Therefore, the density log is used to calculate the porosity of a shale gas reservoir. The other porosity contributions are other sources of secondary porosity like vugs or vugular porosity, effective matrix porosity, and non effective matrix porosity like clay cement porosity. Porosity density log model crains petrophysical handbook. If neutron porosity is significantly less than the density porosity through a good hole section, then a gas effect must be present i. Improved horizontal well logging porosity calculation for a gas. Interpretation of porosity and fluid constituents from well logs using an interactive neutron density matrix scale olabode ijasan 1, carlos torresverdin, and william e.
Jun 24, 2015 in dual porosity reservoirs, the secondary porosity e. Nevertheless, the expression for the bulk density of a shaley, waterbearing formation is. Petrophysical approach for estimating porosity, clay. Notice on the first lithology track, how low is the gr gamma ray signal compared with signal yielded by a neutron porosity minus density porosity estimator vsh. From the first equation, to estimate the effective porosity, some estimate of vd must be made. Crains petrophysical handbook porosity density log model.
As with the density porosity, obtaining accurate neutron porosity depends on using the correct matrix. Even if shale is present, the interpretation merely requires an additional term. Porosity evaluation with acoustic logging petrowiki. Petrophysical relationship to predict synthetic porosity log. Fluid identification using neutron porosity and bulk density logs. Errors as large as 12% porosity can occur, and 3 to 6% are common if incorrect assumptions are made.
The lack of these porosity tools makes the determination of. Wyllies equation can be written in terms of the interval transit time. An approach for estimating porosity from soniclogs in shaly. The edited bulk density b log measurements were then used to calculate sediment porosities in holes 994d, 995b, and 997b using the standard density porosity relation.
These two values correspond to the density of a sandstone with zero porosity and a hypothetical sandstone with a porosity of 100%. Therefore, you need to multiply it by 100 to get the density porosity in %. The standard scale is the limestone compatible where the neutron porosity scale is. Hydrocarbon presence was determined using resistivity log while porosity was determined from density log. This formula is effective even in gas zones and is based on the premise that the neutron log sees deeper into the zone than the density log, and thus has more gas effect. Pdf correlation analysis between sonic and density logs for.
The response equation for the density log in porosity units follows the classical form. Equation 1 secondary porosity index arelative secondary porosity index is sometimes computed as the ratio of the absolute index, defined above, to total porosity. Nov 17, 2020 the formation bulk density is related to formation matrix density r maand formation fluid density r f as. Jul 30, 2009 values of q and the constant are easily derived by plotting the log of porosity vs. In the absence of core measurements, much useful information about porosity can be known by using a combination of at least two of these logs.
Default values for a and m in gardners equation are 0. Moreover, the shallowreading density log frequently is an invadedzone measurement, completely masking the gas effect on it. The matrix density used by the logging program to calculate porosity is different from the actual formation matrix density. Pdf basic well log interpretation mbereobong archibong. Wyllies equation can be written in terms of the inter. The density log corrected for shale is a very good approximation to porosity, but the log was not common before 1965, so sonic or neutron methods may be necessary for wells drilled before that time. The logging device is a contact tool that emits gamma rays from a source. The calculation of porosity from either the neutron, density or sonic logs is keyed to the log response of the matrix. The porosity was estimated using a reliable log porosity, which is typically the density log ellis, 2003. There is two method to measure porosity from density log.
While natural gas is a hydrocarbon, similar to oil, the physical properties of the fluids are very different, making it very hard to correctly quantify the total amount of gas in a formation. Crains petrophysical handbook porosity shaly sand models. It s also useful in the detection of gasbearing formations and in the recognition of evaporites. To fit this the density log has to have its zero limestone point 2. Whereas the total porosity based on kerogen corrected density log and kerogen porosity matches perfectly with the corebased porosity having porosity ranged from 5 to 10%. The neutron density porosity overlay in conjunction with the gammaray log gives an excellent first look at lithologies. Measurement technique edit neutron tools are based on the measurement of a neutron cloud of different energy levels within the investigated volume. Based on a detailed analysis, we proposed a novel porosity calculation model in which the normal acoustic log was. Most density transforms never made it into published chart books. Relationship between porosity and water saturation. This characteristic allows a neutron porosity log to be used with other porosity logs such as a density log to detect gas zones and identify gasliquid contacts.
Rattanachan 15 bet equation t adsorption temperatu re r gas constant 8. The objective of this project is to calculate a neutron log vs. Jun 24, 2015 as mentioned in the general interpretation discussion, the density log rarely sees past the invaded zone, so. Porosity determination using most of the logging devices presented in earlier chapters relies on a knowledge of the parameters related to the type of rock being investigated. Excel spreadsheet to calculate porosity, effective.
The formation density logging tool provides a radioac tivity measurement that. Limitations of the neutron density porosity overlay. In rocks, it is a function of the densities of the individual grains, the porosity, and the fluid filling the pores. Given the neutron porosity log and the densities of clay 2. In contrast, nuclear porosity devices, such as density and neutrons, measure total porosity. Projection of the straight line to a porosity of 1. Petrophysical relationship to predict synthetic porosity. Typically the density neutron separation or the gamma ray logs are used to make this estimate. What three conditions should be met for eyeballing porosity from the neutron density log display. Gardners equation was used to derive pwave becomes density. As the formation density tool is a log that is commonly used to calculate porosity, a pseudo porosity that has been calculated from the density data may be shown.
The porosity data obtained from density logs are considered to be total porosity. Preeg2 abstract neutron and density logs are important borehole measurements for estimating reservoir capacity and inferring saturating fluids. The density log is used to estimate the effective porosity of shaley sands, assuming the shale density is approximately 2. Table 5 chapter 4 lists the photoelectric absorption cross section index, the. Using geophysical logs to estimate porosity, vater resistivity, and. The density logging tool emits gamma rays from a chemical source which interact with the electrons of elements.
The formation density tools are induced radiation tools. In the same way, the simandoux equation overestimated the water saturation with more than 100% sw. Because the presence of oil has little effect on the density log, this tool usually provides the best indication of porosity in liquidfilled holes. Its main use is to derive a value for the total porosity of the formation. Density sonic porosity estimates fracture porosity. The method works well when shale volume and matrix rock density are accurately known. The tool operates by bombarding the formation with high energy neutrons. Note core grain density data, if available, can be used to select a matrix density appropriate for the formation of interest. It is conceptually the easiest of the porosity logs to interpret because, if ever a tool obeyed a linear bulk mixing law, it is the density logs. Both allow the log analyst to visualize the data more. Density estimations using densityvelocity relations and. Using geophysical logs to estimate porosity, vater.
Both neutron and density curves are in porosity referenced to limestone. Across the same intervals, the cnl thermal neutron porosity log reads higher than the neartoarray porosity an epithermal measurement because of thermal absorbers in. Guide to calculate porosity, effective porosity, saturation index, density, specific gravity etc. Neutron tools are based on the measurement of a neutron cloud of different energy levels within the investigated volume. Why porosity from density log is preferred over neutron log. From the sonic log, a sonic derived porosity log sphi may. The porosity log can be estimated by transforming density in the case of there are only a few logs available. Figure 4 shows a crossplot between coretested porosity and the density log. In this research, log transformation method was used so that. Excel spreadsheet to calculate porosity, effective porosity. When a formation contains gas, the density log values are high and the neutron log values are low. Therefore, any log measured bulk density values of less than 1. Its main use is in the determination of the porosity of a formation. Understanding porosity and density university of wisconsin.
As the formation density tool is a log that is commonly used to calculate porosity, a pseudoporosity that has been calculated from the density data may be shown. The matrix travel time is used in interpreting the compressional wave interval transit time. Bulk density log crossplot total porosity using, at this point, the schlumberger cnl or tnph charts as shown below. As an industry standard the neutron porosity log is recorded in limestone porosity units and displayed in either vv units, p. Porosity from the density log the density log is widely used to estimate porosity. Geoloil how to calculate fracture porosity from well logs. Emitted gamma rays collide with formation electrons and scatter. Porosity and lithology from formationdensity log and. Porosity and lithology from formation density log and. Idealized dual porosity log calibrated to limestone matrix and gammaray log 4 3. The data table lists the dry, wet, and grain densities of the samples.
The other input, p fluid, is usually that of water 1 gcm3 appropriate choice of p matrix values, which are often derived from other measurements, is crucial to the density porosity. Secondary porosity is classically estimated from well logs as the difference of total density porosity minus total sonic porosity. The densityneutron log is a combination log that simultaneously records neutron and density porosity. The most common reservoir rocks are sandstone p matrix 2. Because the porosity of a reservoir could be obtained based on resistivity and density, the log transformation method could be used to estimate the porosity log. Densityporosity log interpretation in the standard sequential interpretation process, the analyst determines porosity directly from the density log. Electric log spontaneouspotential and resisitivity logs 5 4. May 06, 2019 rock can have a mix of grains with different density, an assumption on the matrix density could miscalculate the porosity from density log. Energies free fulltext porosity and water saturation. Thus, the fluid density applied in equation 1 should be lower than 1.
Log derived porosity crossplot porosity derived from the crossplot of neutron and density logs, using the appropriate matrix density. The traditional use of density log is to measure the porosity of the reservoir scholes. For clean, shalefree formations, in which the porosity is filled with water or oil, the neutron log measures the fluidfilled porosity. While natural gas is a hydrocarbon, similar to oil, the physical properties of the fluids are very different, making it very hard to correctly quantify the total amount of gas in a formati. In clean water bearing formations, the crossplot porosity total porosity is similar to that derived from the density log. Hence, as a general rule, the density log is used in combination with the neutron log for accurate evaluation of porosity. Formation density log applications in liquidfilled holes onepetro.
While the presence of gas increases the apparent porosity seen by a density log, it decreases the apparent porosity seen by the neutron log. The results from wireline logs revealed that the cored sections are predominantly sandy. If the gas density is unknown for a density porosity formula, what value should be used as the fluid density. If gas is present, the actual f will be porosity will be too high. Shale corrections could create apparent gas crossover and this may be real or an artifact of excessive correction. A detector, located a fixed distance from the tool source, counts the number of returning gamma rays.
Actually, in the industry today, over 20 different models for the shale fraction vd are in use. Total porosity comes from nuclear log density, neutron, or combo of density and neutron. Idealized electric log of shale and sandstone sequence containing fresh and saline water 6 5. Gas porosity is the fraction of a rock or sediment filled with a gas determining the true porosity of a gas filled formation has always been a problem in the oil industry. The effective water saturation sw was also computed using three different models.
An example of a density log run in the dakota is shown in figure 5. This parameter is commonly calculated from the data to compare the results with the known. These matrix density values are used to compute density porosity. Apr 16, 2016 there is two method to measure porosity from density log. However, it has its limitations because the amount of information to be gained is restricted by the number of inputs in this case, three logs. The apparent total porosity thus obtained is then corrected for shale effects to derive the effective porosity. Pdf correlation analysis between sonic and density logs. Using a local fit for sands and shales, differentiated using the gr log, results in values of a and m of 0. The density log measures the electron density of a formation. Neutron log is i personally think is the worst of all. You will learn the basic physics and operating principle behind each tool, the common log displays, and limitations of each tool. The number of returning gamma rays is an indicator of formation bulk density. In the case of the density tool, the density of the rock matrix must be known.
See the log plot below v the functions panel workflow below shows all the functions and equations used, including the volume of shale vsh that was computed combining the contrast. Log analysts evaluate porosity either from core measurements or analysis of porosity tools density. It should be noted that if the bulk volume and dry weight, or the bulk volume, saturated weight and porosity of a rock sample is known, then the grain density can be calculated. The simplest and most common means of combining multiple logs is in a multitrack log display or a crossplot. Similar porosity through interval, but resistivity decreases either gradually or over a shaper transition when moving down the log. Comparing the porosity derived from dresser atlas equation 8 and the suggested equation 7 with those computed using the traditional technique, which is based on the density neutron data via the following equation. An approach for estimating porosity from soniclogs in. These neutrons undergo scattering in the formation, losing energy. Porosity of a gasbearing formation sstvd 6750 6760 6770 6780 6790. Phi dc porosity from the density log the response equation for the density log in porosity units follows the classical form. We usually assume the fluid density f is between 1.
Pdf uncertainty in well log analyses and petrophysical. Petrophysical approach for estimating porosity, clay volume. The regression line suggests that the density porosity is about one porosity unit too low, which is likely caused by having calculated the density porosity using. Interpretation of porosity and fluid constituents from well. D total porosity derived from density log r ma matrix grain density, gmcc r b bulk density of formation, gmcc r f fluid density, gmcc porosity derived from the density log is adversely affected by borehole rugosity, washouts and light hydrocarbons. All the pores open and sealed % true porosity volume of all pores x 100 total vol. Lithology determination from the overlay of density and neutron logs.
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