useful? Simultaneous inversion has value over a post stack inversion because it produces three volumes simultaneously (P-imp, S-imp, and density), preserving the relationship between the three attributes. Simultaneous inversion outputs are attributes that can be used in other processes, ex. Emerge or LithoSI. The outputs can also be used to generate rock property volumes such as Lambda Rho, Mu Rho, and Young s Modulus. The relationships between variables will add stability to problems that are sensitive to noise, and non-uniqueness. 3

4 Minimum Recommended Data Requirements for Simultaneous Inversion Conditioned Angle Gathers 3D, 2D, or Multi 2D Correlated Well Data P-wave, Density, and S-wave logs. Interpreted Horizons Additional Data Options Stacking Velocities can be used to help build the initial model Horizons can be interpreted in HRS-9 Missing log information can be calculated from transforms in HRS-9. 4

5 Opening a Project Once HRS9 launches, select an existing project or create a new one. 5

6 Pre-stack Inversion The Pre-stack inversion process runs on angle gathers. To create angle gathers, select Angle Gather from the Gather portion of the Processes menu. Edit the needed parameters shown in the image to the right and click Specify Velocity Note we are creating an angle range from 0 to 45 degrees. 6

7 Pre-stack Inversion The Angle Gather should appear as shown It is important to note that the pre-stack inversion process requires properly conditioned angle gathers and well logs. The seismic should be in the angle domain with as much noise removed as possible and the event of interest flattened. The logs should be correlated and wavelets should have been carefully selected before starting the inversion process. 7

8 Pre-stack Inversion Once the correct data is available, start the model building process. The Model building tool is found under, Strata Model, in the Processes Tab. Select Build Strata Model. Choose, Build Pre-Stack Model, fill in the needed parameters, and Click OK 8

9 Pre-stack Inversion After the model is built, turn off the trace data (this option can be found under the eyeball icon) to see the P-Impedance model. 9

10 Pre-stack Inversion Before inverting the entire volume use the Pre-Stack Inversion Analysis tool. This tool is used to test inversion parameters before running the process. Select Pre-Stack Inversion Analysis from the Process menu above, then fill in the analysis parameters on the right as shown. Click OK to analyze. 10

11 Pre-stack Inversion The analysis window will appear as shown. The well cross plot menu allows the user to control the log relationships that will be included in the inversion. Be sure all the settings are correct and click OK then click Apply. 11

12 Pre-stack Inversion The resulting Analysis display appears, and we can see there is a sharp drop in the Vp/Vs ratio. You can see the response in the inversion analysis. 12

13 Pre-stack Inversion When the Pre-Stack Inversion Analysis is completed, run the inversion on the entire volume. Select Pre-stack Inversion from the Process menu and fill in the parameters as needed. Note that the advanced parameters are filled in with the corresponding values from the analysis menu. Run the inversion on the volume over a limited time window around the zone of interest to QC the results, then run the entire volume.

14 Pre-stack Inversion When the inversion finishes, a number of windows appear. One of the more interesting windows, is the V P /V S Ratio display. Note the predicted drop in V P /V S at the zone of interest. P-impedance VpVs Ratio 14

15 Interpretation of Results Now that the inversion is finished, we will begin VpVs interpreting the results. Start by projecting a previously selected cross plot zone interpreted on the Zp vs. Vp/Vs logs on the seismic volume. Select the, Cross plot tab from the top tab bar and the log cross plot will be displayed in the resulting screen. The cross plot will appear as shown. Save the cross plot by clicking the disk icon ( ) in the upper left. Zp 15

16 Interpretation of Results After the cross plot is saved, return to the seismic tab and click the eyeball button to get the Seismic View Attributes menu. Select, Cross Plot from the Color Attribute drop down, and fill in the Cross Plot Parameters section as shown. Click OK. 16

17 Interpretation of Results The resulting view clearly shows that the interpreted hydrocarbon zone has been highlighted by the cross plot zone of the Zp against VpVs cross plot. CDP stack w/ cross plot zone VpVs Ratio 17

18 Interpretation of Results Another possible reservoir discriminator is the Lambda Mu Rho (LMR) analysis. Lambda Rho VpVs Ratio 18

19 Interpretation of Results The cross plot below shows how the LMR volumes relate to the reservoir characteristics. Typically the reservoir zone is interpreted along the edge with low LR values which represents areas of relatively low incompressibility. The reason behind this is because gas is more compressible than brine, therefore rocks charged with gas will have higher compressibility, and lower incompressibility, than brine saturated rocks. MR LR 19

20 Interpretation of Results Once the reservoir is interpreted from the cross plots it can be useful to examine the interpreted zone spatially in View3D. Viewing the zone in 3D can give insight into the reservoir structure and its relationship to wells or horizons. 20

21 Simultaneous Inversion Assumptions Linear relationships between ln(zp), ln(zs), and Ln(ρ) are reasonable. A constant value of γ = Vs/Vp is used in Fatti s Coefficients. NMO-stretch can be handled by angle dependent wavelets. 21

22 Simultaneous Inversion Conclusions Simultaneous inversion is preferable to acoustic impedance (post-stack) inversion because it uses the relationships between Zp, Zs, and density to help guide the inversion. The inversion outputs Zp, Zs, Density, and Vp/Vs can be inserted directly into other programs (see EMERGE or LITHOSI examples). Rock property volumes generated from inversion results can be useful in discriminating hydrocarbon reservoirs from surrounding rocks. To generate reliable density volumes using simultaneous inversion, users must have reliable Vs (shear) and density logs (to guide the initial model), and as much quality offset (angle) as possible on the gathers (35-45 degrees is usually the minimum angle required to get reliable density information). 22

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