The heart of a conventional gyroscope is the gyro wheel, which spins at high speed symmetrically about an axis. A perfect gyro wheel will maintain its orientation in inertial space for as long as no external forces act on it to alter the direction of the spin axis.
In conventional gyro directional survey tools this principle is used to measure the change in orientation of the tool, when following the wellpath, relative to the original spin axis orientation at the start of the survey. Consequently the conventional gyro needs aligning of the initial gyro orientation to a known direction.
The term 'conventional' gyro is used for film-type and surface read-out gyroscopic survey tools. Gyro tools are run on wireline and along hole depth measurements are taken from the wireline measuring head. Gyros are delicate complicated instruments and are only run by Survey Engineers. Conventional gyro tools are used in casing to confirm magnetic surveys taken. They can be used for definitive surveys. However, their accuracy and quality control is less than that of North Seeking gyro tools and of solid state magnetic surveys.
Principle of conventional gyroscopic tools
Conventional gyroscopic directional drilling survey tools measure inclination, azimuth and toolface. Inclination is measured with a pendulum. As the pendulum is aligning itself along the gravity vector, it will always sit on the low-side. The projection of the cross hairs of the pendulum on the disc (in the cross-borehole plane) will indicate low-side.
The high-side direction is now determined by the line through the cross hairs and the centre of the disc. Actually, this high-side direction is the projection of high-side on the disc and therefore it is called projected high-side. If the tool's azimuth changes, the position of the pendulum will not change but the position of the centre of the compass card will rotate around the pendulum. This results in the projected high-side following directional drilling survey tool rotations. On the other hand, the gyro pointer which is attached to the gyro wheel will stay oriented to its initial position (initial azimuth) due to the principle of gyros. This results in the azimuth change being the angle between the gyro pointer and projected high-side. In the same way toolface is determined. A pointer fixed to the compass card will sense tool rotation while the gyro pointer is invariant to tool rotations. The angle between the toolface pointer and gyro pointer is toolface. In case the gyro pointer is initially not aligned with True North, an initial azimuth is determined. This is the angle between the gyro pointer and True North.
Basic design of conventional gyroscopic directional drilling survey tools
The gyro wheel is a balanced mass spinning at high speed symmetrically about an axis. To allow the gyro wheel to maintain its original orientation it is mounted in a gimbal assembly with an inner and an outer gimbal. In general the spinning wheel will sense in addition to the tool rotation along the wellpath an artifact due the rotation of the Earth. The rotation of the Earth which is defined along the spin axis of the Earth, can be divided into a borehole component and a cross-borehole component. To allow the spinning wheel to maintain its original orientation, the tool will rotate about the outer gimbal to compensate for the Earth's borehole rotation component. This is called drift. Imperfections of the gyro may also cause the gyro wheel to drift. The tool will also rotate about the inner gimbal to compensate for the rotation felt by the spinning wheel due to the cross-borehole component. This is called tilt. This design is implemented in Tilt Scale Gyros (TSGs). In Level Rotor Gyros (LRGs) the gyro is also mounted in a gimbal assembly but tilting of the gyro is maintained zero using a feedback mechanism. This feedback converts tilt into drift. This reduces saturation of the gyro whereby maximum tilting of the inner gimbal is reached.
Tilt scale gyro directional drilling survey tool
A Tilt Scale Gyro tool (TSG) is a 'free' gyro in which the gyro wheel is mounted in a gimbal assembly, which allows the tool case or housing to be moved about the axis of the spinning rotor without disturbing the attitude of the latter in space. The orientation of the gyroscope is shown by a compass card mounted on the outer gimbal which yields an apparent azimuth.
TSGs are more accurate at low inclinations than LRGs.
Level rotor gyro directional drilling survey tool
A Level Rotor Gyro (LRG) tool is similar in construction to the tilt scale gyro except that the tilt is maintained at zero by means of a mercury or electrolytic level switch mounted on the inner gimbal.
4 Corrections
In general, conventional gyro directional drilling survey tools have to be corrected for drift and cross-borehole projection known as the inter-gimbal correction. Tilt correction is only applicable for TSGs as the tilt is maintained at zero in LRGs.
Drift correction
Drift is the rotation of the outer gimbal due to the Earth's rotation sensed and imperfections of the system. Drift is expressed in (°/hr) and can be in the order of 10 to 100 (°/hr).
Gyro single-shot
The initial gyro azimuth recorded when the gyro was aligned to the foresight is considered as zero. The difference between this initial azimuth and the final azimuth, recorded when the gyro is realigned to the foresight after the survey, is called the total observed drift. This is because it is the drift actually observed. For a gyro single-shot, drift correction is applied by estimating the drift at the time the survey was taken. This is done by calculating the time proportional share of the total observed drift. Often the applied drift correction amounts to about half the final observed drift. Because the actual drift rate is unlikely to be constant (linear) over longer periods, this method is only suitable when the total duration is less than 15 minutes. This implies a depth limitation to the use of gyro single-shots to approximately 800 m (2500 ft). For deeper holes a surface read-out gyro single-shot or a gyro multi-shot shall be used.
Gyro multi-shot
The drift in a gyro multi-shot survey is intermittently sampled during the survey by stopping the tool for a short time and recording the change in azimuth during this period due to gyro drift. Drift checks are usually made for 5 minutes after each 15 minutes for tilt scale gyros and after every 10 minutes for level rotor gyros, which have a higher drift rate.
Inter-gimbal correction
Azimuth observed by the compass card is the azimuth in the cross-borehole plane. However, the true azimuth is defined in the horizontal plane. The difference between the observed and true azimuth depends on inclination. This difference is called the inter-gimbal error. It is present at every survey station and drift check. Consequently a correction is applied to the observed azimuth before applying any other corrections.
Tilt correction
For tilt scale gyros, the Earth's rate of rotation and imperfections in the gyro dynamics, cause a tilt in the vertical direction which results in a tilt error.
When processing a gyro survey the raw azimuth data of each survey station must be corrected for drift, inter-gimbal and if applicable tilt errors. It is the responsibility of the Survey Engineer to apply these corrections. It is the responsibility of the Wellsite Drilling Engineer to check that the Survey Engineer has corrected the preliminary survey data.
5 Types of conventional gyro directional drilling survey tools
Conventional gyro tools can be divided into two types, namely film-type and surface read-out gyro tools.
Film-type gyro directional drilling survey tools
The survey recording mechanism of film-type gyro tools is a camera section which operates in exactly the same manner as those described under conventional magnetic directional drilling survey tools, see also Section 10. Inclination is measured by a plumb bob-type angle unit which attaches on top of the gyro unit. Various survey companies offer film-type gyro tools and they can be used as single and multi-shot tools.
Surface read-out gyro directional drilling survey tools
Surface read-out gyro (SRG) tools have been designed to replace film reading gyro tools. Surface read-out gyros are run on conductor wireline. Their output is fed to a surface computer and printer which continuously give the provisional survey data. These provisional data are corrected at the end of the survey when the drift curve is closed.
There are two types of SRG directional drilling survey tools:
· modified LRGs, where the compass card is replaced by an electronic encoder and the inclinometer by accelerometers. These tools include:
-Baker Hughes INTEQ Sigma 300 and 175;
-Scientific Drilling International Surface Recording Gyro (SRG);
-Sperry-Sun Surface Recording Orientation (SRO).
· specially developed surface read-out gyros that use the gyro in a stabilised platform. This type include:
-Sperry-Sun Borehole On-line Survey System (BOSS)
Surface read-out versus film-type gyro directional drilling survey tools
The advantages of surface read-out over film-type gyro tools are as follows:
·more reliable results;
·survey time is less than for film reading gyro surveys;
·the need for survey film (and processing time) is eliminated;
·data are collected electronically; hence reading uncertainties and depth interpretation uncertainties are removed; instrument uncertainties remain and these can be quantified;
·directional drilling survey tool position co-ordinates are continuously available;
·all readings are directly printed out at the surface; hence excessive variations in drift rates can be observed and remedial action can be taken;
·real time gyro performance monitoring;
·saturation of gyro can be obtained immediately;
·instrument temperature is displayed on the surface; hence the risk of total loss of surveys due to excessive temperature is eliminated;
·tool failure can be recognised immediately, hence reducing the amount of lost survey (rig) time in case of failure.
In addition, where a magnetic steering tool cannot be used due to a disturbed magnetic environment, a surface read-out gyro may be used to orient the mud motor.
The disadvantages of SRGs are:
·the survey can be lost if there is a power failure;
·conductor wireline is needed.
8 Processing conventional gyro surveys
Processing gyro single-shot surveys
Gyro single-shot surveys are processed by the survey company engineer. The single-shot discs are read in a similar way to magnetic single-shot discs. After reading, the inter-gimbal, tilt and drift corrections should be calculated and applied to the raw azimuth as read from the disc.
It is recommended to make a rerun if:
·the observed drift is greater than 5°;
·the survey results are suspect (check all data and procedures before a rerun);
·the disc is unreadable.
If the disc is unreadable, the following are possible faults:
·disc black; overexposure;
·disc transparent; bulbs did not light (faulty bulbs or batteries, or timer not working);
·disc blurred; tool and/or string moving (timer incorrectly set). Excessive gyro drift due to rough handling.
Processing gyro multi-shot surveys
The film recorded in the survey is developed. Rerun the survey if:
·the film is foggy, overexposed, blurred or transparent;
·pictures are missing, stacked-up, or parts are missing.
Load the film in the reader. Identify the start case, end case and drift checks. Rerun in case of:
·excessive observed drift;
·excessive and/or erratic drift rates within and between drift checks.
Read the complete film and record inclinations and raw azimuths on the worksheet.
After reading, the inter-gimbal, tilt and drift corrections should be calculated and applied to the raw azimuth as read from the film. In addition, the survey may be corrected for foresight offset or misalignment correction.
Processing surface read-out gyro surveys
Processing of surface read-out gyro surveys is performed by the Survey Engineer. The raw data should be corrected for drift and inter-gimbal corrections.
Quality control
In general, quality control of the of conventional gyro surveys is difficult as processing of the survey is heavily operator dependent. It is therefore recommended to have a second Survey Engineer (office) to independently reread and reprocess the survey. In addition, the following quality checks for GSS and GMS are recommended:
For GSS directional drilling survey tools the observed drift should not exceed 5°. For GMS the following acceptance criteria should not be exceeded:
·the total observed drift rate must not exceed 10° per hour of survey;
·the closure rate must not exceed 2° per hour of survey;
·for inclinations over 10° the azimuth in-run/out-run comparison must not exceed 1°;
·the inclination in-run/out-run comparison must not exceed 0.25°.
If any one of the quality parameters is not met the survey should be rejected.
Uncertainties of conventional gyroscopic directional drilling survey tools
The major sources of uncertainty of conventional gyro surveying are:
·foresight azimuth uncertainties;
·reading uncertainties (only film-type gyro tools);
·misalignment uncertainties;
·depth uncertainties;
·uncertainties due to incorrect corrections, i.e. drift, tilt and inter-gimbal correction.
System uncertainties such as misalignment and depth uncertainties are discussed in Section 15.
Foresight azimuth uncertainties
Any uncertainty in the foresight azimuth will cause a systematic azimuth uncertainty throughout the survey.
It is the responsibility of the Topographic Department to ensure that foresight azimuths are provided to an accuracy of 0.05°. In case the foresight is missing or removed, then a temporary foresight marker shall be used and resurveyed as soon as possible. In addition, a foresight offset correction shall be applied to the survey.
Reading uncertainties
To reduce inclination and azimuth reading uncertainties, an angle unit suitable for the borehole inclination over the interval being surveyed should be used. For plumb bob-type angle units, this may necessitate two or more runs in a deviated well.
Good quality gyroscopic instruments make use of vernier scales, which enable the drift and inter-gimbal corrections to be read more accurately, hence reducing the total survey uncertainty.
To reduce human reading uncertainties, two Survey Engineers must independently read each survey film. One surveyor may be in the Suco base.