Assessing quantitative risk

You use the Quantitative Risk Assessment (QRA) form (Wellbore Stability> Predict Mud Weight > Predict Mud Weight Window > QRA) to examine the sensitivity of the selected input parameters of your geomechanical model on the predicted required mud weight window, either at a certain depth or for a particular wellbore section. The output will be a probability distribution for the drilling success, where achievement of success is defined by engineering design. This information is extremely useful when prioritizing data collection or when determining the new data you need to increase the confidence in the wellbore stability analysis results. You can perform QRA at a specific depth or for an entire section or casing.

To assess quantitative risk

  1. Click Show to open the Predict Mud Weight view for reference.

  2. Specify whether to perform the analysis for a specific depth or a casing or section range.

    Analyze at depth  Select to apply the analysis to the hole section at a specified depth only. Specify a depth in MD or TVD for which you will specify the uncertainty inputs in the next step.

    Analyze for casing  Select to apply the analysis to a particular casing section (selected in Casing parameter of the form). After you select the casing section, the critical depth for lower and upper bound is automatically updated.

  3. In the Specify Inputs area, you define all of the inputs before running the analysis. Inputs are organized by tabs, with the available tabs and fields varying, depending on the type of analysis selected. If Analyze at depth is selected, inputs are specified on either the Bounds or Parameters tab. If Analyze for casing is selected, available tabs are Maximum Lower Bound, Minimum Upper Bound, and Parameters. The analysis of these parameters provides probabilistic assessments of the upper and lower-bound safe mud weights for the selected open-hole section.

    If you choose Shmin as the upper bound for the mud window, Shmin is the only parameter which influences the uncertainties of the upper bound as Shmin is not calculated from the other parameters.

    For each parameter, the application provides a most likely value. The default minimum and maximum values are the most likely value minus and plus 5% or 10%, depending on the parameter (this value is listed in the table on the respective tab). The default angular uncertainty (for hole deviation, azimuth, breakout width, and azimuth of SHmax) is ±10 degrees. The application handles azimuths that extend from west of north to east of north, either by negative values for the azimuth or by values close to 360 degrees. For example, you can enter N10°W either as –10 or as 350.

    You can edit the minimum, most likely, and maximum values directly or by changing the deviation from the most likely value in Diff column. If the minimum and maximum values are symmetrical around the most likely value, the application assumes a Gaussian distribution. If not, it assumes a log-normal distribution.

    If the most likely value does not lie between the minimum and maximum values, the application displays a warning. In addition, when you edit the minimum or maximum value, the deviation value is removed and the background of the corresponding deviation field turns gray to indicate that the deviation is not in accordance with the current minimum and maximum values. You can reset the minimum and maximum values by typing a new value in the Diff deviation column. The minimum and maximum values adjust to fit the deviation.

    The values in the Min, Most likely, and Max columns are automatically populated based on the specified depth.

    Random  Indicates that the parameter has an uncertainty associated with it. You can select only eight of the available parameters for uncertainty analysis.

    Name  The name of the analysis parameter.

    Min  The minimum value for the input as defined by the deviation specified in the Diff column or manually specified.

    Most likely  The value calculated based on the minimum and maximum values for the parameter. This value can be changed manually or by changing the deviation value in the Diff column.

    Max  The maximum value for the input as defined by the deviation specified in the Diff column or manually specified.

    Unit  The unit associated with the input parameter. You can right-click in the column to change the unit. Changing the unit also changes the unit selection for all of the other parameters that share a common unit. For example, SV, Shmin, and SHmax all use MPa. If the unit for Sv is changed to GPa, the units for Shmin and SHmax (as well as any other parameter using MPa) also change to GPa.

    Diff  The default deviation in % for the Min and Max values from the Most likely value.

    The following options can be used (if you do want to use one or more of the options, select the checkbox to activate it):

    Lock "Diff" and "%"  If you change any of the entries in the Diff and/or % column with this option on, changing the MD or casing section will not update these columns in the table back to the default values. Instead, the new settings you have entered and selected will still be used.

    Tie SHmax to Shmin for all values  Sets SHmax equal to Shmin.

    Consider Stress Ratios  Adds the following stress ratios as uncertain parameters:

    (Shmin-Pp)/(Sv-Pp)

    (SHmax-Pp)/(Sv-Pp)

    You can narrow the final mud window by defining the following parameters on the Parameters tab. Entering a positive number indicates a pressure increase due to surging when pipe is moving downward. Entering a negative number indicates a pressure decrease due to swabbing effects when the drill string is pulled upward.

    Swab  Use to include the effect of swab pressures. Entering a negative swab pressure shifts the lower mud bound to the right

    ECD effect  Use to enter an ECD pressure value. The application assumes that the increase in wellbore pressure due to the ECD effect occurs only while circulating. When circulation stops, the mud pressure drops to the equivalent static density.

    Surge  Use to define a surge pressure. Entering a positive ECD and surge pressure shifts the upper mud bound to the left.

  4. Click Re-Calculate to perform the analysis and click Show to open the QRA Results view.