Inertial Navigation Systems (INSs) are capable of accurately measuring the position of boreholes e.g. the Ferranti Inertial Navigational Directional Surveyor (FINDS) and the Ring laser Inertial Guidance System (RIGS).

An INS comprises a triad of accelerometers and gyroscopes respectively. The system is designed to measure the tool movement from its reference point. This is achieved by measuring acceleration with the accelerometers and rotation with the gyroscopes. Integrating with respect to time of acceleration measured yields change in velocity, and a second integration gives change in position. These displacements can be used to calculate inclination, azimuth and along hole depth of the borehole.

FINDS and RIGS can be used for definitive surveys.

FINDS (Baker Hughes Inteq)

FINDS is a stable platform system in which the accelerometers and gyroscopes are mounted on a gimballed platform. There are a limited number of tools in use and their availability is usually limited. The large diameter limits its use to 13-3/8" casings and larger.

Advantages and disadvantages

·Advantages:

-can be a very accurate system, but actual accuracy is operator dependent;

-can be road tested at surface.

·Disadvantages:

-No downhole communications;

-Very sensitive to shocks, e.g. bumping the BOP is enough to have the system shut off;

-Measures only gyro drift at the end of the survey, i.e. no intermediate drift compensations;

-Battery powered, which limits the operating time and the final drift check can only be carried out if sufficient power is left;

-Temperature limited, e.g. premature switching off may occur because of overheating of the tool. This precludes proper QC based on inrun/outrun comparison;

-Can only be used in 13-3/8" casing and larger;

-The system only measures North, East and Vertical. Along hole depth, inclination and azimuth have to be back-calculated using a method which may introduce uncertainties;

-Data processing is heavily dependent on operator intervention.

Quality assurance

Baker Hughes Inteq is responsible for routine maintenance of FINDS. At the end of ten surveys, or every two months, whichever occurs first, the tool should be shipped to base for the maintenance. Tools are calibrated before and after every survey job at the district office. Part of the calibration check procedure consists of a road verification. The tool is put on a truck and taken along a pre-surveyed trajectory.

Pre-survey checklist

Wellsite Engineer:

• Check the values of latitude and longitude to the nearest 0.1 nautical mile.
• Ensure that the Survey Engineer has been supplied with the well status, including:

1.Interval to be surveyed, and depth reference, e.g. rotary table. See also Section 5.3.

2.Casing size and weight, and any restrictions in the casing or wellhead.

3.Maximum bottom hole temperature.

4.Mud weight and viscosity.

5.Wellhead co-ordinates.

6.Target azimuth (Grid or True North).

7.Elevation from the datum of:

-rotary table;

-top of casing head housing or bottom flange;

-top of cellar (if applicable);

-mudline suspension (if applicable);

-sea bed (if applicable).

8.Maximum expected inclination and intervals where the dogleg severity is expected to exceed 1°/10 m (3°/100 ft).

9.Grid convergence if applicable.

• Check availability of heave compensators on floating rigs.
• Verify that the Survey Engineer has checked the equipment and tool.

Survey Engineer

• Check that crossovers are available for the wireline to be used.
• Check that the fishing neck dimensions are known. A sketch of the tool in use must be available on the well site.
• Check that the container of the tool is certified for the particular location.
• Check the pressure rating. If this is likely to be exceeded, only survey down to the limit depth.
• Check the temperature rating. If the temperature limit is exceeded, circulate hole to cold mud.
• Check correct size and stiffness of available centralisers.
• Check availability of line wiper. Visually inspect the wireline unit, including the accuracy of the wireline measuring wheel. The tool can also be run on a sandline but this is not recommended because of poor depth control. Check the strength of the wireline and that the wireline unit is capable of pulling an 800 kg (1800 lb.) tool at 60 m/min (200 ft/min). Check availability of 2275 kg (5000 lb.) weak point. The tool may also be run on drillpipe.
• For deep surveys, estimate the running time so that the transit time may be chosen to allow the survey to be undertaken without the batteries running down.
• Check availability of 9-5/8inch casing slips and drill collar safety clamp.

Running procedures

FINDS surveys are run by Baker Hughes Inteq engineers. It is their responsibility to run FINDS to give optimal quality performance. FINDS may be used in deep holes, high inclination wells or run on drillpipe. For these situations the accuracy will be less.

Data processing

During surveying, velocity data from the first integration is sampled and stored at a rate of 20 lines/minute. After data have been transferred to the surface computer, a hard copy is printed out. The Survey Engineer then:

·checks velocities to identify fixes;

·checks for parity errors to ensure correct data transfer;

·checks for inertial navigation platform tilt;

·selects the best minimum number of fix lines and enters their line numbers;

·runs a computer program which calculates the inertial error velocities from station fixes and Best estimate Heading error (BeHe) data. These inertial error velocities are used to correct the transit velocities prior to the second integration to give the displacement.

The survey results are provided as North, East and True Vertical Depth co-ordinates. A back calculation should be carried out to derive the corresponding azimuth, inclination and along hole depth values from the North, East and True Vertical Depth co-ordinates.

Quality control

The data processing of FINDS is operator dependent. Processing of the data may vary between different Survey Engineers. Specially selecting fixes requires in depth knowledge and experience. Quality control is limited to inrun/outrun comparison which should be less than 1 per 1000 in all directions.

Uncertainties of FINDS

The uncertainties of FINDS are expressed in directional uncertainties. The uncertainty in each direction is 1 per 1000.

RIGS (Baker Hughes Inteq)

RIGS is a strapdown system, meaning that the accelerometers and gyroscopes are strapped rigidly to the body of the tool and consequently have 'mathematical gimbals'. The strapdown system, using ring laser gyros, has been introduced by Baker Hughes Inteq. RIGS uses the wireline depth and the tool calculated along hole depth to make a 'cable-aided' drift correction for the tool as it is continuously surveying. The tool can be run in casings/liners down to 7". The advantage of this tool is that accurate surveys can be obtained in casings smaller than 13-3/8", however, the laser gyros have an absolute maximum temperature limit of 100 °C, above which they suffer irreversible damage.

RIGS is a cable-aided strapdown inertial navigation system. It consists of a wireline depth measurement system (Kerr) and a surface computer. The probe is housed in a 5.25" diameter pressure barrel. It contains three accelerometers, three ring laser gyros, a processor and related electronics. The gyros and accelerometers are strapped down, which means that they are fixed to the body of the tool. Consequently there is a 'mathematical gimbal system'.

A ring laser gyro is temperature sensitive and therefore this system has an absolute maximum temperature limit of 100°C. Exceeding temperature limits causes irreversible damage to the gyro.

However, fast running speeds allow wells with bottom hole temperatures greater than 100°C, typically up to 120°C, to be surveyed. Moreover, a heat shield is available for high temperatures or slow running speeds.

Advantages and disadvantages

·Advantages:

-The system is more rugged than a mechanical-type gyro and is better suited to strapdown systems.

-Surveying on the move, with a surveying speed of 200-300 ft/min.

-Final survey data is real time transmitted to the surface and displayed on a system computer.

-The system offers good quality control.

-The system is not largely operator dependent.

-Accurate system and its survey quality easy to check.

-Measuring six components, i.e. along hole depth, inclination, azimuth, North, East and True Vertical Depth. By measuring six components no minimum curvature calculations are needed to compute positions from directions, hence final well path does not necessarily follow the minimum curve trajectory approximation resulting in a more accurate trajectory.

·Disadvantages:

-Wireline depth control is very critical for all measured components; however, wireline slack is detected and flagged.

-Difficult to test complete system and hence no full pre- and post-system check possible.

-RIGS is limited to 7" casing/liner.

Quality assurance

To ensure the quality of the survey the alignment should be made with the tool in a preferred toolface orientation. Each tool has a chart giving the preferred toolface orientations for the alignment phase and this should be checked to ensure that the alignment was made with the tool in a preferred toolface orientation.

Pre-survey checklist

The pre-survey checklist for RIGS is identical to that of FINDS described above.

Running procedures

A typical survey begins with a 12 minute alignment phase and a 3 minute drift check, performed downhole. A survey is made while running continuously into the hole at between 200 and 300 ft/min or 60 and 91 m/min. The surface computer displays the data continuously (updated every 2 seconds). Data are printed and stored at any pre-selected AHD increment, which is typically every 10 m or 20 ft. A drift check is taken on bottom, and another survey taken while pulling out of the hole. A third drift check is taken at the top of the survey. These drift checks are used purely for quality control of the survey data and are not used to correct the survey results. During the survey, real time quality control parameters can be displayed on the surface computer and warnings are displayed for unacceptable values.

Quality control / Uncertainties of RIGS

The main uncertainties of RIGS are the alignment heading uncertainty and wireline depth uncertainty. Alignment heading uncertainty is controlled by pre- and post-calibration checks and aligning the tool in a preferred tool face orientation. Significant wireline depth uncertainties may occur for high inclined wells. RIGS will detect any significant wireline (>0.001 AHD/AHD) depth uncertainties and the corresponding survey stations are flagged.