Wellsite Seismic Logging: A Basic Guide for Wellsite Geologists

Introduction:

Seismic logging is an important part of wireline logging operation, particularly? on exploration wells. It?provides valuable subsurface information that aids in reservoir characterization,?well placement,?and drilling decisions.?This article delves into the critical aspects of wellsite seismic logging,?specifically focusing on check shots and Vertical Seismic Profile (VSP).?It aims to equip junior wellsite geologists with the knowledge and understanding required to successfully plan,?execute,?witness,?and QC these operations.?

Check Shots:

Check shots are a fundamental type of wellsite seismic logging that involve recording the arrival times of seismic waves generated at the surface and received by a downhole receiver (geophone).?The primary purpose of check shots is to establish a time-depth relationship,?which allows for the correlation of seismic data with the wellbore.?This correlation is vital for accurate depth conversion of seismic reflections and the identification of subsurface geological formations encountered in the wellbore.

Planning and Execution

Successful check shot acquisition requires careful planning and execution.?The first step is to determine the optimal depth intervals for recording check shots, which is done in town by G&G team.?Their decision is influenced by factors such as the target formation depths, seismic data quality,?and wellbore conditions.?On rig, wellsite geologists coordinates the logistics of equipment and crew, as well as in the ?execution of plan with wireline engineer. Once the depth intervals are received,?the tool (downhole receiver) is positioned at the desired depth,?and the seismic source is activated at the surface.?The receiver records the arrival times of the seismic waves,?which are then used to calculate the time-depth relationship.

Witnessing and QC

Wellsite geologists play a crucial role in witnessing and QCing check shot data.?During acquisition,?they monitor the real-time data to ensure that the signals are clear and free of noise.?They also verify that the receiver depths are accurately recorded and that the seismic source is functioning correctly.?After acquisition,?the data is processed to generate a check shot survey,?which is then carefully reviewed by the geologist to confirm its accuracy and reliability.

VSP

Vertical Seismic Profile (VSP) is another important type of wellsite seismic logging that involve recording seismic waves generated at the surface and received by a series of downhole receivers (an array of geophones) positioned at various depths in the wellbore.?VSP provides a wealth of subsurface information,?including:

• High-resolution images of subsurface structures and geological formations along wellbore
• Improved seismic data quality,?particularly in areas with complex geology or poor surface seismic data
• Correlation of seismic data with wellbore logs,?facilitating accurate depth conversion and formation identification
• Identification of fluid contacts and reservoir properties

Types of VSPs

There are several types of VSPs,?each with its own advantages and applications.?Some of the most common types include:

• Zero-offset VSP:?In this configuration,?the seismic source is located directly above the wellhead,?providing a vertical image of the subsurface directly beneath the wellbore.
• Offset VSP:?In this configuration,?the seismic source is located at some distance from the wellhead,?providing an image of the subsurface at an angle to the wellbore.?This type of VSP is useful for imaging subsurface structures that are not directly beneath the wellbore.
• Walk-away VSP:?In this configuration,?the seismic source is moved along a line away from the wellhead, providing a series of images of the subsurface at different angles to the wellbore.?This type of VSP is usually performed on highly deviated and horizontal wells. It is also useful for generating a 3D image of the subsurface.

Planning and Execution

VSP acquisition is a complex operation that requires meticulous planning and execution.?From wellsite geologist’s point of view, It is again managing the logistics that include equipment, crew as well a boat (or a truck in case of a land rig). An in-depth planning would also include? the weather forecast, as it can negatively affect the source positioning and thus the quality of data. Once the survey design is finalized in town,?the downhole receivers are deployed in the wellbore,?and the seismic source is activated at the surface.?The receivers record the seismic waves,?which are then processed to generate the VSP data.

Witnessing and QC

Wellsite geologists are actively involved in witnessing and QCing VSP data acquisition.?They monitor the real-time data to ensure signal quality and verify receiver depths and source functionality.?Post-acquisition,?the data is handed over to G&G team in office, who participate in data processing and interpretation,?correlating VSP data with well logs and surface seismic data to create a comprehensive subsurface image.

Basic Interpretation of VSP Data

While detailed interpretation of seismic logs is typically performed by specialized geophysicists,?wellsite geologists should possess a basic understanding of interpretation principles to make informed decisions on the rig.?Key interpretation aspects include:

• Identifying primary seismic events:?Recognizing direct arrivals,?reflections,?and multiples.
• Correlating seismic events with well logs:?Matching seismic events to formation tops and lithological changes observed in well logs.
• Recognizing structural features:?Identifying faults,?folds,?and other geological structures in seismic images.
• Estimating formation velocities:?Calculating interval velocities from check shot and VSP data to improve depth conversion accuracy.

Additional Considerations

Beyond the knowledge of check shots and VSPs,?several other factors contribute to successful wellsite seismic logging.?These include:

• Safety:?Adhering to strict safety protocols to mitigate risks associated with seismic operations on oil rigs.
• Communication:?Maintaining clear communication with the logging crew,?drilling team,?and other stakeholders to ensure seamless operations.
• Continuous Learning:?Staying updated on the latest advancements in seismic logging technology and interpretation techniques.

Conclusion

Wellsite seismic logging is a powerful tool that provides invaluable subsurface information for well planning,?drilling and reservoir management.?By understanding the principles of check shots and VSPs,?as well as their planning, execution,?QC,?and basic interpretation,?wellsite geologists can contribute significantly to the success of drilling operations and optimize hydrocarbon production.

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