Magnetic Shielding During MWD Azimuth Measurements and Wellbore Positioning

Abstract

Abstract Magnetic contamination of the drilling fluid may contribute significantly to errors in directional surveying of wellbores. Such contaminations shields the field measured by the magnetic sensors in MWD directional tools. The resulting error often exceeds typical sensor uncertainties, and so the accuracy of the inferred wellbore position is severely degraded. Although the magnetic interference can be reduced by careful attention to procedures for transport, pumping and handling of drilling fluids and their associated additives, the problem has not yet been eliminated. In order to understand the magnetic shielding effect in a well-defined setting, we have initiated a series of laboratory measurements, where magnetic material of known properties are added to a well defined non-magnetic drilling fluid, and measuring the resulting magnetic shielding with a fluxgate magnetometer immersed in the fluid. In our experiments, powdered magnetite was added to a mixture of xanthan gum in water to obtain a mixture of nominal magnetic susceptibility typical of drilling fluids containing metal from pipe wear and erosion. Immediately after stirring, the vertical component of the Earth's magnetic field inside the liquid was measured and found to be significantly damped. Without touching the system, the field was subsequently logged. It was found to continue to decay for the next hour, or so, reaching a fairly sharp minimum. The magnetic field then started to increase slowly and irregularly again over the next few days. The observed effect, combined with analysis of directional data, can explain errors encountered in directional surveying. Introduction The drilling fluid is one of the factors that may contribute significantly to errors in directional surveying of wellbores. The fluid can contain magnetized contaminations which shield the Earth's magnetic field as recorded by the magnetic sensors used in measurement while drilling (MWD) directional tools. According to Wilson and Brooks (2001), Torkildsen et al. (2004) and Amundsen et al. (2006), this shielding effect typically causes azimuth errors of 1 - 2 degrees, but under unfavourable conditions, errors even five to ten times larger may occur for certain wellbore directions. Thus, the drilling fluid may degrade the borehole position accuracy sufficiently to reduce the possibility of hitting the planned target. The commonly used model for estimating wellbore position accuracies (see Ekseth (1998) or Williamson (1999)), does not include the effect of magnetic contamination of the drilling fluid. Torkildsen and Lotsberg (2001) found that for some directions the presence of the phenomenon can be inferred by using appropriate acceptance criteria. Nevertheless, there is a need for a general approach to quantify and reduce this troublesome influence on the magnetic sensor readings. The magnetic properties of a drilling fluid is known to vary significantly, depending on such as mechanical wear of downhole and surface equipment, the mineralogical composition of the weight material and the bulk material transferring procedures of the drilling fluid additives, and the circulation properties of the drilling fluid as reported by Torkildsen et al. (2004). In this contribution we shall show that it can also depend significantly on operational aspects of the drilling and data logging processes.