Analyzing Wellbore Location Uncertainty

Display an Ellipse of Uncertainty (EOU) along the wellbore as it progresses from heel to toe. The purpose of an EOU at any point along the borehole is to quantify the possible locations of that point. The actual location of the point can be anywhere within the ellipse. Calculations for each point are based on the MD, azimuth, and inclination values.

EOU can be used to optimize wellbore placement and used within the FPZ workflow to locate potential inter-wellbore communication concerns between neighboring wells that are undergoing hydraulic fracture.

EOU is defined at a specified number of standard deviations (Number of Sigmas) using the Highside, Lateral, and Alonghole (HLA) system, which are defined as follows:

• Highside - this axis is normal to the along hole axis in the vertical plane, positive upwards.
• Lateral - this axis is normal to the along hole axis in the horizontal plane, positive to the right when viewed looking downhole.
• Alonghole - this axis is aligned with the local wellpath orientation, positive with increasing measured depth.

Measure the proximity between either a single adjacent well, or all of the wells in the display area, and if measuring the proximity for all of the offset wells, output the results to Microsoft Excel.

To analyze wellbore location uncertainty

1. With the subject well displayed on the map, launch Wellbore 3D Viewer for the subject well.
   The Wellbore 3D Viewer dialog box appears.
2. Using the Wellbore Uncertainty & Proximity Scanning tab, select from the following options:

• Confidence interval in number of sigmas - a sigma is a standard deviation. You can select up to three standard deviations for your analysis. 68% of the values lie within one standard deviation from the mean. 95% of the values lie within two standard deviations from the mean. 99.7% of the values lie within three standard deviations from the mean.
  
  The number of Sigmas in AccuMap 3D Viewer is initially set to the default value 2, which corresponds to a 95% confidence interval for a single dimension of the HLA system. The size of the EOU increases along with the MD as the uncertainty accumulates at each survey point. The last EOU is rendered with an attached label showing the size of the EOU and the corresponding MD.
  
  For example, the label 2.00HL (17.4, 58.2) @5900MD shows the following parameters:
  
  - 2.00 – Number of standard deviations (Number of Sigmas) used to define EOU HL size;
  - 17.4 meters semi axis is the Highside Uncertainty as 2.00 * SigmaH, where SigmaH equals the Highside Standard Deviation Error (in the horizontal leg, the SigmaH is usually close to the TVD or Subsea error);
  - 58.2 meters semi axis is the Lateral Uncertainty as 2.00 * SigmaL, where SigmaL equals the Lateral Standard Deviation Error.
  You can also set the individual Error Model Scaling Factor for the concrete well by right-clicking the wellbore.
• Error model scaling factor - globally applied to the subject well and offset wells. To set a unique error model scaling factor to specific offset wellbores, right-click the wellbore, and select Set Wellbore Uncertainty Params from the context menu.

3. Using the Wellbore Uncertainty & Proximity Scanning tab, click Scan Proximity to Offset Wells ().

• To output a detailed report to Microsoft Excel, click Proximity Report ().

4. Click the 3D Proximity Log check box to display the log in the main display.
5. Click Refresh 3D Scene () to apply your selections to the display.

Calibrating EOU to Drilling

EOU is typically calculated by the Drilling Subcontractors based on the Instrument Performance Model (IPM) for the concrete tool used in each well (also known as the Tool Code).

Since AccuMap doesn't have access to this type of data, Wellbore 3D Viewer generates the expected EOU using the generalized error propagation model.

Using the Error Model Scaling Factor, you can calibrate EOUs to be close to the Drilling Subcontractor EOUs.

To calibrate Wellbore 3D Viewer data to drilling data

1. Set the Error Model Scaling Factor to 1.
2. Find SigmaL from the last EOU label.
3. Request the SigmaL_IPM for the last EOU MD from the subcontractor.
4. To calibrate EOUs, use Error Model Scaling Factor = SigmaL_IPM / SigmaL

Related Topics