Basic well log interpretation


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Basic well log interpretation

  2. 2. BASIC WELL LOG INTERPRETATION3.1 INTRODUCTIONThe continuous recording of a geophysical parameter along a borehole produces a geophysicalwell log. The value of the measurement is plotted continuously against depth in the well. Welllogging plays a central role in the successful development of a hydrocarbon reservoir. Itsmeasurements occupy a position of central importance in the life of a well, between twomilestones: the surface seismic survey, which has influenced the decision for the well location,and the production testing. The traditional role of wireline logging has been limited toparticipation primarily in two general domains: formation evaluation and completionevaluation.The goals of formation evaluation can be summarized by a statement of four questions ofprimary interest in the production of hydrocarbons: Are there any hydrocarbons, and if so are they oil or gas?First, it is necessary to identify or infer the presence of hydrocarbons in formations traversedby the wellbore. Where are the hydrocarbons?The depth of formations, which contain accumulations of hydrocarbons, must be identified. How much hydrocarbon is contained in the formation?An initial approach is to quantify the fractional volume available for hydrocarbonin the formation. This quantity, porosity, is of utmost importance. A second aspect is toquantify the hydrocarbon fraction of the fluids within the rock matrix. The third concerns theareal extent of the bed, or geological body, which contains the hydrocarbon. This last item fallslargely beyond the range of traditional well logging. How producible are the hydrocarbons?In fact, all the questions really come down to just this one practical concern. Unfortunately, itis the most difficult to answer from inferred formation properties. The most important input isa determination of permeability. Many empirical methods are used to extract this parameterfrom log measurements with varying degrees of success. Another key factor is oil viscosity,often loosely referred to by its weight, as in heavy or light oil.Formation evaluation is essentially performed on a well-by-well basis. A number ofmeasurement devices and interpretation techniques have been developed. They provide,
  3. 3. principally, values of porosity, shaliness and hydrocarbon saturation, as a function of depth,using the knowledge of local geology and fluid properties that is accumulated as a reservoir isdeveloped. Because of the wide variety of subsurface geological formations, many differentlogging tools are needed to give the best possible combination of measurements for the rocktype anticipated. Despite the availability of this rather large number of devices, each providingcomplementary information, the final answers derived are mainly three: the location of oil-bearing and gas-bearing formations, an estimate of their producibility, and an assessment of thequantity of hydrocarbon in place in the reservoir.3.2 APPLICATIONSIn the most straightforward application, the purpose of well logging is to provide measurements,which can be related to the volume fraction and type of hydrocarbon present in porous formations.Measurement techniques are used from three broad disciplines: electrical, nuclear, and acoustic.Usually a measurement is sensitive either to the properties of the rock or to the pore-filling fluid.Uses of well logging in petroleum engineering. (Adapted from Pickett)Logging applications for petroleum engineeringRock typingIdentification of geological environmentReservoir fluid contact locationFracture detectionEstimate of hydrocarbon in placeEstimate of recoverable hydrocarbonDetermination of water salinityReservoir pressure determinationPorosity/pore size distribution determinationWater flood feasibilityReservoir quality mappingInterzone fluid communication probabilityReservoir fluid movement monitoring
  4. 4. 3.3 Well Log Interpretation: Finding the HydrocarbonThe three most important questions to be answered by wellsite interpretation are:1. Does the formation contain hydrocarbons, and if so at what depth and are they Oil or gas?2. If so, what is the quantity present?3. Are the hydrocarbons recoverable?3.4 INTERPRETATION PROCEDUREThe basic logs, which are required for the adequate formation evaluation, are: 1. Permeable zone logs (SP, GR, Caliper) 2. Resistivity logs (MFSL, Shallow and Deep resistivity logs) 3. Porosity logs (Density, Neutron and Sonic).Generally, the permeable zone logs are presented in track one, the resistivity logs are run intrack two and porosity logs on track three.Using such a set of logs, a log interpreter has to solve the following problems,(I). Where are the potential producing hydrocarbons zones?(II). How much hydrocarbons (oil or gas) do they contain?First step: The first step in the log interpretation is to locate the permeable zones. Scanning thelog in track one and it has a base line on the right, which is called the shale base line. This baseline indicates shale i.e., impermeable zones and swings to the left indicate clean zones- e.g.,sand, limestone etc. The interpreter focuses his attention immediately on these permeablezones.Next step: To scan the resistivity logs in track 2 to see which of the zones of interest giveshigh resistivity readings. High resistivities reflect either hydrocarbons in the pores or lowporosity.Next step: Scan the porosity logs on the track 3 to see which of the zones have good porosityagainst the high resistivity zones. Discard the tight formations. Select the interesting zones forthe formation evaluation.
  5. 5. 3.5 FORMATION EVALUATIONDetermining Geothermal GradientThe first step involved in determining temperature at a particular depth is to determine thegeothermal gradient (gG) of the region. Temperature increases with depth, and the temperaturegradient of a particular region depends upon the geologic, or tectonic, activity within thatregion. The more activity, the higher the geothermal gradient. Geothermal gradients arecommonly expressed in degrees Fahrenheit per 100 m (°F/100m).If the geothermal gradient of an area is not known, then it can be determined by chart or byformula.gG= (BHT- Tms/TD) x100Where:BHT = bottom hole temperature (from header)TD = total depth (Depth-Logger from header)Tms = mean surface temperatureDetermining Formation Temperature (Tf)Once the geothermal gradient (gG) has been established, it is possible to determine thetemperature for a particular depth. This is often referred to as formation temperature (Tf).Where:Tms = mean surface temperaturegG = geothermal gradientD = depth at which temperature is desiredEnvironmental CorrectionsIn actual logging conditions, porosity (Ø) and the "true" resistivity of the uninvaded zone (Rt)cannot be measured precisely for a variety of reasons. Factors affecting these responses mayinclude hole size, mud weight, bed thickness, depth of invasion, and other properties of thelogging environment and formation. Many of these effects have strong impacts on analysis andmust be corrected prior to evaluating the formation. Several types of corrections and the toolsfor which these corrections are necessary are illustrated in table 3.1
  6. 6. Table 3.1: Required Environmental CorrectionsCorrecting Resistivity for TemperatureResistivity decreases with increasing temperature, and therefore any value of Rmf and/or Rwdetermined at one depth must be corrected for the appropriate formation temperature (Tf)where those values will be used to calculate water saturation (Sw). It is vital that formationwater resistivity (Rw) be corrected for temperature. Failing to correct Rw to a highertemperature will result in erroneously high values of water saturation (Sw). Therefore, it ispossible to calculate a hydrocarbon-bearing zone as a wet zone if the temperature correction isnot applied.Correction may be applied through the use of a chart (GEN-5) or an equation(Arps equation).Where: R2 = resistivity value corrected for temperature R1 = resistivity value at known reference temperature (T1) T1 = known reference temperature T2 = temperature to which resistivity is to be corrected k = temperature constant k = 6.77 when temperature is expressed in °F k = 21.5 when temperature is expressed in °C
  7. 7. Density porosityFormation bulk density (ρb) is the function of matrix density, porosity, and density of the fluidin the pores (salt mud, fresh mud, or hydrocarbons). To determine density porosity, either bychart or by calculation the matrix density and the type of fluid in the borehole must be known.The formula for calculating the density porosity is:Where; ρma = matrix density of formation. ρb = bulk density of the formation. ρf = pore fluid density in the borehole.Cross-Plot PorosityThere are a variety of methods--visual, mathematical, and graphical--used to determine thecross-plot porosity of a formation. Porosity measurements taken from logs are rarely adequatefor use in calculating water saturation. There are two methods for the determination ofporosity:1. Cross-Plot Porosity EquationWhere: ΦD = density porosity ΦN = neutron porosity2. Cross- Plot Porosity from ChartThe proper Cross-Plot Porosity (CP) chart is determined first by tool type, and second by thedensity of the drilling fluid.
  8. 8. SONIC POROSITYSonic Tool Cross-Plot ChartsThe "Sonic versus Bulk Density" and "Sonic versus Neutron Porosity" charts may beinterpolated and extrapolated in the same manner as the "Bulk Density versus NeutronPorosity" charts. These charts may be used as an alternative to the neutron-density cross-plots,or an additional method for providing more information on the possible lithology of aformation.Wyllie-Time Average Equation:Consolidated and compacted sandstones:Unconsolidated sands:Where: ∆tlog = travel time from the log. ∆tma = formation matrix travel time. ∆tf = fluid travel time Cp = compaction factor.Determining Formation Water Resistivity (Rw) by the Inverse Archie Method:Determining a value for formation water resistivity (Rw) from logs may not always providereliable results; however, in many cases logs provide the only means of determining Rw. Twoof the most common methods of determining Rw from logs are the inverse-Archie method andthe SP method. Another method of Rw determination is by means of Hingle plot.INVERSE ARCHIE METHOD: RwaWhere: Rt = resistivity of the uninvaded zone Φ = porosity
  9. 9. Sw Calculations:Water saturation may now be calculated for those zones that appear to be hydrocarbon bearing.The water saturation equation for clean formations is as follows:Archies EquationWhere: Sw = water saturation n = saturation exponent a = tortuosity factor. Φ= porosity. m = cementation exponent. Rt = formation resistivity Rw = formation water resistivityAmong the most difficult variables to determine, but one which has a tremendous impact uponcalculated values of water saturation (Sw). Often best obtained from the customer, but can beobtained from logs under ideal conditions. Other sources include measured formation watersamples (DST or SFT), produced water samples, or simply local reservoir history.Moveable Hydrocarbon Index (MHI)One way to investigate the moveability of hydrocarbons is to determine water saturation of theflushed zone (Sxo). This is accomplished by substituting into the Archie equation thoseparameters pertaining to the flushed zone.Where: Rmf = resistivity of mud filtrate. Rxo = resistivity of flushed zone.
  10. 10. Once flushed zone water saturation (Sxo) is calculated, it may be compared with the value forwater saturation of the uninvaded zone (Sw) at the same depth to determine whether or nothydrocarbons were moved from the flushed zone during invasion. If the value for Sxo is muchgreater than the value for Sw, then hydrocarbons were likely moved during invasion, and thereservoir will produce.An easy way of quantifying this relationship is through the moveable hydrocarbon index(MHI).
  11. 11. SHALYSAND INTERPRETATIONThe presence of shale (i.e. clay minerals) in a reservoir can cause erroneous results for watersaturation and porosity derived from logs. These erroneous results are not limited tosandstones, but also occur in limestones and dolomites.Whenever shale is present in the formation, porosity tools like, (sonic and neutron) will recordtoo high porosity. The only exception to this is the density log. It will not record too high aporosity if density of shale is equal to or greater than the reservoir’s matrix density. In addition,the presence of shale in a formation will cause resistivity logs to record lower resistivity.Calculation of Vshale:The first step in the shalysand analysis is the calculation of volume of shale from a gamma raylog. Volume of shale from gamma ray log is determined by the chart or by the followingformulas:Where:IGR = gamma ray indexGRlog = actual borehole-corrected GR response in zone of interestGRmin = minimum borehole-corrected GR response against clean zonesGRmax = maximum borehole-corrected GR response against shale zonesDetermining Effective Porosity (Φe):The second step of shaly sand analysis is to determine the effective porosity of the formationi.e. determining porosity of the formation if it did not contain clay minerals.Effective Porosity from Neutron-Density Combinations: Φn-corrected = Φn - (Vcl x Φnsh) For Neutron Φd-corrected = Φd - (Vcl x Φdsh) For Density
  12. 12. These values of neutron and density porosity corrected for the presence of clays are then usedin the equations below to determine the effective porosity ( effective) of the formation ofinterest.Determining Water Saturation (Sw) :( Indonesian Equation)There are many different equations by which water saturation (Sw) of a clay-bearing formationmay be calculated. However, the most suitable equation is the Indonesian Equation, which is asfollowWhere: Rt = resistivity of uninvaded zone Vcl = volume of clay Φe = effective porosity Rcl = resistivity of clay Rw = resistivity of formation water