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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 measurements 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 measurement 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 real-time delivery of critical information. LOGGING MEASUREMENT TYPES Logging measurements are quite sophisticated. The prime target is the measurement of various Geophysical properties of the subsurface rock formations. Of particular interest is Porosity and Permeability . In the oilfield, formation porosity indicates the amount of space found between the granules that form the rock. It is this space that contains the oil and gas. Permeability is a measure of the interlinking of the rock pores, a description of the passages between the pores through which the oil and gas may pass. The higher the porosity, the higher the possible oil and gas content of a rock reservoir. The higher the permeability, the easier for the oil and gas to travel from far out in the formation toward the wellbore. While logging tools don't measure these quantities directly, they provide measurements that allow for the mathematical Interpretation of these quantities. Beyond just the porosity and permeability, various logging measurements allow the interpretation of what kinds of fluids are in the pores – oil, gas, brine. In addition, the logging measurements are used to determine mechanical properties of the formations. These mechanical properties determine what kind of enhanced recovery methods may be used ( Tertiary Recovery ) and what damage to the formation (such as erosion) is to be expected during oil and gas production. The types of instrumentation deployed in well logging is quite broad. Historically, logging measurements consisted of basic electrical logs (resistivity) and spontaneous potential (SP) logs, introduced by the Schlumberger Brothers in the 1920's. Tools later became available to estimate porosity via sonic velocity and nuclear measurements. Tools are now more specialized and better able to resolve fine details in the formation. Radiofrequency transmission and coupling techniques are used to determine fluid Conductivity (brine is more conductive than oil or gas). Sonic transmission characteristics (pressure waves) determine mechanical integrity. Nuclear Magnetic Resonance can determine the properties of the hydrogen atoms in the pores (surface tension, etc.). Nuclear scattering ( Radiation Scattering ), Spectrometry and absorption measurements can determine Density and Elemental Analysis or composition. High resolution electrical or acoustical imaging logs are used to visualize the formation, compute formation dip, and analyze thinly-bedded and fractured reservoirs. In addition to sensor-based measurments above, robotic equipment can sample formation fluids which may then be brought to the surface for laboratory examination. Also, controlled flow measurements can be used to determine in situ viscosity, water and gas cut (percentage), and other fluid and production parameters. 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: Some logging companies are:
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