Page | 51
value would be used. This mobile condition had implications for both the start and end reference for the survey and its effect on Drift Curve closure and survey reference accuracy.
When setting the Gyro Orientation Reference for higher inclination surveys the gyro would be set with the spin axis perpendicular (oriented across) to the wellbore path, with due account taken of the expected gyro drift rate and any change in the wellbore azimuth trajectory. Setting the spin axis in this attitude, provides the most stable orientation for gyro operation as the gimbals tilt with inclination and reduces the potential for Gimbal Lock (System Bearing Jam) and subsequent gyro spin out and survey miss-run. The offset reference orientation value (relative to True or Grid North) applied in this technique was accounted and corrected for by applying a baseline shift to the calculated Drift Curve. 9.5.2 Gyro Drift – Precession Correction The vast majority of gyro surveying performed for the industry, even today, still utilises rotor systems. A spinning gyro rotor tends to keep its axis pointing in the same direction. This is called Gyro Rigidity. If a force is applied which tends to change the direction of the spin axis, the axis will move at right angles to the direction of the applied force. If the spin axis is horizontal and you try to tilt it, the axis will turn. If the axis is horizontal and you try to turn it, the spin axis will tilt. This second characteristic of a gyro is called Precession. In normal operational use, conditions such as bearing wear, Temperature Coefficients of Expansion (Inertial Mass Distribution – C of G) and System Attitude Change (related to a given well profile) all interact to generate a net force which acting on the Gyro Spin Axis cause the gyro to precess (drift from its initial orientation reference). The gyro precession experienced during a survey has historically been corrected with a Drift Curve constructed with drift data samples recorded during the in-run and outrun survey. The frequency and duration of drift checks has tended to change over time but the basic premise has always been to take samples related to time, attitude change and temperature (particularly for deeper, hotter surveys). Drift checks were normally taken at least every 15 minutes, for a sample duration of 5 minutes with film systems. However, with the introduction of later Vernier scale readings, SRG and digital data, sampling criteria tended to change to 10 minute intervals with 3 minute sample duration. Drift Checks during the survey were no longer relevant or required with the advent of earlier discreet sampling North Seeking Systems (more later). However, they are beneficial and recommended when operating the current North Seeking Systems in continuous Dynamic Mode where the individual or calculated survey sample stations are not determined by discreet north seek sensing. The drift correction data is then either applied on its own or as a super set to the Gyro Calibration Model (current systems). The calculated real-time drift correction curve is tied and closed to the start and end reference data, be it a sight observation or north seek reference as in current technology systems. 9.5.3 True Centre Correction (or Offset Centre Correction) From an early stage it was recognised that a gyro survey tool had inherent misalignments associated with the modular structure of the Instrument Stack and more particularly the vane type centralization (Weatherford) which was universally adopted for running in both open and cased hole. These centralizers were not a precision piece of manufactured equipment and variants of this centralizer design, all be it with improved centralization capability, continue to be used today for some applications. These misalignments result in small errors in both the inclination and azimuth values calculated and recorded for each survey point. In 1936 the Sperry-Sun Well u y ng omp ny d lop d ‘ rue Centre Correction’ methodology which still forms the basis of cu n ppl c ons. h ‘ u c n ’ nd ‘co c d alues’ are calculated using a Polar Coordinate Method by representing each Station data set as a radial and angular coordinate. Provided sufficient tool rotation has taken place between the in-run and out-run samples, the common intercept of each sample vector pair denotes the True Centre Correction Value which can then be applied to each individual survey sample. True Centre Correction is particularly important in shallow, low angle, multi-well applications where small errors in Inclination can seriously misrepresent current well position and hence adjacent well displacement.
Made with FlippingBook flipbook maker