Well logging

Introduction

Well logging, also known as borehole logging, is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface (geological samples) or on physical measurements made by instruments lowered into the hole (wireline logging). Some types of geophysical well logs can be done during any phase of a well's history: drilling, completing, producing, or abandoning. Well logging is performed in boreholes drilled for the oil and gas, groundwater, mineral and geothermal exploration, as well as part of environmental and geotechnical studies.

History

The history of well logging dates back to the 19th century with the first well logs made by hand. In the 1920s, the first modern log was created by Conrad and Marcel Schlumberger. Their electric log recorded resistivity, which is the opposition to the flow of electric current through a substance (electrical resistivity). This was a significant advancement in the field as it allowed for a continuous recording down a wellbore to be made, providing a more complete picture of subsurface conditions.

Types of Well Logging

There are several types of well logging that can be performed, each providing different information about the subsurface conditions. These include:

Resistivity Logging

Resistivity logging measures the subsurface's resistance to the flow of electricity. This can provide information about the subsurface's porosity, fluid saturation, and lithology. The two main types of resistivity logging are laterolog and induction logging.

Porosity Logging

Porosity logging measures the pore spaces within subsurface formations. This can provide information about the formation's ability to store and produce hydrocarbons. The three main types of porosity logging are neutron logging, density logging, and sonic logging.

Formation Testing

Formation testing involves taking fluid samples from the formation and measuring their properties. This can provide information about the formation's permeability and fluid composition.

Spectral Logging

Spectral logging involves measuring the energy spectrum of gamma rays emitted by subsurface formations. This can provide information about the formation's mineralogy.

Well Logging Equipment

Well logging equipment typically consists of a downhole tool, surface equipment, and a recording system. The downhole tool, also known as a sonde, is lowered into the wellbore on a wireline. The sonde contains one or more physical property sensors which make measurements that are transmitted to the surface equipment. The surface equipment consists of a winch to lower and raise the sonde, and a recording system to display and record the data.

Applications of Well Logging

Well logging has a variety of applications in the oil and gas industry, groundwater exploration, mineral exploration, geothermal exploration, and environmental and geotechnical studies. These include:

Oil and Gas Industry

In the oil and gas industry, well logging is used to determine the presence of hydrocarbons, the type of hydrocarbons present, and the quantity of hydrocarbons that can be produced from a formation. It is also used to determine the best methods for extracting these hydrocarbons.

Groundwater Exploration

In groundwater exploration, well logging is used to determine the depth and yield of water-bearing formations. It is also used to determine the quality of the water.

Mineral Exploration

In mineral exploration, well logging is used to determine the presence and concentration of minerals. It is also used to determine the best methods for extracting these minerals.

Geothermal Exploration

In geothermal exploration, well logging is used to determine the temperature and pressure conditions at depth. It is also used to determine the best methods for extracting geothermal energy.

Environmental and Geotechnical Studies

In environmental and geotechnical studies, well logging is used to determine the subsurface conditions and the presence of contaminants. It is also used to determine the best methods for remediation.