MAGNETIC INTERFERENCE EFFECTS ON ACCELEROMETER AND MAGNETOMETER DATA:DETECTION,QUALITY CONTROL AND CORRECTION

摘要

Accurate orientation is required for both hole trajectory determination and for information gleaned from borehole images,acoustic waveforms and other azimuthally acquired data.Orientation quality depends on high quality acquisition of accelerometer and magnetometer information.Anomalous magnetization within the wellbore and formation can influence accelerometer and magnetometer measurements. Detection and proper correction for these magnetic anomalies is important to maintain the integrity of the orientation data and consequently the results derived from the data.Using a fast robust method,quality control and corrections for these magnetic field disturbances has improved the quality of the directional components. At least four different sources of well magnetic anomalies have been observed:1)geological remnant magnetization,2)magnetic materials near,but not in the well,3)magnetic field below the casing shoe and 4) magnetic debris in the well.Geologic remnant magnetization and magnetic materials near the well are of interest to geologists and drillers,e.g. paleomagnetism and collision avoidance,respectively. Repeated multiple log passes from a test well provides proof that the orientation module is sensitive enough to detect and correct certain magnetic sources. Comparisons to gyro data have further provided verification of results in several wells. Several data sets are illustrated that suffer from both accelerometer and magnetometer bias and tool string magnetization.Corrections have been applied to remove the accelerometer and magnetic biases and tool string magnetization effects.Additional processing has been applied to extract(and correct for)the local magnetic anomalies.These local anomalies are described by three curves containing the anomalous magnetic field strength,its inclination and azimuth. Example data was diverse and varied from conventional sediments to geothermal volcanics along with anomalies from tool string magnetization to remnant magnetic fields. Data with a high rotation rate were also tested,analyzed and corrected.The limitations of the orientation module,the acquisition system and processing are summarized.