Well logging
Different industries, as mining, oil and gas uses wireline logging to obtain a continuous record of a formation's rock properties, also, groundwater consultants.
[1] Wireline logging can be defined as being "The acquisition and analysis of geophysical data performed as a function of well bore depth, together with the provision of related services."
The measurements are made referenced to "TAH" - True Along Hole depth: these and the associated analysis can then be used to infer further properties, such as hydrocarbon saturation and formation pressure, and to make further drilling and production decisions.
Logging tools developed over the years measure the natural gamma ray, electrical, acoustic, stimulated radioactive responses, electromagnetic, nuclear magnetic resonance, pressure and other properties of the rocks and their contained fluids.
The two data sets are then merged using the common time base to create an instrument response versus depth log.
"Open hole logs" are run before the oil or gas well is lined with pipe or cased.
[3] Wireline logs can be divided into broad categories based on the physical properties measured.
[4] In 1931, Henri George Doll and G. Dechatre, working for Schlumberger, discovered that the galvanometer wiggled even when no current was being passed through the logging cables down in the well.
The SP effect was produced naturally by the borehole mud at the boundaries of permeable beds.
Nuclear logging was initially developed to measure the natural gamma radiation emitted by underground formations.
More recent engineering developments by NUMAR (a subsidiary of Halliburton) in the 1990s has resulted in continuous NMR logging technology which is now applied in the oil and gas, water and metal exploration industry.
MWD logs use mud pulse technology to transmit data from the tools on the bottom of the drillstring to the processors at the surface.
Specific applications are fracture identification,[10] analysis of small-scale sedimentological features, evaluation of net pay in thinly bedded formations, and the identification of breakouts (irregularities in the borehole wall that are aligned with the minimum horizontal stress and appear where stresses around the wellbore exceed the compressive strength of the rock).
A sonic log provides a formation interval transit time, which is typically a function of lithology and rock texture but particularly porosity.
A log of the natural radioactivity of the formation along the borehole, measured in API units, particularly useful for distinguishing between sands and shales in a siliclastic environment.
[15] This is because sandstones are usually nonradioactive quartz, whereas shales are naturally radioactive due to potassium isotopes in clays, and adsorbed uranium and thorium.
The magnitude of this deflection depends mainly on the salinity contrast between the drilling mud and the formation water, and the clay content of the permeable bed.
Therefore, the SP log is commonly used to detect permeable beds and to estimate clay content and formation water salinity.
[18][19] The chief application of the NMR tool is to determine moveable fluid volume (BVM) of a rock.
On modern tools, both CBW and BVI can often be seen in the signal response after transforming the relaxation curve to the porosity domain.
As far back as 1955, an acoustic detector was proposed for use in well integrity analysis to identify casing holes.
[21] Over many years, downhole acoustic logging tools proved effective in inflow and injectivity profiling of operating wells,[22][23] leak detection,[24][25] location of cross-flows behind casing,[26] and even in determining reservoir fluid compositions.
[28] Throughout the life of the wells, integrity controles of the steel and cemented column (casing and tubing) are performed using calipers and thickness gauges.
[30] In LWD, measured data is transmitted to the surface in real time via pressure pulses in the well's mud fluid column.
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 networked or wired drillpipe that deliver memory quality data in real time.
"[34] The current oil industry standard mud log normally includes real-time drilling parameters such as rate of penetration (ROP), lithology, gas hydrocarbons, flow line temperature (temperature of the drilling fluid) and chlorides but may also include mud weight, estimated pore pressure and corrected d-exponent (corrected drilling exponent) for a pressure pack log.
