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Practical Depth Conversion with Petrel

Depth conversion (domain conversion) of seismic time interpretations and data is a basic skill set for interpreters. However, there is no single methodology that is optimal for all cases since the available seismic and geologic control varies in quantity and quality within each project. To design an effective approach to depth conversion, the first part of this course prioritizes understanding the nature of velocity fields and practical approaches to velocity representation. Next, appropriate depth-conversion methods are presented in case history and exercise form. Single-layer and more sophisticated multi-layer approaches are reviewed, along with depth-error analysis and the impact on formation top prognoses and volumetrics.

Depth conversion must also embrace the process of database validation. Poorly positioned wells, miscorrelated horizons, and inconsistent formation tops can introduce distortions in the implied velocity field and result in false structuring. Database validation is addressed via the formation of synthetic seismograms to confirm horizons correlation and the formation of basic Seismic Time vs. Formation Top QCs.

Prestack depth migration is now commonplace, and there is always the need to calibrate the depth volumes with well control. The basic QCs and methods used for depth conversion will also be applied to validating the ties between the formation tops and the surfaces used for calibration. This is particularly important during anisotropic depth migration where inconsistencies between well control and the seismic interpretation impact the estimation of anisotropic parameters, resulting in a compromised depth image.

This course emphasizes the formation of velocity models consistent with the well control. This is in context to creating Petrel Models suitable for reservoir simulation employing depth-calibrated inversion and other attribute cubes precisely integrated with the well information.

Day 1

Module 1:  Overview of Depth Conversion

* Learning Objectives and Importance:    

  • Discuss goals for vertical time-to-depth conversion

* Topics:          

  • Accuracies needed for  relative structure, well prognoses, volumetric estimates, and reservoir models
  • Database validation
  • Indicators for prestack depth migration (PSDM)

* Exercises:  Discussions on student goals and experiences with time-to-depth conversion


Module 2:  Sources of Velocity

* Learning Objectives and Importance:    

  • Review sources of velocity information

* Topics:          

  • Sonic logs, check shots, and VSPs
  • Seismic (refraction, reflection)
  • Inversion

* Exercises:  Analysis of various velocity data types


Module 3:  Defining Velocity Types

* Learning Objectives and Importance:    

  • Review definitions and characteristics of velocities

* Topics:          

  • Types of velocities
  • Conversion of velocity types
  • Compactional and layered geologies
  • Velocity gradients

 * Exercises:  Various problems on relating velocity types and conversions. Petrel exercises.


Module 4A:  Functional Representation of Velocities

* Learning Objectives and Importance:    

  • Define velocities fields using vertical functions

* Topics:          

  • Velocity as a function of time and depth
  • Implicit velocity representation via T-D functions
  • Petrel Velocity Models with time and depth functions

* Exercises:  Various problems defining velocity fields in various domains

Day 2

Module 4B:  Gridded Representation of Velocities

* Learning Objectives and Importance:    

  • Define velocities fields using grids

* Topics:          

  • Spatial velocity variations (lateral gradients)
  • Creating an edited PSTM velocity model in Petrel

* Exercises:  Import SEG Y velocities to Petrel and forming a gridded model


Module 5:  Well and Seismic Data Integration

* Learning Objectives and Importance:    

  • Understand methods for linking well and seismic information in Petrel

* Topics:          

  • Sonic and density log editing
  • Checkshot loading
  • Establishing seismic data polarity and phase
  • Database validation and QCs
  • Creating synthetic ties

* Exercises:  Problem sets and interactive work sessions

Day 3

Module 6:  Vertical Time-to-Depth Conversion (Single Layer)

* Learning Objectives and Importance:    

  • Implement basic depth conversion using T/Z functions and/or Petrel Velocity Models

* Topics:          

  • Simple, intuitive depth conversion (no velocity model)
  • QC methods that define Time Depth Relationships (TDRs)
  • Basic Petrel Velocity Models

 * Exercises:  Problem sets and interactive work sessions


Module 7:  Vertical Time-to-Depth Conversion (Multi-Layer)

* Learning Objectives and Importance:

  • Explore advanced depth conversion with layer-based Petrel Velocity Models

* Topics:

  • Geologic and data-driven modeling considerations
  • Multi-layer options
  • “Simple” thrust models
  • Advanced Petrel Velocity Models

* Exercises:  Problem sets and interactive work sessions

Day 4

Module 8:  Well/Seismic Database Validation

* Learning Objectives and Importance:    

  • Appreciate the need to review and correct the database prior to  incorporating well control

* Topics:          

  • Confirm database settings
  • Review seismic data polarity, phase, and synthetic correlations
  • Using basic depth-conversion QCs to encounter data discrepancies

* Exercises:  Extensive exercises on detecting and correcting errors and inconsistencies in the database


Module 9: Petrel Models and Uncertainty Analysis

* Learning Objectives and Importance:    

  • Implement domain conversion and uncertainty analysis with Petrel Velocity and 3D Models

* Topics:          

  • Evaluating depth uncertainty
    • Mean and standard deviation workflow (new)
    • Hidden well workflow
    • Stochastic velocity modeling
  • Impact of structural uncertainty on volumetrics in 3D Models

* Exercises:  Various Petrel exercises


Module 10:  Pitfalls of Vertical Depth Conversion

* Learning Objectives and Importance: 

  • Understand accuracy of vertical time-to-depth methods and when they fail

* Topics:          

  • QCs and validity of vertical depth conversion
  • Extreme geologic regimes
  • Alternatives to vertical depth conversion

* Exercises:  Problem sets and interactive work sessions

Day 5

Module 11:  Anisotropy and Depth Migration

* Learning Objectives and Importance:    

  • Appreciate the impact of anisotropy on seismic velocities and depth imaging

* Topics:          

  • Seismic anisotropy
  • Parameterizing anisotropy (Vz, delta, epsilon, VTI/TTI)
  • Industry approaches to anisotropic prestack depth migration (APSDM)
  • Problems and promise of anisotropy for velocity definition and reservoir attributes

* Exercises:  Discuss impact of anisotropy on depth conversion and imaging


Module 12:  Calibration of Depth Migration with Wells

* Learning Objectives and Importance:    

  • Learn basic approach for stable integration of depth-domain seismic (PSDM) with well control

* Topics:          

  • Working in the time domain
  • Updating the time/velocity model
  • Conversion of time data to calibrated depth
  • Optional: Map migration for dynamic calibration and uncertainty

* Exercises:  Various Petrel calibration exercises


Learning activity mix

This course is of importance to geoscientists involved in seismic interpretation and subsequent time-to-depth conversion. The course is also oriented towards persons performing well calibration of depth migration data and other attributes prior to reservoir modeling.

Attendees will gain an understanding of depth conversion methodologies, QCs for validating the methods employed, and tools for quantitative error estimation. They will also learn how to:

  • Review types of velocity data and methods of representation and application
  • Investigate the compatibility between well information and seismic data
  • Evaluate the polarity and phase of the seismic data with synthetic seismograms
  • Use well-to-seismic tie information to define time-to-depth functions
  • Perform basic and advanced depth conversion of time data with depth-errors analysis
  • Validate and correct databases via well-to-seismic ties and error-analysis QCs
  • Create structural-uncertainty estimates using various methods

Petrel Fundamentals and Petrel Geophysics courses.

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Client Testimonials

Fabulous course ! excellent instructor ! It's got a great number of reviews and updates of seismic inversion techniques as well as very practical Petrel exercises.
Fabulous course ! excellent instructor ! It's got a great number of reviews and updates of seismic inversion techniques as well as very practical Petrel exercises.

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