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It is a means of determining properties for a section of well under consideration. The physical property of interest is measured by various means and plotted against the well depth. From physical characteristics of a borehole by caliper log to oil & gas detection by complex analogies, logging applications have a tremendous potential in navigating reservoir evaluations. .
The log is based on physical measurements made by instruments lowered into the hole. Well logging is done when drilling boreholes for oil and gas, groundwater, minerals, and for environmental and geotechnical studies.
A successful logging program, along with core analysis, can supply data for subsurface structural mapping, define the lithology, identify productive zones and accurately describe their depth and thickness, distinguish between oil and gas, and permit a valid qualitative and quantitative interpretation of reservoir characteristics such as fluid saturation, porosity and permeability.
How a log is run
A datum is selected such as Kelly Bushing(KB), Ground Level(GL) or Drilling Rig Floor(DF)
Go to the bottom with electric log or any other tool and record on the way back tacking out slack of line, uniform stretch and controlled speed.
Repeat by going to the bottom and recording again.
If a second log is to be run, go to the bottom, pull a short section, adjust depth to match the original log and repeat the procedure.
A schematic view of the formation is shown below.
Evolution of the concept
Resistivity logs: Resistivity logging is a method of well logging that works by characterizing the rock or sediment in a borehole by measuring electrical resistivity. Resistivity is a fundamental material property which represents how strongly a material opposes the flow of electric current. In these logs, resistivity is measured using 4 electrical probes to eliminate the resistance of the contact leads. The log must run in holes containing electrically conductive mud or water.
Resistivity logging is sometimes used in mineral exploration and water-well drilling, but most commonly for formation evaluation in oil- and gas-well drilling. Most rock materials are essentially insulators, while their enclosed fluids are conductors. Hydrocarbon fluids are an exception, because they are almost infinitely resistive. When a formation is porous and contains salty water, the overall resistivity will be low. When the formation contains hydrocarbon, or contains very low porosity, its resistivity will be high. High resistivity values may indicate a hydrocarbon bearing formation.
Usually when drilling, drilling fluids invade the formation, changes in the resistivity are measured by the tool in the invaded zone. For this reason, several resistivity tools with different investigation lengths are used to measure the formation resistivity. If water based mud is used and oil is displaced, "deeper" resistivity logs (or those of the "virgin zone") will show lower conductivity than the invaded zone. If oil based mud is used and water is displaced, deeper logs will show higher conductivity than the invaded zone. This provides not only an indication of the fluids present, but, at least quantitatively, whether the formation is permeable or not.
Self potential log: The spontaneous potential log, commonly called the self-potential log or SP log, is a measurement taken by oil industry well loggers to characterize rock formation properties. The log works by measuring small electric potentials (measured in millivolts) between depths in the borehole and a grounded voltage at the surface.
The change in voltage through the well bore is caused by a buildup of charge on the well bore walls. Clays and shales (which are composed predominantly of clays) will generate one charge and permeable formations such as sandstone will generate an opposite one. This build up of charge is, in turn, caused by differences in the salt content of the well bore fluid (drilling mud) and the formation water (connate water). The potential opposite shales is called the baseline, and typically shifts only slowly over the depth of the borehole. Whether the mud contains more or less salt than the connate water will determine which way the SP curve will deflect opposite a permeable formation. The amplitudes of the line made by the changing SP will vary from formation to formation and will not give a definitive answer to how permeable or the porosity of the formation that it is logging.
The SP tool is one of the simplest tools and is generally run as standard when logging a hole, along with the gamma ray. SP data can be used to find:
Where the permeable formations are
The boundaries of these formations
Correlation of formations when compared with data from other analogue wells
Values for the formation-water resistivity
The SP curve can be influenced by various factors both in the formation and introduced into the wellbore by the drilling process. These factors, mentioned below, can cause the SP curve to be muted or even inverted depending on the situation.
Formation bed thickness
Resistivities in the formation bed and the adjacent formations
Resistivity and make-up of the drilling mud
The depth of invasion by the drilling mud into the formation
The drilling mud salinity will affect the strength of the electromotive forces (EMF) which give the SP deflections. If the salinity of the mud is similar to the formation water then the SP curve may give little or no response opposite a permeable formation; if the mud is more saline, then the curve has a positive voltage with respect to the baseline opposite permeable formations; if it is less, the voltage deflection is negative. In rare cases the baseline of the SP can shift suddenly if the salinity of the mud changes part way down hole.
Mud invasion into the permeable formation can cause the deflections in the SP curve to be rounded off and to reduce the amplitude of thin beds.
A larger wellbore will cause, like a mud filtrate invasion, the deflections on the SP curve to be rounded off and decrease the amplitude opposite thin beds, while a smaller diameter wellbore has the opposite effect.
Sonic log: Sonic logging shows a formation's interval transit time, designated Dt. It is a measure of a formation's capacity to transmit sound waves. Geologically, this capacity varies with lithology and rock textures, notably porosity.
Quantitatively, the sonic log is used to evaluate porosity in liquid filled pores. The sonic tool is only capable of measuring travel time. Many relationships between travel time and porosity have been proposed, the most commonly accepted is the Wyllie time average equation. The equation basically holds that the total travel time recorded on the log is the sum of the time the sonic wave spends traveling the solid part of the rock, called the rock matrix and the time spent traveling through the fluids in the pores.
Gamma ray log: Gamma ray logging is a method of measuring naturally occurring gamma radiation to characterize the rock or sediment in a borehole. It is sometimes used in mineral exploration and water-well drilling, but most commonly for formation evaluation in oil and gas well drilling. Different types of rock emit different amounts and different spectra of natural gamma radiation. In particular, shales usually emit more gamma rays than other sedimentary rocks, such as sandstone, gypsum, salt, coal, dolomite, or limestone because radioactive potassium is a common component in their clay content, and because the cation exchange capacity of clay causes them to adsorb uranium and thorium. This difference in radioactivity between shales and sandstones/carbonate rocks allows the gamma tool to distinguish between shales and non-shales.
The gamma ray log, like other types of well logging, is done by lowering an instrument down the hole and recording gamma radiation at each depth. In the United States, the device most commonly records measurements at 1/2-foot intervals. Gamma radiation is usually recorded in API units, a measurement originated by the petroleum industry. Gamma logs are affected by the diameter of the borehole and the properties of the fluid filling the borehole, but because gamma logs are most often used in a qualitative way, corrections are usually not necessary.
Three elements and their decay chains are responsible for the radiation emitted by rock: potassium, thorium and uranium. Shales often contain potassium as part of their clay content, and tend to absorb uranium and thorium as well. A common gamma-ray log records the total radiation, and cannot distinguish between the radioactive elements, while a spectral gamma ray log (see below) can.
An advantage of the gamma log over some other types of well logs is that it works through the steel and cement walls of cased boreholes. Although concrete and steel absorb some of the gamma radiation, enough travels through the steel and cement to allow qualitative determinations.
Sometimes non-shales also have elevated levels of gamma radiation. Sandstone can contain uranium mineralization, potassium feldspar, clay filling, or rock fragments that cause it to have higher-than usual gamma readings. Coal and dolomite may contain absorbed uranium. Evaporite deposits may contain potassium minerals such as carnallite. When this is the case, spectral gamma ray logging can be done to identify these anomalies.
Gamma ray absorption/ Density log: Density logging is a well logging tool determining rock bulk density along a wellbore. This is the overall density of a rock including solid matrix and the fluid enclosed in pores. Geologically, bulk density is a function of the density of the minerals forming a rock (i.e matrix) and the enclosed volume of free fluids (porosity).
A radioactive source applied to the hole wall emits medium-energy gamma rays into the formation so these gamma rays may be thought of as high velocity particles which collide with the electrons in the formation. At each collision the gamma ray loses some of its energy to the electron, and then continues with diminished energy. This type of interaction is know as Compton scattering. The scattered gamma rays reaching the detector, at the fixed station from the source, are counted as an indication of formation density.
The number of Compton scattering collisions is related directly to the number of the electron density of the formation. Consequently, the electron density determines the response of the density tool. Electron density is related through equation
where is the atomic numbers of all the atoms making up the molecules in the compound, and M is the molecular weight of the compound.
Inferring porosity from bulk density
Assuming that the measured bulk density (Ïbulk) only depends on matrix density (Ïmatrix) and fluid density (Ïfluid), these values are known along the wellbore; porosity (Ï†) can be inferred by the formula
Common values of matrix density Ïmatrix (in g/cmÂ³) are:
Quartz sand - 2.65
Limey, arkosic, or shaly sand - 2.68
Limestone - 2.71
Dolomite - 2.87
A fluid bulk density Ïfluid of 1 g/cmÂ³ is appropriate where the water is fresh; highly saline water has a slightly higher density. For flushed gas or oil reservoirs, even lower Ïfluid values should be assumed depending on the hydrocarbon density and residual saturation. In some applications hydrocarbons are indicated by the presence of abnormally high log porosities.
Neutron log: Inelastic scattering, elastic scattering and absorption are the basic phenomena that occur after a fast faced neutron is introduced in the formation. The fast paced neutrons are first slowed by inelastic scattering and then by elastic scattering. The neutrons eventually slow to the level of energy at which they coexist with the formation nuclei in the thermal equilibrium. Neutrons in this state are called thermal neutrons. Thermal neutrons continue to scatter off of the formation nuclei elastically and diffuse through the formation nuclei elastically and diffuse through the formation. Each thermal neutron eventually is captured by one of the nuclei. The nucleus instantaneously emits gamma rays, called capture gamma rays. Neutrons that have slowed down to the thermal energy level yet energetic enough to evade capture are called as epithermal neutrons.
Energy and momentum conservation of elastic collisions dictates that the presence of hydrogen in the formation dominates the slowing process. The reason is that the mass of hydrogen nucleus is approximately equal to that of the incident neutron. Consequently, at a point sufficiently removed from the source, formations with high hydrogen content displays low concentrations of epithermal and thermal neutrons and vice versa. Because most of the hydrogen is a part of the fluids located in the pores space, this concentration is inversely related to porosity.
Types of detectors: Most neutron porosity tools use chemical sources containing about 16 curies of americium and produce roughly 4x107 neutrons/sec. The most efficient are the epithermal neutron detectors, since the spatial distribution for these neutrons is the function of the slowdown length and not of the capture characteristics. Epithermal neutron tools are characterized by short source to detector spacing, which signifies a shallow depth of investigation.
Borehole measurements provide an array of recorded or calculated parameters. The parameters usually available are true formation resistivity Rt, flushed zone resistivity Rxo, formation bulk density measurements provide an array of recorded or calculated parameters. The parameters usually available are true formation resistivity Rt, flushed zone resistivity Rxo, formation bulk density ï²b, formation density porosity ï¦D, formation neutron porosity ï¦N and natural gamma ray radioactivityï§.
Pattern recognition interpretation techniques are based on plotting one of these parameters against other. The plot is known as Crossplot. Cross-plotting is a very useful technique. An ambiguous relationship between two parameters can be clarified by crossplotting the parameters. Pg.144