Borehole logging or well logging is the procedure in which a detailed record (also known as well log) of different natured geological formations is obtained by a borehole. It is also known as the graphical representation of subsurface strata. The subsurface information deviates from the well plotted on strip chart or graph paper is known as the well log. These detailed logs can be based on visual inspection of the rock sample brought on to the surface, or on the physical measurements by different instruments lowered into the borehole. These logs can be run during or after the completion of wells. These well logs are performed for different purposes including groundwater, oil and gas, mineral and geothermal exploration and geotechnical studies. The first practical tool that could make a well bore recording of the earth’s natural radiations was developed in 1953.
- Objectives of Well Logging
- To study the well and its geometry
- To analyze geological formations and fractures
- To know Reservoir Properties
- Permeability Fluid Pressure
- Hydrocarbon Saturation
- Radioactive Material
- To know source, reservoir and seal rock properties
- To co-relate the nearby wells
- Basic Instruments Used In Well Logging
- Recording Truck
- Wire Line
At one end of the wire line, Sonde is attached which is being sent down in the bore hole and at the other end is attached with a recording truck which records the continuous data of borehole receiving from the Sonde. There are different types of well logs which are given below:
- Drill Time Log
- Sonic Log / Acoustic Log
- Mud Log
- Density Log
- SP Log
- Lithology Log
- Gamma Ray Log and Neutron Log
The petroleum industry consists of following sectors:
The first step is the determination of surface and subsurface structures (e.g. surveying). For this purpose, there are different kinds of surveys which are used in the industry e.g., Seismic Survey, Electrical Resistivity Survey and Magnetic Survey. The Seismic survey is based on the principle of seismic waves and it has two common phenomena which are reflection and refraction. The seismic reflection survey is mainly used in the petroleum industry. In the magnetic survey, the magnetic properties (susceptibility) of the subsurface rocks. The electrical resistivity survey is based on the electrical resistivity of subsurface rocks which are determined by the resistivity meters / ABEM SAS Terrameter 1000 / 4000 / LS (these are different models of Terrameter which is widely used in electrical resistivity survey and tomography). There are different components of rig including derrick, Swivel, Draw Work, Kelley, Crown Box, Drill Collar, Travelling Block, Drill Pipe, Hock, Bit, Rotary Table, Cat Walk and Mouse Hole etc. There are also some common tools of production which are Packer, Tubing, Production Casing etc. Main systems of rigs are e.g., Hoisting System, Rotary System, Mud Circulating System, Dower System and BOP system.
- To analyze where are the Formation tops?
- Are the potential zones porous or not?
- To analyze the results of seismic data and other surveys.
- To interpret the results of the Synthetic Seismic Section?
- At what depths, there are the Formation tops?
- Is this environment suitable for Hydrocarbons?
- What type of Hydrocarbons are present there in the Formation?
- Are the Hydrocarbons present in a commercial quantity?
- What is the total reserve?
- Logs Types on Basis of Logging Tools
- Geological Log
Geophysical logging is used for collecting data at the surface rather than by going down hole instruments. It includes time logs, core logs and sample logs /mud logs.
It records the time required to drill a given thickness of the rock formation. The change of rock formation, different rocks cause variations in the drill time log. This is expressed in minutes per feet (also called “time per thickness covered”).
It is the solid cutting of Formation up to 10 or 20 feet and gives detailed information about the different properties of rocks. Generally, there are two types of core logs which are (a) whole core and (b) sidewall core.
This log is made after the complete examination of the cutting which comes on the surface by drilling fluids which were initially pumped into the well.
In the mud log, different features are recorded which are lithology, gas, hydrocarbon flow, time, temperature, mud weight, weight of bit and most significantly it is used to detect the “Kick”.