Tornado Chart

Test Your Knowledge

Tornado Chart Quiz

Instructions: Choose the best answer for each question.

1. What is the primary purpose of a Tornado Chart in oil and gas exploration?

a) To visualize seismic data and identify potential hydrocarbon traps. b) To analyze resistivity log data and assess fluid invasion in formations. c) To map the distribution of different rock types in a geological formation. d) To calculate the volume of hydrocarbons present in a reservoir.

2. Which of the following is NOT a typical resistivity value represented on a Tornado Chart?

a) Initial Water Resistivity (Rw) b) Deep Resistivity (Rt) c) Shallow Resistivity (Rs) d) Permeability (K)

3. What does a large difference between the Shallow Resistivity (Rs) and Deep Resistivity (Rt) values indicate?

a) A highly porous and permeable rock formation. b) A significant amount of fluid invasion from drilling mud. c) The presence of a hydrocarbon reservoir. d) A good quality reservoir with minimal fluid invasion.

4. How can Tornado Charts aid in reservoir characterization?

a) By identifying the location of faults and fractures in the formation. b) By determining the pressure and temperature conditions in the reservoir. c) By identifying potential hydrocarbon zones, porous and permeable layers, and non-productive formations. d) By estimating the recovery factor of the reservoir.

5. What is a key advantage of using Tornado Charts for analyzing resistivity log data?

a) They provide a quantitative measure of the volume of hydrocarbons present. b) They offer a detailed analysis of the composition of the drilling mud used. c) They present a clear and intuitive visual representation of fluid invasion patterns. d) They predict the future performance of a production well.

Tornado Chart Exercise

Scenario: You are analyzing a Tornado Chart from a recent oil well exploration. The chart shows the following values:

• Rt: 20 Ohm-m (consistent throughout the formation)
• Rs: 5 Ohm-m (in the upper 50 meters of the formation)
• Rw: 0.1 Ohm-m

Task:

1. Describe the fluid invasion pattern observed in this formation.
2. Based on the information provided, would you consider this to be a good quality reservoir? Explain your reasoning.
3. What potential challenges might this fluid invasion pose for production?

Techniques

Chapter 1: Techniques for Constructing Tornado Charts

This chapter delves into the specific techniques employed in creating Tornado Charts from resistivity log data.

1.1 Data Acquisition and Pre-processing:

• Resistivity Log Data: The foundation of a Tornado Chart lies in high-quality resistivity log data obtained during well logging operations. This data typically includes measurements from various resistivity tools, such as the Induction, Laterolog, and Micro-resistivity logs.
• Data Cleaning and Correction: Before analysis, raw data often requires cleaning and correction to account for environmental factors like temperature variations and tool calibration errors. This step ensures the accuracy of the final chart.

1.2 Calculating Key Resistivity Values:

• Initial Water Resistivity (Rw): This value represents the resistivity of the formation water before any invasion by drilling mud. It is typically determined from a combination of laboratory analysis of formation fluids and interpretation of resistivity logs in known water zones.
• Deep Resistivity (Rt): This value represents the true resistivity of the rock formation, free from the influence of invading fluids. It is calculated using various methods, including the "deep induction" log, the "Laterolog deep" log, and sophisticated analytical techniques like the "Archie equation" and its variants.
• Shallow Resistivity (Rs): This value represents the resistivity close to the borehole where invading drilling mud filtrate has altered the original fluid content. It is determined using logs that measure resistivity at shallower depths, such as the "Micro-resistivity" log or the "Shallow Induction" log.

1.3 Data Visualization and Chart Construction:

• Depth as the Vertical Axis: The depth of the formation is plotted along the vertical axis, representing the vertical profile of the wellbore.
• Resistivity Values as the Horizontal Axis: The resistivity values (Rw, Rt, Rs) are plotted on the horizontal axis.
• Graphical Representation: Each resistivity value is represented by a separate line on the chart, with distinct colors or line styles for easy differentiation. The area between Rs and Rt lines represents the "invasion zone."

1.4 Additional Techniques:

• Log Interpretation Software: Specialized software packages are readily available to streamline the process of data acquisition, processing, and chart generation. These tools offer automated calculation routines, visualization options, and advanced analytical capabilities.
• Overlaying Other Logs: Tornado Charts can be further enriched by overlaying additional logs like gamma ray, sonic, or density logs to provide a more comprehensive understanding of the formation. This integration aids in identifying potential hydrocarbon zones, facies changes, and other geological features.

1.5 Conclusion:

Mastering the techniques for constructing Tornado Charts allows geologists and engineers to effectively visualize and analyze resistivity log data, ultimately leading to better interpretations of subsurface formations and informed decision-making in oil and gas exploration and production.