the estimation of porosity and permeability values for a...

International Journal of Engineering Technology, Management and Applied Sciences

www.ijetmas.com April 2018, Volume 6, Issue 4, ISSN 2349-4476

41 Pshtiwan Tahsin Mohammed Jaf & Huner Kareem Haias

The Estimation of Porosity and Permeability Values for a Well

at North Sea through Different Techniques; Well-Logging,

Well-Testing and Core Analysis

Pshtiwan Tahsin Mohammed Jaf 1 & Huner Kareem Haias 2

1 Petroleum Engineering Department, Faculty of Engineering,

Koya University, Koya KOY45, Kurdistan Region-Iraq

2 Ministry of Natural resources, Erbil, Kurdistan Region-Iraq

ABSTRACT

Studying the porosity and permeability in a particular formation from a well in North Sea will bean address in this

paper.The name of the field is not mentioned in this work due to the confidentiality of publication and will be named as X

well. Porosity and permeability are the most important physical reservoir rock properties which play a key role during

the exploration of oil and gas fields in terms of the total volume estimation of hydrocarbon in place and the ability of the

reservoir rock for production which are required for field development plan.

The data from three different techniques; well-logging, well-testing and core analysis have been assessed and examined

to estimate the value of porosity and permeability. Each of these formation evaluation methods has been applied in

different stages in the life of the well. The problem here is the lack of data for some intervals from each technique; this

led to make the analysis being more complex.

The result indicates that a wide range in the value of porosity and permeability for each technique at different intervals.

In some cases, permeability has been defined through the porosity value since there was not a direct value of

permeability.

Keywords Porosity, Permeability, Well Logging, Well Testing, Core Analysis.

INTRODUCTION

The main objective of this paper is to interpret the productive zone in the well X through a comparison

between core, wireline and well test data analysis. The well Xis an appraisal well in the North Sea Field, UK.

The total depth being drilled is 3010 m. The openhole-well goes under three main tests; well-test, well-log and

coring for the purpose of analyzing and charactering the productive zone (reservoir). The Porosity and

permeability values were being identified and compared that obtained from the three test categories. The

productive zone interval is between (8540 – 8820 ft).

WIRELINE ANALYSIS (WELL-LOGGING)

According to Zaki B. (1994) [3] the main objectives of well log interpretation includes the determination of

physical rock characteristics such as porosity and permeability. In this paper, the porosity logging tools have

been focused on such as density, sonic (Acoustic), and neutron to define the formation porosity. These logs do

not measure porosity directly. Since, porosity is calculated from logging measurements such as electron

density, sonic travel time and hydrogen index which are measured by density, sonic and neutron tool

respectively [1].

The following essential equations have been applied to calculate formation porosity:

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𝜑𝑑𝑒𝑛𝑠𝑖𝑡𝑦 =𝜌𝑚𝑎𝑡𝑟𝑖𝑥 − 𝜌log(𝑏𝑢𝑙𝑘)

𝜌𝑚𝑎𝑡𝑟𝑖𝑥 − 𝜌𝑓𝑙𝑢𝑖𝑑 … … … … … … (1)

𝜑𝑠𝑜𝑛𝑖𝑐 =∆𝑡𝑙𝑜𝑔 − ∆𝑡𝑚𝑎

∆𝑡𝑓 − ∆𝑡𝑚𝑎 … … … … … … (2)

To accurately calculate porosity, the lithology and fluid type must be known. So, it is clear from the available

data that the lithology tends to be sandstone with probably around 30% shale content and the fresh fluid has

been used. Therefore, the matrix and fluid density are 2.65 gm/cc and 1 gm/cc, while the transit time

is55.4sec/ftfor sandstone and 189 sec/ft fresh fluid.

Porosity is calculated and compared among several models (Density, Acoustic (Raymer-Hunt), Neutron-

Density, and Neutron – Sonic crossplot). The results are compared with porosity from the core data. Porosity

calculated from density displays the best match with the core porosity as expected as the density porosity

accounts for formation total porosity (primary and secondary); whereas acoustic only reflects the primary

porosity and the crossplot gives the average porosity.

Table -1: Summary of Petrophysical Data Based on Manual Log Analysis (well X)

The analysis carried out on the wireline data is compared to core data collected and well test analysis in order

to scrutinize the results. This evaluation will reduce uncertainty and ensure the most representative values are

chosen. There are differences due to in-situ and surface conditions that will need to be considered.

CORE DATA ANALYSIS

According to David G. K. (1986) [4] the primary objectives of conventional core analysis is to identify

physical rock and fluid properties include porosity, permeability and saturation; also compare the results with

the outcomes from log and well-test analysis to get a reliable value and reduce uncertainty.

Permeability and porosity data from obtained coreswas provided for well X at varying points within the

reservoir section (table.2). These data were analyzed statistically with a number of models including various

different averages.

The core data has been corrected for overburden pressure. This correction reduced the permeability and

porosity of the samples as it assumes compression of the cores to reservoir pressure. This was necessary as the

cores were tested at surface conditions. Core data has also had to be depth matched (Figure.1). This was

carried out due to evidence collected at the core store, it gave a better match with the log data, and this error

could have arisen from the stretch of the tubing on which the coring tool was run.

Parameters Well X

Top Interval (ft) /TVD 8540

Bottom Interval (ft) /TVD 8820

Gross Thickness 280

Porosity 0.2

Oil Saturation 0.34

Net/Gross 0.78





Figure -1: Representative Log (Well X)

Table -2: Porosity and permeability from the core data for appraisal wellX

Interval (ft) 8540 – 8820

Number of Samples 26

k (md) ɸ (%)

Arithmetic Average 209.7 21.1

Geometric Average 33.8 21.6

Harmonic Average 0.150 22

Standard Deviation 377.3 7.68

Coefficient of Variation 11.17 0.364





Figure - 2: Core sample (well X)

Cut off value of the porosity was indicated from the porosity- permeability crossplot of the core data and the

well test results. Cut off value of porosity is defined to be 0.12 which corresponds to the permeability of 1 md

(figure.3).

WELL-TEST ANALYSIS

According to Earlougher Robert C., (1977) [2] the primary objectives of the well test operation carried out

was to provide an estimate of average permeability across the reservoir, associated skin factor, reservoir

pressure and the presence of possible boundaries (type and location).

The results from well test analysis conducted were then compared to parameters obtained from both

petrophysics and core analysis. The use of the Weatherford PanSystem software highlighted in well X the

presence of a nearby discontinuity barrier but it cannot be concluded as a fault due to lack of information from

the other sources such as 3-D seismic. The skin value is around 7.4 for the pay zone being tested. Estimated

permeability equals to 176 mD. A summary of the results obtained are shown in (Table.3).

Figure -3: ∅-k to define cut off value for porosity (well X)





Table -3: Well-test summary data

Appraisal Well No. Well X

Test Interval (ft) 8540 - 8820

Permeability (mD) 176

Reservoir Pressure (Psia) 3887

Kh (mD-ft) 12510

Skin 7.4

Distance to boundary Not seen

By making a comparison the above results with the core analysis outcomes, it can be observed that the

permeability value from well testing is close to the arithmetic average permeability values from core analysis

which is giving an indication that the reservoir most likely layered. The reservoir pressure is close to 3887

psia for the purpose of this study. The skin value is probably high, the fact that the well under analysis is

appraisal well which will provide a guide for the upcoming development wells and the completion strategy,

all with an aim of reducing the level of formation damage during the development phase.

From this available well test data, only the build-up was interpreted because a constant rate drawdown was

not established during the test period. By looking at the log-log diagnostic plots for the DSTs conducted

shows wellbore storage not to be a major concern.

An early plateau on the diagnostic plot was identified and defined as the middle time region (MTR), from

which a zero-slope line was fitted, and permeability and skin values derived[5]. Based on the following

equation which has been applied to derive the value of permeability and skin factor values:

K = −162.6 qs B μ

mh … … … … … (3)

S = 1.1513 [Pwf (∆t=0) − P∗

m− log10

ktp

∅Ctrw2

+ 3.2275] … … … . (4)

Figure-4: Build up Diagnostic Plot for Well X





With reference to the results of the well test analysis, it is necessary at this point to take into account possible

uncertainties that could have affected the results of such an analysis: for instance, possible errors during the

data acquisition process resulting from gauge resolution, improper identification of the flow regimes and

uncertainty on input parameters e.g. porosity averages.

CONCLUSION

The pay zone is mainly within the zone between 8540 – 8820 ft. The results from the interpretation of core,

well-test and wireline data to identify the porosity and permeability values show that the most representative

permeability is most likely around 175 – 210 mD (average= 195 mD) beside that the average porosity value

almost close to 21 per cent. The comparison has been made between the results from the data set analysis to

take into account possible uncertainties which impact on the results of such analysis.

REFERENCES

[1] Ali, A., (2012) ‘Formation Evaluation’, Edinburgh: Heriot-Watt Institute of Petroleum Engineering.

[2] Earlougher Robert C., (1977) ‘Advances in Well Test Analysis’, SPE

[3] Zaki B., (1994)‘Theory, Measurement, and Interpretation of Well Log’, SPE

[4] David G. Kersey (1986) ‘The Fundamentals of Core Analysis’, Western Atlas International

[5] Stewart G. and Jami M., (2013) ‘Well Test Analysis’, Edinburgh: Heriot-Watt Institute of Petroleum Engineering


the estimation of porosity and permeability values for a...

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