Chapter_5-OCW

Core Analysis

 Can be divided into two categories:  Conventional Core Analysis.

Special Core Analysis.

Conventional Core Analysis.

The core is usually slabbed, cut lengthwise to make the structure visible.

Provides information on lithology, residual fluid saturation, ambient porosity, ambient gas permeability and grain density.

Core Analysis

 Special Core Analysis :

Provides the following information:

Porosity and permeability at elevated confining stress.

Electrical properties such as formation factor and resistivity index.

Capillary pressure.

Wettability and relative permeability.

Mechanical rock properties such as compressibility.

Waterflood sensitivity for injectivity and well performance.

Open-hole Logging

Open-hole logging, also known as well logging is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole.

Open hole logs are run before the oil or gas well is lined with pipe or cased.

Principal of Well Logging

A well log is a record of certain formation data versus

depth.

The appropriate downhole logging tools instrument called ‘sonde’, about 3.5 inches in diameter is lowered into mud-filled hole on logging cable.

This tools will measure the electrical, acoustic, and radioactive properties of the formation.

The result will be analyzed to determine which of the layers are porous and permeable, and likely to contain hidrocarbon.

A depth calibration wheel records the length of cable in the hole.

Electrical Logs

Developed by Conrad & Marcel Schlumberger (who founded Schlumberger Limited), and intoduced to the US in 1929.

Can be divided into two main types: measurement of natural electrical current in the rock (SP Log), and measurement of induced electrical current (Resistivity Log and Induction Log).

(2)Resistivity Logs

Use to measure the resistivity of the formation, and

thus the possibility of hc shows.

A sonde sends an electrical signal through the formation and relays it back to a receiver at the surface (induced electricity). The surface detector will measure the formation’s resistance to the current.

A rock which contains an oil and/or gas saturation will have a higher resistivity than the same rock completely saturated with formation water.

(3)Induction Logs

Use to measure the conductivity of the formation, and thus the possibility of hc shows.

 A rock which contains an oil and/or gas saturation will have a lower conductivity than the same rock completely saturated with formation water.

 Induction logs use an electric coil in the sonde to generate an alternating current loop in the formation by induction.

 Induction tools t give best results when mud resistivity is high with respect to formation resistivity, i.e., fresh mud or non-conductive fluid. In oil-base mud, which is non conductive, induction logging is the only option available.