PETROPHYSICAL ANALYSIS OF THE THAMAMA GROUP, ABU DHABI, UAE by Atul Narsinh ...

190 Pages·2015·5.6 MB·English

PETROPHYSICAL ANALYSIS OF THE THAMAMA GROUP,

ABU DHABI, U.A.E.

by
Atul Narsinh Rathod
A thesis submitted to the Faculty and Board of Trustees of the Colorado School of
Mines in partial fulfillment of the requirement for the degree Master of Science
(Petroleum Engineering).

Golden, Colorado
Date ________________

Signed: _____________________________
Atul Narsinh Rathod

Approved: _____________________________
Dr. Ramona M. Graves
Thesis Advisor

Approved: _____________________________
Dr. Neil F. Hurley
Thesis Co-Advisor

Golden, Colorado
Date ________________
_______________________________
Dr. Craig W. Van Kirk
Professor and Head
Department of Petroleum Engineering

ii
ABSTRACT

When dealing with carbonate reservoirs, vuggy porosity can be a major concern.
Conventional wireline logs do not always recognize vugs. Thus, a systematic
petrophysical analysis to quantify vuggy porosity becomes essential. Electrical borehole
images, neutron porosity, and 700 ft (213 m) of core from Thamama Group, Abu Dhabi,
U.A.E., were available for this study. This study has three objectives: a) minipermeability
measurements on core, b) vuggy porosity quantification from core and borehole images,
and c) flow unit definition using neutron porosity, core-based pixel-count porosity and
minipermeability.

The first phase involved permeability data acquisition along the entire length of
core using a pressure-decay minipermeameter. After cleaning and cataloging the core,
permeability measurements were obtained every 1 inch (2.5 cm) down the center of the
core. The core is dominantly limestone with different types of porosity (vuggy, moldic,
interparticle, intercrystal) and local microfractures. Due to these features, the core
showed a wide variation in permeability (0.002 mD to 8 D). Even after filtering of very
high permeability values, many core intervals had permeability values close to 1 Darcy.
iii
The permeability dataset thus obtained will be useful for flow unit determination and
future analyses.

The second phase of this study quantified vuggy porosity from core and borehole
images. For core-based vuggy porosity, the core was first polished, coated with
fluorescent paint to highlight vugs, and photographed under ultraviolet light to obtain
high-resolution black and white photographs. The photographs were scanned and stacked
to form a single continuous core image, then analyzed using pixel-counting techniques to
obtain core-based vuggy porosity. Using the borehole images, different thresholds were
applied to the static FMI log. Frequency histograms of FMI pixel counts were then
compared with the core pixel counts for vuggy porosity extraction. The comparison
showed that the FMI-based vuggy porosity follows a similar trend, consistently lower
than the core-based pixel-count vuggy porosity in the upper 400 ft (122 m) of the cored
interval. In the rest of the core, FMI-based vuggy porosity shows higher values when
compared to the core-based pixel-count vuggy porosity. This reversal in trend is
attributed to differences in hydrocarbon and water saturations in those intervals,
respectively.

The third phase of this study was to characterize flow units using neutron
porosity, core-based pixel-count vuggy porosity, and minipermeability. A flow unit is
defined as a reservoir rock with a unique porosity-permeability relationship that affects
iv
fluid flow. Flow units in this study were defined by two methods: a) cumulative storage
capacity (h) and cumulative flow capacity (kh) plots versus depth, and b) a Stratigraphic
Modified Lorenz Plot (SMLP) which plots cumulative storage capacity (h) versus
cumulative flow capacity (kh). Both methods recognized thirteen flow units which occur
at different depths.

v
TABLE OF CONTENTS

ABSTRACT ..................................................................................................................... iii
TABLE OF CONTENTS ................................................................................................. vi
LIST OF FIGURES .......................................................................................................... x
LIST OF TABLES ......................................................................................................... xix
ACKNOWLEDGEMENTS ........................................................................................... xxi

CHAPTER 1. INTRODUCTION .................................................................................. 1
1.1 Background ......................................................................................... 1
1.2 Research Methods ............................................................................... 3
1.3 Research Objectives ............................................................................ 4
1.4 Research Contributions ....................................................................... 5

CHAPTER 2. GEOLOGIC SETTING ......................................................................... 7
2.1 Stratigraphy of the Thamama Group .................................................. 7
2.1.1 Stratigraphy of the Shuaiba Formation ................................. 11
2.1.1.1 Lower Member.................................................... 12
2.1.1.2 Upper Member .................................................... 14
vi
2.1.1.3 Bab Member........................................................ 16
2.1.2 Stratigraphy of Kharaib Formation ....................................... 16
2.2 Local Stratigraphy ............................................................................. 17
2.3 Regional Structural Geology ............................................................. 22

CHAPTER 3. MINIPERMEAMETRY ...................................................................... 24
3.1 Minipermeameter .............................................................................. 24
3.1.1 Minipermeameter Theory .................................................. 25
3.1.1.1 Steady-State Permeametry .................................. 25
3.1.1.2 Pressure-Decay Permeametry ............................. 28
3.1.2 Minipermeameter Design ................................................... 30
3.1.3 Minipermeameter Operation .............................................. 32
3.2 Experimental Methods ...................................................................... 34
3.3 Permeability Data Calibration........................................................... 35
3.4 Results ............................................................................................... 40
3.4.1 Permeability Measurements on Study Core ....................... 40
3.4.2 Permeability Contours ....................................................... 49
3.5 Discussion ......................................................................................... 60

vii
CHAPTER 4. VUGGY POROSITY QUANTIFICATION ...................................... 64
4.1 Carbonate Rocks ............................................................................... 64
4.1.1 Classification of Carbonate Rocks ........................................ 66
4.1.2 Carbonate Pore Systems ....................................................... 67
4.2 Vuggy Porosity Detection ................................................................. 76
4.2.1 Vuggy Porosity Detection Using CT Scans .......................... 76
4.2.2 Vuggy Porosity Detection Using NMR ................................ 79
4.2.3 Vuggy Porosity Detection - Core and Borehole Images ....... 80
4.2.3.1 Formation MicroScanner (FMS)......................... 80
4.2.3.2 Formation MicroImager (FMI) ........................... 81
4.3 Method: Vuggy Porosity Quantification from Core ......................... 87
4.3.1 Sample Preparation for Photography .................................... 88
4.3.2 Photography .......................................................................... 89
4.3.3 Digitization: Scanning and Stacking of Images .................... 92
4.3.4 Image Analysis for Extraction of Vuggy Porosity Data ....... 93
4.4 Method: Vuggy Porosity Quantification from Borehole Images ...... 94
4.5 Results ............................................................................................. 101
4.6 Discussion ....................................................................................... 109

viii
CHAPTER 5. FLOW UNIT DEFINITION ............................................................. 118
5.1 The Concept of Flow Unit .............................................................. 118
5.2 Flow Unit Definition by Various Researchers ................................ 119
5.3 Methodology ................................................................................... 128
5.4 Results ............................................................................................. 131
5.5 Discussion ...................................................................................... 147

CHAPTER 6. CONCLUSIONS ................................................................................. 151
REFERENCES ............................................................................................................. 155
APPENDIX A: Status of the Core ................................................................................ 167
APPENDIX B: Core to Log Depth Converter .............................................................. 167
APPENDIX C: Vuggy Porosity Comparison of Well A .............................................. 167

ix
LIST OF FIGURES

Figure 2.1: Map of U.A.E. showing oil and gas fields. ................................................. 8

Figure 2.2: Arabian Peninsula showing major geological structures including Rub al
Khali basin. .................................................................................................. 9

Figure 2.3: General stratigraphic chart of the Cretaceous in Abu Dhabi, U.A.E. ....... 10

Figure 2.4: Generalized stratigraphic column of U.A.E. showing the Shuaiba
stratigraphy. ............................................................................................... 13

Figure 2.5: Log correlations showing the lateral variations within the Shuiaba
Formation. Vertical scale: 1 inch = 100 ft. ................................................ 15

Figure 2.6: Well composite plus display of conventional openhole logs from the study
area. ........................................................................................................... 18

x