Lecture Notes: Reservoir Characterization in Clastics and Carbonates

Introduction

• Speaker: Dr. Mustafa Araravi
• Experience: 25 years in the petroleum industry
• Education: Bachelor's and Master's from Alexandria University, Ph.D. from North Carolina State University
• Positions: Various roles in academia and industry

Lecture Structure

• Two parts:
  1. Addressing major questions
  2. Open Q&A

Major Questions Addressed

1. Porosity in Gas Bearing Reservoirs

• Gas Effect on Porosity:
  • Gas has fewer hydrogen atoms than oil and water
  • Neutron tool measures hydrogen atoms, thus reads lower porosity in gas zones
• Neutron Tool Behavior:
  • In gas reservoirs, neutron log reads lower porosity
  • Less hydrogen = lower neutron porosity
• Density Tool Behavior:
  • Gas has lower density than oil and water
  • Density tool measures bulk density
  • In gas zones, bulk density decreases, increasing calculated porosity
• Neutron-Density Log Combination:
  • Gas zones: neutron and density logs separate significantly
  • Oil zones: minimal separation
  • Limestone overlays in oil zones but separates in gas zones
  • Dolomite shows reversed separation due to density differences
• Identification:
  • Significant separation between neutron and density logs indicates gas zones
  • Average neutron-density porosity is still used for accurate porosity estimation

2. Identification of Salt and Anhydrite Beds

• Salt (Halite - NaCl):
  • No hydrogen: neutron log reads zero porosity
  • Low density: density log reads low
  • Non-radioactive: gamma ray reads low
  • High resistivity: resistivity log reads high
  • Signatures: Low gamma ray, zero neutron porosity, low density, high resistivity
• Anhydrite (CaSO4):
  • No hydrogen: neutron log reads zero porosity
  • High density: density log reads high
  • Non-radioactive: gamma ray reads low
  • High resistivity: resistivity log reads high
  • Signatures: Low gamma ray, zero neutron porosity, high density, high resistivity

3. Variogram Processing

• Purpose: Quantify variations based on distances between wells
• Process:
  1. Identify well locations (x, y coordinates) and measure properties (e.g., porosity)
  2. Calculate distances between all well pairs
  3. Determine the variance (gamma) based on property differences
• Steps:
  • List wells with x, y coordinates and property values
  • Calculate distances between wells
  • Arrange distances in ascending order
  • Divide range into bins
  • Calculate variance for each bin
  • Plot distance vs. variance to identify zones of correlation and non-correlation
• Applications:
  • Identify spatial variations in properties
  • Determine zones of correlation within a reservoir

Q&A Highlights

Differences between Clastics and Carbonates

• Clastics: Mainly sandstone (SiO2), can include minor amounts of limestone (calcareous sand)
• Carbonates: Mainly limestone or dolomite, named based on majority composition
• Naming: Based on dominant lithology, e.g., calcareous sandstone, dolomitic limestone

Required Parameters for Reservoir Characterization

• Skills: Understanding geological environment, log responses, core-log integration
• Data Analysis: Multi-well processing, statistical variations
• Tools: Utilize logging and geological data, integrate core analysis

Core Helium Porosity vs. Fluid Porosity

• Fluid Porosity: Measured by heating cores and extracting fluids (often underestimated)
• Helium Porosity: Measures porosity using helium gas, more accurate

Histogramming Data

• Focus: Use net pay section for accurate analysis, avoid including non-reservoir data

Identifying Gas Bearing Reservoirs

• Separation: Significant neutron-density separation in gas zones (greater than in oil zones)
• Rule of Thumb: Separation > 6 pu indicates gas zones

Conclusion

• Exam Information:
  • Available times: Friday at 2 PM, 8 PM, and Saturday at 2 AM (Egypt time)
• Recording: Lecture will be uploaded to PI Petrol YouTube channel