ISSN: 0016-7975 / 1011-9565

GEOMINAS, Vol. 51, N° 91, agosto 2023

Petrophysics/Petrofísica

 

 

Total and effective porosity derived from density and gamma ray logs. What are the most common insides and errors, their consequences and how to mitigate them


Porosidad total y efectiva derivadas de los perfiles de densidad y rayos gamma. Cuáles son las insidias y errores más comunes, sus consecuencias y como mitigarlos


Porosidade total e efetiva derivada de perfis de densidade e raios gama. Quais são as armadilhas e erros mais comuns, suas consequências e como mitigá-los

 

Roberto Barbato

Ing°Geó°, PhD, Barbato, Training, Consulting and Services, Correo-e: r_barbato@msn.com

Alfonso Quaglia

Ing°Geó°, PhD. Inter-Rock, C. A., Correo-e: quagliaa@inter-rock-ca.com

Recibido: 29-6-23; Aprobado: 26-7-23

Abstract

Porosity is a critical parameter in the oil industry and its determination has evolved over time due to the incorporation of new tools and technologies. With the incorporation of interpretation software, enormous progress was made in the efficiency of calculations and now with the incorporation of artificial intelligence and “data science” in general, another big step in efficiency has been made. However, this by itself contains a big risk that has to do with the mechanization/automation of processes; and this ranges from the log quality control, selection of interpretation methods, parameters and understanding of the associated uncertainties. In this project, the theoretical bases of porosity calculations from the density log will be reviewed and finally, using real data and parameter selection processes and results will be simulated with different sets of parameters to visualize and measure the potential ranges of difference in results.  

Resumen

La porosidad es un parámetro crítico en la industria petrolera y su determinación ha evolucionado con el tiempo por la incorporación de nuevas herramientas y tecnologías. Con la incorporación de los softwares de interpretación se avanzó enormemente en la eficiencia en los cálculos y ahora con la incorporación de la inteligencia artificial y el data science en general se ha avanzado de manera sorprendente. Sin embargo, esto mismo encierra una insidia que tiene que ver con la mecanización/automatización de los procesos; y esto abarca desde el control de calidad de los perfiles, selección de métodos de interpretación, parámetros y comprensión de las incertidumbres asociadas. En este proyecto se revisarán las bases teóricas de cálculos de porosidad a partir del perfil de densidad y finalmente, utilizando data real, se aplicaran los diferentes métodos, procesos de selección de parámetros y se simularán resultados con diferentes sets de parámetros para visualizar y medir los potenciales rangos de diferencias de los resultados. 

Resumo

A porosidade é um parâmetro crítico na indústria petrolífera e a sua determinação tem evoluído ao longo do tempo através da incorporação de novas ferramentas e tecnologias. Com a incorporação de softwares de interpretação, enormes avanços foram feitos na eficiência dos cálculos e agora com a incorporação da inteligência artificial e da ciência de dados em geral, o progresso foi feito de forma surpreendente. No entanto, isso em si contém uma insidiosidade que tem a ver com a mecanização/automação de processos; E isso vai desde o controlo de qualidade dos perfis, seleção de métodos de interpretação, parâmetros e compreensão das incertezas associadas. Neste projeto, as bases teóricas dos cálculos de porosidade a partir do perfil de densidade serão revistas e, finalmente, usando dados reais, os diferentes métodos, processos de seleção de parâmetros serão aplicados e os resultados serão simulados com diferentes conjuntos de parâmetros para visualizar e medir os potenciais intervalos de diferenças nos resultados.

Palabras clave/Keywords/Palabras-chave:

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Citar así/Cite like this/Citação assim: Barbato y Quaglia (2023) o (Barbato y Quaglia, 2023).

Referenciar así/Reference like this/Referência como esta:

Barbato, R., Quaglia, A. (2023, agosto). Total and effective porosity derived from density and gamma ray logs. What are the most common insides and errors, their consequences and how to mitigate them. Geominas 51(91). 67-77.

Introduction

Porosity is a key parameter in the development of hydrocarbon reservoirs, it is critical for the calculation of resources and reserves, determination of the economic value, planning of the production strategies of the fields, for the monitoring and analysis of the production, for the evaluation of individual wells and well testing and completion decisions. Porosity also has a generic but NOT direct relationship with permeability, which is another fundamental parameter in the development of hydrocarbons and used with other parameters is essential for practically all Geological, Geophysical, Geomechanical, Reservoir Simulation and Production analysis and forecasts. This importance applies to any type of reservoir, regardless of their level of complexity. From shallow reservoirs such as those of the Orinoco oil belt in Venezuela or the bituminous reservoirs of Canada where porosity borders on the maximum theoretically possible (close to 40%) to the most recent unconventional reservoirs where it is absolutely critical for geomechanical characterization and for therefore fracturing, which is the production method in 99% of cases. Going through Carbonates and even (rare) reservoirs in igneous and metamorphic rocks. There are different methods and ways to obtain porosity from well logs and cores, based on different characteristics and properties of the rock, each one has its pros and cons. In this paper, one of the most important tools in the calculation of porosity is analyzed individually, such as the density log, highlighting the positive aspects and showing the risks or attention points so that the magnitude of the risks is understood. Many new interpreters have been carried away by the automation and mechanics of the processes, executing procedures without being clear about all the assumptions that are behind the processes and the range of variability of the results as a consequence of these. In this study, the case of porosities is analyzed for the density log and the assumptions, uncertainties and possible pitfalls that arise during the interpretation process are highlighted.

Basic theory and definitions

Porosity

The porosity of a rock is the part of the volume of void spaces over the total volume of the rock. Those voids include pores, fissures, fractures, vogues, inter- and intra-crystalline spaces.

The porosity is described with the following relationship:


Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Where:

Vpores = Pore Volume

Vtotal = Total Volume of the sample

Vmatrix= Volume of the matrix

ρdry matrix= Total weight of dry rock

ρmatrix= Matrix density

Below are the most commonly used definitions in the petroleum environment:

• Total Porosity: The total void volume of a rock over the total rock volume.

• Effective porosity: The interconnected void volume of a rock over the total rock volume; this is the definition used by Reservoir Engineers, however, for Petrophysicists, log analysts, etc. Effective porosity is the total porosity minus the porosity occupied by the clays. Unfortunately, these terms are often used interchangeably and can cause severe communication problems between the different stakeholders if the differences are not well understood.

Both are represented in figure 1. Total porosity includes all fluids, while effective only mobile fluids. The methods of calculating from logs will be discussed in the methodology.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 1. Schematic representation of Total and Effective Porosity for log analysis. 

Metodology

This work is mainly based on an investigation of the application of Density and Gamma ray logs to calculate total and effective porosity. It will consist of simulations of porosity calculation varying the parameters to:

The general initial general characteristics of the reservoir are as follows:

Results

For this study, there were three wells in the area, one with a core but with poor quality logs and two with a complete set of logs. For the calculation part, a thickness of 250 feet was selected from one of the wells with a sequence of quartz sands and shales, which will be sufficient to meet the objective. The same sequence presented in the methodology will be followed.

1. Total and effective porosities calculations from density log.

i. Total Porosity

There is a core in one well in the area and it was used to select the range of matrix density in the cleanest zones. Figure 2 shows a log with grain density (column C) and core porosity (column D); in red the cleanest zones; a crossplot of the two parameters and a histogram of the grain density distribution. The zones highlighted in red correspond to the cleanest zones and are the ones that will be used as clean matrix in this analysis. To calculate the total porosity, the formula (2) will be used.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 2. Core grain density analysis.

In the crossplot, some grain density values above 2.70 g/cc are observed, which probably correspond to carbonate intercalations. The rest of the points are concentrated between 2.65 and 2.67 g/cc. The matrix of this well is very simple and therefore the results to be obtained will be minimal compared to complex matrices. The values used for the simulation will be 2.65, 2.66, 2.67 and 2.70 (the latter simulating a slightly more complex matrix composed of 98% quartz and just 2% pyrite) and a fluid density of 1.0 g/cc.

Figure 3 shows the results obtained. In column C the density curve of the density log and successively, from left to right, the calculations with matrix densities equal to 2.65 (D), 2.66 (E), 2.67 (F) and 2.7 (g/cc respectively). From column D to G, the porosity of the core was also plotted, which for the areas in red (clean areas, without clay) is considered equal to the total porosity.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 3. Porosity calculation from density log changing the value of the matrix.

An immediate observation is that column D (matrix density = a 2.65 g/cc) shows the porosity that best correlates with the core results.

As the plot moves to higher matrix density values, the correlation becomes less consistent. Before continuing with the analysis of the porosity calculation results, the correlation between the core porosity data and the porosity calculated with the density log using a grain density of 2.65 g/cc were analyzed, since this is the one that showed a better correlation in the log. Figure 4 shows the difference between the core values and those calculated from the density log. The blue histogram represents all the samples, and the red represents the cleanest sands in the area. It can be seen that despite looking very good in the log (column D), the differences range from almost -5 to 5.0 porosity units’ difference; that means that the error is enclosed in a total of 10 porosity units in the extreme cases, but more conservatively in a range of 5.0 porosity units. This in itself is an “alarm” that should be in all studies; many times, with seeing a “good” correlation in the graph of the log, it is taken as automatic that the error is low... well, that assumption is wrong!!

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 4. Comparison of core porosities vs density log porosities . 

Figure 5 will be used to analyze the difference between the porosities of the density log calculated with different matrices. The porosity differences were plotted using the density of 2.65 g/cc as a reference, which was the one that showed the best correlation with the core; highlighted in red the clean sands. From left to right are presented the differences between 2.66 g/cc and 2.65 gr/cc (blue histogram); 2.67 g/cc and 2.65 g/cc (light blue histogram) and finally 2.70 g/cc and 2.65 g/cc (green histogram, the latter simulating a mixed matrix of 98% quartz and 2.0% pyrite. The blue histogram already shows a difference of approximately half a porosity unit with a change of just 0.01 g/cc, the light blue histogram shows a change of one porosity unit for a variation of 0.02 g/cc and finally the green histogram shows between 2 and 2.5 units of porosity for a grain density change of 0.05 g/cc. At first sight these variations appear small, however, for this reservoir with porosities of approximately 25% (see core values in figure 6) these values represent changes in 1.25, 2.5 and between 5 and 10% of the total porosity. It is easy to understand the impact this can have, for example, in reserves calculation.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 5. Difference of porosities obtained by using different matrix values. 

Now the effect of changing fluid density will be analyzed. Figure 6 shows the porosity values calculated with a 2.65 g/cc matrix (which will always be used from now until the end of the work) and varying the density of the fluid between 1g/cc (equivalent to a freshwater based mud filtrate) and 1.1 g/cc (equivalent to a saltwater based mud filtrate). The crossplot shows the two curves (blue line) and the difference between them (black line). It can be seen that the greater the porosity, the greater the change that, in the histogram, can be up to 2 (two) porosity units.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 6. Effects on porosity due to variation in the density value of the water filtrate.

Figure 7 shows the comparison between a mud filtrate of 1g/cc (fresh water) and one of 0.86 g/cc (diesel). The general observations are similar to the previous ones, but the difference now is up to 2.5 porosity units. The differences shown in these two cases would be the most extreme; however, they would be easier to manage in the sense of selecting the parameters because the salinity values or characteristics of the diesel are found in the heads of the wells. But the difficulty lies in knowing the real saturation in the invaded area, especially in this last case. For this, it is necessary to have resistivity logs that obtain information in the same area of the density log and, again, this could be relatively easy in “simple” reservoirs, but much more complicated for complex reservoirs.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 7. Effects on porosity using fluid densities 1.1 g/cc (salt water) and 0.86 g/cc (diesel). 

As a conclusion in the case of total porosities from the density log, the selection of the matrix value is very critical and the only way to minimize the effects is to know the matrix density values of the rock, better if coming from core and/or rock samples studies.

ii. Effective Porosity from Density log.

The following formula will be used to calculate effective porosity:

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Where:

ρb= Density value from density log

ϕe= Effective porosity

Vsh = Volume of shale calculated from Gamma ray log

ρma= Matrix density

ρe = Density of fluid in matrix

ρsh= Shale density

To calculate the effective porosity from the density log, a clay value is needed. Common practice is to use the gamma ray log. There are various criteria on how to select the parameters; the first one, illustrated in figure 9, is to select the maximum and minimum values of the gamma rays as shown in the Histogram; this criterion is used by some software that automatically detects these values. However, in the histogram it can be seen that these are the least representative in terms of abundance and, therefore, probably not the most appropriate. This can also be observed in the log where the only point of 100% shale is located at 11,990 ft and on the clay distribution histogram where the 0 and 100% values are much underrepresented.

A second criteria is to use “average” values. An example is shown in figure 8. The yellow line shows the value considered for the “clean” matrix or Vsh equal to 0; the green line represents the shale value equal to 100%. Any point that falls to the left of the yellow line will result in a negative clay value and anything that falls to the right of the green curve will result in clays greater than 100%. This can be seen in column “C”. Both cases are volumetrically impossible, therefore they are generally taken to “0” and “100” %. Software do it automatically. The most competent evaluators use the result curve without the adjustment as a control of the quality of the interpretation because they can measure how much the amount of adjustment is.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 8. Vsh calculation using maximum values.

Expert evaluators use a minimum Gr value even lower than the minimum value of gamma rays of the log, under the theory that, except in very special environments, there are no totally clean reservoirs. This is based on knowledge of the area and mineralogical analysis of the reservoir rocks.

In the central histogram of figure 9 the selected values are presented, and it can be seen how many values will result in a negative calculation (left cutoff of 20API) and how many will exceed 100% (to the right of 75API); this can be observed also in the histogram of clay distribution, on the right.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 9. “Average” values of gr max and gr min. 

Figure 10 shows the difference in results between the two applied methods and it can be seen that, despite the fact that the variations in the parameters are not very large, the range of differences is 20% in clay, from -10 up to 10%. This is the linear method and, as explained before, an extra step should be taken to adapt it to the known area.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 10. Calculation of vsh using “average” values.

For this study, the results obtained by applying the method of “average” parameters will be used. Having the shale zone defined, we proceed to determine the shale density parameters using the shale that was selected to obtain the GR maximum and GR minimum parameters. Some evaluators lose this connection and use shale bodies different from the one selected as representative of the VSH, committing a misinterpretation concept. In figure 11, the first column is the calculated VSH and the shale zone that was used to calculate the VSH in the well is highlighted in yellow in the log, in the second column the density log was plotted; the histogram shows the distribution of density values for that interval. The parameters 2.23 and 2.27 g/cc were selected following the criteria of average values used previously. It is important to note that this is a fairly simple case, since big changes in the density of the shale are not present. We could say that it is a homogeneous shale, and, as a result, the differences between the calculations of porosity using the two densities will be “small” compared to other scenarios.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 11. Difference between vsh using highest and average parameters.

Figure 12 shows the results obtained for the effective porosity using a shale density of 2.3 g/cc. In the log graph, from left to right, the gamma ray curve (column B), the VSH curve (column C), the density curve (column D), the total and effective porosity curves (column E) are presented and the difference between the two porosities. The reference shale is highlighted in yellow, the clean sands in light blue, and the clayey sands in light blue. These colors are maintained in the crossplot and the histogram. This presentation model will be maintained from now on for the rest of the work. The crossplot shows the difference between total and effective porosity. It can be observed, as expected, that the clean (orange) sands present little difference between the two values and are concentrated in the part with the highest porosities; while the shalier sands are scattered throughout the rest of the graph and present differences between 10 and 15 porosity units as can be seen in the histogram.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 12. Selection of clay density parameters. 

Figure 13 shows the results obtained using the 2.7 g/cc as shale density and they look very similar to those obtained using a 2.3 g/cc.

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 13. Difference between total and effective porosity calculation using shale density of 2.3 g/cc.

The comparison between the two results is presented in figure 14. Columns A, B, C and D are the same as the two previous figures; in column E, the total porosity and the two effective porosities and in column F the difference between the two effective porosities were plotted. Column E shows, as expected) a significant difference between the total porosity (leftmost curve) and the effective ones in the shaly sands (highlighted in light blue). In column E the difference between the two effective porosities shows a difference of approximately one porosity unit as is also seen in the distribution histogram of the differences between effective porosities. It might seem little, but it must be remembered that this difference can represent 5% of the total volume of a reservoir with a total porosity of 20%!

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 14. Difference between total and effective porosity calculation using shale density of 2.27 g/cc. 

Core, densidad de fluido, densidad de lutita, densidad de matriz, densidade da matriz, densidade do fluido, densidade do xisto, effective porosity, fluid density, Gamma ray, Log. density, matrix density, núcleo, perfil de densidad, perfil de densidade, perfil de raios gama, perfil de rayos Gamma, porosidad, porosidade, porosidad efectiva, porosidade efetiva, porosidad total, porosidade total, porosity, shale density, total porosity, volume of shale, volumen de lutitas, volume do xisto, VSH.

Figure 15. Difference between effective porosity calculation using shale densities of 2.27 g/cc and 2.23 g/cc. 

It is interesting, and expected, to note how in the crossplot of the effective porosities, the clean sands (orange) show a closer high correlation in the higher porosity part of the graph, whereas when going down to the lowest effective porosities (with higher clays) the correlation tends to become much more dispersed.

Conclusions

The integration of mineralogical information in the interpretation and calculation of porosities is essential to reduce the uncertainty inherent to the methods.

The selection of the parameters is the most critical part of the interpretation. The use of different sets of parameters for the same method can give differences between the results that varied between 1 and 5 porosity units.

Mechanized/Automatized processes contain a great risk if the variability of the results depending on the method and parameters are not previously analyzed.

Data quality control is essential for any interpretation process.

The use of adjacent shales assumes that the material in the shales is the same as that in the reservoirs, which is not necessarily true.

No well log directly reads the porosity of formations. All of them read properties that allow calculating the porosity.

Experience, both in terms of managing petrophysical concepts and knowledge of the areas, plays a fundamental role in the interpretation of logs.

Recomendations

Before beginning any log interpretation, analyze the geological setting you are working in.

Integrate whenever possible the results of core data.

Use mineralogical analyzes of any kind (description of thin sections, SEM, etc) and of any origin (cores, well samples, etc) to rule out minerals that do not exist in the reservoirs of interest or to take precautions in cases that there are critical minerals that, even in small quantities, greatly affect the readings, such as pyrite for density.

For the areas of interest it is necessary to have well defined what the “adjacent clays” are. The use of mineralogical descriptions would help to reduce the risks.

Acquire cores and sets of multiple logs at the beginning of the exploration of the reservoir to create the most consistent models that serve as a reference at the time of the most massive drilling and where sets of less logs have already been acquired.

Perform several computations, varying the parameters (keeping them within logical ranges) to determine the range of uncertainty associated with it.

Bibliografy

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