1.0 Reasons for Directional Surveys
Surveys are taken to better define the position of the wellbore, generally relative to the surface location. The reasons for wanting this information include:
a. Directional Control: To ensure the wellbore will reach its target.
b. Well control: provide information for drilling relief wells.
c. Reservoir development/exploitation: accurate description of the well’s position in the reservoir.
d. Unitisation issues: As for b. where a field is unitised between several operators.
e. Future sidetracks or adjacent wells: required for planning purposes to reach a target or ensure an adjacent wellbore is not intersected accidentally.
2.0 Determining Survey Requirement
All surveys have a degree of inaccuracy. A wellbore surveying programme is developed after deciding an acceptable tolerance for the positional accuracy of the wellbore. Since the position of a vertical well will be directly below the surface location, the typical survey programme for vertical wells is merely a check that their inclination remains low.
A deviation of one degree from surface to 3500m will result in a horizontal displacement of 61 metres, and a reduction in TVD of 0.5 metres. Assuming that the deviation is in one direction.
Positional uncertainty of the wellbore increases dramatically with deviation, necessitating survey requirements to be reviewed.
Magnetic ranging tools typically need to be within 75m of a target well before they can pick up a signal to direct a relief well to one with a well control problem. Exact distances are dependant upon the volume of steel in the target wellbore.
3.0 Survey Types
There are three types of tool used to gather survey information about a wellbore: inclination only, magnetic survey tools & gyroscopic tools.
Inclination only tools do not give any information on azimuth and thus cannot provide full survey information. These tools are used to confirm vertical wellbores remain within limits.
Magnetic tools measure the strength of the earth’s magnetic field at a given depth in the wellbore. Surveys must be taken in open hole away from any magnetic interference from previous casing strings. To ensure that the drillstring does affect the earth’s magnetic field a non-magnetic drill collar (NMDC) is run in the drillstring and the survey is taken from inside this. Two basic types of tools exist, i.e. conventional photo-mechanical (MSS) and solid state tools (EMS). Conventional photo-mechanical tools contain a compass which is photographed at a given depth. Multishot and single shot tools are available. For conventional tools a length of NMDC must be selected such that the interference from the steel drillstring is reduced to an acceptable level. Solid state survey tools measure the strength of the earth’s magnetic field and calculate azimuth. For solid state survey instruments, rather than magnetic error reduction by full NMDC length, a shorter length of NMDC is used and the magnetic interference from the drillstring measured and corrected for. Magnetic interference from adjacent wells or objects e.g. buried BHA’s, cannot be corrected for and will produce erroneous azimuth readings. In all situations, for accurate magnetic surveys, the Earth's magnetic field must be stable during the survey and appropriate values of magnetic field must be available. Magnetic surveys are not reliable during periods of high ‘sun spot’ activity!
Gyroscopic tools come in three variants, conventional, rate integrating and inertial. The older Conventional Gyroscope instrument takes stationary readings referenced to an initial surface alignment. This alignment is preserved by mechanical gimbals as the tool travels along the wellbore. A camera in the instrument photographs a plomb bob in relation to the gyro compass thereby collecting inclination and directional data. The more accurate, Rate Integrating Gyroscope, measures the tool’s response to the earth’s rotation. Measurements are electronic and allow inclination and direction to be calculated for each survey point. The tool can be setup to take stationary readings or readings as it moves along the wellbore. An improvement to the rate gyro is a North Seeking Rate Gyroscope, which references itself to True North and thus requires no initial surface alignment. The most accurate survey tool available is the Inertial Navigation Gyroscope and is designed to measure the tool movement from its reference point at surface. This is achieved by measuring acceleration with three accelerometers and rotation with three 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.
Since Gyro compasses are not affected by magnetism, gyroscopic instruments do not require NMDC’s and can be run in cased holes and drillpipe.
Survey information can be retrieved in three ways:
Memory: electronic (EMS, Gyrodata RGS-BT), physical (Totco) or photographic (MMS).
Surface readout: real time survey information via electric line (GPIT).
Mud Pulse Telemetry: information transmitted via drilling fluid (Teledrift, MWD)
AAC have experience of the following types of surveys:
a. Totco inclination single shot.
b. Baker electronic multishot (EMS).
c. Slim1 (SEA-AD), Anadrill’s (EMS), which is actually the internals of an MWD tool configured to run in memory mode.
d. GPIT Schlumberger magnetic survey obtained during UBI run.
e. Gyrodata RGS-BT: Battery operated, North seeking, rate gyro system.
Since almost all of Company’s wells are vertical, as long as the inclination remains low, the wellbore positional uncertainty is minimal. This inclination is monitored by taking Totco surveys.
4.0 Essential Header Information for Survey Reports
All directional survey reports, should contain the following information:
Wellname / field /wellhead co-ordinates / rig / date / names of personnel / hole TD / last casing seat & size. All surveys are to be referenced to Grid North, since this is required for calculation of UTM co-ordinates and magnetic North varies with time.
Magnetic Surveys (e.g. EMS, Schlumberger GPIT)
Tool type / probe No. / grid convergence (defined in words & drawing) / wellhead co-ordinates.
North Seeking Gyro Tools (e.g. Gyrodata drop system, GWS-BT)
Tool type / probe No. / operating mode (gyrocompassing or continuous mode) / pre – post calibration data / quality control parameters / if continuous mode, survey duration and drift time. Details of conversion from True North to Grid North.
5.0 Tool Dimensions and Parameters
GWS-BT (Gyrodata’s battery drop gyro)
Tool OD = 1.75”, no heat shield
Max temperature = 125 deg C, no heat shield, normal batteries.
Heat shield OD = 2.60”
Max temperature with batteries heat shielded = 150 deg C.
Max temperature with tool & batteries heat shielded = 260 deg C
Anadrill EMS (Slim1, SEA-AD)
Tool OD = 1.75”
Max temperature = 150 deg C
Further information: Anadrill Drilling Services Catalogue, 1993, page 36.
Baker EMS (Electronic Magnetic Surveyor)
Tool OD = 1.75”, no heat shield.
Max temperature = 125 deg C, no heat shield, normal batteries.
Max temperature with Lithium batteries = 140 deg C.
Heat shield OD = 2.00”
Max temperature with heat shield = 260 deg C, (for 8 hrs).
Further information:
Schlumberger GPIT
Tool OD = 4.50”
Max temperature = 175 deg C.