Prediction of Wellbore Position Accuracy When Surveyed With Gyroscopic Tools


This paper describes a new method for estimation of well bore position accuracies, when using gyroscopic tools. The developed method represents a solution to the industry’s need for a general and flexible error model which is applicable for all gyroscopic surveying tools and services.

The general gyro error model consists of a new set of error terms and a mathematical description of how the different error sources contribute to position uncertainties dependent on sensor configurations and operational modes. The model is suitable for appropriate modelling of most gyro surveying services.

The error propagation mechanisms are chosen to be identical to those in the ISCWSA’s MWD error model (SPE 67616), which has become an industry standard during the last five years. Thus future standardisation and software implementation are simplified. The description of the model and the attached numerical examples should be sufficient to implement the model.

The paper is a product of a collaborative work in ISCWSA (Industry Steering Committee on Wellbore Surveying Accuracy).


Work in recent years by a group of industry experts, members of the ISCWSA, culminated in the publication of an error model for magnetic Measurement While Drilling (MWD) survey tools1 which has become widely accepted and used within the oil industry. The work described here was born out of a desire to extend that model to encompass the full range of surveying techniques available to the industry, and, specifically, to include gyro survey tools. The formative work, which has led to this paper, took place within the meetings of the ISCWSA, with subsequent detailed development being undertaken by a Gyro Working Group within that committee.

Gyro tools are widely used for completion surveys and to control the drilling of well bores in regions of high magnetic interference, where the magnetic tools become less reliable. Recent advances in gyro technology have led to the application of gyro survey tools during drilling operations; the MWD gyro.

This paper contains a description of a gyro survey tool error model, generated to provide:

  • estimation of well bore position accuracies
  • a standardised and generalised model for the oil industry
  • a model that is easy to implement in well planning and survey management software


The model described here has been generated in response to a demand for a single model which is adaptable to the broad range of gyro based systems and services available to the oil industry, both now and in the foreseeable future. The formulation, as described, has sufficient flexibility to model the growth of survey errors in such systems, taking into account the types of sensor used, the sensor configuration and any operational procedures which will influence the performance of the system. The model also attempts to provide a fair representation of the physical processes which influence the propagation of errors, whilst avoiding unnecessary mathematical complexity. A balance between these two objectives has been sought in the selection of an acceptable format for the model.

There follows a description of the model for the derivation of inclination and azimuth errors in both stationary and continuous modes of survey operation. This description includes the definition of terms, a statement of the assumptions made in the preparation of the model and details of the gyro and accelerometer error terms that have been included. A subsequent section illustrates the application of the model to some example survey tools. A simplified derivation of the error model coefficients and software implementation details are given in the Appendices.