Well Tie view in g-Space is a powerful tool that allows you to visualize and analyze well data, generate synthetic seismograms, and perform seismic-to-well tie operations.

Preparing for Well-to-Seismic Tie

To perform a seismic-to-well tie, you must have the following data loaded:

• Seismic Data: 2D lines or 3D volumes.
• Well Information: This includes check shots, sonic logs, density logs, and optionally, impedance data.

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In this chapter, we will guide you through the well-to-seismic tie process using the Demo project for the Teapot oil field.

We will focus on the well 67-1-TpX-10, which is equipped with sonic and density logs, as well as checkshot data. The goal is to tie this well to a 3D seismic volume by generating a synthetic seismogram, comparing it to the seismic composite trace at the well location, and then analyzing the outcomes.

Let's have a look at the data

Well Correlation View (left panel) is used to visualize the well logs, including the sonic and density logs for well 67-1-TpX-10. The logs are plotted against depth and time. Markers  are annotated alongside the logs, aiding in the correlation with seismic data.

Location Map displays the position of well 67-1-TpX-10 within the seismic survey area.

Inline Seismic View shows a vertical slice of the 3D seismic volume passing through the well location.

Checkshot Table View (right panel) provides a checkshot data for well 67-1-TpX-10, which includes both True Vertical Depth (TVD) and One-Way Travel Time (OWT). Well correlation view automatically recalculates and displays TWT values along its axis.

Please verify the 3D seismic reference level and replacement velocity in the Module Properties. Ensure that the Datum is set to 5500 feet and the Replacement Velocity is set to 9000 ft/s.

Navigate to the Well Tie Bar, which will be the workspace for this process.

Setting Up the Well Tie Bar

The Well Tie Bar is divided into three main sections: Seismic, Synthetic, and Cross-correlation.

Seismic section (highlighted in red) allows the user to select the seismic lines or volumes that are near the selected well. It displays options to choose specific seismic data from the drop-down menu, such as the seismic volume or group. In this Demo project, we will be performing the well tie to a 3D seismic volume.

The Nearest Seis Distance field displays the distance from the selected well to the nearest seismic line or volume. For example, in this case, the distance is 53.7 feet.

The user can define a specific distance for the nearest seismic data using the Nearest Seis Distance setting. For instance, if a distance of 1,500 feet is specified, only seismic lines within this radius from the well will be considered.

Nearest Seis Width defines the number of seismic traces to be displayed around the well location. In this case, we set it to 50 traces.

Synthetic Section (highlighted in green) is dedicated to generating and analyzing the synthetic seismogram. It includes tools for creating synthetic traces based on the sonic and density logs and performing wavelet analysis.

To create the synthetic curve, you need to specify the type of data that will be used in the process. In our scenario, we have both acoustic (sonic) and density logs available. To proceed, in the drop-down menus (highlighted in orange), select the appropriate curves that correspond to the available data:

• For the Sonic log, choose the curve labeled DT
• For the Density log, select the curve labeled RHOZ

Use anti aliasing filter we will set by default, checked.

Synthetic Seis Width allows you to define the width of the synthetic seismogram by specifying the total number of seismic traces to include. For our project, we will set this value to 1 trace to generate a single synthetic trace.

To visualize the well-to-seismic tie process, start by creating a new workspace in the View Manager.

To display the tie, you'll need to include a Location Map where the wells will be displayed. As you choose a well with time-depth data, it will be visualized at Well tie View.

Following this, create a Well Tie View by clicking  from the Well tie bar. This function is also available in Views Bar, choose the same button Well tie.

The default Well Tie View will open in front of you. Let's review and configure it.

The default template shows the following data:

• Two Way Time
• Interval velocities
• Reflectivity
• Synthetic trace
• A seismic section with the synthetic trace overlaid at the well intersection

The default template also sets how these data are drawn: the interval velocity curve is filled with the Rainbow palette, Reflectivity is displayed on a fixed scale from -1 to 1, the seismic gather is drawn with the Seismic palette under black wiggle traces, and the synthetic gather is drawn with black wiggles only.

You can also check all the settings in the Module Properties panel.

Let's configure the template and adjust it to a view that suits your preferences. The full settings description for Well Tie View are available here. The curve settings are similar to the settings in Well correlation

By right-clicking, you can access global settings, where you can easily add scales, such as TVD, which is especially useful for horizontal wells.

For this example, the seismic section was configured to display as wiggles, and the number of synthetic traces was adjusted for easy visual comparison. Markers were also visualized along with the well logs to enhance the interpretation.

To save the settings as a template, right-click in the well name field and select Save Template.

To apply the template, navigate to the Well Tie Bar and select the desired template from the drop-down menu.

The Wavelet analysis option lets you select an existing wavelet or create one for the synthetic seismogram.

The Wavelet analysis window is not modal, so you can keep working in g-Space while it is open. It is divided into a Wavelet side, where you choose the input and set the parameters, and a Spectrum side, which plots the spectrum of the current wavelet.

When the project contains seismic data, the Seismic data group at the top of the window lists the available datasets. Type in the search box above the list to filter it by name, then select the dataset the wavelet is estimated or extracted from.

By default, the system uses a Ricker wavelet. Select Create new wavelet, then choose one of the following Wavelet type options:

1. Theoretical: Creates a Ricker, Gaussian, or Ormsby wavelet from the specified parameters.
2. Statistical: Estimates a wavelet from the selected seismic data.
3. Extracted: Extracts a wavelet from seismic traces at a chosen location.
4. From well log: Derives a wavelet from the current well's sonic and density logs and the seismic trace at the well location.

When Wavelet type is set to Statistical, the Data range group defines which seismic traces the wavelet is estimated from. The start and end fields accept only values within the extent of the dataset selected under Seismic data, and are pre-filled with its full extent.

Leave Limit data range cleared to estimate the wavelet from the seismic trace at the current well location. Select it to enable the range fields and estimate the wavelet from every trace inside the range instead:

• For a 3D volume, set Inline start, Inline end, Xline start and Xline end.
• For a 2D line, set CDP start and CDP end; the Xline fields do not apply and are hidden.

When Wavelet type is set to Extracted, the Extracted parameters group defines how the wavelet is taken from the selected seismic data:

• Phase type: select Constant phase to give the whole wavelet a single estimated phase, or Variable phase to keep the phase as extracted from the data.
• Trace location: Wellhead uses the trace nearest to the well, Average in radius averages the surrounding traces within the number of traces set in Radius, traces, Composite trace uses the composite trace along the wellbore, and Manual uses the trace at the Inline and Crossline you enter. The additional fields appear only for the location that needs them.

Set the length of the wavelet with the Wavelet length slider or its value box, and rotate its phase with the Phase slider or value box; click Estimate phase to estimate that phase from a tie to the current well. For the Statistical, Extracted and From well log types, Detect time start and Detect time end limit the time interval the wavelet is estimated from and also set the window that Estimate phase correlates over.

Estimate phase keeps the wavelet you have configured and looks only for its constant phase. It rotates that wavelet through every phase from -180° to 180° in steps of 1°, convolves each rotation with the reflectivity of the current well to build a synthetic trace, compares each synthetic trace with the seismic trace at the well, and writes the phase that correlates best into Phase. The wavelet and its spectra are redrawn immediately. Nothing else changes: no wavelet is extracted from the seismic data, and the wavelet type, dominant frequency, length, sample interval, shape and amplitude spectrum stay as you set them. The result is an absolute value rather than an additional rotation, so clicking the button again with the same settings returns the same phase.

The estimate therefore needs a dataset selected under Seismic data, a current well whose sonic and density logs and checkshot give a reflectivity, and a valid seismic trace at that well in the selected dataset. If one of them is missing, or the correlation window is too short for the length of the wavelet, g-Space describes what to correct in a message and leaves Phase unchanged.

The Spectrum side plots the amplitude spectrum of the current wavelet. Select Normalize to normalize the amplitudes and Phase spectrum to add the phase curve with its own Phase, deg axis on the right. Use the mouse wheel over the plot to change its scale.

Additionally, users can save the synthetic trace to a LAS file for further analysis.

After changing parameters, click Update to recalculate the wavelet preview. Click Apply to use the current wavelet in the well tie and refresh the synthetic trace, Save wavelet as... to save the wavelet with a name, or Close to dismiss the window.

Click Save to workflow to add the selected input and parameters as a Wavelet analysis job in the current workflow without running it immediately. The window remains open; the wavelet is created when the workflow runs.

To edit a saved job, open it from the Workflows window. The window restores the saved settings and shows Save and Cancel; click Save to update the job or Cancel to leave it unchanged.

Cross-correlation section handles the cross-correlation process between the synthetic seismogram and the seismic data. It includes settings for the time range (start and end times) and the lag in the Well tie bar. The user can adjust these settings to optimize the match between the synthetic and seismic data.

Note

For accurate results, it is recommended to set the cross-correlation calculation window to at least 2-3 wavelet lengths.

Let's move on to Seismic to Synthetic Comparison and Time-Depth Adjustment

Begin by identifying and matching known reference reflections between the seismic and synthetic data.

Activate the anchor change function. Then, select a reference reflection and double-click the left mouse button to place an anchor on it. Anchors act as tie points, allowing users to align the seismic data with the synthetic trace. Hold the left mouse button to drag and position the anchor at the desired interval.

For optimizing the correlation, use the auto-shift function, which automatically adjusts to achieve the maximum cross-correlation coefficient.

If you are working on a specific interval, we recommend reducing the correlation interval and placing a series of anchors over a larger scale. Visually inspect the alignment of key reflectors between the synthetic and the seismic trace. And correlate them by using anchors (highlighted in red on picture below).

Note that as the curve changes, you see changes in the cross-correlation graph and in the time-depth table. There are two new columns for Result TVD and result OWT.

Drift Curve

As you edit the anchors, g-Space automatically calculates a Drift curve, defined as the difference in two-way time (ΔTWT) between the adjusted (edited) checkshot and the original base checkshot at each depth. The drift shows how far the calibrated time-depth relationship departs from the original checkshot and is a useful quality-control indicator of the tie.

The Drift curve is refreshed automatically each time the checkshot is edited and is displayed in the well tie view together with the other checkshot-derived curves. It is also stored as a LAS curve (in milliseconds) under the corresponding well in the Data Manager, so it can be reviewed or exported like any other well log.

To delete the current version of your checkshot, select Remove All Anchors from the top menu. This will also update the checkshot curve.

In g-Space, you can upload multiple checkshots. To perform a well tie, select the active checkshot in the Data Manager by double-clicking on it.

Creating a check-shot from sonic log

If the wells with Sonic logs are not loading for well-tie, please perform the following steps:

1. Create a new checkshot for this well in the Data Manager; make sure that this checkshot is active.

2. Go to Location map and select "Current well on map" icon

Click on the well and it will display a new window as shown below

Now set the curve corresponding to the sonic from the drop-down list, in our case DT log, and make sure the units are correct.

Set Step for checkshot and First velocity value

Notes

1. The process of generating a checkshot from a sonic log involves using the sonic log data to estimate the travel time of seismic waves through the subsurface. A sonic log provides interval transit times (Δt), which is the time it takes for a sound wave to travel through a unit length of rock. By integrating these transit times over the depth of the well, we can calculate the cumulative travel time and create a synthetic checkshot.
2. However, the sonic log often does not cover the uppermost part of the well, leaving a gap in the data. In such cases, the user must manually set a velocity value for the first available depth in the log. This user-defined velocity serves as an approximation for the travel times through the upper layers that are not recorded in the sonic log.

User can see the created checkshot in the well folder in the data manager and open it as a table

Once satisfied with the tie, finalize it by saving the synthetic seismogram and the adjusted Time-Depth curve.

Notes

1. Datum Level Consideration. The datum level represents a reference point, usually the mean sea level or another fixed elevation, from which depths are measured. Ensuring that all wells and seismic data are tied to the same datum level is essential to avoid misalignments.
2. Understanding Different Data Scales and Resolutions. Be aware of potential mismatches in resolution when correlating seismic and well data. For example, subtle geological features visible in well logs may not be as apparent in seismic data.
3. Reviewing Log Data for Errors. Log data may contain errors, such as spikes, shifts, or inconsistent readings, which can lead to incorrect synthetic seismograms and false reflections. A thorough review of the log data by a petrophysicist is recommended before proceeding with the well-to-seismic tie.
4. Calibration with Checkshot Data. Always calibrate your well logs against checkshot data, if available. Checkshots provide more accurate time-depth information and help correct any discrepancies between the log-derived and actual time-depth relationships. Cross-verify the time-depth data with multiple sources, such as VSP (Vertical Seismic Profiling) data, to ensure consistency and accuracy.