Compaction analysis and predicting pore pressure

Pore pressure is defined as the pressure acting within the pore space of a rock at depth. In the absence of any other processes, the pore pressure is called hydrostatic, which is the pressure associated with a static column of fluid from the surface to the depth of interest. For water, hydrostatic pore pressure increases with depth at the rate of 10 MPa/km or 0.44 psi/ft (depending on salinity).

Pore pressure can exceed hydrostatic values in a confined pore volume at depth. This phenomenon is called overpressure. Overpressure origin and magnitude varies from basin to basin, depending on the rock and fluid properties. Mechanisms that are related to the overpressure processes include: volume expansion, compaction disequilibrium, fluid movement, and buoyancy among others.

Also known as the vertical related stress, undercompaction or compaction disequilibrium occurs due to the combined effect of the high rate of sedimentation and the low ability of the rock to drain the pore fluids during burial. Methods like Eaton, Equivalent depth and ratio accounts for this process. The Bowers method considers not only undercompaction but fluid or volume expansion mechanisms, which are based on the assumption that the effective stress decreases even when the burial process continues.

Currently, the application focuses on effective stress methods such as Eaton, Equivalent Depth, Ratio method, and the Bowers method. It also accounts for centroid (buoyancy effect) and depletion and injection mechanisms to accurately predict pore pressure.

Predicting pore pressure

Using the three buttons provided in the Compaction Analysis workflow (1D MODEL > Pore Pressure) you can:

  • Select the available composite logs with the Input Logs form: Density, Acoustic, Resistivity, Velocity, and Drilling Exponent and shale filter using either lithology modeling or user defined logs.
  • Shale filter using either lithology modeling or user defined logs. Smooth shale filtered logs by using boxcar parameters. On the Filter Parameters form, you can select which type of scale depending on which type of normal compaction trend will be used.
  • Define and optimize normal compaction trend lines for the selected composite logs.
  • Calculate pore pressure curves from the shale filtered or boxcar filtered logs using the compaction trend lines and the overburden stress log you previously generated.
  • Define a Pore Pressure Profile from the various calculated (or edited) pore pressure curves.

To predict pore pressure

  1. Under the Input Logs and Filtering button, open the Input Logs form (1D MODEL > Pore Pressure > Compaction Analysis > Input Logs and Filtering > Input Logs).
  2. Select the composite curves to use for the pore pressure prediction calculation from the drop-down lists. Click OK to go to the Filter Parameters form (other option under the Input Logs and Filtering button).
  3. On the Filter Parameters form, the selected logs can be filtered prior to calculating the pore pressure. It is useful, however, to first open the Pore Pressure Prediction view so that you can observe the effects of your filtering selections. To do this, click Show at the base of the Filter Parameters form.
  4. On the Fill Parameters form, the Filter Type section is used to apply any kind of filtering to the input logs:
    • None  No filtering is applied to the input logs.
    • Lithology Model  The default selection for filtering is the lithology model created in the Lithology sub-strip. Only the high-gamma lithologies are used for pore-pressure prediction using normal compaction trends; the application applies the filter, passing on and using for further calculations only those sections of the composite with high-gamma lithology.
    • PPP Lithology Model  A second lithology model can be calculated and used as a filter instead of the original lithology model. Click Show to open the Lithology Model for PPP form, and see Lithology model for PPP for more details.
    • Logs  If you wish to use a log (such as vshale) to perform the filtering, as opposed to the lithology model, you can select the Logs option and specify an up to two logs and parameters with which the filtering should be applied. These logs and parameters are entered into an if-and mathematical expression.
  5. Boxcar filter  You can adjust the boxcar filtering windows for each of the input logs. If the lock icon in the Calculate section is set to the Open posture you will immediately see the effects of the adjustments on the boxcar logs, otherwise, you will need to click the (Re)Calculate button.
  6. In the Display section of the form, the Show Shale Filtered Logs and Show Boxcar Logs checkboxes are checked by default. For each log, select the type of scale to use, choose between Logarithmic and Normal, to define the normal compaction trends.
  7. Click Show (or Refresh when you have the Pore Pressure Prediction view already open) to open the Pore Pressure Prediction view displaying the selected logs.

  8. When selecting a logarithmic or normal scale for your logs, you will be able to define a normal compaction linear trend or non-linear trend respectively for all logs.

  9. Click OK to close the form.

  10. Open the Linear Model form (Compaction Analysis > Linear Trends > Linear Model). With this form you can calculate the linear model based on custom linear trends, and access the linear Model table. You can calculate the linear Model based on manually drawn linear trends or you can import a normal linear Model.

    11. Export/Import Use to import or export linear NCT Model data. This functionality allows you to share linear NCT among wellbores or cases.

    (Re)Create If the lock icon is in the open posture, the linear NCT lines are calculated automatically. If the lock icon is in closed posture, click (Re)Calculate to calculate the Linear custom lines.

    Show Opens the Linear Model view so that you can view and refine the linear custom data.

  11. When you graphically create the trend lines, the trend line properties (start/stop depth, calculation method, slope etc.) are entered in the NCT Model table. Depending on the lock icon setting, the linear Model table is automatically created and updated. For more information, see Linear Model view.

  12. Click OK to close the form.
  13. Open the Pore Pressure Calculation form.

    14. Specify Input Data

  14. Pore Pressure source log - Select which logs to use in order to calculate the pore pressure curve. Selecting Shale Filtered will calculate the pore pressure logs based on shale filtering only. Selecting Boxcar will calculate the pore pressure based on the logs filtered by the lithology and boxcar parameters.

  15. Overburden Stress  - Select the Overburden Stress log to use in the pore pressure calculation.

  16. Pore Pressure source trend - Select the type of trend previously defined in the PPP view. You can select multiple trends at the same time.

    17. Specify Non-Linear Trend Parameters

    When non-linear trend has been specified in the Specify Input Data section of this form, this section is used to define the calculation method and parameters used for the non-linear trends. To know more about the methods used, please refer to section Pore pressure prediction methods.

    The custom trend or linear trends will continue to use the Linear Model Table to define the methods and parameters. NCT lines are TVD objects, therefore, care must be given when working with horizontal or highly deviated wells.

    Calculate EMW Pore Pressure - Select which source log will be used based on the Overburden Stress log and the compactions trends previously defined.

    (Re)Calculate  If the lock icon is in open posture, the pore pressure is automatically calculated. If the lock icon is in closed posture, click (Re)Calculate to calculate the pore pressure.

    To change the default name of the created log, double click in the log field. The background turns blue, indicating you can edit the text. When done, click the green check mark to apply the changes.

    Interpret Pore Pressure

    Show  - Displays the calculated pore pressure in a new track in the Pore Pressure Prediction view.

    Hydrostatic down to - For the first part of the interval below sea level, no pore pressure can be calculated. Select this option and specify a depth (positive is below Sea Level). Over the specified depth interval pore pressure is considered equal to hydrostatic pressure.

    (Re)Calculate  If the lock icon is in closed position and changes are made to the trend lines from which the pore pressure is calculated, click this button to update the calculated pore pressure curve. To honor changes, you must also update the calculation of the pore pressure (either automatically or by clicking (Re)Calculate in the Calculate EMW Pore Pressure section). If the lock icon is in the open position (and the Calculated pore pressure in the Calculate EMW Pore Pressure is updated as well), the button is inactive.

    Show  The second Show button adds the resulting Interpreted Pore Pressure log to the view.

  17. Click OK to close the form.