Logging:Logging or Well logging is a techniqueused in the Oil and Gas Industry for recording the properties of the rocks andtheir fluids in the well to find the potential hydrocarbon zones in thegeological formations within the earth’s crust.It is a technique of making petrophysical measurements in the sub-surface formations through the drilled borehole to determine both the physical and chemical properties of the rocks andthe fluids they contain. It is also called as Formation Evaluation.These physico-chemical measurementsare then interpreted to locate and quantify the potential depth zones where oiland gas can be found.
There are different LOGGING TOOLS usedto measure the electrical, acoustic, radioactive, electromagnetic, and other propertiesof the rocks and fluids containing the rock cuttings.Well Logging is very useful as itprovides detailed geological information of drilled holes in a very costeffective manner.Logging or Formation Evaluation is carried out by:1. MudLogging2. CoreLogging3. ElectroLogging1.
Mud Logging:Here the rock cuttings, brought to thesurface by the circulating drilling fluid / mud, are examined thoroughly to find out anytraces of subsurface hydrocarbon gases & oil when the well is being drilled.2. Core Logging:Here a cylindrical shaped rock sample(core) is taken out from a particular depth and examined for various propertieslike porosity, permeability, fluid saturation, etc and recorded in the welllog. This activity of core logging is carried out in between the drillingoperations by pulling out the drilling assembly and inserting a specialassembly which comprises a core bit & a core barrel to remove a core samplefor analysis. Here the drilling operation is stopped to remove the core.
Sothis core logging involves extra rig time and hence core logging increases theexploration cost. The cores removed at different depths are numbered and keptin a sequential way as per the depthfrom where they have been removed and can be used in case of any futureanalysis if required.3. Electro loggingor Wire Line Logging:Here a logging tool called as”Sonde” is attached at the end of a cable or wireline & loweredinto a well to measure the rock and fluid properties like electrical,electromagnetic, acoustic, radioactive properties of the formation.
Themeasured data is then transferred through the cable and recorded in a logging unit on the surface.All the informations and measurementsobtained through different logging techniques are recorded and interpreted to locate the potentialdepth zones where hydrocarbon oil and gas can be found out. Description of Natural Gamma RayLogging:It is one type of Electro Logging orWire line Logging method based on the principle that different types of rocksemit different amounts and different spectra of natural gamma radiation.
Usually shales emit more gamma raysthan other sedimentary rocks, such as sandstones stones. Thisis because of the presence of radioactive potassium in the clay content ofshales and the cation exchange capacity of the clay causes them to absorb uranium and thorium. Thisdifference in radioactivity between shales and sandstones/limestone rocksallows the gamma ray logging tool to distinguish between shales and non-shales.This gamma ray logging, like othertypes of Electro Logging, is done by lowering the probe, called as Sonde, downthe drill hole and recording the variation in gamma radiation at different depths.
The gamma ray log is plotted in a graph with low radioactivity to the left andhigh radioactivity to the right. Accordingly, the shales kick to theright showing maximum radioactivity while sandstones and limestones kick to theleft showing minimum radioactivity. Gamma radiation is usually recorded in API units.A typical Gamma Ray response is asbelow.
An advantage of the gamma log overother types of well logs is that it works through the steel and cement walls ofcased boreholes where other logging methods do not respond properly.Although concrete and steel absorbsome of the gamma radiation, enough travels through the steel and cement toallow qualitative determinations of radioactivity. Typical application of GammaRay Logging includes Indication of lithology, determination of reservoirthickness, correlation between wells and estimation of Shale volume in the wellbeing drilled.