well placement final

survey tools

• pipe tally: measures MD for TVD

• accelerometer: measures inclination for northing

• magnetometer: measures azimuth for easting, vertical section & DLS

surveying

defining a point in space or along the path of the wellbore

purpose of survey

• drilling

  • ensure a safe well path to the target

  • ensure the target is hit

  • prepare for relief well

  • locate DL & allow DLS calculation

  • avoid collision

  • locate the TF orientation of deflection tool/ steerable system

• reservoir/ production

  • provide a good log position/ reserves estimation

• report data to regulators

• conduct forensics investigation

survey tool classification

• magnetic → use earths magnetic field to determine the direction of the wellbore. used in drilled open-hole sections & placed in NMDC

• gyroscopic → use gyro to determine hole direction for where M-interference prevents the use of magnetic tools, in DP or casing strings

Survey Tools Data Gathering Techniques

• photographic film

• memory modules

• wireline

• mud pules telemetry

• electromagnetic telemetry

• wired drill pipe

magnetic survey tools

• compass-based tools

  • magnetic single shot

  • magnetic multishot

• electronic tools

  • electronic magnetic multishot

  • steering tool

  • measurement while drilling (MWD)

gyroscopic survey tools

• single/ multi shot

• rate/north seeking gyro

• ring laser gyro

• inertial grade gyro

inclinometer/ drift indicator

Inclination only tools measure only the hole inclination and give no indication of hole azimuth

• MD Totco deviation (Single)

• Teledrift (Multi)

magnetic single shot

Records simultaneously the magnetic direction and inclination of an uncased well bore on a single film disc. Used as:

• check shot at section TD

• bit trip

• WD has failure

magnetic multishot

Records simultaneously the magnetic direction and inclination of an uncased hole on a film strip at multiple stations.

• when BHA is being tripped out of the hole

Solid State Magnetic Survey Tools (Electronic)

Measure Earth's gravity and magnetic field by using sets of three orthogonal (i.e. mutually perpendicular) solid state accelerometers and magnetometers, respectively. Used as:

• single-shot tools (ESS)

• multi-shot tools (EMS)

• wireline steering tools

• MWD tools

electronic survey tool process

1. can record survey data downhole on a computer chip or transmit the data to the surface by a wireline or mud pulse telemetry.

2. A surface computer initially set up and configure the tool prior to the survey and also to recover and process the data after the survey.

Electronic Magnetic Multishot (EMS)

Uses a sensor array of accelerometers and magnetometers housed in a rugged electronics probe. The data is recorded downhole on a memory chip and then transferred to computer disc for processing when the tool is retrieved at surface or data sent via wireline to surface.

→ confirms MWD surveys

steering tool

Give continuous surface readout of survey data while drilling with a downhole motor and bent sub assembly.

• A solid state electronics probe plus spacer bars + a mule-shoe

steering tool process

1. The raw data from the probe is transmitted to surface via the conducting wireline.

2. A surface computer decodes the signals and calculates the survey data.

steering tool limitations

• Pulling and running the steering tool for each connection takes a long time.

• With side entry sub, time could be saved however the wireline cable might be damaged while making connection.

• The drill string cannot be rotated while the steering tool is in the hole (CT?).

MWD tools

Incorporated as part of the downhole drilling assembly, use magnetometer and accelerometer sensors and transmit the recorded sensor data to surface via a series of pulses sent through the column of drilling mud. The pulses are detected as pressure differentials by surface interface panels and thereafter derived into the required directional information. Measures:

• directional survey

• drilling mechanics data

MWD components

• Downhole sensor package (Microelectromechanical systems (MEMS)).

• Downhole power source.

• Downhole computer (microprocessor and electronics for controlling and monitoring the downhole system).

• Method to transmit data from downhole to surface.

• Surface sensors (for reception of data signals from downhole).

• Surface computer to receive data and convert it to a usable format.

MWD systems

• collar based: All the sensors and electronics are built directly into the body of NMDC. → Allows full wellbore of the NMDC to be used, specially when LCM are expected to be pumped

• probe based: In this system , the tools are built in separate barrel. This barrel then sits inside the ID of the NMDC

MWD downhole assembly

• power supply

• sensors

• directional sensors: triaxial accelerometer → all 3 reads some value of the vector G (orthogonal set) determines inclination and tool face

• orientational sensor: triaxial magnetometer → all 3 read some value of the vector H (orthogonal set) determines the azimuth

• mud pulse telemetry

• electromagnetic telemetry

• wireline telemetry

power supply

Batteries, or downhole turbine, supply power to the tools → allow the tools to operate without the flow of mud, but the operating time and sensor power output is limited.

sensors

• There are two types of directional sensors:

  • PM (Position Monitor): sensor used in conjunction with the negative pulse telemetry system

  • PCD (Pressure Case Directional): sensor used with the positive pulse and EMT (electromagnetic telemetry)

field acceptance criteria

• B-total: total field strength of the local M-field

• G-total: total field strength of the earth’s G-field

• G = Reference +/- 2.5 milli g

• H = Reference +/- 6 counts (300nT)

mud pulse telemetry

1. Information is transmitted to the surface through the mud by way of a data signal created downhole.

2. The surface equipment decodes the data signals of the measurements so that the driller can make adjustments.

The three common types of signals generated are:

• Positive pressure pulses

• Negative pressure pulses

• Continuous pressure waves

mud pulse telemetry Shortcomings

1. Transmission medium must be incompressible

2. Slow data transmission rates (1 to 3 bits/sec)

3. Advanced signal processing techniques are required to reduce the effects of distortion and noise within the telemetry band

Electromagnetic Telemetry

An electromagnetic wave is created, and it is transferred through the formation used when compressible drilling fluids are used & fore onshore mainly

Electromagnetic Telemetry characteristics

• No continuous fluid column requirements

• No LCM restrictions

• Real-time use can be influenced by the vibration

• Data transmission rate is slow, but possible while making a connection (save time)

• Only batteries are the source of power (Usage life).

• Two-way communication

Electromagnetic Telemetry advantages

• No restriction on drilling fluid characteristics

• Reduced survey/connection time

• No moving parts

Factors effecting the signal of electromagnetic telemetry

• Formation impedance (Higher than 500 ohms , or less than 10 ohms, not possible)

• TVD (Signal loss due to pipe, solution Repeater)

• MD (Signal loss due to pipe, solution Repeater)

• Drilling fluid (OBM)

• Casing effect (75 ft below casing shoe)

• Batteries life time

wireline telemetry

Data can also be sent to the surface through a wire attached to the MWD tool (steering tools). With an attached wire, the drill-string cannot be rotated.

Today, wireline is used in conjunction with coiled tubing, where the drill string is a continuous length of metal pipe fed into the wellbore from a drum and cannot be rotated.

MWD tool operation

1. Surveys are taken when the tool is in stationary mode

2. Pump must be stopped for 30 to 60 second

3. Turn the pump back

4. Flow begins

5. Tool powered up

6. 30 second warm up period

7. Running pulses start

8. Pulses are measured by the transducer and encoded by the surface computer

MWD surface processing

1. A transducer (or sensor) at the surface receives the pressure pulses and converts them to electrical signals.

2. Surface computers decode the electrical signals from the transducer and turn the digital information into engineering values and survey computations.

3. The data produced by the MWD tool is processed and used to provide information about the well. This information is used to make critical decisions about the drilling process, such as the well direction.

4. Monitors display data in real time on the drilling floor so that the driller can make well steering decisions

Gyroscopic Survey Tools

Provide an accurate means of surveying a borehole free from drill string and/or casing steel interference. Run (on wireline ) after a casing has been cemented as a verification survey to measure AHD.

→ must be centralised

Categories of Gyroscopic Survey Tools

1. Conventional Gyroscopic Survey Tools

2. North seeking Gyroscopic Tools

3. Inertial Grade Gyro (Inertial Navigation System)

4. Gyro while drilling (coming soon)

conventional gyroscopic tool

Spinning gyro determines azimuth using the difference between the orientation of the gyro (of known direction aka foresight) and the orientation of the case containing the gyro. Does not measure inclination independently.

conventional gyroscopic tool disadvantages

• Drift: It is the rotation of outer gimbal due to earth rotation (time dependence)

• Reference misalignment

• Centralization

North seeking Gyroscopic Tools (Rate Gyro)

Based on the measurement of the horizontal component of the Earth rotation vector, which becomes smaller for increasing latitudes. It orients itself to true north, which eliminating the human error associated with Foresight and reduce the error due to drifting.

North seeking Gyroscopic Tools operation modes

• Gyro-compassing Mode: the tool is held stationary, and the azimuth is calculated independently at each survey station by measuring the component of the Earth's rate of rotation vector in the horizontal plane.

• Continuous Mode: At the start of the survey interval, the tool is referenced to True North by gyro-compassing. Following this, the tool is run continuously, with the tool's azimuthal change determined and its integration resulting in the actual azimuth.

North seeking Gyroscopic Tools limitations

• Sensitive to motion

• The maximum latitude of operation is approximately 80° N/S due to the reduction of the horizontal component of the Earth rotational vector, reducing the ability of tools to North seek.

• For gyro compassing, the survey time might be longer, depends on survey interval.

Inertial Grade Gyro (Inertial Navigation System)

The system measures the change in direction of the platform and the distance it moves. It not only measures the inclination and direction of the well but it also determines the depth. Uses three rate gyros and three accelerometers mounted on a stabilized platform.

Inertial Grade Gyro (Inertial Navigation System) types

• FINDS (Ferranti) → stable gimballed platform; accumulated error 1 ft/1000 ft; requires ~40 min surface initialization; limited to 13⅜" casing or larger

• RIGS (Ring Laser) → strap-down system; usable in 7" casing/liners; accumulated error 2 ft/1000 ft; inclination limited to <45°; regular recalibration required

Gyroscopic Survey Advantage and Limitation

• advantages:

  • Increased Accuracy : Improves the ellipse of uncertainty

  • Not Affected By Magnetic Fields: interference, e.g. batch setting conductors, casing string, drill-string, fish, formations, magnetized mud/cuttings or magnetic variations (daily, storms)

  • Resurveys: old wells, re-entries.

  • Surveying: in cased hole/tubing/Pipe , where magnetic survey tools can not be used

• limitations:

  • very delicate and vulnerable in tough drilling environment

  • only run on wireline (or dropped) during drilling interruptions

Factors Influencing Survey Tool Selection

• Target Size

• Latitude of Well

• Target Direction

• Type of Drilling Installation

• Rig Costs

• Maximum Inclination Planned

• Formation and Hole Conditions

• Survey Depths

• Open or Cased Hole

Survey Tool Selection Criteria

• Application

• Accuracy

• Cost

• Physical Constraints

• Availability

• Reliability