Author: Felix Segovia
Product Owner, Seismic Analysis
Seismic well calibration is a fundamental component within the seismic interpretation workflow. It is done to match the geological information observed in depth (at the well) to the seismic images acquired from the surface in time.
The calibration is typically done by acquiring well logs - with the sonic (acoustic signal transit time) and rock density logs being the most relevant. Sometimes a Vertical Seismic Profile (VSP) is also acquired, however, not all the information acquired from the VSP is usually leveraged for the well tie and seismic interpretation workflows.
With the latest DecisionSpace® 365 Geosciences Suite, the Seismic Analysis software enables the VSP corridor stack and the time-depth table to be conditioned and used for seismic-well calibration and seismic interpretation.
As well as being available on the cloud, the same great software is also available as part of our on-premise equivalent, DecisionSpace 10ep. Whatever works best for you.
Let’s look at some of the new developments in the latest release.
The new workflow offered by Seismic Analysis software (Figure 1) can use VSP corridor stacks instead of synthetics to perform seismic-well calibration. The VSP corridor stack can be loaded into OpenWorks® software from a SEGY file and the time-depth curve from an ascii file. This can then be visualized in the Geosciences Suite. The VSP corridor stack can be conditioned using a bandpass filter, phase rotation, bulk shift, or match filter, and can be applied as part of the Well-Tie Workflow. This ultimately enables a more accurate seismic well calibration, since the VSP data provides the ideal seismic trace at the well.
Figure 1: Using a VSP corridor stack (instead of a synthetic) in the well tie workflow
The VSP corridor stack can be displayed in section view in conjunction with the surface seismic (Figure 2). The surface seismic typically has lower frequency content than the original VSP corridor stack and the phase is often different. After the corridor stack is tied with the surface seismic, both can be displayed and compared to correlate geologic events to assist the seismic interpretation. This workflow offers a key advantage - the corridor stack contained a real seismic wavelet and better vertical resolution than the surface seismic.
Figure 2: The image on the left side shows the original VSP corridor stack and the image on the right side shows the VSP corridor stack after a Bandpass filter, and phase rotation has been applied. Dataset - Courtesy of Equinor
The VSP corridor stack can be displayed and processed in the Seismic Analysis software, as part of the well tie workflow. The sonic and density logs can still be leveraged for reference purposes.
If required, the logs can be selected to compute the acoustic impedance with the reflection coefficient, which can then be used to perform wavelet extraction using the SeisWell® method.
A cross correlation is computed between the VSP corridor stack and the surface seismic, and values for bulk shift and phase rotation are calculated which can be applied to maximize the cross correlation (Figure 3).
A key step for seismic-well calibration using a VSP corridor stack is frequency spectral analysis. The surface seismic and the VSP corridor stack typically have different frequency content which makes it difficult to calibrate between them. Plotting the frequency spectrums together allows us to design the optimum filter which can be applied to the VSP corridor stack to match the surface seismic frequency (Figure 4).
Figure 3: Original VSP Corridor stack displayed in the A-surface track and cross correlated with the surface seismic. Dataset - Courtesy of Equinor
Figure 4: Left - Bandpass Filter (F1: 4 Hz, F2: 8 Hz, F3: 32 Hz, F4: 64 Hz) and phase rotation ( -88 degrees) applied to the VSP corridor stack. Right - Frequency spectral analysis from original VSP corridor stack in red, Surface seismic in blue and the filtered VSP corridor stack in black. Dataset - Courtesy of Equinor
The VSP corridor stack with bandpass filter applied, phase rotated, and bulk shifted now ties with the surface seismic. The geological events previously identified and interpreted on the well logs, can now be correlated between the VSP corridor stack reflectors and surface seismic reflectors (Figure 5). The VSP corridor stack can be used to improve seismic interpretation with a more accurate time-depth relationship. Certain deeper seismic events from the wellbore data can now be analysed (e.g., after 2500 ms (Figure 5)). All the data created during the seismic-well calibration is saved in OpenWorks software and can be visualized in the Geosciences Suite (Figure 1).
Figure 5: VSP corridor stack and Surface seismic with similar frequency content and calibrated showing the best correlation coefficient. Dataset - Courtesy of Equinor
The new workflow in the Seismic Analysis software within the Geosciences Suite maximizes VSP corridor stack use for seismic-well calibration and seismic interpretation by utilizing the time-depth functions from the VSP and the VSP corridor stack. The frequency spectral analysis (between the surface seismic and VSP corridor stack), allows us to design the optimum filter to match the VSP corridor stack with the surface seismic frequency. The cross-correlation process provides the best phase rotation and bulk shift application guidelines.
The geological events previously identified and interpreted on the well logs can now be corelated between the VSP corridor stack reflectors and surface seismic reflectors. This allows us to analyze seismic events deeper than the wellbore. The edited VSP corridor stack can also be used to improve our seismic interpretation with a more accurate time-depth relationship.
To learn more about these new capabilities, request a demo today.