electric survey

Test Your Knowledge

Electric Surveys Quiz:

Instructions: Choose the best answer for each question.

1. What is the primary purpose of electric surveys in oil and gas exploration?

a) To measure the temperature of the Earth's crust. b) To locate underground water sources. c) To provide a detailed understanding of subsurface geological formations. d) To determine the age of rocks.

2. Which of the following properties is NOT measured by electric surveys?

a) Resistivity b) Porosity c) Magnetic field strength d) Sonic velocity

3. Which type of electric survey is specifically used to determine the ease of fluid flow through rock formations?

a) Resistivity Logs b) Porosity Logs c) Permeability Logs d) Density Logs

4. Electric surveys are used in which stages of drilling and well completion?

a) Only during exploration. b) Only during drilling. c) Only during well completion. d) Throughout the entire process, from exploration to production.

5. What type of electric survey uses radioactive sources to measure subsurface properties?

a) Resistivity Logs b) Porosity Logs c) Nuclear Logs d) Sonic Logs

Electric Surveys Exercise:

Instructions: Imagine you are an oil and gas exploration geologist. You are analyzing electric survey data from a newly drilled well. The data reveals the following:

• Resistivity Logs: High resistivity readings in a specific interval.
• Porosity Logs: High porosity readings in the same interval.
• Permeability Logs: Moderate permeability readings in the same interval.

Based on this information, describe the potential of this interval as a hydrocarbon reservoir. Justify your answer.

Techniques

Electric Surveys: A Comprehensive Guide

Chapter 1: Techniques

Electric surveys, or well logging, employ various techniques to measure the physical properties of subsurface formations. These techniques rely on lowering specialized logging tools into the borehole, which then transmit data to the surface. The key techniques used include:

• Resistivity Logging: This technique measures the ability of the formation to resist the flow of electrical current. Different methods exist, including:

  • Induction logging: Uses an alternating electromagnetic field to measure resistivity. Effective in conductive formations.
  • Laterolog logging: Employs focused electrical currents to minimize the influence of borehole effects, providing more accurate measurements, especially in high-resistivity formations.
  • Microresistivity logging: Uses small electrodes to measure resistivity very close to the borehole wall, providing high-resolution images of the formation.

• Porosity Logging: Determining the pore space within the rock is crucial. Techniques include:

  • Neutron porosity logging: Uses a radioactive source to measure hydrogen content, which is directly related to porosity.
  • Density porosity logging: Measures the bulk density of the formation using gamma rays. Porosity is calculated by comparing the bulk density to the matrix density.
  • Sonic porosity logging: Measures the travel time of sound waves through the formation. Porosity is inferred from the relationship between velocity and porosity.

• Permeability Logging: While direct permeability measurement in a wellbore is difficult, permeability can be estimated indirectly through various logs. Techniques used often incorporate other log data for calculation.

• Formation Density Logging: Gamma-ray density tools measure the bulk density of the formation, providing information about lithology and porosity. These logs are calibrated using known density values of various rocks.

• Sonic Logging: Acoustic logging tools measure the speed of sound waves through the formation, providing insights into lithology, porosity, and fracture detection.

• Nuclear Logging: This encompasses several techniques using radioactive sources: Gamma ray logging identifies radioactive elements in the formation; neutron logging measures hydrogen index and porosity; and spectral gamma ray logging differentiates various radioactive elements for more detailed lithological analysis.

Chapter 2: Models

Interpreting electric survey data involves using various models to translate raw measurements into geological information. These models account for factors such as borehole effects, invasion of drilling mud, and variations in rock properties. Key models employed include:

• Archie's Equation: A fundamental equation relating resistivity, porosity, water saturation, and formation factor. This allows estimation of water saturation from resistivity logs.

• Porosity-Permeability Relationships: Empirical relationships are used to estimate permeability from porosity measurements, considering rock type and other factors. These relationships are often specific to a reservoir.

• Geological Models: Three-dimensional geological models integrate log data with seismic and other geological information to create a comprehensive subsurface picture. These models often involve complex algorithms for reservoir characterization.

• Electromagnetic Modelling: Sophisticated numerical techniques solve Maxwell’s equations to model the response of electromagnetic tools in complex geological scenarios, enhancing interpretation accuracy.

Chapter 3: Software

Analysis of electric survey data relies on specialized software packages that provide tools for data visualization, processing, and interpretation. Key features of this software include:

• Data Import and Quality Control: Import of various log data formats, including LAS files. Tools for detecting and correcting data errors.

• Log Processing: Functions for smoothing, filtering, and correcting for borehole effects and other environmental influences.

• Log Interpretation: Built-in modules for running standard interpretations such as Archie's equation and porosity calculations.

• Visualization Tools: Advanced plotting capabilities for displaying logs, cross-sections, and 3D models. Integration with GIS software.

• Reservoir Simulation Integration: Some packages directly connect with reservoir simulation software, allowing seamless integration of log data into reservoir models.

Popular software packages include Petrel (Schlumberger), Kingdom (IHS Markit), and others developed by individual logging companies.

Chapter 4: Best Practices

Ensuring accurate and reliable results from electric surveys requires adherence to best practices throughout the process:

• Pre-Survey Planning: Careful planning of the survey, including selecting appropriate logging tools, optimizing the logging sequence, and defining clear objectives.

• Data Acquisition: Maintaining consistent logging parameters, implementing quality control during data acquisition, and documenting all procedures meticulously.

• Data Processing and Interpretation: Employing appropriate correction techniques, using calibrated tools and models, and applying sound geological principles in the interpretation process. Independent verification and validation are critical.

• Data Management: Establishing a robust data management system to ensure data integrity, accessibility, and long-term preservation.

• Health and Safety: Strict adherence to safety protocols during all phases of well logging operations is essential for the well-being of personnel and the integrity of the wellbore.

Chapter 5: Case Studies

Illustrative examples showcasing the application and interpretation of electric surveys:

• Case Study 1: Reservoir Characterization: A case study demonstrating how resistivity, porosity, and density logs were used to delineate the extent of a hydrocarbon reservoir, estimate its hydrocarbon saturation, and guide well placement for optimized production.

• Case Study 2: Formation Evaluation in a Complex Geological Setting: A case study illustrating the challenges and techniques used for formation evaluation in a geological setting with complex lithologies and faulting. This shows how multiple log types were integrated to overcome these challenges.

• Case Study 3: Monitoring of Enhanced Oil Recovery (EOR): A case study explaining how electric logs were used to monitor the effectiveness of an EOR process (e.g., waterflooding or steam injection), demonstrating changes in reservoir properties over time.

• Case Study 4: Wellbore Integrity Assessment: Examples of how various logs (including imaging logs) can identify zones of wellbore instability or potential casing failure.

These case studies would provide specific details on the logging tools used, the data interpretation techniques, and the key findings. Real-world examples with data visualizations would enhance understanding.