Well logging
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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 within 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.
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[edit] 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 instruments 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.
[edit] 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 do not 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 measurements 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.
[edit] Mud logging
Another logging technique used is known as 'Mud-Logging'. A Wellsite Geologist (usually called a "Mudlogger" or "Mudlogging Field Geologist") works to describe cuttings, monitor formation gas and general drilling operations while drilling is in progress. The geologist/mudlogger analyzes the rock samples (cuttings) coming out of the circulating mud/fluids off of the 'flow line' from the drill string/pipe. Cuttings are sampled as they move across the shale shakers at drilled depth intervals (10', 20', 30', 50') described by the chief geologist. The 'Mud Log' is prepared by a Mud-logging company that has been hired by an operating company. A Mud Log displays formation gas (gas units and ppm), rate of penetration (ROP in min/ft); lithological sample descriptions; interpretive geology based upon ROP, formation gas/oil cut-stain-fluorescence, and gas curves including a total gas (gas units = ppm/1000) curve and Methane through Pentane (ppm). A mud log often also displays bit information, drilling parameters, mud weights, deviation surveys, and formation tops.
[edit] Information use
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 and quality of hydrocarbons present.
