A Methodology for Portraying Three-Dimensional Positional Uncertainty Using Along-Hole Depth, Inclination, and Azimuth Measurement Accuracies
Authors:   DwpD Ltd., Depth Solutions, Bolt Harald , Harbidge Philip ,   PDHWellplacement
Published: November 2024

Along-hole depth (AHD) is the most fundamental subsurface measurement made. AHD, together with inclination (I) and azimuth (A), are used to describe the three-dimensional (3D) position of the wellbore and hence the 3D position of the recorded subsurface parameters. This is used to describe the well geometry in 3D, drilled geological horizon locations, reservoir descriptions and models, and fluid contacts and gradients. Operators are presented with a model for managing their 3D positioning and positional uncertainty based on AHD, I, and A measurement values and accuracies. Geo- and petrophysical data, depending on rig state, can be logged to measured depth (MD) and then matched to specific 3D positions while providing bespoke 3D positional uncertainty. The operator’s 3D positional uncertainty is determined and managed by defining the well survey parameters, the measurement method chosen, and equipment accuracy specifications. A wellbore is considered as a sequential series of intervals; these approximated to straight-line descriptions of AHD, I, and A. Using basic geometry, vertical (V), North (N), and East (E) positions are defined per interval. AHD, I, and A calibration and observation, correction, and model-fit accuracies are used to define the interval uncertainties. These interval uncertainties are used to derive the separate interval V, N, and E contributory uncertainties. These are sequentially concatenated, resulting in V, N, and E positional uncertainties. The 3D positional uncertainties are influenced by AHD, I, and A measurement accuracies, well geometry, conveyance specifications, and interval spacing. AHD accuracy is the most important contributor to V uncertainty. The MD can be derived using drillpipe as well as wireline conveyances and corrected to AHD. Individual V, N, and E positional uncertainties are shown to be dependent on well geometry, sampling interval spacing, conveyance specifications, as well as AHD, I, and A measurement accuracy choices. The 3D positional data and positional uncertainty are compared against results using conventional MD and high-accuracy corrected AHD. Four example well geometries demonstrate that each well has its own unique, and quite different, 3D positional uncertainty profile. Geological modeling and reservoir descriptions are provided with specific uncertainties. Fluid levels and gradients are defined against V using auditable uncertainties. The measurement process and accuracy decisions can hence be tuned to meet the operator’s 3D positional uncertainty requirements for well placement and reservoir description. This results in better data and decisions and improved asset value. Operator 3D positional uncertainty requirements are used to define the well survey plan, including optimization of equipment used and measurement procedures. These choices define V, N, and E positional uncertainties. Each well has its own unique description of the 3D position and positional uncertainties, the same way as the subsurface is uniquely described. Optimization of accuracy choices arrives at a value-for-money approach to AHD, I, and A measurement, resulting in 3D positional uncertainty tailored to operator needs.