Positional uncertainty can be calculated anywhere along the wellpath, from tie-on to total depth.
A well-understood fact: Absolute location accuracy is nearly impossible because of the inherent positional uncertainty (commonly referred to as “errors”) in measurements associated with sensors, depth measurements, axial or cross-axial interference, and geomagnetic field uncertainty, for example. In extended reach wells using conventional MWD techniques, applying poor or inappropriate tool error models can add positional uncertainty of tens of feet larger than a high quality tool with advanced applications. Correct tool applications, such as short collar corrections, in-field referencing, wellpath orientation, multi-station analysis, and survey intervals help minimize the uncertainty the most.
Well Trajectory Calculations: Visualize the Uncertainty
CoViz 4D and WellArchitect
With WellArchitect, engineers and geologists can calculate positional uncertainty values along the entire planned wellpath while accounting for different surveying tools and programs. Seeing these uncertainty models presented in a 3D subsurface environment, via the integration of CoViz 4D, makes it easier for multidiscipline team members to work collaboratively to answer questions such as:
- Can the target location be reasonably achieved?
- Does the proposed wellpath risk collision with existing or planned wellpaths?
- Is the wellpath being planned too close to a lease line?
- How accurate is the geological model?
Calculating Positional Uncertainty
To begin the process, engineers calculate the path’s positional uncertainty associated with survey tools in combination with the vertical and horizontal uncertainty of the surface hole location. When assessing the uncertainty of a planned wellpath, the designers can utilize two modes: one as if the wellpath has been drilled (for when the plan is used as an offset), and one for while the plan is being drilled (for when the plan is used as a reference). As a reference, the eas-drilled (or “while drilling”) surveys are used, whereas positional uncertainty as an offset utilizes post-drilling data, survey corrections, or even resurvey run. Three factors determine the accuracy of the positional uncertainty calculation: confidence level, declination data, and survey model.
Confidence level (number of standard deviations)
A default of two standard deviations provides a 74% confidence level (in three dimensions) regarding the location of the wellpath in the three-dimensional subsurface space. This means that the positional uncertainty calculated will be smaller value, but will also only likely cover 74% of the volume. Setting to three standard deviations (97% confidence) provides a more conservative estimate of uncertainty, and is appropriate when evaluating collision avoidance, although some operators may feel comfortable with a higher or lower confidence level. Note that when performing clearance calculations, the confidence level is set by the anti-collision rule itself to ensure that the positional uncertainty of all paths are dealt with appropriately.
Geomagnetic Model
Positional uncertainty is affected by the magnetic declination at the date and location of when and where the survey was taken.
WellArchitect accommodates numerous sources for the declination calculations including HDGM (NOAA’s High Definition Geomagnetic Model), BGGM (British Geological Society’s Global Geomagnetic Model), MVHD (MagVar’s High Definition model), as well as the IGRF model, provided by the US National Oceanic and Atmospheric Association (NOAA), and user-entered values. (Note: Some models require separate licenses from the appropriate vendor.)
Positional uncertainty model used to survey the wellpath
The positional uncertainty model chosen depends on the type of tool and surveying method to be used, as well as the expected post-survey corrections. WellArchitect can be used to calculate uncertainty based on a wide variety of vendor’s tools, as well as all of the Operator Well Survey Group’s and ISCWSA’s tool types and models.
Positional uncertainty is accumulated along the path, and can be queried anywhere along the wellpath, including at total depth (TD). WellArchitect can display the ellipsoids in 3D, as well as showing the dimensions of the ellipsoid of uncertainty using:
- length of the major semi-axis
- length of the minor semi-axis
- vertical semi-axis
- angular direction (relative to True or Grid North) of the minor semi-axis
Based on the number of standard deviations, the declination, and the tool type and model selected, WellArchitect calculates the ellipsoid of uncertainty (EOU) at each survey station (or intended surveying station) along the wellbore trajectory. When displayed in 3D, these ellipsoids form a twisted elliptical cylinder that is easily depicted.
A planned wellpath with ellipsoids of uncertainty, geologic (blue) targets, and surrounding actual wellpaths.
Trajectory Location Is an Estimate, but Additional Data Improves Geologic Understanding
Integrated Data, Uncertainty Calculations, and Visualization for Reduced Risk
Technical advancements in surveying and drilling technologies are providing volumes of data to help well planners and drilling engineers better understand and navigate the complexity of subsurface environments. When these data are integrated and presented in a 3D environment, depicting relevant geological formations, as well as existing and future wells, reservoir teams can then apply positional uncertainty models and collaboratively evaluate conditions to plan and execute drilling strategies that minimize drilling risk.
CoViz 4D, a data visualization and analytics software from Dynamic Graphics, Inc., gives reservoir teams the ability to plan and drill wells with greater confidence. Powerful visualization capabilities and positional uncertainty calculations provide a more accurate understanding of wellbore placement in context of geologic formations. WellArchitect, also from Dynamic Graphics, Inc., offers a wide range of powerful visualization, analysis, and planning tools, promotes a more accurate understanding of hydrocarbon assets and facilitates better well planning. To learn more about CoViz 4D contact our team.
