Along-hole Depth (AHD) is the most fundamental subsurface wellbore measurement made. Well depth is the main descriptor of wellbore position, measured from zero depth point (ZDP). This is translated into vertical depth (V) using inclination (I). V is the main descriptor subsurface wellbore events and then North (N) and East (E) act as the Qualifying descriptions of V.

Well depth is commonly described as measured depth (MD) and is used to describe well construction, navigation and collision avoidance, drilled geologies, reservoir properties, fluid gradients and interfaces, production, and subsurface positioning of well services. AHD is a calibrated and corrected well depth measurement defined using a specific rig-state and can deliver improved subsurface position and positional uncertainty.

Well depth, I and azimuth (A) are used to calculate subsurface 3D position. Well depth is measured at surface, represented by drillpipe or wireline length. I and A are subsurface measurements referenced to the provided well depth. These together provide the navigation information required to arrive at the 3D positions of N, E, and V. These positions are used to define the location of subsurface events such as well placement, geological horizons and fluid contacts.

This paper outlines a method ("3D method") for defining 3D subsurface positional locations using "way-points" (Bolt 2021). Way-points represent a sequential series of specific, calibrated, 3D positional locations each defined by calibrated and corrected AHD. Based on Pythagorean geometry using AHD, I, and A measurements, these are converted into N, E, and V positions. Each way-point has a specific N, E, and V positional uncertainty.

Four component accuracies are used to describe the individual AHD, I and A measurement uncertainties at each way-point: calibration and observation, applied correction, model-fit, and a fixed-term applicable to all observations. AHD, I, and A measurement uncertainties which are converted into individual interval N, E, and V positional uncertainties and sequentially concatenated.

The method provides a simplified yet accurate solution to 3D positional and positional uncertainty. The calculations demonstrate the dependency of the positional and positional uncertainty results on both interval spacing between way-points and measurement accuracy. The example results demonstrate that each well has its own specific and unique N, E and V positional uncertainty description. Specific positional uncertainty requirements of operators can be answered to through instrumentation accuracies and way-point interval spacing defined in the well survey program. Well placement can be more easily portrayed, reservoir characteristics more confidently reported, and asset volume estimation improved.

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