Directional surveys acquired by Measurement While Drilling (MWD) are subject to many errors that are not easily recognized by traditional Quality Control (QC) procedures. This commonly leads to inaccurate wellbore placement and greater positional uncertainty. Common sources of MWD survey error are inaccurate geomagnetic references, localized distortions in the natural magnetic field, poor instrument calibration, random sensor noise, magnetic mud, and human error. Often times, such errors go unrecognized due to limitations in traditional single-station QC tests. This is a significant problem because wellbore collision avoidance, geological modeling, and reservoir drainage are all greatly affected by wellbore placement accuracy. Fortunately, most sources of MWD error can now easily be identified and corrected through implementation of robust independent survey quality control processes. By using web-based systems to facilitate this process, drillers can benefit from the most powerful quality assurance practices which can be standardized across the industry regardless of service provider or vendor specific technologies.
Well placement by MWD employs the use of orthogonally positioned accelerometers and magnetometers to measure the orientation of the bottom-hole assembly (BHA) relative to the Earth's gravitational and magnetic fields as shown in Figure 1. Taking survey measurements at regular intervals along the well path enables computation of the wellbore trajectory through minimum curvature interpolation.
Standard MWD surveying is subject to numerous error sources which can lead to inaccurate wellbore placement. These sources of error are divided into three categories: gross, random, and systematic. Gross errors occur from human mistakes, instrument failure, or environmental factors that cannot be predicted or estimated. Random and systematic errors occur with some measure of predictability and can therefore be estimated and quantified. The standard approach for estimating positional uncertainty in the wellbore caused by random and systematic survey error is to use instrument performance models called tool codes. Tool codes provide the mathematical framework to compute Ellipsoids of Uncertainty (EOUs) which represent positional uncertainty evaluated at a particular sigma, or confidence level (Grindrod 2016). Figure 2 shows how EOUs form an elliptical tunnel when propagated along the well path which characterizes the statistical distribution of where the actual wellbore could exist. Quantifying positional uncertainty is a critical step in the well planning and drilling processes because it enables drillers to evaluate collision risk and understand wellbore placement.