The Combination of Solid-State Gyroscopic and Magnetic Surveys Provides Improved Magnetic-Survey Data and Enhanced Survey Quality Control

Abstract

Summary The use of advanced solid-state gyroscopic sensors has become both a viable and practical option for high-accuracy wellbore placement, with the potential to outperform traditional mechanical gyroscopic systems. In this paper, we describe how the contributions of the new gyroscope technology are causing service providers to reconsider current survey practices, and to examine how the new gyroscopic-survey tools can best be used for wellbore surveying and real-time wellbore placement. The simultaneous application of multiple survey tools, largely made possible as a result of the unique attributes of solid-state gyroscopic sensors (including small size and significant power reduction), has clear benefits in terms of enhanced well placement, reliability, and the detection of gross errors in the survey process. Further advantages accrue through the combination of different but complementary survey methods. In this paper, we focus mainly on the benefits of combining gyroscopic and magnetic measurements to reduce or remove the known errors related to the Earth's magnetic field to which magnetic-survey systems are susceptible: errors in total magnetic field, declination, and dip angle. In this context, the use of statistical estimation techniques depending on the performance models of the survey systems used is described. For post-drilling surveys (using drop-survey tools or wireline-conveyed tools, for example), post-run analysis of the data using least-squares estimation techniques is appropriate. Alternative methods capable of achieving real-time data correction during drilling are also described, and results are presented to demonstrate the potential for enhanced magnetic-survey performance. The principles described can be used when running basic magnetic measurement-while-drilling (MWD) systems and for systems that use field correction methods, such as the various in-field referencing (IFR) techniques, that are frequently used. The proposed methodology is of particular benefit in the former case, allowing enhanced magnetic surveying to be achieved without the need for expensive and complex magnetic-field correction procedures. The potential also exists either to identify or to correct possible errors in the IFR data when such methods are used. The particular strength of the methods described in this paper is in the estimation and correction of errors in the Earth's magnetic-field parameters while drilling, or after a section of the well has been drilled. Currently, all the magnetic models are dependent on magnetic measurements alone and at the surface or in space. Although there are techniques to estimate the downhole values, such techniques are difficult to quality control (QC) or validate. These updated magnetic-field data allow further sections of the same well, or additional wells in the same region, to be drilled using MWD alone, without the need to use field referencing techniques to achieve accurate well placement. The consequent rig-time and cost savings are expected to be significant.