Well Logging

Practical definition of a log

• Log is an indirect measurement of formation properties exposed by the well bore acquired ‐by lowering a device or a combination of devices in the well bore.

• Oldest and most used methods that depends on the geophysical properties of rocks.

• Good when surface outcrops are not available.

• Seismic can be used to determine the subsurface structure but a direct sample of the rock is needed to be sure of the lithology.

• The most practical way is to drill a well and record all the useful information possible from a core.

• Continuous core recovery is far too expensive and is not done except when great detail is needed.

Objectives

• To give an insight in the basic well logging methods used to derive petrophysical properties for hydrocarbon exploration

• Well logs provide continuous, in situ measurements of parameters related to porosity, lithology, presence of hydrocarbons, and other related rock properties of interest

Well Logging History• The first electrical log was introduced in 1927 in France.• The first commercial electrical resistivity tool in 1929 was

used in Venezuela, USA and Indonesia.• SP was run along with resistivity first time in 1931• Schlumberger developed the first continuous recording in

1931• GR and Neutron logs was started in 1941• Microresistivity array dipmeter and lateralog were first

time introduced in 1950’s• The first induction tool was used in 1956 followed by

Formation tester in 1957, Fomation Density in 1960’s, Electromagnetic tool in 1978 and most of Imaging logs were developed in 1980’s

The “First” Log recorded in 1927

Well in Pechelbronn - France Surface Recording Instrument

Purpose of Well Logging

• Are there any hydrocarbons? Oil? Gas?• Where are the hydrocarbons?• How much of the hydrocarbons is

contained in the formation?• How producible are the hydrocarbons?

Uses of Well Logging• Detect presence of hydrocarbons - Gas, oil or water– Estimation of total reserves– Estimation of productive zones

• Zone correlation• Isopach mapping• Defining physical rock properties:– Lithology– Porosity– Pore geometry– Permeability

• Determination of depth and thickness of zones

• Correlation with seismic data• Structural and sedimentological studies• Formation boundaries and stratigraphic

correlation• Quantitative fluid saturation• Reservoir modelling• Reservoir monitoring

Uses of Well Logging

1. Rock type

• The basic concentration during the interpretation of well logs is the rock type or Lithology. It could be either,

• Sandstone• Limestone• Shale (unconventional)

2. Fluids

a. What are the fluid type– Water– Oil– Gas b. Saturation– Water saturated, oil saturated, gas saturatedc. Pressure– Fluid pressure

ROCK PROPERTIES

– Porosity– Permeability– Lithology / Mineralogy– Density– Velocity– Pore Fluids (Water, Gas, Oil) / Pore Fluid Ratio– Thickness and Depth of the Formation– Fracture– Temperature

How to measure these properties?

• Few of these parameters can be directly measured!

• Derived or inferred from measurements of other physical parameters, such as:– Resistivity– Bulk density– Interval transit time– Spontaneous potential– Natural radioactivity

Types of Logs

Lithological logs Porosity logs Fluid indicator logs

Gamma ray Sonic Resistivity

SP Neutron Induction

Litho-density Density

Neutron-density

• Calliper logs for bore-hole properties

Wireline Logging Logging while Drilling