Positional uncertainty can be calculated anywhere along the wellpath, from tie-on to total depth.
The greater the wellbore length the greater the degree of uncertainty in determining the actual trajectory location. A well-understood fact: Absolute accuracy is nearly impossible because of the inherent errors in measurements (sensors, measured depth, axial or cross-axial interference, geomagnetic field uncertainty). In extended reach wells using conventional MWD techniques, poor tool error models can create positional uncertainty that can be tens of feet larger than a high quality tool. Correctly calibrated survey tools and best-practices in-field survey techniques can help minimize the uncertainty.
Whether the goal is reaching the target for an extended-reach well or safely navigating the maze of existing wells in an established field, reservoir teams can benefit from optimized well design which minimizes factors than can contribute to the wellpath’s positional uncertainty.
Well Trajectory Calculations: Visualize the Uncertainty
Two tightly-integrated software products are helping well planners, drilling engineers, and reservoir teams to minimize the uncertainty of well trajectories. Powerful data integration and visualization capabilities (CoViz 4D) and the positional uncertainty models available in the Integrated Well Designer, well planning software, can be used to interactively design wellpaths in relation to any geologic models and offset wellpaths displayed in a 3D visualization environment.
CoViz 4D and Integrated Well Designer
With the Integrated Well Designer, engineers and geologists can calculate positional uncertainty values along the entire planned wellpath. Seeing these uncertainty models presented in a 3D subsurface environment makes it easier for well planners 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 positional uncertainty associated survey tools and its application to the trajectory. This is combined with the uncertainty of the surface hole location. When utilizing the uncertainty of the reference wellpath, the designers can utilize two models; one that has been drilled, and one that is being drilled. The one that is being drilled will utilize as-drilled surveys whereas the one that has been drilled can utilize post-drilling data and survey corrections. 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 three standard deviations provides a 97% confidence level (in three dimensions) regarding the location of the wellpath in the three-dimensional subsurface space. Three standard deviations are relatively conservative and appropriate when evaluating collision avoidance, although some operators may feel comfortable with a higher or lower confidence level.
Declination date of the survey
Positional uncertainty is affected by the magnetic declination at the date and time the survey was taken. Declination is calculated in degrees between Magnetic and True North. The magnetic dip angle is the angle of the magnetic field horizontal to earth’s surface respective to the drilling location. Field strength displays the calculated or specified field strength in nanoTeslas (nT).
Source for the declination calculation can be either HDGM High Definition Geomagnetic Model, BGGM British Geological Society’s BGGM, IGRF model which is provided by the US National Oceanic and Atmospheric Association NOAA, or user-entered. Each declination model covers specific dates.
Positional uncertainty model used to survey the wellpath
Model chosen depends on the type of tool and surveying method to be used. Note: WellArchitect, Dynamic Graphics’ directional drilling/survey management software, can be used to calculate uncertainty for a wide variety of vendor’s tools, as well as all of the Operator Well Survey Group’s tools.
Positional uncertainty can be calculated anywhere along the wellpath, and at total depth (TD). IWD 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 model selected, IWD calculates the ellipsoid of uncertainty (EOU) at each location along the wellbore trajectory. When displayed in 3D, these ellipsoids form a twisted elliptical cylinder that is easily depicted by CoViz 4D.
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
Regardless of the tools and surveying methods used, positional uncertainty calculations should be considered an estimate. However, even with positional uncertainty calculations the information acquired during drilling can aid in the understanding of subsurface models. For example, often multiple wells target a specific geologic marker at a certain depth; but one well may appear to come in much deeper than a second well. With only one measurement, the marker could be anywhere within an individual well’s ellipsoid of uncertainty. With two measurements, the uncertainty is reduced, knowing that the marker is somewhere within the overlapping ellipsoids, giving drilling engineers a more accurate estimate of the depth to target.
Continually Improve the Understanding of Reservoir Conditions
As additional data are obtained during the drilling process, geological models can quickly and easily be updated to improve the placement of well trajectory. If during the drilling process, a horizon or fault is encountered sooner than expected (perhaps the horizon was determined by (inaccurate) picks in an adjacent well) the geologic model can be updated to correct the horizon depth and reposition the wellbore within the context of the updated model. Detecting significant differences between the expected and the actual horizon depth allows critical calculations such as KOP, build rate, and tangent angle to be corrected, leading to better wellbore trajectory placement.
Detecting significant differences between the expected and the actual horizon depth allows critical calculations such as KOP, build rate, and tangent angle to be corrected, leading to better wellbore trajectory placement.
Integrated Data, Uncertainty Calculations, and Visualization for Reduced Risk
Technical advancements in survey 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 planned wells, reservoir teams can then apply positional uncertainty models and collaboratively evaluate conditions to plan and execute drilling strategies that minimize drilling risk.
