WEBINAR INDEX 2003 - 2011WELLOGRevised 09-05-2011All Rights ReservedPart I:Geophysical ConsiderationsRock typesReservoir characteristicsPrimary PorositySecondary PorosityPermeabilitySaturationThe BoreholeThe Borehole environmentHydrostatic pressureWater loss of mudDifferential pressureTimePorosityPermeabilitySymbolsWell log FormatLog HeadingAPI Log GridTracksColumnsDepth ScalesGrid ScalesWell EvaluationThe base logCorrelationLithologySpontaneous PotentialGamma RayShale VolumeResistivity LoggingResistivity conceptsResistivity ToolsJump to Part IIJump to Part IIIJump to Part IVGeophysical Considerations

ROCK TYPES:A thorough understanding of reservoir characteristics is an important part of evaluating oil, gas or water bearing formations. This understanding makes it possible to understand how geophysical measurements made by the many types of logging tools are related to reservoir parameters.What type of rock is a reservoir rock and what type is not?How do we quantify the value of a reservoir rock and the three important primary rock parameters; porosity, permeability and water saturation?RESERVOIR ROCKS:Igneousrocks are volcanic in origin and rarely contain oil, gas or water.Metamorphicrocks are sedimentary rocks that have been recompressed thru a combination of extreme heat and pressure into solid rock material. Metamorphic rocks are not a favorable reservoir rock.Sedimentaryrocks are formed thru erosion of igneous and metamorphic rocks or thru organic deposition.Most reservoir rocks are sedimentary in origin.The three general sedimentary rock types of importance are Sandstone, Limestone and Dolomite.Clastic sediments are composed of broken and worn particles of pre-existing minerals, rocks and shells. These sediments are transported and eventually deposited in successive layers.Non-clastic (carbonates) are characteristically Limestone or Dolomite. The rock matrix is generally composed of once living organisms found in sea water.Fossil remains of this sea life are found in the rock matrix.RESERVOIR CHARACTERISTICS:Complete analysis of a reservoir requires three pieces of data: porosity, permeability and saturation.Porosity is the capacity of a formation to contain fluids. By definition, porosity is the percent of void volume divided by the total volume.The symbol for porosity is the Greek symbol.Porosity ()= void volume / total volumePRIMARY POROSITY:Primary porosity results from the void space between inter-granular rock fragments and particle grains after their accumulation as sediments. The theoretical maximum porosity based on spherical sand grains stacked on top of each-other is 46.7 percent. Primary porosity is a function of the depositional environment, compaction, and cementation.SECONDARY POROSITY:Secondary porosity results from leaching of sediments or other actions that remove material and form fractures, channels, caverns or vugs in a formation. Carbonate rocks are frequently found to contain secondary porosity in the form of vugs, solution cavities or channels.In general, porositydecreases with depth.As depth increases, the increasing pressure causes compaction. The older more cemented formations exhibit lower porosity. A shallow, younger formation may have a porosity of 25 percent and an older deeper formation may have less than 10 percent porosity.

PERMEABILITY:Permeability defines the ability of a reservoir to allow flow or passage of fluids.The symbol for permeability is the letter (K).Permeability is measured in darcies, a numerical expression named after the French Engineer Henry d Arcy, who in 1865devised a method for quantifying permeability.Most producing reservoirs have permeabilities less than 1 darcy.Permeability of rocks is measured in millidarcies (md). Permeability may vary from 5000 md for an unconsolidated sand to .1 md for some carbonates.Permeability is one darcy when 1 sq. cm of rock releases1 cc of fluid of unit viscosity in 1 second under a pressure differential of one atmosphere/cm.Determination of permeability using achart

Porosity and permeability of selected oil sands: (Note: actual values may vary from those given here)

Sandstone formation:Porosity:Permeability:Clinch, Lee County, VA9.6.9Bartlesville, Anderson County, KS17.525.0Nugget, Fremont County, WY24.9147.5Woodbine, Tyler County, TX22.13390.0The rule of thumb for classifying permeability:Excellent:> 1000 mdVery good:250 1000 mdGood:50 250 mdModerate15 50 mdPoor to fair:< 1 15 mdPermeability in reservoir rocks is directional property.Formations exhibit both horizontal and vertical permeability.Horizontal permeability has the greatest affect onproduction. The ratio of Kh to Kv is between 1.5 and 3.0 depending on deposition, grain type, size and shape.Actual permeability can be measured by core analysis.The results depend on the testing method used for determination of permeability.Gas and fluid permeability are two different values.Core samples having permeability measured under surface conditions without the pressure of overburden are more willing to liberate fluids. The result is permeability that is optimistic by25 % to 1000 %.Permeability, a primary rock parameter, can be indicated by but not measured directly with logging tools which measure secondary rock parameters like resistivity and porosity.

Permeability of sandstone and shaly sandschartGo to our page on permeability for completepermeabilityinterpretation.

SATURATION:The fluid saturation of a rock is the ratio of the volume of fluid filled porosity to the total porosity. Fluid saturations are expressed as a percent of total pore volume. For example, a water saturation of 20 percent means that 20 percent of the pore volume is water filled.In a hydrocarbon reservoir, other fluids usually hydrocarbons fill the remaining pore space.Due to differences in specific gravity, fluids having lower specific gravity become segregated in ascending layers in a reservoir.Gas will move upward until it reaches a layer of rock that is impermeable. A formation layer of this type is referred to as a trap. Oil will occur below the layer of gas and water will be in a layer below the oil.Some of the oil cannot be produced. The non-producible oil is referred to as RESIDUAL or IRREDUCIBLE saturation. Residual hydrocarbons may be producible using secondary recovery techniques including the use of steam or chemicals or other methods.The portion of pore space that does not contain formation water is assumed to contain hydrocarbons.Hydrocarbons are in the form of oil or gas.

Hydrocarbon saturation (Sh) = 1 Water Saturation (Sw)

TheBorehole

THE BOREHOLE ENVIRONMENT:Theborehole environmentbegins with the fluid within the borehole which is usually drilling mud but can be air or water. The resistivity of the borehole fluid is referred to as Rm.

As the borehole fluid is forced into the surrounding formation, a mud cake having resistivity (Rmc) and thickness (hmc) is formed on the wall of the borehole.Fluid from the borehole that enters into the immediate surrounding rock formation and which flushes that part of the formation has resistivity (Rxo) and also is called mud filtrate (Rmf) resulting in saturation of the flushed zone (Sxo).Continuing outward from the borehole, the invaded zone having resistivity (Ri) and (Rz) is saturated with water and is defined as (Swi).Beyond the invaded zone is a zone that is not invaded by borehole fluid. This zone is called theuninvadedzone. Theuninvadedzone contains fluid (water) having resistivity (Rw) and total resistivity (Rt) with water saturation (Sw).HYDROSTATIC PRESSURE:Hydrostatic pressure is a measure of the pressure at any given point in a well. The pressure is a function of the weight of the fluid in the well and the depth. Water has a weight of approximately .4 lbs. PSI per foot.A waterfilled well would have a pressure of 400 PSI at 1000 feet. Drilling fluid may have heavy chemicals added to increase the hydrostatic pressure. If the hydrostatic pressure is greater than the pressure exerted by the surrounding formations then positive hydrostatic pressure occurs. When a well is drilled using rotary drilling methods, it is customary to maintain the weight of the drilling fluid at a weight that gives positive hydrostatic pressure.The advantage of positive hydrostatic pressure is that the formation fluids do not escape. It is also an advantage that the pressure is greater in the well that fluid flows from the well into the surrounding formation. As fluid flows into a formation, a mud cake is formed on the sides of the well.WATER LOSS OF MUD:A well having positive hydrostatic pressure will have water loss as water within the well invades into the surrounding formations. The higher the water loss, the deeper the invasion will be.DIFFERENTIAL PRESSURE:Invasion is a function of Differential Pressure. As the Differential Pressure increases, the amount of invasion also increases. Drilling fluids are designed to minimize water loss thru the process of creating a mud cake that limits invasion of fluids into the formation.TIME:The length of time that a formation is exposed to the forces of a mud column in a well also affects the amount of invasion. It is important to know that the longer a formation is exposed to invasion, the deeper the invasion.POROSITY:Under given conditions, a formation having greater porosity will invade LESS deeply than a formation having lower porosity.PERMEABILITY:Normally, the permeability of the mud cake is low and it controls the amount of invasion. When a formation is highly permeable, it may have greater control over the amount of invasion than the mud cake. It is interesting to note in a given well, the differences in filtrate invasion and resulting thickness of mud cake from one formation to another.BOREHOLE TEMPERATURE:Borehole temperature increases from a surface average temperature (Tsurf) to a maximum borehole temperature (TTD) assumed to be at the termination depth or TD. This differential is usually measured in degrees per 1000 feet and is referred to as geothermal temperature gradient.Often times it is necessary to calculate the temperature at an intermediate depth (Tf) in the well.

If the average surface temperature is 70 degrees F and the Temperature at TD of 10000 feet is 170 degrees, What is the temperature at 3500 feet?The temperature differential is 100 degrees over 10000 feet or 10 degrees F per 1000 feet.Tf =(TTD-Tsurf)/TD *Fd+TSurf)Tf= (170 -70 )/10000 * 3500 + 70 = 105 degrees at 3500 feet.Temperature versus depth is often obtained from achart.

SYMBOLS:A few symbols used in Well Drilling, Well logging, and Well Log Analysis: (this is not a complete list) d = diameter of the holePorositydi= diameter of invasion= density h = thickness of a formation bedt = Delta Thmc= thickness of the mud cake Ra = Apparent ResistivityRi= Resistivity of the invaded zoneRm= Resistivity of the mudRmc= Resistivity of the mud cakeRmf= Resistivity of the mud filtrateRt= True Formation ResistivityRw= Resistivity of the WaterRwa= Apparent Water ResistivityRwe= Equivalent Water Resistivity Rxo = Resistivity of the flushed zoneSw=Water SaturationSxo= Water saturation of the flushed zone F = Formation factor m = Cementation factorn= Saturation exponent

Well Log Formats

A well log is a permanent record of the geophysical information measured at the time the log was performed.It is important to future analysis that the heading information be complete and thorough.

The log heading should contain all of the information that is necessary to analyze the log traces.Because auxiliary documents are frequently unavailable to other users of the log, all of the critical information concerning the log should be on the final log heading.The header information should also be included in the same computer file as the log data!If information is not available or not applicable it should be noted on the heading!The following information should be included:Background well information:Owner of well and address. Location of well; date; logging contractor and address; Logging operator; drilling contractor and address; client and address; observer and address; elevation of top casing and distance above ground; and drilling history, methods etc.Borehole conditions:Casing description; description of log depth datum; elevation of log depth datum; type of drilling fluid; resistivity and temperature of borehole fluid; depth of origin of borehole fluid samples; fluid level; time since last circulation; bottomholetemperature; and problems and unusual conditions.Equipment data and logging parameters:Description of probe reference point; model and manufacturer of logging tools; logging company tool number; date and type of last calibration; date, type, and response of field standardization; top and bottom of logged interval; logging speed and direction; vertical depth error after logging; time constant or the time interval of digital samples; identification of the disk containing digitized logs; and equipment problems.Specific information for nuclear logging probes:source description, initial source strength and date determined; source to detector or receiver spacing; detector description; and data filtering or enhancement parameters.Specific information for acoustic and electric logging probes:Source or transmitter description and signal output; source or transmitter to detector or receiver spacing; detector or receiver description; and data filtering or enhancement parameters.(Reference: ASTM)API LOG GRID:Well logs are graphic representations of tool response with reference to depth.On occasion a log is made at a fixed depth (station) with reference to time.As a log Analyst, it is important to properly interpret the graphically indicated numeric response from the log.The API Log Grid is the standard format used for recordingWellLogging measurements.Other formats that may be acceptable can be seen athttp://www.rockware.com.TRACKS:The log is organized into three tracks in the following format:From left to right;A single track (on the left) (track 1), a depth track, and two tracks called track 2 and track 3.Each track is 2.5 inches wide.The depth track is .75 inches wide.Each track is divided or scaled.COLUMNS:Tracks appear as columns. Each track may contain one or more curves representing the logged data at a given depth.DEPTH SCALES:The Depth scale is established in terms of inches of log per 100 feet of well. Standard scales are:1 inch per 100 feet-called 1 inch2 inches per 100 feet-called 2 inch5 inches per 100 feet-called 5 inchGRID SCALES:The divisions within a track are referred to as the grid scale.Three types of grid scales are available; Linear, Logarithmic, and Split Grid.Data that is linear in nature is recorded on a linear scale. For example porosity on a scale from 0 to 40 percent across 10 linear divisions provides 4 percent porosity per division.Data that needs a larger dynamic scale for exampleresistivity,may be indicated on a logarithmic scale. A typical logarithmic scale may be two cycle meaning the scale is from 1 to 10 to 100 ohm-meters or it might be on a four cycle scale .2 to 2.0 to 20.0 to 200 to 2000 ohm-meters depending on the desired ranges.LOG ASCII FILES:Digital logging systems record logs in a text file using a format called log ASCII, abbreviated LAS. These text files use a .lasextension. You may learn more about LAS format at the Canadian Well Logging Society web sitewww.cwls.org.

Most oil producing states in theUnited Stateshave websites that contain LAS files on wells drilled in that state.Later in this webinar, log interpretation including import to Microsoft Excel spreadsheets. Charting is performed on LAS files.

Well Evaluation an OverviewWELL EVALUATION:The reason for running a Well Log is to help locate an Oil or Gas bearing formation.Other reasons may be related to defining bed thickness of Coal or other minerals and defining aquifers for production of water.In order to analyze a formation, three types of logs are available. The types are; Lithology, Resistivity, and Porosity. Each will be discussed in detail in subsequent parts of this webinar.When a well logging unit arrives on location, a geologist or engineer approaches the evaluation task in the following manner:The first step is to run a base log ,normally a resistivity log, which is used to correlate formations with previous logs ran in the local area. This base log helps establish the structural position of the well.Information from the base log that indicates lithology or formation type is examined to determine which zones have sufficient porosity and permeability to be of most interest for production.The resistivity anomalies are then further evaluated.No definite conclusions can be made at this point regarding the commercial value of the well. Further information on porosity must be obtained in order to make a quantitative evaluation.Note: The porosity logs only respond to variation in porosity. Therefore logging engineers are advised that the porosities calculated from tool response may be subject to correction after further evaluation.When Lithology, resistivity, and porosity logs are available then the analyst has sufficient information to proceed with a numerical analysis of porosityandsaturation.This data combined with other geological information provides the basis for determination of the commercial value of a well.WHAT ASTM says about LogInterpretation:The American Society for Testing and Measurement is actively involved in testing a measurement processes and setting acceptable standards.ASTM, founded in 1898, is a developer and publisher of technical information designed to promote the understanding and development of technology and to ensure the quality of commodities and services and the safety of products.WELLOGstrongly recommends the following ASTM publication:ASTM standards on Ground water andVadoseZone investigations Drilling, Sampling, Geophysical logging, Well Installation and Decommissioning.Stock #: Drill99The website ishttp://www.astm.orgQuoting ASTM:The full potential of a logging program cannot be realized until the logging measurements are interpreted. Log interpretation should start at the time of data acquisition and should continue as an iterative process through-out the project.

Lithology Identification

LITHOLOGY IDENTIFICATION:A logging tool that could measure lithology and produce a lithology Log would be a valuable tool! When software is applied to multiple logs in a well defined area, methods have been demonstrated that give lithological representations.

One tool that is considered by many to measure lithology is the Photoelectric Density tool. The measurement of bulk density when plotted with a measurement of atomic cross-section comes very close to providing rock type identification.MINEROLOGY:Lithology is associated with certain mineralogy. Sandstone is composed largely of quartz minerals. Limestone formations are composed of calcite and other related calcium minerals. Dolomite is another common type of lithology.Major physical differences in these mineral types allow analysts to identify the mineralogy.Mineral:Density:Cross-section:Dolomite2.850 gm/cc4.78Sigma matrix (x1021Barns/cc)Sandstone2.655 gm/cc8.66Limestone2.690 gm/cc8.72Anhydrite2.950 gm/cc12.30Mineral densities Table 1Mineral densities Table 2SPONTANEOUS POTENTIAL:One of the first logging measurements ever recorded, Spontaneous Potential, or SP provides information that infers lithology.In addition, SP can infer permeability. It is possible to perform Lithology identification using multiple logs.

From Schlumberger, C. & M., (1934), Doll, H. G., (1948), Wyllie, M. R. J., (1949) (1951) & othersSpontaneous potential is a measurement of the natural voltage that is created from current produced in the earth because of electrochemical action. It is normally recorded in wells drilled with water.Formations having permeability are invaded by mud filtrate from the drilling mud. The result is electrochemical action that causes current flow in the formation.Shale formations have very low or non-existent permeability and therefore no current flow and low spontaneous potential.

The SP curve is recorded in track 1 (left-hand track) of the well log. The intensity of the Spontaneous potential can bedetermined bycharts using the resistivity of the mud filtrate (Rmf) and the Formation water resistivity (Rw).

SP is expressed as:SP =-(60 + .133T) log10 (Rmf/Rw)Where:T = temperatureRmf= Resistivity of the mud filtrateRw= resistivity of the formation water.SSP =-(K) log10 (Rmfe/Rwe)Where:T = temperatureRmfe= Resistivity of the mud filtrate effective.Rwe= resistivity of the formation water effective.RmfeandRweare obtained fromcharts

EXERCISE 1:Generate an MS excel spreadsheet with 4 columns.Calculate SP in the 4thcolumn.Use the following values:Temp:Rmf:Rw:SP:110100.5?110100.5?View a copy of an example spreadsheet(sp.xls).Since SP is not a zero based curve, its deflection is measured from a shale base line or predominant right most deflection.Shale formations have little or no permeability.Sandstone, limestone and dolomite do have some degree of permeability. The SP is useful in detecting permeable beds, locating bed boundaries, determining water resistivity, and as a shale indicator.In formations containing hydrocarbons, SP is depressed because of the reduction of conductive ions.SP curves may be calibrated using a fixed voltage calibrator.CORRECTION CHARTS:Charts are used to predictSP fromRwe.

SP measurements can be corrected forbed thicknessandRmandRs.

GAMMA RAY:Clean sandstones and carbonates are low in gamma radiation.In contrast formations containing shale are higher in gamma radiation.Gamma radiation is statistical in nature because the radioactive decay of radioisotopes is random.Because radioactive isotopes tend to concentrate in shale or clay formations and clean sandstone and carbonate formations are low in radioactive isotopes, the Gamma ray tool may be used to infer lithology.Gamma tools should be calibrated with a reference test source in order to perform in a standardized manner.Spontaneous Potential and Natural gamma ray curves are positioned in track one of the log and indicate sandstone or carbonate formations when at the extreme left of the scale and indicate shale or clay at the extreme right side of the scale.CORRECTION FOR SHALE:Certain porosity logs require correction for shale volume (Vsh). Neutron porosity is optimistic in shale.Acoustic porosity is optimistic in shale. When shale is present, effective porosity, (phi subscript e), can be used to more accurately determine water saturation (Sw) .Using information from the natural gamma log;Shale Volume =Vsh=(GrGrcs) / (GrshGrcs)Where:Gr= Gamma ray counts in the zone of interestGrcs= Gamma ray counts in a clean sandGrsh= Gamma ray counts in a shale zoneUsing information from the SP log;Shale Volume =Vsh= (sp spcs) /spshspcs)Where:SP = SP in zone of interestSpcs=sp in clean sandSpsh= sp in shale zoneWhich shale volume equation should be used?Use SP for shale volume calculation for instances of highRmf/Rw.Use Gamma ray for shale volume calculation for instances of lowRmf/Rw.

Resistivity Logging

RESISTIVITY CONCEPTS:Resistivity can be defined as the degree to which a substance resists the flow of electric current.

Resistance from Ohms Law relates to current and voltage as follows:R = V/IWhere:R = ResistanceV = VoltageI = CurrentThemost simplegalvanic measurement is Resistance.A Resistance log is performed by connecting one electrode to the surface (ground) and another electrode to adownholetoolthat is immersed in borehole fluid.Applying a constant current and measuring voltage allows calculation of resistance. This type of log is called a single-point resistance log.If both electrodes are placed on the tool then a differential resistance log is produced.Multiple-electrode arrays extend the depth of investigation. A better representation of True Formation resistivity (Rt) is obtained. Formation Resistivity can be measured when four electrodes are used. Two electrodes one on the surface and one down-hole on the tool are used to generate an electrical current in the formations in and around the electrodes. The surface electrode is referred to as B and the down-hole electrode as A. The voltage measured between two points referred to and M and N is then calculated as follows:VMN= R x I/4px((1/rAM-1/rAN)-(1/rBM-1/rBN))Viewof resistivity model.The Resistivity (R) (in a homogenous medium) is determined by:R = V / I x GThe apparent Resistivity (ra) (in a heterogeneous medium) is determined by:ra = V / I x GWhere:G = Geometric array factorV = VoltageI = CurrentNote aboutsymbology:Thegreeksymbol (r)is commonly used in geophysics and (R) is used in the well logging industry for Resistivity.The meaning is the same in both cases.Calculation of Geometric Factor (G):Normal array:G = 4 xpx (1/rAM 1/rAN 1/rBM+ 1/rBN)-1Simplified:G = 4 xpx MNFor example: 16Normal; 16 = .4 meters; G = 12.56 x .4 = 5.02.Where:MN = distance between MN electrodes in meters for ohm-metersOrMN = distance between MN electrodes in feet for ohm-feetLateral array:G = 4px (1/rAM 1/rAN)-1Resistivity is a physical property and is independent of size and shape.Resistivity, (R) is expressed in units of ohm-meter2/ meter abbreviated ohm-meters or ohms.Conductivity is the reciprocal of resistivity.Conductivity = 1 / RConductivity is frequently expressed in units of micro-mhos/cm.Conductivity in micro-mhos/cm = 10000/RWhere Resistivity (R) is in units of ohms meter2/ meter (also ohm-meters).Also, conductivity is expressed in units ofmilli-mhos per meter or simplymilli-mhos. Another unit ismilli-siemens.Visit this web page for more information on the units ofsiemens and mhos.Conduction in liquids is controlled by ion flow.Ions are created whensodium chloride(orNaClequivalent i.e. Potassium) are present in drilling and formation waters.The higher the sodium chloride concentration the higher the conductivity and lower the resistivity.Ion flow is controlled by fluid viscosity and therefore temperature affects the flow of ions and conductivity.Resistivity is affected by temperature.As temperature increases, conductivity increases and resistivity decreases.Determination of Rw:This step is often overlooked!Heresa couple of rulesBefore any interpretation of resistivity data can take place, Rw must be known.As mentioned previously, the value of Rw is affected by temperature. If a water sample is taken and Rw is measured, it is equally important toNote the temperature of the water sample!Determination of Rw from SP:Resistivity of formation water is related to the SP curve.Rw may be obtained from achart.Geothermal gradient:Geothermal gradient is a measure of temperature increase with depth. Geothermalgradientsare normally 1.0 to 1.7 degrees per 100 feet.For example: If a well has a surface temperature of 75 degrees F and bottom hole temperature is 175 degrees F at a depth of 10,000 feet, the geothermal gradient is 1.0 degrees per 100 feet.Evaluation of a formation using Rw should always be performed using a corrected Rw at formation temperature.Rw @ temperature should be documented on the log heading.A FREE CALCULATOR:When interpretation is performed on resistivity IT MUST BE AT IN-SITU TEMPERATURE. For example: given Rw at 70 degrees F. What is Rw in the well at 200 degrees F?Heres a Resistivity at T2calculator.Resistivity related to porosity:The amount of water contained in a formation is directly related to porosity. Porosity therefore affects formation resistivity. As the volume of water increases, the capacity for ions increases. More ions mean more conductivity. Conductivity and Resistivity are inversely related as previously mentioned.Resistivity of a formation 100 percent water saturated (Ro) = Formation resistivity factor (F) times Resistivity of the water (Rw).Formation resistivity is affected by three factors: Salt Concentration, Temperature, Pore volume (porosity).Formation Resistivity Factor isa proportionalityconstant based on the ratio of Ro to Rw.The equation is:F = Ro/RwKnown as the Archie equation.Ro is resistivity of a 100 percent water filled formation and Rw is resistivity of the water.Given Rw = .05,If Ro = 5.0 then F = 100If Ro = 1.25 then F = 25If Ro = .55 then F = 11Formation resistivity Factor(F) is related to Porosity (f) as follows:F = a /fmThe variables (a) and (m) are related to lithology. Cementation factor (m) ina cementedsandstone or a porous limestone is 2.0 and (a) is equal to 1.0.Resulting in the equation:F = 1 /f2Calculation of Formation factor from porosity:Porosity of 10 percent results in a Formation resistivity Factor of 100Porosity of 20 percent results in a Formation resistivity Factor of 25Porosity of 30 percent results in a Formation resistivity Factor of 11Notice these three Formation Resistivity factors are the same as calculated with F = Ro/Rw above.RESISTIVITY TOOLS:Water saturation and hydrocarbon saturation affect formation resistivity.The measurement of resistivity is therefore one of the most important measurements to be made in logging a well. A resistivity tool is most useful if it measures two or more characteristics of formation resistivity. Resistivity measurements combined with porosity measurements and estimations of permeability allow a complete analysis of a well to be performed.ELECTRIC LOG (E-LOG):The Electric Logging tool was originally introduced by Conrad and Marcel Schlumberger in 1927 inPechelbronnFrance.[First Log][E-log page]Mono-electrode configuration.The concept of operation of the electric logging tool is as follows:When two electrodes are placed inaoil or water filled well and voltage is applied to them, a current will flow through the well fluid and formation fluids.If additional electrodes are placed in the vicinity of the current producing electrodes, a voltage can be measured.The voltage measured is directly related to the resistivity of the surrounding formation fluids.Electric logging tools generate an alternating current and measure the resulting alternating voltage at measurement electrodes.The depth of measurement is directly related to the spacing or separation between electrodes.The depth is approximately equal to of the distance from the measure electrode and the midpoint between the two current electrodes.Different electrode configurations yield different depths of investigation.Thenormal electrode configurationis as follows:One current electrode (A) on the tool down-hole and the other current electrode (B) located at the surface. Measurement electrodes (M) are spaced from the down-hole current electrode at 8 inches, 16 inches, 32 inches or 64 inches above the A electrode depending on tool design. The reference electrode (N) is on the surface. The most common configuration is 16 inch (short normal) and 64 inch (long normal) spacing.This configuration results in a shallow resistivity and deep resistivity measurement.Thelateral configurationuses a current electrode (A) down-hole on the upper part of the tool or on an electrode bridle and the other current electrode (B) on the surface.Two lower electrodes (M) (N) measure the lateral voltage which is representative of a much deeper formation resistivity. Lateral measurements can be from 72 inches to 18 feet or more depending on electrode spacing and tool design. SeeAMN Lateral configuration. Also a configuration referred to asMABelectrode configuration.The advantage of short spacing is better thin bed definition.The advantage of longer spacing is a deeper measurement of true formation resistivity.Comparison of deep and shallow resistivity giveinformation about invasion.If shallow and deepresistivity arethe same, no invasion has occurred.If there is separation, the most probable reason is that invasion has occurred causing the shallow (invaded) and deep water resistivities to differ.The electric logging tool requires a fluid filled borehole in order to have a complete electrical path.CONSIDERATIONS:All logging methods have limitations to consider.Bed thickness effect: The curves produced by the normal devices are affected by bed thickness and resistivity (Lynch 1962).View a chart for bed thickness correction for16 normal.View a chart for bed thickness correction for64 normal.Formation transitions:Where the resistive bed is more than6 AMspacings thick, logging up hole, there is a gradual increase in resistivity until the M electrode on the sonde enters the bottom of the bed. This level of resistivity is maintained until the A electrode enters the bed. As the sonde continues there is a gradual increase in resistivity until the midpoint of the bed is reached. Thereafter a gradual reduction occurs in resistivity, which is symmetrical with the curve below the midpoint of the bed, until the sonde passes out of the bed. The recorded resistivity approaches but does not fully equal the true resistivity of the bed. The bed also appears to be1 AMspacing thinner than it actually is, the major resistivity deflections occurring AM above the bed bottom and AM spacing below the bed top. As the bed thickness decreases, the resistivity peak at the center decreases in amplitude. Further thinning to AM or less than AM causes the resistivity deflection to disappear entirely, and the curve actually reverses. The resistive bed now appears to be more conductive than the surrounding formations.Although the radius of investigation increases as the electrode spacing increases, the use of AM spacing greater than 64 inches is not practical because thinner beds are not only shown at less than true resistivity but may be recorded as conductive beds if their thickness is less than or equal to the AM spacing.Focusedresistivity tools overcome this limitation.INVERSION METHODS:Recently, software has been developed for improving resistivity log interpretation. Old logs and new are being subjected to inversion processing that removes the effect of surrounding formations. These techniques will make electrical resistivity a more accurate viable logging method well into the future.INDUCTION LOG:Induction toolsoperate on the concept of electromagnetic induction.A transmitter coil is energized at a frequency of 20,000 cycles per second (20 KHz).The electromagnetic field is coupled through the surrounding formations. Variation in formation fluid resistivity causes phase shifting of the transmitted signal. The formation produces a secondary electromagnetic field.A receiver coil having a fixed spacing receives the transmitter signal and the phase shifted secondary signal related to conductivity is converted into resistivity.Depth of investigation is directly related to coil spacing. The induction resistivity tool does not require conductive fluid in the borehole because it uses electromagnetism.The induction tool will not operate in steel casing.DUAL INDUCTION LOG:Because depth of investigation is related to coil spacing, the Dual Induction tool was developed in order to get two depths of investigation.The Dual Induction tool has one or more transmitter coils and two receiver coils at two fixed positions from the transmitter. Focusing is performed thru the addition of other coils. Focusing of the electromagnetic field reduces the effect of borehole signal.Invasion profiles are obtained fromchartsavailable from the logging service company.GUARD LOG:In wells containing highly conductive drilling fluids, guard tools are used.Afocusedguard tool offers the function of having a focused current path into the formation.Electrodes surrounding the current electrode are used to focus the tool current outward into the surrounding formation and not allow the current to travel through the conductive borehole fluid.Properinterpretation of focused logging tool measurements involveuse of correction charts.OTHER RESISTIVITY TOOLS:Many specialized varieties of resistivity tools are available.Micro-resistivity[Wall]devices, for example,micro-log, mini-log,FoRxo, Contact and others that measure resistivity of the borehole mud cake and flushed zone. One such tool has a depth of investigation of 2 inches for example.Micro-resistivity provides a measurement ofRxoandRmf. This information is valuable for the purpose of determination of permeability. Permeability is established by calculation of the saturation of the flushed zone (Sxo).Sxo= (Rmf/Rxo)1/2Determine porosity from micro resistivity using thischart.Recently added Electric and Induction tools can perform a synthetic aperture measuring at a great many different depths into the surrounding formation. Such tools give a more precise profile of resistivities surrounding the borehole.