Investigation of Reservoir Rocks

The application of geophysics, geology, economics, and other disciplines to determine the identity, character, value and dimensions of the subsurface features is called “formation evaluation”. Today the major tools of formation evaluation are borehole geophysics, petrology, rock mechanics, subsurface geology, and core analysis.

Mud Logging

Mud logging, more precisely referred to as hydrocarbon mud logging, is a process whereby the circulating mud and cuttings in a well are continuously monitored as the well is being drilled. This process involves physical examination and description of the cuttings, as well charting various drilling parameters.

Coring

A number of methods can be used to cut cores in a wellbore. Conventional cores are cut using a special core bit, and are retrieved in a long core barrel. Using this method, the recovered core sample may undergo physical changes on its journey from the coring depth to the surface, where it will ultimately be analyzed. More sophisticated coring mechanisms are able to conserve either the orientation, the pressure, or the original fluid saturations of the rock sampled. An awareness of these changes and sampling methods is essential for understanding core analysis results.

Other coring methods have been devised to obtain additional rock samples after the well has been drilled, but before it has been cased. These methods require wireline tools that cut core plugs from the sides of the wellbore.

With suitable drilling tools, rock samples (mud samples, drill cores) are obtained from the reservoir rock zone and can subsequently be examined comprehensively. Special geophysical measurement (logging) and the first, brief productivity tests (drillstem tests) are also carried out. Continuous coring is impossible for economic reasons. Geophysical borehole measurements allow the continuous determination of rock parameters, especially the porosity, the original saturation conditions, the nature of the pore fluid, the type of rock, the mineral composition, and the bed thickness. The laboratory examinations of the core samples taken point-by-point, however, take longer.

Open-Hole Logging

Openhole logging provides the great meeting place for all formation evaluation methods. Only through openhole logging can a continuous record of formation properties such as porosity, water saturation, and rock type be obtained. In particular, wireline logs can record formation self potential, electrical resistivity, conductivity, bulk density, natural and induced radioactivity, hydrogen content, and elastic properties. Most wells drilled for hydrocarbons are logged with these types of instruments.

Unfortunately, the full potential of the logs may not always be utilized, or the logs may be incorrectly analyzed. This is caused either by a lack of training on the part of the analyst or a lack of understanding of where wireline logs fit in relation to the other formation evaluation tools.

Log Interpretation Objectives

The objective of log interpretation depends very much on the user. Quantitative analysis of well logs provides the analyst with values for a variety of primary parameters, such as:

1. porosity

2. water saturation, fluid type (oil/gas/water)

3. lithology

4. permeability

Gamma Ray Log

Gamma Rays are high-energy electromagnetic waves which are emitted by atomic nuclei as a form of radiation. Gamma ray log is measurement of natural radioactivity in formation versus depth. It measures the radiation emitting from naturally occurring uranium, thorium, and potassium and is also known as shale log. GR log reflects shale or clay content. GR log can be run in both open and cased hole.

Spontaneous Potential Log (Sp)

The spontaneous potential (SP) curve records the naturally occurring electrical potential (voltage) produced by the interaction of formation connate water, conductive drilling fluid, and shale. The SP curve reflects a difference in the electrical potential between a movable electrode in the borehole and a fixed reference electrode at the surface. Though the SP is used primarily as a lithology indicator and as a correlation tool, it has other uses, such as permeability indicator, shale volume indicator, porosity indicator.

Neutron Logging

The Neutron Log is primarily used to evaluate formation porosity, but the fact that it is really just a hydrogen detector should always be kept in mind. It is used to detect gas in certain situations, exploiting the lower hydrogen density, or hydrogen index.

The Neutron Log can be summarized as the continuous measurement of the induced radiation produced by the bombardment of that formation with a neutron source contained in the logging tool that has sources emitting fast neutrons that are eventually slowed by collisions with hydrogen atoms until they are captured.

The Density Log

The formation density log is a porosity log that measures electron density of a formation. Dense formations absorb many gamma rays, while low-density formations absorb fewer. A radioactive source applied to the hole wall emits medium-energy gamma rays into the formation so these gamma rays may be thought of as high velocity particles which collide with the electrons in the formation. At each collision the gamma ray loses some of its energy to the electron, and then continues with diminished energy. The scattered gamma rays reaching the detector, at the fixed station from the source, are counted as an indication of formation density.

Resistivity Log

Resistivity measures the electric properties of the formation. The ability to conduct electric current depends upon the volume of water, the temperature of the formation, the salinity of the formation. Resistivity logs measure the ability of rocks to conduct electrical current and are scaled in units of ohm-meters.

Acoustic Log

Acoustic tools measure the speed of sound waves in subsurface formations. While the acoustic log can be used to determine porosity in consolidated formations, it is also valuable in other applications, such as:

1. Indicating lithology;

2. Correlation with other wells;

3. Detecting fractures and evaluating secondary porosity;

4. Evaluating cement bonds between casing, and formation;

5. Determining mechanical properties (in combination with the density log).