Editing Well logging (section)

==Logging while drilling==
{{main|Logging while drilling}}
In the 1970s, a new approach to wireline logging was introduced in the form of [[Logging While Drilling|logging while drilling (LWD)]].  This technique provides similar well information to conventional wireline logging but instead of sensors being lowered into the well at the end of wireline cable, the sensors are integrated into the [[drill string]] and the measurements are made in real-time, whilst the well is being drilled.  This allows drilling engineers and geologists to quickly obtain information such as porosity, resistivity, hole direction and weight-on-bit and they can use this information to make immediate decisions about the future of the well and the direction of drilling.<ref>{{Cite web|url=https://www.rigzone.com/training/insight?insight_id=297&c_id=1|title=How Does Logging-While-Drilling (LWD) Work?|website=www.rigzone.com}}</ref>
 
In LWD, measured data is transmitted to the surface in real time via pressure pulses in the well's mud fluid column.  This mud telemetry method provides a bandwidth of less than 10 bits per second, although, as drilling through rock is a fairly slow process, data compression techniques mean that this is an ample bandwidth for real-time delivery of information.  A higher sample rate of data is recorded into memory and retrieved when the drillstring is withdrawn at bit changes.  High-definition downhole and subsurface information is available through [[IntelliServ#Technology|networked or wired drillpipe]] that deliver memory quality data in real time.<ref>{{cite journal|last=Ali|first=T.H. |author2=M. Sas |author3=J.H. Hood |author4=S.R. Lemke |author5=A. Srinivasan|title=High Speed Telemetry Drill Pipe Network Optimizes Drilling Dynamics And Wellbore Placement|journal=Society of Petroleum Engineers|year=2008|url=http://www.onepetro.org/mslib/app/Preview.do?paperNumber=SPE-112636-MS&societyCode=SPE|access-date=25 September 2012}}</ref>

===Memory log===
This method of data acquisition involves recording the sensor data into a down hole memory, rather than transmitting "Real Time" to surface. There are some advantages and disadvantages to this memory option.
*The tools can be conveyed into wells where the trajectory is deviated or extended beyond the reach of conventional Electric Wireline cables. This can involve a combination of weight to strength ratio of the electric cable over this extended reach. In such cases the memory tools can be conveyed on Pipe or Coil Tubing.
*The type of sensors are limited in comparison to those used on Electric Line, and tend to be focussed on the cased hole, production stage of the well. Although there are now developed some memory "Open Hole" compact formation evaluation tool combinations.  These tools can be deployed and carried downhole concealed internally in drill pipe to protect them from damage while running in the hole, and then "Pumped" out the end at depth to initiate logging. Other basic open hole formation evaluation memory tools are available for use in "Commodity" markets on slickline to reduce costs and operating time.
*In cased hole operation there is normally a "Slick Line" intervention unit. This uses a solid mechanical wire (0.072 - 0.125&nbsp;inches in OD), to manipulate or otherwise carry out operations in the well bore completion system. Memory operations are often carried out on this Slickline conveyance in preference to mobilizing a full service Electric Wireline unit.
*Since the results are not known until returned to surface, any realtime well dynamic changes cannot be monitored real time. This limits the ability to modify or change the well down hole production conditions accurately during the memory logging by changing the surface production rates. Something that is often done in Electric Line operations.
*Failure during recording is not known until the memory tools are retrieved. This loss of data can be a major issue on large offshore (expensive) locations. On land locations (e.g. South Texas, US) where there is what is called a "Commodity" Oil service sector, where logging often is without the rig infrastructure. this is less problematic, and logs are often run again without issue.