Reservoir characterization using porosity ...

Reservoir characterization using porosity– permeability relations and statistical analysis: a case study from North Western Desert, Egypt Emad A. Eysa, Fatma S. Ramadan, Mohamed M. El Nady & Nermin M. Said

Arabian Journal of Geosciences ISSN 1866-7511 Volume 9 Number 5 Arab J Geosci (2016) 9:1-9 DOI 10.1007/s12517-016-2430-x

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Author's personal copy Arab J Geosci (2016) 9:403 DOI 10.1007/s12517-016-2430-x

ORIGINAL PAPER

Reservoir characterization using porosity–permeability relations and statistical analysis: a case study from North Western Desert, Egypt Emad A. Eysa 1 & Fatma S. Ramadan 1 & Mohamed M. El Nady 2 & Nermin M. Said 1

Received: 9 July 2015 / Accepted: 9 March 2016 # Saudi Society for Geosciences 2016

Abstract The main objective of this work is to evaluate the petrophysical characteristics of Alam-El Bueib reservoirs (AEB-3E, AEB-3D, and AEB-3A) in North Western Desert, Egypt, where they are important targets in this area. Core plugs laboratory measurements (porosity and permeability) of about 275 core samples of these reservoirs in TUT-1X well are used to achieve reservoir petrophysical evaluation. The core data analysis involves the construction of histograms, relations, and statistical analysis for porosity and permeability of these reservoirs. The statistical analysis and histograms of AEB-3E porosity indicate a good to very good homogenous reservoir with mean values 16 and 18 % for helium and fluid summation porosities, respectively. The horizontal and vertical permeability are very good with mean values 835.5 and 429.25 md, indicating high-quality reservoir. The study of AEB-3D indicates good and heterogeneous reservoir with mean values 10 %, 80.66 and 59.28 md for porosity, horizontal, and vertical permeability, respectively. AEB-3A reservoir reflects a good to very good homogeneous porosity and very good to excellent permeability with mean values 14 %, 299.75 and 184.32 md for porosity, horizontal, and vertical permeability. Concerning the three reservoirs, the helium-fluid porosity relations and the horizontal-vertical permeability relations give positive trends with strong correlation coefficients

* Emad A. Eysa [email protected]

1

Geology Department, Faculty of Science, Zagazig University, Al Sharkia, Egypt

2

Exploration Department, Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt

ranging from 0.51 to 0.93 which give the ability to calculate one parameter from the other. Keywords Porosity . Permeability . Statistical Analysis . Alam El Bueib Formation . Northern Qattara Depression . North Western Desert

Introduction Core analysis is a critical part of an overall reservoir evaluation as it provides direct evaluation of reservoir properties and furnishes a basis for calibrating other evaluation tools such as well logs. Core analysis makes it possible to recognize the structure of the reservoir, determines its physical characteristics such as porosity and permeability, and estimates production possibilities of exploratory wells. Core data play a vital role in exploration programs, well completion, work over operations, and in well and reservoir evaluations (Gluyas and Swarbrick 2004). As coring is expensive and time consuming, oil companies are keen to maximize the information which can be obtained from the core samples. It provides rock properties and descriptive data that cannot be determined from any other source. It is also valuable to use established relations and linear regression equations between different parameters in order to predict some of them. It saves time and high costs of coring, which is one of the main targets of this kind of work. Improved reservoir characterization is usually done from permeabilityporosity correlations developed from cored wells. The study area is located in the northern part of the Western Desert between latitudes 30° 36′ and 30° 54′N and longitudes 26° 51′ and 27° 06′E to the north of Qattara Depression (Fig. 1). It lies at 4–5 km north and northwest of Salam field at the northern edge of the major Safir-Salam-TUT ridge in the

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Arab J Geosci (2016) 9:403

Fig. 1 Location map of the studied TUT-1X well, North Qattara Depression, Northern Western Desert, Egypt

northern part of the Western Desert. In this paper, the interpretation of core analysis involves the construction of porosity and permeability histograms their relations and statistical analysis of Alam-El Bueib reservoirs (AEB-3E, AEB-3D and AEB-3A). Core data analysis of Alam-El Bueib reservoirs in TUT-1X well, northern part of Qattara Depression at northern Western Desert, is used statistically to determine the reservoir characterization.

Geologic settings The subsurface geology of the northern part of the Western Desert has been investigated by many workers such as Norton (1967), El Gezeery et al. (1972), Rebortson (1982), unpublished; Berglund et al. (1994), El-Kadi et al. (1998), Shalaby et al. (2000), and El-Khadragy et al. (2010). The stratigraphic column of the northern Western Desert is thick and includes most of the sedimentary succession from Pre-Cambrian Basement Complex to Holocene (Fig. 2). The cumulative sedimentary section of the Western Desert ranges from Lower Paleozoic to Holocene. According to Said (1990), four major sedimentary cycles occurred which is comparable with maximum southward transgression during the Carboniferous, Upper Jurassic, Middle and Late Cretaceous, and Pliocene times. Alam El-Bueib Formation is predominantly white to yellow sandstones with siltstones and few amounts of gray shales (Rebortson 1982, unpublished). Its lithology has been subdivided into six units from bottom to the top:

AEB-6, AEB-5, AEB-4, AEB-3, AEB-2 and AEB-1, whereas the AEB-3 unit was subdivided into six sub units: “G”, “F”, “E”, “D”, ”C”, and “A”. In the present study, AEB-3A, AEB-3D, and AEB-3E of Alam-El Bueib reservoirs are adapted for this work. The dominate lithology are thick massive sandstones, argillaceous, and calcareous limestone intervals. The environment of deposition is shallow marine with more continental influence toward the south. Sharaf and El Nady (2003) recognized that Alam El-Bueib Formation has poor to fair potential to generate oil at optimum maturity in south Umbraka area. Ramadan et al. (2012) recognized that Alam El Bueib source rock in Tut oil field varies from poor to very good organic richness with kerogen of type III and characterized by immature to mature rocks. The best reservoirs in this formation are the fine-grained sandstones which produce oil and gas in several oil fields.

Materials and methods The materials used in this study include core data from one well (TUT-1X) for deducting the petrophysical parameters of Alam El-Bueib reservoirs in the study area. The following methods are done: 1. The studied samples of Alam El-Bueib Formation in Tut oil field were cut with a diameter of one inch and a length of 1.5 in.

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Fig. 2 Generalized lithostratigraphic column of North Western Desert, Egypt (after Khalda 2012, unpublished)

2. Core cleaning using organic solvents has been done. 3. The cleaned samples of Alam El-Bueib Formation in the studied area were dried by vacuum controlledtemperature oven utilizing a maximum temperature of 93 °C for 3 h. 4. Thereafter, porosity is determined on the samples. The porosity of a rock controls the storage capacity in the rock and expressed either as decimal fraction or as a percentage. It is conventionally symbolized by the Greek letter

Phi (ϕ). Porosity can be determined mathematically using this equation: ϕ ¼ V p =V b ¼ ð1−V m Þ=V b ¼ V b –V g =V b ¼ V p = V p þ V g

ð1Þ where ϕ the porosity (fraction) Vp the void space volume of the rock



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Vb the bulk volume of the reservoir rock of the rock Vm the solid matrix volume of the rock Vg the grain volume of the rock In the most reservoir types, the porosity ranges from 5 to 30 %, and the porosity of reservoir can be classified, according to Levorsen (1967) into five categories as follows: & & & & &

Negligible porosity 0–5 % Poor porosity 5–10 % Fair porosity 10–15 % Good porosity 15–20 % Very good porosity 20–25 %

(b) Good if 10 < k < 100 md (c) Very good if 100 < k < 1000 md (d) Excellent if k > 1000 md

Results and discussion Reservoir characterization of Alam-El Bueib reservoirs (AEB-3E, AEB-3D, and AEB-3A) using the interpretation of porosity and permeability relations and statistical analysis revealed the following: Alam El-Bueib-3E reservoir

5. Finally, permeability estimation has been done on conditioned core samples. The permeability of rock is a measure of the ease with which a fluid of a certain viscosity can flow through it, under pressure gradient (Lynch 1962). Darcy’s work was further developed by Muskat and Botest (1931) and Muskat (1937). They developed Darcy’s fluid flow equation which expressed in empirical formula for permeability measurements as follows: μQL AðP1−P2Þ

K¼

ð2Þ

where K permeability, Darcy μ viscosity of the fluid, centipoises Q rate of flow (volume of fluid passing through in time unit) L length of the sample, cm A surface area of the sample, cm2 P1-P2 pressure at the terminate of the segment of sample of length (L) According to Levorsen (1967), the quality of a reservoir rock can be classified according to their permeability values as follows: (a) Fair if 1 < k < 10 md Table 1

The helium porosity values range from 6 to 21 %, while the mean of these readings is about 16 %. On the other hand, the fluid summation porosity ranges from 7 to 23 % with mean value of 18 % (Table 1). The standard deviation values are 3 % for helium and fluid summation porosity, indicating homogeneity in the porosity of Alam El-Bueib-3E which can be considered as a good reservoir. The horizontal permeability values range from 0.98 md to 3740 md, with mean value 835.51 md. On the other hand, the vertical permeability ranges from 1.4 to 4660 md, and the mean value is 429.25 md (Table 2). The standard deviation values are 624.64 and 563.47 md for horizontal and vertical permeabilities respectively, indicating heterogeneity of the reservoir (Table 2). Alam El-Bueib-3E reservoir in the studied well can be described as a high-quality reservoir with a very good permeability. The histogram and cumulative frequency of helium and fluid summation porosity for Alam El-Bueib-3E reservoir in TUT-1X well are represented in Fig. 3a. It is observed that 3.39, 34.75, and 61.86 % of the samples represent fair, good, and very good helium porosity respectively; while 3.39, 8.47, and 88.14 % of the samples represent fair, good, and very good fluid summation porosity (Fig. 3a). About 61.86 % of the studied samples have helium porosity values larger than 15 % which considered as

Descriptive statistical analysis for the porosity of Alam El-Bueib reservoirs (3E, 3D, and 3A)

Descriptive statistics

Mean Median Mode Standard Deviation Minimum Maximum Count

AEB-3E reservoir

AEB-3D reservoir

AEB-3A reservoir

Helium porosity (ϕh), %

Fluid porosity (ϕf), %





14.15 16 16 4.59 00 20 116

10 9 15 05 01 21 43

10 10 4 05 02 19 43

18 18 18 03 07 23 118

16 16 16 03 06 21 118

14.51 15.8 15.8 4.34 2.4 22.7 116

Author's personal copy Arab J Geosci (2016) 9:403 Table 2

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Descriptive statistical analysis for the permeability of Alam El-Bueib reservoirs (3E, 3D, and 3A)

Descriptive statistics

Mean Median Mode Standard deviation Minimum Maximum Count

AEB-3E reservoir

AEB-3D reservoir

AEB-3A reservoir

Horizontal permeability (Kh), md

Vertical permeability (Kv), md





835.51 680 1020 624.64

429.25 236 218 563.47

80.66 1.85 0.16 169.67

59.28 0.28 0.04 143.52

299.75 191.5 70 304.64

184.32 92.5 66 215.42

0.98 3740 118

1.4 4660 118

0.02 771 43

0.01 648 43

0.06 1780 114

0.05 1030 114

good to very good reservoir. On the other hand, about 88.14 % of the studied samples have fluid summation porosity values larger than 15 %. These high values of helium and fluid summation porosity indicate that Alam El-Bueib-3E reservoir is represented by good to very good reservoir. The histogram and cumulative frequency of horizontal and vertical permeability for Alam El-Bueib-3E reservoir in TUT1X well are represented in Fig. 3b. From these figures, it is observed that 68.64 and 27.97 % of the samples represented

by very good and excellent horizontal permeability respectively, while 76.27 and 6.78 % of samples represented by very good and excellent vertical permeability. It is noticed that about 96.61 % of the studied samples in horizontal permeability and about 83.05 % of the studied samples in vertical permeability have values larger than 100 md. It means that most of sandstone samples of the Alam El-Bueib3E reflect very good to excellent permeability in the study area.

Fig. 3 a Histogram and cumulative frequency curve of helium (left) and fluid summation porosity (right). b Histogram and cumulative frequency curve of horizontal (left) and vertical permeability (right) in Alam El-Bueib-3E reservoir, TUT-1X well, North Western Desert, Egypt


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Fig. 4 a Histogram and cumulative frequency curve of helium (left) and fluid summation porosity (right). b Histogram and cumulative frequency curve of horizontal (left) and vertical permeability (right) in Alam El-Bueib-3D reservoir, TUT-1X well, North Western Desert, Egypt

Alam El-Bueib-3D reservoir There are approximately 43 core samples, which were used to study the porosity of Alam El-Bueib-3D reservoir (Table 1). The helium porosity values range from 2 to 19 %, with mean value 10 %. On the other hand, the Fig. 5 a Histogram and cumulative frequency curve of helium (left) and fluid summation porosity (right). b Histogram and cumulative frequency curve of horizontal (left) and vertical permeability (right) in Alam El-Bueib-3A reservoir, TUT-1X well, North Western Desert, Egypt

fluid summation porosity ranges from 1 to 21 % with mean value 10 %. The standard deviation value 5 %, for both helium and fluid porosity, indicates that the porosity of Alam El-Bueib-3D reservoir is partially heterogeneous and this reservoir could be described as a fair to good reservoir.


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Fig. 6 Relationships between helium porosity and fluid summation porosity in Alam El-Bueib-3E reservoir (I), Alam El-Bueib-3D reservoir (II), and Alam El-Bueib-3A reservoir (III) in TUT-1X well, North Western Desert, Egypt

From Table 2, the horizontal permeability ranges from 0.02 to 771 md, with mean value 80.66 md. On the other hand, the vertical permeability ranges from 0.01 to 648 md with mean value 59.28 md, indicating a good reservoir. The median, mode, and standard deviation values of the horizontal permeability are 1.85, 0.16, and 169.67 but for vertical permeability, these are 0.28, 0.04, and 143.52, respectively. The difference in values of mode, median, and standard deviation reflects the heterogeneity of this reservoir and gives indication of a symmetrical distribution of this permeability. The histogram of helium and fluid porosity is represented in Fig. 4a. It shows that about 20.93 % of the studied samples have helium porosity values larger than 15 % and about 23.3 % of the studied samples have fluid summation porosity values larger than 15 %. These values indicate that this reservoir is represented by fair to good reservoir. The horizontal and vertical permeability histograms and cumulative frequency for the studied samples of this reservoir

are represented in Fig. 4b. About 11.62 and 23.26 % of the samples represent good and very good horizontal permeability respectively, while 11.63 and 16.28 % of the samples represent good and very good vertical permeability respectively. Alam El-Bueib-3A reservoir It is represented in Table 1 by approximately 116 core samples, where the helium porosity values range from 0 to 20 %, with mean value 14.15 %. On the other hand, the fluid summation porosity values range from 2.4 to 22.7 % with mean value 14.51 %. The standard deviation values 4.59 and 4.34 for helium and fluid porosity respectively indicate that the porosity of Alam El-Bueib-3A reservoir is nearly good and homogeneous. From Table 2, the horizontal permeability values range from 0.06 to 1780 md, with mean value 299.75 md. On the other hand, the vertical permeability ranges between 0.05 and 1030 md with mean value 184.32 md, where it can be



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Fig. 7 Relationships between horizontal and vertical permeability in Alam El-Bueib-3E reservoir (I), Alam El-Bueib-3D reservoir (II), and Alam ElBueib-3A reservoir (III) in TUT-1X well, North Western Desert, Egypt

described as very good reservoir. The permeability values are characterized by different values of mode, median, and mean indicating a symmetrical distribution, where the standard deviation values 304.64 and 215.42 md for horizontal and vertical permeability reflect the heterogeneity. The histogram of helium and fluid summation porosity for the studied samples are represented in Fig. 5a. This figure shows that about 53.85 and 56.9 % of the helium and fluid porosity values respectively which is larger than mean value

of 15 %. These high values of helium and fluid summation porosity indicate that Alam El-Bueib-3A reservoir is good to very good reservoir. The horizontal and vertical permeability histograms are represented in Fig. 5b. About 65.49 and 48.7 % of the horizontal and vertical permeability respectively have values larger than 100 md. It means that most of Alam El-Bueib-3E reservoir has very good to excellent permeability.

Table 3 Relations between measured porosities in Alam El-Bueib reservoirs

Table 4 Relationships between vertical and horizontal permeability of Alam El-Bueib reservoirs

Reservoir

Linear regression equation

Correlation coefficient(r)

Figure

Reservoir

Linear regression equation

Correlation coefficient (r)

Figure

Alam El-Bueib-3E Alam El-Bueib-3D Alam El-Bueib-3A

ϕf = 0.756 ϕh + 0.058 ϕf = 0.998 ϕh − 0.002 ϕf = 0.783 ϕh + 0.035

r = 0.64 r = 0.93 r = 0.83

(I) (II) (III)

Alam El-Bueib-3E Alam El-Bueib-3D Alam El-Bueib-3A

Kv = 4.894Kh0.609 Kv = 0.280Kh0.943 Kv = 0.959Kh0.866

r = 0.51 r = 0.76 r = 0.82

(I) (II) (III)

ϕf fluid summation porosity, ϕh helium porosity

Kv vertical permeability, Kh horizontal permeability


Relations between helium and fluid porosity The relations between helium and fluid summation porosity in Alam El-Bueib reservoirs (3E, 3D, and 3A) are represented by positive trends as illustrated in Fig. 6 (I, II, and III). It shows fluid summation porosity will increase with increase in helium porosity. These relations illustrate that the helium porosity of the studied samples is greater than its fluid summation porosity. It reflects the effect of bedding pores and fractures on the increasing of helium porosity over fluid summation porosity and also reflects the variation of the pore space in the studied samples, which are micropores (less than the water droplet diameter) and macropores (more than the water droplet diameter). The high correlation coefficients of these relations (Table 3) reflect the ability to calculate one parameter from the other. Relations between vertical and horizontal permeability The relationships between vertical permeability (Kv) and horizontal permeability (Kh) of Alam El-Bueib reservoirs in Tut1X well are illustrated in Fig. 7 (I, II, and III). The linear regression equations and correlation coefficients are represented in Table 4. These relations reflect moderate to strong correlation coefficients varying from 0.51 to 0.82, indicating strong relation between the two types of permeability. The horizontal permeability is greater than the vertical permeability in all reservoirs that reflect the effect of bedding pores and fractures on the increasing of horizontal permeability over vertical permeability. It gives also an indication of high reservoir quality.

Conclusions The proved reservoir characterization of Alam-El Bueib reservoirs (AEB-3E, AEB-3D, and AEB-3A) using the interpretation of porosity and permeability relations and statistical analysis revealed the following: 1. The relationships between helium and fluid summation porosities of Alam El-Bueib reservoirs show positive trends. This means fluid summation porosity will increase

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with increase in helium porosity of Alam-El Bueib Formation. 2. The obtained high values of porosity indicate that Alam El-Bueib Formation is a high quality reservoir. 3. Statistical analysis of the permeability dataset indicates that the reservoirs are very good to excellent category.

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