University of Kerala, India. Exploration Geology Well Logging Omer M. Ahmed MSc, 2ed Year, 2016 University of Kerala, India.

Introduction Petrophysic of the rocks It is the study of the physical and chemical properties of the rocks related to the pores and fluid distribution Porosity, is ratio between volume of void to the total voids of the rock. Permeability, is ability of a porous material to allow fluids to pass through it. Electric, most of the sedimentary rocks don’t have conductivity. Radiation, clay rocks have 40K, radiate alpha ray. Hardness, it depends on the cementing material and thickness of the sediments.

WELL LOGGING The systematic recording of rock properties and it’s fluid contents in wells being drilled or produced to obtain various petrophysical parameters and characteristics of down hole sequences (G.E Archie 1950). The measurement versus depth or time, or both, of one or more physical properties in a well. These methods are particularly good when surface outcrops are not available, but a direct sample of the rock is needed to be sure of the lithology.

A wide range of physical parameters can be measured. In some cases, the measurements are not direct, it require interpretation by analogy or by correlating values between two or more logs run in the same hole. Provide information on lithology, boundaries of formations and stratigraphic correlation. Determine Porosity, Permeability, water, oil and gas saturation. Reservoir modeling and Structural studies… etc.

Types of Well Logging Logs can be classified into several types under different category Permeability and lithology Logs Gamma Ray log Self Potential [SP] log Caliber log Porosity Logs Density log Sonic log Neutron log Electrical Logs Resistivity Log

Permeability and lithology Log

Gamma Ray Log (GR) Measures the radioactivity of the formation and evaluation of radioactive minerals, such as potassium and uranium deposits in clay rocks, like shale. Gamma ray penetrates steel, so the log can be run in cased holes. The log has robust lithostratigraphic indicator that can be used for correlation and facies studies. Unaffected by fluids Evaluation of shale content.

Sand GR Log Scale 0 ------- 150 The base line (shale line) is center line = 75 100 Clay Permeable zone 150 200 Sand ---- Depth (M)

Evaluation of the shale content Where; V(sh)= shale volume (%), GR(log)= GR value from log, GR(max)=GR value from log at shale line, GR(min)= GR value from log at sand line

Spontaneous Potential (SP) Log Measuring the difference in electrical potential between two electrodes, a grounded electrode and an electrode on the sonde (logging tool). The SP log is very similar in shape to GR log (but does not identify thin beds), and both can be used alternatively for correlation purposes. If the fluid is better conductor than the drilling mud (salt water), the curve will deflect to the left. If the fluid is poor conductor (fresh water or oil), it will deflect to the right.

SP Log Scale, 1 line = 20 mV The base line = 0 Friable rock Fresh water Clay Hard rock Saline water Not clay

Caliber Log The logging system provides a continuous recording of borehole diameter versus depth. Can be used in both soft and hard formations, run in uncased wells. The main indicator of the log is: Determine hole and casing diameter, Locate caved zones, Recognition of mud cake When a hole diameter less than the bit size is an excellent indicator of permeability.

Caliper log Base line is bit size Friable rock Hard rock permeable zone

Determine the porosity, Porosity Log Determine the porosity, And Type of fluids

Density Log The log measures the bulk density of the formation, by tool contains a radioactive source emitting gamma rays. Measure how much radiation returns to a sensor and that controlled by rock density, porosity, composition of the formation and fluids. The main use of the Log is to determine a value of porosity. It s also useful in the detection of gas-bearing formations and recognition of evaporites.

Density Log Scale = 0.1 Add the value from left to right Or subtract from right to left

Sonic (acoustic) Log Is a continuous record of the specific time required versus depth for a compressed wave to travel through a given distance of formation in the borehole. (ms/ft.) or (s/ft.) The acoustic travel time in a formation depends upon lithology and porosity, the more dense the lower the travel time (∆t) because sound waves travel through the rock it self rather than the mud The main uses, is estimation of porosity (primary) and the calibration of regional seismic data.

Common lithological travel time

Sonic Log ∆t log = Reading from sonic log in µs/ft ∆t ma =Transit time of matrix material ∆t f = fluid velocity ~189 s/ft.

Neutron Log The log design from the fact, hydrogen atom is consist of proton and electron so it is very needful to the neutron. Using a radioactive source emitting neutrons which is collide with the hydrogen nuclei of the formation material. The amount of returning radiation will show the total of the lost, a dry layer will reject all the neutron and if it is contain a fluids the lost will be more. Most of the gas field discovered by the neutron Log (when the density is low compare with other Logs for the water and oil).

Neutron Log Displayed as (NPHI, PHIN, NPOR) For the limestone porosity is direct from the log EX: 14% Whereas other lithology is from the other chart

Electrical Logs EX: Resistivity Log For determine water and Hydrocarbons

Electrical Logs Are the most important tools available and first logs to be used by Marcel and Conrad Schlumberger 1927 Widely used in exploration work for water, minerals , hydrocarbons and engineering purposes. The main use of the electrical tools is to calculate The water saturation of a reservoir formation Electrical conductivity or resistivity of the materials And give an idea about hydrocarbon.

Resistivity Log Resistivity of the material is resistant in ohms of a unit cube, (ohms/S) or (ohms/M) Most of the rocks are porous and moisture (sandstone), and other are compacted which they are containing moisture in the component minerals (granite) Resistivity decrease with increase in the moisture content and for rock with low moisture content resistivity determined by the component minerals. Weathered rocks has lower resistivity than fresh.

Types of Resistivity 1/ Micro for saturated zone 2/ Shallow for transition zone 3/ Deep for the zone which is not effected by drilling mud

Resistivity Log M<S<D Saline water M>S>D Fresh water Micro: red Deep: black Shallow: blue M<S<D Saline water M>S>D Fresh water M<S<<D Petroleum Dry layer when they shows overlap between them When the D and M only overlapping that give an idea about the reservoir

CONCLUSION Depending on the Petrophysic of the rocks different type of logging have been used for determine different properties , porosity, The importance thing in the log is heading and base line. Combinations of two or more logs better than one log for determine lithology, porosity, and hydrocarbon content.

REFERENCES Andrew D. Miall (2016), Stratigraphy: A modern Synthesis, Springer International Publishing Switzerland, 454 P. Baker Hughes INTEQ (2001), Advanced Wireline & MWD Procedures Manual, Baker Hughes INTEQ Technical Publications Group, 192P. http://www.see.leeds.ac.uk/ https://www.spec2000.net/

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