Learning Objectives
After completing this course “Porosity”, you will be able to:
- Interpret LWD and wireline density logs to determine porosity.
- Use neutron porosity logs in formation evaluation.
- Incorporate sonic log data when evaluating the porosity of formations.
- Combine data from two or more logging tools for improved porosity determination.
- Adjust log porosity calculations for the effects of shale in the formation.
- Correct for the effects of hydrocarbons on well log porosity measurements.
- Understand the limitations of using resistivity logs for porosity determination.
Understanding Porosity
Well logging is a technique used in the oil and gas industry to gather vital information about subsurface formations. It involves the measurement and analysis of various parameters, including porosity. Porosity serves as a key indicator of a reservoir’s ability to store and transmit hydrocarbons. Understanding porosity and accurately determining its value is essential for reservoir characterization and optimal hydrocarbon extraction.
Introduction
Porosity is that fraction of the total rock volume which is filled with water, gas or oil. A reservoir formation may range in composition from a very loose, unconsolidated sandstone to a very hard, dense sandstone, limestone or dolomite. The cement that fills some of the pores and binds the grains together may consist of silica, calcite, dolomite, clay or other minerals. This mixture of rock matrix material, cement and pore-filling clays can result in a large range of porosity values and porosity types within subsurface formations.
The three main logging while drilling (LWD) and wireline logs used to determine the formation porosity are the density, neutron and sonic, or acoustic, logs. Each of these logs can be used alone to estimate a rock’s porosity with reasonable accuracy when the formation is “clean”—it contains little to no shale and clay—and is water-bearing. However, the commercial emphasis is always on accurate porosity evaluation in hydrocarbon-bearing reservoirs. Resistivity logs can also occasionally be used in certain specific circumstances to give approximate estimates of the formation porosity, although there are significant limitations to the use and applicability of resistivity logs for porosity estimation.
Of the well logs used to estimate porosity, the density is generally the most useful and reliable, primarily because the effects of shale and gas content in the logging tools’ zones of investigation are relatively less harmful to an accurate density porosity determination than when using the neutron or sonic logs alone.
Neutron logs are strongly affected by any gas or shale within the zone of investigation, rendering the porosity estimation in such cases highly inaccurate. Sonic tools tend to measure only the primary porosity and are also significantly affected by the formation’s shale content, particularly at shallower well depths.
Petrophysicists almost always use the density-neutron and sonic logs in a variety of combinations to overcome many of the interpretation challenges of porosity determination, particularly the effects of both shale and hydrocarbons on the log porosity measurements.
Types of Porosity
There are two primary types of porosity encountered in well logging:
Primary Porosity
Primary porosity refers to the original pore spaces present in the rock at the time of its formation. It includes intergranular porosity (between individual grains) and intragranular porosity (within the grains). The primary porosity is largely a result of sedimentary processes and the depositional environment.
Secondary Porosity
Secondary porosity develops after the rock formation due to various geological processes such as fracturing, dissolution, or other alteration mechanisms. Secondary porosity can significantly enhance the overall porosity of a reservoir and improve its hydrocarbon storage capacity.
