Well logging
Well logging is a technique used in the oil and gas industry for recording rock and fluid properties to find hydrocarbon zones in the geological formations below the Earth's crust. A logging procedure consists of lowering a 'logging tool' on the end of a wireline into an oil well (or hole) to measure the rock and fluid properties of the formation. An interpretation of these measurements is then made to locate and quantify potential depth zones containing oil and gas (hydrocarbons). Logging tools developed over the years measure the electrical, acoustic, radioactive, electromagnetic, and other properties of the rocks and their contained fluids. Logging is usually performed as the logging tools are pulled out of the hole. This data is recorded to a printed record called a 'Well Log' and is normally transmitted digitally to office locations. Well logging is performed at various intervals during the drilling of the well and when the total depth is drilled, which could range in depths from 300 m to 8000 m (1000 ft to 25,000 ft) or more.
Wireline and "While Drilling" Well Logging
Well logging usually refers to downhole measurements made via instrumentation that is lowered into the well at the end of a wireline cable. The wireline consists of an outer wire rope and an inner group of wires. The outer rope provides strength for lowering and lifting the heavy insturments and the inner wiring provides for transmission of power to the downhole equipment and for data telemetry uphole to the recording equipment on the surface.
In recent years a new technique, Logging While Drilling (LWD), has been introduced which provides similar information about the well. Instead of sensors being lowered into the well at the end of wireline cable, the sensors are integrated into the drill string and the measurments are made while the well is being drilled. While wireline well logging occurs after the drill string is removed from the well, LWD measures geological parameters while the well is being drilled. However, because there is no high bandwidth telemetry path available -- no wires to the surface -- data is either recorded downhole and retrieved when the drill string is removed from the hole or the measurment data is transmitted to the surface via pressure pulses in the well's mud fluid column. This mud telemetry method provides a bandwidth of much less than 100 bits per second. Fortunately, drilling through rock is a fairly slow process and data compression techniques mean that this is an ample bandwidth for delivery of critical information.
Mud Logging
Another kind of logging technique used is 'Mud Logging'. A Mud Logger works when drilling is going on. The Mud Logger analyzes the rock samples coming out of the circulating mud/fluids off 'flow line' from the drill string/pipe. Similar to a Well Log, a 'Mud Log' is prepared by the Mud logging company. A mud log displays the Gas present in the formation by using Gas Chromatograph techniques. A mud log also describes the formation geology.
Use of Information
The well log and the mud log are usually transferred in 'real time' to the operating company, which uses these logs to make operational decisions about the well and to make interpretations about the quantity of hydrocarbons present.
Major well logging and mud logging companies
Both these jobs are performed by Oil & Gas Service companies. Well Logging companies are:
External links
Some Mud logging companies are:
- Geoservices,
- Baker Inteq (part of Baker Hughes),
- Datalog,
- [1].