Well logging is the process of maintaining real-time data during a drilling project.
Sometimes called ‘borehole logging’, well logging is a vitally important part of drilling operations. The nature of the geologic formations being penetrated need to be examined and understood so the operation is not drilling blindly.
Well logging is conducted during oil and gas drilling, mineral excavation, water well drilling, as well as geological and environmental exploration.
Well logging is a rather broad term that includes many subtypes of logging or data collection during the drilling operation. Here we will briefly cover a few of the main types.
This is the lowering of a tool or set of tools calibrated on a ‘string’ into the borehole to take measurements of the geological makeup and the possible fluids or gases that lie within.
The information received through the tools is recorded either at the surface (in real-time) or ‘in the hole’, also called a memory log. Memory logs are created by a process called LWD, or logging while drilling.
Data is recorded in the hole and analyzed when the unit is brought to the surface. Well logging is usually performed by a separate contractor which offers the well logging equipment and analysis to the drilling company.
Some of the types of logging data include:
- borehole imaging – advanced imaging technology to view borehole integrity
- porosity log – measurement of pore volume in formations
- resistivity log – measurement of electrical conductivity in formations
Mud logging is the analysis of the drilling mud during operations. Rock fragments in the drilling mud are brought to the surface and identified. Other measurements that are taken are the rate of drilling penetration, flow line temperature, weight of the drilling mud, viscosity, pump pressure, etc.
A very important aspect of mud logging is the data associated with gas levels. A close eye is kept on what types of gases are present and to what concentration, as this can be vital in avoiding a well blowout.
Similar to LWD, a system called MWD (measurement while drilling) is used to keep real-time data on certain aspects such as gas levels. This is accomplished by sensors installed in the borehole that constantly send data to the surface with mud pulsing.
LWD and MWD data are combined to create a complete well log.
This is when actual samples of the rock are taken from the borehole to examine fully. This is either done by taking a full core sample which is more involved as the drilling needs to cease in order to obtain a full core sample, and then there is sidewall sampling which is taken from the side of the borehole after the bit has passed through the section of rock.