Periodo de realización

The high side is the top of the hole viewed along the bore hole axis. Assuming the hole has an inclination, the low side is the path, that a small, heavy ball would follow if it is rolled slowly down the well (Refer to figure 12.f).

Figure 12.F - Definitions of Inclined Hole

During a kick off or correction run, the measurement of greatest value is tool facing since it indicates the orientation of the bent sub. When a MWD or steering tool is used to control the deviation, tool face is referred to the high side of the hole when sufficient inclination exists (over 5^o) or to magnetic North for low inclinations (up to 5^o). The gravity tool face angle (GTF) is the projection onto a plane perpendicular to the hole axis of the angle between high side of the hole and tool face.

The magnetic tool face angle (MTF) is the projection onto horizontal plane of the angle between magnetic North and tool face(Refer to figure 12.g)

Figure 12.G - Magnetic Tool Face

HIGH SIDE

LEFT RIGHT

LOW SIDE

ROLLING BALL

HIGH SIDE

ROLLING BALL

a

a

LOW SIDE VERTICAL

TOOL FACE MAGNETIC NORTH

45°

TOOLFACE

LEFT RIGHT

HIGH SIDE

LOW SIDE

Steering the mud motor by means of magnetic toolface Bit and mud motor trying to kick off in the direction of 45° magnetic azimuth

Steering the mud motor by means of gravity toolface Bit and mud motor trying to build angle and turn well to the right

12.4.1. Magnetic Surveys

Length Of Non Magnetic Drill Collar

Magnetic instruments must be run inside a sufficient length of non-magnetic drill collars (NMDC or Monel Collar) made of special nickel alloy to allow the instrument to respond to the earth's magnetic field, by isolating it from the magnetic influence of the drill string.

The required length of NMDC is determined by taking into account the following factors:

• The geographical area of operations. Since the earth's horizontal magnetic intensity varies geographically, a zone selection map is used to determine which set of empirical data should be used for a given area.

• The proportion of steel drilling tools below the NMDC.

• The direction and inclination of the well.

The Directional Drilling Contractor shall provide updated indication of magnetic intensity related to the area of operation.

Compass spacing is generally recommended to be at or below the centre of the non-magnetic collars.

Magnetic Single Shot Surveys

Prior to use, the instrument should be thoroughly checked out and tested to ensure it is in good working condition. After loading, the timer is set and synchronised with a watch on the surface.

The time required for the instrument to fall is approximately 1,000ft per minute for inclinations up to 40^o and 800ft per minute for inclinations over 40^o. A safety margin of 5 mins shall be added to the calculated running time. Mud weight and viscosity are important factors to be considered, as are drill string restricted internal diameters.

For high inclinations (over 60) sinker bars should be used and the survey barrel may need to be pumped down. The mud pump rate should be very low, giving just sufficient pressure to break circulation. The drill string may be rotated slowly (not however, if running the survey on wireline) and reciprocated to prevent sticking and assist the survey tool in reaching bottom.

Drill pipe movement and pumping (if used) should be continued until a minute or so before the timer is due to operate..

If run on wireline, it should be taken into account the time the instrument generally takes longer to assemble and to run. Sandlines are quicker to run but can cause higher wear on drill pipe protective linings. Whichever wireline is used, thread protectors should be installed on the tool joint.

Magnetic Multishot Surveys

Magnetic multishot surveys are generally run prior to running casing as a check on the single shot surveys taken while drilling. This survey may be run either as an in run or running outrun survey, although it is generally run on the outrun wiper trip before casing.

This gives an opportunity for the instrument to be retrieved at the casing shoe and checked whilst the trip back to bottom is being made. A second opportunity is then available if necessary.

As the name implies, the magnetic multishot provides a series of single shot surveys. The camera of the instrument, instead of carrying one single shot disc, contains a length of photographic film. The film is exposed and advanced continuously, at a set time interval, from the time the instrument is started until stopped. The interval between exposure is generally 20secs but it is altered on some instruments.

The survey is normally made by dropping the instrument into the drill string and allowing it to get to bottom before pumping the slug and commencing the trip out of the hole.

As the drill string becomes stationary in the slips after each stand is broken off, the time since starting the instrument is recorded together with the number of stands out of hole.

This enables the survey picture to be correlated to instrument depth. With an instrument set on a twenty second sample rate, good practice is to ensure there are a minimum of two surveys taken at each depth by remaining stationary.

Steering Tool (with mud motor)

Steering tools use a system of magnetometers and accelerometers to measure the Earth's magnetic field and gravity in order to determine inclination and direction.

The tool is run on a conductor wireline which provides power for the sensors and returns the signal to the surface computer where it is decoded and relayed to the rig floor read out.

The tool may be operated on one of two modes displaying tool face with respect to North (Magnetic Tool Face) or relative to the high side of the hole (Gravity Tool Face). The magnetic tool face mode is used in vertical or near vertical wells for kick off in the desired direction. As the inclination is increased above about 5^o the tool is switched to gravity tool face.

The advantages of steering tools over single shot orientation are in the continual read-out of the tool face whilst drilling and in saving time in situations where orientation problems may require repeated single shot surveys.

One of the drawbacks of the system is the time required to pull the tool out of hole for making pipe connections.

The steering tool system is used only in specific situations, i.e. KOP in a high temperature zone.

When a motor is used for kick off or correction runs (operations not requiring rotation of the drill string), a side entry sub may be used. This sub prevents the need to pull the tool to make connections. The wireline passes through the entry sub enabling the drill pipe to be added to the string in the normal manner.

Measurement While Drilling (MWD)

Measurement While Drilling is a technique which takes various downhole measurements and transmitting these data to the surface for decoding and display. The most common transmission media is mud pulse telemetry in which the flowing column of drilling mud is modulated periodically by some mechanical means within the downhole assembly. The intermittent pressure pulses are transmitted from downhole to the surface where they are detected by a pressure transducer mounted in the standpipe. The transducer converts the mud pulses into electrical signal that is then transmitted to the surface computer. The computer decodes and displays this transmitted information.

There are three distinct types of MWD transmission systems currently available, all using mud column as their transmission medium:

• The positive system uses a plunger type valve that momentarily obstructs mud flow thus creating a positive, transient pressure pulse.

• The negative pulse system utilises a valve that momentarily vents a portion of the mud flow to the borehole annulus, resulting in a negative, transient pressure pulse.

• The continuous wave system utilises a spinning, slotted rotor and slotted stator that repeatedly obstructs mud flow. This operation generates a continuous low frequency fluctuation in standpipe pressure of approximately 50psi.

One of the most common applications for a directional MWD system is to orient downhole motor/bent sub assemblies when changing the course of the well path. Sensors located immediately above the bent sub, taking measurements while the bit is drilling on bottom, provide immediate data (inclination, azimuth and tool face) to the Directional Driller.

As already discussed in the description of steering tool systems, tool face may be referred to magnetic North or high side of the hole, depending on hole inclination.

12.4.2. Gyroscopic Surveys

Gyro instruments are used when the proximity of casings or other magnetic interference precludes the use of magnetic tools.

Gyro Single Shot Surveys

Gyro single shot surveys are run on wireline. Since gyroscopes are delicate instruments, running speeds should be within that recommended and the tool stopped and started off gently.

The gyro instrument has the same mule shoe feature as the magnetic single shot used for orientation and, although it uses a different system, the data obtained is the same, (i.e. hole direction, inclination and tool face).

The maximum depth to which they can be effectively run is approx. 1,300ft about 400m.

This is a limitation imposed by the time taken between orienting the gyro on surface, running into hole, taking the survey, pulling out of hole and checking the orientation.

The difference in azimuth between the initial orientation and final check on return to surface is the amount the gyro has drifted or wandered off its true north orientation. The drift is assumed to be constant for the time interval between initial and final orientation. The correction is calculated by simply determining the proportion of drift occurring in the time from the initial orientation to the survey picture being taken. Gyro drift is approx. 4^o per hour in static conditions and 8^o per hour in dynamic conditions.

Gyro Multishot Surveys

The gyroscopic multishot is the survey tool for surveying extended intervals inside casing or drill pipe without a non-magnetic drill collar. The tool comes in two sizes. The smaller one can be run in completed wells or through drill pipe. The larger one is a more rugged tool and is used to run surveys inside casing. Depending on the length of survey run, it will be a number of hours before the calculated survey data are available.

Gyro multishot drifts are the same as that of the single shot gyro.

Surface Read-out Gyroscopes

Surface read-out gyroscopes are used for the same purposes in single shot and multishot data collection. The instrumentation is more sophisticated and requires a conducting wireline to power the tool and transmit the information back to the surface for decoding by computer. With a surface read-out multishot gyro, the drift can be constantly monitored to ensure the tool is performing well and the calculated survey is produced shortly after completing the log run.

Gyrocompass (North Seeking Gyroscope)

These instruments use the principle of earth rate gyro compassing to define true azimuth and inclination in near vertical parts of the borehole. Then, as the hole builds angle to above 15° it switches to a continuous integrating mode. This dual mode makes the tool accurate in either vertical and deviated borehole where it eliminates the inaccuracies that gyrocompass based instruments have at high latitude, high inclination or in the East/West axis. The rugged construction makes these tools capable of steering and surveying while drilling (Gyro While Drilling).

12.4.3. Survey Calculation Methods

When drilling on a cluster, the co-ordinates of the centre of the 30" conductor shall be used on the rig for computations of each individual well.

The centre of the cluster may be used by the Company Drilling Office for mapping, planning and reporting.

There are a number of methods of calculating the wellbore trajectory from the survey data.

The most common are:

• Average angle method: It assumes the borehole is parallel to the simple average of both the drift and bearing angles between two survey stations. It is fairly accurate and calculation is simple enough for field use with a non programmable scientific calculator.

• Radius of curvature: Using sets of angles measured at the upper and lower ends of sections along the surveyed course length, it generates a space curve representing the wellbore path. For each survey interval, it assumes that the vertical and horizontal projections of the curve have constant curvature.

• Minimum curvature method: shall be used on the rig, in Company Drilling office and Directional Drilling Contractor office for survey computations. It assumes the borehole is a spherical arc with minimum curvature (maximum radius of curvature) between survey stations. It is the most accurate for most boreholes, however it requires very complex calculations using a programmable calculator or computer.

Average Angle method