electric log

Test Your Knowledge

Quiz: Unraveling the Secrets of the Subsurface

Instructions: Choose the best answer for each question.

1. What is the primary function of a logging sonde in electric logging? a) To measure the temperature of the wellbore. b) To send electrical signals into the surrounding rock formations. c) To drill deeper into the subsurface. d) To extract samples of rock for analysis.

2. Which type of electric log is used to determine the amount of empty space in the rock? a) Resistivity Log b) Porosity Log c) Density Log d) Sonic Log

3. What information can be derived from the interpretation of electric logs? a) The type and saturation of fluids in the rock formations. b) The size and shape of reservoirs. c) The permeability and porosity of the rock. d) All of the above.

4. Which of the following is NOT a benefit of using electric logs in well completion? a) Optimizing well placement for maximum production. b) Identifying potential problems during well production. c) Determining the best drilling fluid for the well. d) Monitoring well performance over time.

5. What is the significance of digital logging in the future of electric logs? a) It allows for the collection of higher resolution data. b) It enables faster processing times for data analysis. c) It improves the accuracy of measurements. d) All of the above.

Exercise:

Scenario: You are a geologist working on an oil exploration project. You have obtained electric logs from a recently drilled well. The resistivity log shows a high resistivity zone, indicating a potential reservoir. The porosity log shows a relatively low porosity value in this zone.

Task: Based on this information, explain the potential implications for the reservoir's characteristics and productivity.

Techniques

Unraveling the Secrets of the Subsurface: A Guide to Electric Logs in Drilling and Well Completion

Chapter 1: Techniques

Electric log acquisition involves deploying a logging sonde—a sophisticated instrument package—into the wellbore. This sonde contains various sensors that measure different physical properties of the formations surrounding the borehole. The process generally involves lowering the sonde at a controlled rate while continuously recording the data. Key techniques employed include:

• Wireline Logging: The most common method, where the sonde is connected to the surface via a conductive cable. This cable transmits power to the sonde and receives the measured data. Wireline logging allows for precise depth control and facilitates bidirectional communication.
• Logging While Drilling (LWD): This technique involves incorporating the measurement sensors directly into the drillstring. Data is transmitted to the surface in real-time, enabling immediate decisions during the drilling operation. LWD is advantageous in directional drilling and deepwater applications.
• Measurement While Drilling (MWD): While often used interchangeably with LWD, MWD primarily focuses on directional drilling parameters like inclination and azimuth, while LWD encompasses a broader range of formation evaluation measurements.
• Caliper Logging: Measures the diameter of the borehole, crucial for correcting other log responses for variations in hole size.
• Formation Testing: While not strictly part of standard electric logging, formation testers are often used in conjunction with logs to obtain pressure and fluid samples directly from the formation. This provides valuable information about reservoir pressure and fluid properties supplementing the log data.

Chapter 2: Models

Interpreting electric logs involves understanding the relationships between the measured parameters and the petrophysical properties of the formation. This often requires the use of petrophysical models:

• Porosity Models: These models relate porosity (the volume of pore space in the rock) to measurements from density, neutron, and sonic logs. Common models include the density porosity and neutron porosity equations, which consider the matrix and fluid densities.
• Water Saturation Models: These models estimate the amount of water in the pore space. Archie's equation is a widely used empirical model that relates water saturation to resistivity, porosity, and formation water resistivity. Other models, such as the Simandoux equation, account for the effects of clay content.
• Lithology Models: Determining the rock type (sandstone, shale, limestone, etc.) often involves cross-plotting different log responses. Specific log combinations and characteristic responses help differentiate between various lithologies.
• Permeability Models: Permeability (the ability of a rock to transmit fluids) is difficult to directly measure with logs. Empirical models, often based on porosity and other log parameters, are used to estimate permeability.

These models often require calibration using core data and other well tests to account for local geological variations.

Chapter 3: Software

Specialized software is essential for processing, analyzing, and interpreting electric log data. These software packages offer a range of functionalities:

• Data Processing: This includes corrections for borehole effects, environmental corrections, and data quality control.
• Log Display and Visualization: Software allows for viewing logs individually or in combination, facilitating visual interpretation of log patterns.
• Petrophysical Calculations: Automated calculations of porosity, water saturation, lithology, and permeability using various models.
• Log Correlation: Comparing logs from different wells to establish stratigraphic relationships and identify reservoir continuity.
• 3D Visualization: Creating 3D models of the reservoir based on log data, enhancing the understanding of reservoir geometry and heterogeneity.
• Integration with other data: Incorporating seismic data, core data, and production data to provide a comprehensive reservoir characterization.

Examples of commonly used software packages include Petrel, Kingdom, and Schlumberger's Petrel and Techlog.

Chapter 4: Best Practices

Effective use of electric logs requires adherence to established best practices:

• Quality Control: Ensuring the quality of data acquisition and processing is paramount. This involves regular calibration checks, proper well logging procedures, and thorough data review.
• Calibration: Comparing log responses with core measurements and other well test data is crucial for accurate interpretation.
• Environmental Corrections: Correcting for borehole effects (like mud cake and rugosity) and temperature variations is essential for obtaining accurate measurements.
• Proper Log Selection: Choosing the appropriate suite of logs based on the specific geological setting and objectives of the well.
• Integrated Interpretation: Combining log data with other geological and geophysical information for a comprehensive understanding of the reservoir.
• Documentation: Maintaining detailed records of all aspects of the logging operation, including the logging parameters, processing steps, and interpretation results.

Chapter 5: Case Studies

Numerous case studies showcase the application of electric logs in various scenarios:

• Reservoir Characterization: Case studies demonstrating the use of logs to delineate reservoir boundaries, assess reservoir quality, and estimate hydrocarbon reserves.
• Well Completion Optimization: Examples where log data has been used to optimize well placement, casing design, and completion strategies.
• Production Monitoring: Case studies illustrating how logs help in monitoring well performance, identifying production problems, and guiding remedial actions.
• Enhanced Oil Recovery (EOR): Examples of how electric logs are used to monitor the effectiveness of EOR techniques.
• Geological Studies: Applications of logs in regional geological studies to understand stratigraphic relationships and basin evolution. Specific examples could involve identifying unconformities or characterizing specific depositional environments.

These case studies highlight the diverse applications of electric logs and demonstrate their critical role in successful exploration and production operations. Each case should illustrate the specific techniques, models, and software used, as well as the challenges and solutions encountered.