Deviation Survey

Test Your Knowledge

Quiz: Navigating the Underground: Understanding Deviation Surveys

Instructions: Choose the best answer for each question.

1. What is a deviation survey in the oil and gas industry? a) A geological analysis of the rock formations encountered during drilling. b) A detailed record of the drillhole's trajectory and its deviation from the intended path. c) A safety inspection of the drilling equipment. d) A method for predicting the location of oil and gas reservoirs.

2. Which of the following is NOT a benefit of using deviation surveys? a) Increased drilling efficiency b) Improved safety and cost control c) Accurate target location d) Predicting the price of oil and gas.

3. What does the "deviation angle" measure in a deviation survey? a) The distance between the drill bit and the intended target. b) The angle between the drillhole's axis and the vertical. c) The speed of the drill bit. d) The type of rock formations encountered.

4. Which type of deviation survey uses gyroscopes to determine the drillhole's orientation? a) Wireline surveys b) Magnetic surveys c) Gyro surveys d) Laser surveys

5. What is one way that deviation survey data is used to optimize production? a) By predicting the amount of oil or gas that will be extracted. b) By ensuring the well is properly positioned within the reservoir. c) By identifying new oil and gas deposits. d) By controlling the temperature of the drilling fluids.

Exercise: Analyzing Deviation Survey Data

Scenario: A drilling team is using a wireline survey to track the path of a wellbore. The data collected shows the following:

• Depth: 1000m
• Deviation Angle: 5 degrees
• Departure: 20m

Task: Based on this data, calculate the True Vertical Depth (TVD) of the wellbore at this point.

Instructions:

1. Understand the Concepts:
   • TVD: The vertical distance from the surface to a specific point in the wellbore.
   • Deviation Angle: The angle between the wellbore's axis and the vertical.
   • Departure: The horizontal distance from the wellbore's vertical projection to the actual wellbore location.
2. Use Trigonometry: You can use the cosine function to calculate TVD:
   • TVD = Depth * cos(Deviation Angle)
3. Calculate TVD: Plug in the values from the data and solve for TVD.

Techniques

Navigating the Underground: Understanding Deviation Surveys in Oil & Gas

Chapter 1: Techniques

Deviation surveys utilize various techniques to measure and record the trajectory of a wellbore. The choice of technique depends on factors such as accuracy requirements, well conditions, and cost considerations. Here are some prominent techniques:

• Magnetic Surveys: These surveys utilize a magnetic compass to measure the inclination and azimuth of the wellbore. They are relatively inexpensive and quick but are susceptible to magnetic interference from the drill string and surrounding formations, resulting in lower accuracy. They are often used as a preliminary survey or in shallower wells.

• Gyroscopic Surveys: Gyroscopic surveys employ gyroscopes to measure the wellbore's orientation. Gyroscopes measure the rate of rotation, which can be integrated to determine the orientation relative to the Earth's rotation axis. These surveys offer higher accuracy than magnetic surveys, especially in areas with magnetic interference, but can be affected by high-speed drilling and are generally more expensive.

• Inertial Surveys (Inertial Measurement Units - IMUs): IMU tools use accelerometers and gyroscopes to measure both angular rates and linear accelerations. This allows the tool to continuously calculate the orientation and position of the drill string. IMU surveys provide real-time data and high-accuracy measurements over longer drill string lengths, minimizing the need for frequent surveys.

• Multi-Shot Surveys: These surveys involve taking measurements at multiple points along the wellbore. By combining measurements from multiple depths, a more comprehensive and accurate picture of the wellbore trajectory can be obtained. This improves the accuracy of the overall survey and reduces the uncertainty associated with individual measurements.

• Wireline Surveys: While not a survey technique per se, wireline tools are often used to deploy the measuring instrument (magnetic, gyro, or IMU) downhole. This method allows for accurate measurements and data retrieval after drilling a section of wellbore.

Chapter 2: Models

The data acquired from deviation surveys is processed using various models to generate a three-dimensional representation of the wellbore trajectory. These models account for measurement errors and the Earth's curvature:

• Minimum Curvature Method: This is a widely used method that assumes the wellbore follows a smooth curve with minimum curvature. It is relatively simple and efficient but may not accurately represent highly deviated or complex well trajectories.

• Radius of Curvature Method: This model uses the radius of curvature at each measurement point to build the wellbore trajectory. It is more accurate than the minimum curvature method for highly deviated wells but is computationally more intensive.

• Weighted Average Method: This method combines multiple survey measurements to reduce the impact of individual measurement errors. Weights are assigned based on the reliability and accuracy of each measurement.

• Kalman Filtering: A sophisticated method that incorporates prediction and correction steps based on sequential data from the survey, accounting for uncertainties and noise. It yields a smoother and more accurate wellbore path than simpler methods.

Chapter 3: Software

Specialized software packages are crucial for processing and interpreting deviation survey data. These software applications facilitate:

• Data Import and Validation: Import data from various survey instruments, validate for errors and inconsistencies.
• Trajectory Calculation: Apply different mathematical models (minimum curvature, radius of curvature, etc.) to compute the 3D wellbore trajectory.
• Visualization: Create 2D and 3D representations of the wellbore path, showing inclination, azimuth, and other relevant parameters.
• Report Generation: Generate comprehensive reports containing all essential survey data, plots, and interpretations.
• Well Planning Integration: Seamlessly integrate with well planning software for optimized drilling strategies.

Examples of such software include Petrel (Schlumberger), Kingdom (IHS Markit), and specialized deviation survey processing packages from various service providers.

Chapter 4: Best Practices

Maintaining accuracy and reliability in deviation surveys requires adherence to several best practices:

• Regular Surveys: Frequent surveys are crucial, especially in complex geological formations or during high-angle drilling. The frequency should balance accuracy requirements with operational efficiency and costs.
• Calibration and Maintenance: Regular calibration and maintenance of survey instruments are essential for ensuring measurement accuracy.
• Quality Control: Rigorous quality control procedures, including data validation and error checks, should be implemented throughout the survey process.
• Standardized Procedures: Following standardized procedures for data acquisition, processing, and interpretation ensures consistency and minimizes errors.
• Experienced Personnel: Using experienced and trained personnel is vital for proper instrument handling, data acquisition, and interpretation.
• Environmental Considerations: Following environmental regulations when conducting surveys is crucial for maintaining environmental protection.

Chapter 5: Case Studies

Several case studies demonstrate the importance of accurate deviation surveys:

• Case Study 1: Avoiding a Collision: A detailed deviation survey allowed operators to prevent a potential collision between two wells by detecting a significant deviation from the planned trajectory.
• Case Study 2: Optimizing Reservoir Contact: Precise deviation surveys enabled operators to optimize the well's position within the reservoir, increasing hydrocarbon production.
• Case Study 3: Mitigating Wellbore Instability: Early detection of excessive deviation through frequent surveys allowed engineers to adjust drilling parameters and prevent wellbore instability issues, reducing non-productive time and costs.
• Case Study 4: Successful Horizontal Well Completion: Accurate deviation surveys played a critical role in successfully drilling and completing a long horizontal wellbore in a challenging geological setting.

These case studies showcase how deviation surveys can help prevent costly accidents, optimize drilling operations, and enhance overall production efficiency. Specific details of the case studies would require confidential data from real-world projects which is unavailable here.