Determining Downhole Magnetic Interference On Directional Surveys
Authors: Scott A.C. 
Published: December 2007

A magnetometer inclinometer system developed to orient mud motors and produce survey data, additionally provides data on magnetic field strength and inclination. Observations from field operation of this system have initiated a detailed investigation of magnetic conditions during drilling. An analysis of a field data set is used to support the belief that magnetic pole strength changes can be predicted. A procedure to quantify these changes is described.

Introduction

Magnetic interference can be defined as the compass error resulting from interaction of the earth's magnetic field and a field produced by one or more separate sources. In drilling operations, these sources can be categorized as follows:Magnetized steel sections in the drilling assembly.Adjacent casing strings or sidetracked drilling assemblies.Geological sources.Upper atmosphere disturbances.

This paper concerns itself primarily with the first source, since its effect can be controlled by the correct selection of non-magnetic collar length and compass spacing.

The application of classical magnetic theory, together with a better understanding of the changes in the magnetic properties of the drilling assembly as drilling progresses, predicts the ability to analyze specific drilling assemblies and make good estimates of their effects on survey accuracy for the particular geographic location and wellbore path under consideration.

The complexity of the problem and the need to complete wells with a minimum of fuss has led to the current empirical guidelines on non-magnetic collar and spacing requirements. These are based on simplified, "worst case" assumptions and experience. This is the common sense approach for field operations.

The rewards for a better understanding of the magnetic properties in a drilling assembly are high. Not only will it be possible to verify "worst case" assumptions, but it will also be possible to analyze surveys in retrospect as well possible to analyze surveys in retrospect as well as in advance of drilling. The above has obvious implications, for example, in the drilling of relief wells.

Although qualitative support is often present in field results, good numerical data to test the theory is hard to find, since the survey device is deliberately positioned so as to minimize the effects we are now seeking to measure. However, the analysis presented is one example of a case where the magnetometers of an orientation probe were close enough to a mud motor assembly and far enough from the heavy weight drill pipe above; that the analysis is simplified. Also in this example, the effects of casing and of geological sources were apparent.

In addition to the field results, a procedure is described to accurately determine the quantities involved under controlled conditions. Now that the problem is more clearly understood, a limited number of such measurements should provide good data to apply to the general case.

THEORY

The theory is founded upon the basis that we are dealing with the phenomenon of induced magnetism, i.e., the steel sections of the drilling assembly will become magnetized by the presence of the earth's field. For purposes of presence of the earth's field. For purposes of calculation, this can be treated as the appearance of magnetic poles of the magnitudes mu and ml on the ends of the steel sections.