Well logging

Well logging, also known as borehole logging is the practice of making a detailed record (log) of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface (geological logs) or on physical measurements made by instruments lowered into the hole (geophysical logs). Well logging is done when drilling boreholes for oil and gas, groundwater, minerals, and for environmental and geotechnical studies.

Geophysical well logs
The oil and gas industry records 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.

History
The first geophysical well log was a self-potential log recorded in 1927 in a well in oil field of Pechebronn, Alsace France

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 &mdash; no wires to the surface &mdash; 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 are porosity, permeability, and fluid content. Porosity is the proportion of fluid-filled space found within the rock. It is this space that contains the oil and gas. Permeability is the ability of fluids to flow through the rock. 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 flow toward the wellbore. Logging tools 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 &mdash; 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 instruments used in well logging are quite broad. The first 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 (NMR) 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.

Geological logs
In the petroleum industry, geological logs are commonly called mud logs, because they are made by examining cuttings, which are bits of rock circulated to the surface by the drilling mud in rotary drilling. The wellsite geologist, or mudlogger, describe cuttings, monitors traces of natural gas in the mud, as well as rig operations during both drilling and non-drilling phases. The geologist analyzes the cuttings which have travelled up the wellbore suspended in the drilling fluid or mud which was pumped into the wellbore via the drill string/pipe and returns to the surface via the 'flow line'. Cuttings are then separated from the drilling fluid as they move across the shale shakers and are sampled at drilled depth intervals (determined by the operator, but 10', 20', 30' and 50' intervals are commonly used at different sections of the wellbore), analyzed and described by the logging geologist on duty. The mud log is prepared by a company contracted by the operating company. A typical mud log displays formation gas (gas units or ppm), rate of penetration (ROP, in units of inverse velocity, or T/L); 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 usually also displays drillbit information, drilling parameters, mud weights, directional information about the wellbore (deviation surveys), casing shoe depths, and formation tops.

Information use
The well and mud logs 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. Specialists involved in well log interpretation are called log analysts.