MATQUS, A Survey Quality System for Solid State Magnetic Survey Tools

Abstract

Abstract This paper describes a survey quality system for solid state magnetic survey tools. The system objective is, by use of quality control (QC) parameters, to determine a best estimate or so-called actual inclination, azimuth and upward, lateral borehole position uncertainties of tool and magnetic interference azimuth corrections excluding ruling conditions. This would allow the omission of costly verification surveys in a safe manner prvviding that additional running conditions control measures are taken. MATQUS is divided into planning, execution, analysis and computation. Main elements of MATQUS are MAgnetic Tool Error Model (MATEM) and Shell's patented SUrvey COrrection Program (SUCOP). MATEM is used in the planning phase to determine the theoretical inclination and azimuth uncertainties based on residual sensor uncertainties including the effect of magnetic interference correction as applied by SUCOP. The SUCOP corrections are applied in the execution phase. Furthermore, MATEM determines tolerances for QC parameters being the Earth's relative gravity field strength, magnetic field strength and dipangle. These QC parameters are used in the analysis phase for quality control. The QC parameters are also used in the computation phase to determine the acual inclination and azimuth uncertainties. From these upward and lateral uncertainties are determined. An example survey processed by the MATQUS concept is included. Introduction Magnetic surveys are in general perceived to he less trustworthy then gyroscopic surveys. Three main reasons are the basis for this perception. Firstly the Earth's magnetic field, the quantity the survey tool measures to determine Magnetic azimuth, is subjected to predictable / unpredictable variations. For example declination, the angle between Magnetic North and True North, will cause an azimuth uncertainty if definitive surveys are reported in True azimuth. But also magnetic field strength and dipangle vary. Secondly, the non-ideal conditions, i.e. magnetic components of the BHA surrounding the magnetic sensors will result in considerable magnetic interference, hence azimuth uncertainty. Thirdly, correction methods applied to correct magnetic surveys for magnetic interference, introduce azimuth uncertainties. The latter seems to be a contradiction but is easily explained by the fact that most of the correction methods use local Earth's field data as input, i.e. magnetic field strength and dip angle to correct the measurement of the along-the-borehole axis magnetic field component, which is the most severe magnetically disturbed one of the triad. However, the Earth's field being subjected to variations will as input for correction methods introduce other azimuth uncertainties. These uncertainties are usully not easy to predict due to the complexity of the correction method applied. It is due to this complexity that determination of azimuth' uncertainties and quality control of surveys of solid state magnetic tools is a problem. Quality is here defined as the ratio of theoretical and actual tool performance. However, knowing the actual azimuth uncertainty of solid state tools used in MWD application is very rewarding as:real time drilling position corrections can be made rather than after a verification survey is run enabling to hit the geological target the first time rightreduction in costs of verification surveys as the quality of the magnetic survey can he predicted hence verification surveys can he omittedthe uncertainties can he controlled and customised e.g. if the effect of sensor performance on azimuth is known then specifications for sensors can be derived to meet the survey requirements.