Integrated Understanding of Enhanced Reservoir Properties through Fracturing and Dissolution

Abu Dhabi Gas Development Company Ltd. CONFIDENTIAL Page 1

Integrated Understanding of Enhanced Reservoir Properties through Fracturing and Dissolution

Fatima Al Darmaki1, David A. Lawrence1, Richard P. Singleton1, Budideti A. Reddy1, Georgeta Popa1, Pascal Cortes2

Case Study from the Late Jurassic Arab Formation in a Large Gas Field, Onshore UAE

1 Al Hosn Gas, Abu Dhabi 2 Badley Ashton & Associates, Abu Dhabi


Outline of Presentation• Introduction & Field Overview

– Arab Formation Depositional Model – Well Location Map and 2D Seismic Section

• Fracture Analysis in Arab Members – Fracture Orientation Trends Within Arab Members– Fracture Density Variations in Arab Members

• Impact of Diagenesis & Dissolution in Arab Main Reservoirs• Porosity-Permeability Analysis in Arab Main Reservoirs• Layering Control on Fracturing in Arab Formation• Conclusions & Implications







Introduction & Field Overview

• SW of Abu Dhabi city• Field discovered in 1966• Oil production from Upper

Cretaceous• Target: gas & condensate

from Late Jurassic Arab Formation

• A total of 7 appraisal wells drilled into Arab Reservoir

• Development drilling started on NW flank

• Results from first development wells showed enhanced properties in the main reservoirs







Arab Formation Depositional Model

Chronostratigraphic setting of the Arab Formation

Adapted from Azer and Peebles (1998)

SW NE

Carbonate Ramp Setting

Arab DArab C

Arab AB

Palaeo-environments of the Arab Formation

• Oolitic grainstone

SOGAS Arab Depositional Model

• lime mudstones and anhydrites

• Dolomitic limestone

Main Field area Reservoir Data

• Reservoir Ave Porosity Ave Perm

• U.Arab AB 10 -11% 1.0 – 2.0md

• L. Arab C 17-20 % 0.7 – 8.0 md

• L. Arab D 6-8% 0.05 – 1.0 md

Outer RampMid Ramp

SubtidalIntertidalSabkha

0 2 km2 ft

Inner Ramp

• Marine lime-mudstone• Minor Wackestone to

Packstone







Well location Map and 2D Seismic Section

A1-1V

A2-1VD3-1&2H

D2-1

H

D4-1H

D1-1

D

Arab C Structural Map

A1-1V

A 6-1V

A 3-2S

A 4-1V

A5-1D

A2-1V

A 7-1V

BHI Log

BHI Log

BHI Log

IL 7300

Vertical exaggeration: 2:1

Arab

NW SE

0 10 km

0 1.25km







Fracture Analysis in Arab A/B Members

Fracture Analysis from Resistivity Image Data Correlated with other Open Hole Logs

11.5

ft

Discontinuous Resistive Fractures

Gamma Ray Resistivity Neutron/ Density

Whole Core

Conti. Conductive Frac.Conti. Resistive Frac.DisCont. Cond. Fra.

DisCont. Resistive. Fra.Mixed Fracture


Fracture Analysis in Arab C Member

Discontinuous Resistive Fractures

Continuous Resistive Fractures

5.5

ft

Fracture Analysis from Resistivity Image Data Correlated with Core Image and Open Hole Logs


Conti. Conductive FractureConti. Resistive FractureDisCont. Cond. Fracture

DisCont. Resistive. Fra.Mixed Fracture Whole

Core


Fracture Analysis in Arab D Member

Fracture Analysis from Resistivity Image Data Correlated with Core Image and Open Hole Logs

Discontinuous Conductive Fractures

Continuous Conductive Fractures

6 ft

Whole Core

Conti. Conductive FractureConti. Resistive FractureDisCont. Cond. Fracture

DisCont. Resistive. Fra.Mixed Fracture








Fracture Trends in Arab A/B Members

A1-1V

A 6-1V

A 3-2S

A 4-1V

A5-1D

A2-1V

A 7-1V

D4-1H

D2-

1HD1

-1D

D3-1&2H

• Few fractures are observed in Arab A/B including conductive, resistive and mixed features

• Varying strike orientations were interpreted, mainly NNW-SSE and WNW-ESE

DisCont. Resistive Fracture

Mixed Fracture

Conti. Conductive Fracture

Conti. Resistive Fracture

DisCont. Conductive Fracture

No fractures have been interpreted through Arab AB Members

0 10 km

N= 22

N= 6


• The majority of the interpreted fractures in Arab C were resistive

• Few conductive and mixed fractures were interpreted in the study wells

• Several strike orientations were interpreted in Arab C and show dominant strikes along NNW-SSE and WNW-ESE

• Subordinate trends in broadly NNE-SSW and ENE-WSW directions and parallel to the main fault trends

Fracture Trends in Arab C Member

A1-1V

A 6-1V

A 3-2S

A 4-1V

A5-1D

A2-1V

A 7-1V

D4-1H

D2-1

HD1

-1D

D3-1&2H

0 10 km


Mixed Fracture




N= 50

N= 33

N= 6


• Highest fracture intensity observed in Arab D and most were continuous and discontinuous conductive fractures

• Relatively few resistive fractures were also interpreted in this interval.

• Strike orientations of fractures in Arab D are dominantly NNW-SSE and WNW-ESE.

• Subordinate trends in broadly NNE-SSW and ENE-WSW direction and parallel to the main fault trends

Fracture Trends in Arab D Members

A1-1V

A 6-1V

A 3-2S

A 4-1V

A5-1D

A2-1V

A 7-1V

D4-1H

D2-1

HD1

-1D

D3-1&2H


Mixed Fracture




0 10 km

N= 544

N= 75

N= 67







Fracture Density Variations in Arab Members

A1-1V

A 6-1V

A 3-2S

A 4-1V

A5-1D

A2-1V

A 7-1V

D1-1D

D2-1H

D3-1&2HD4-1H

0 10 km

D1-1D D2-1H A7-1V

TD in Arab D







Stylolitic bedding and Stylo-Fracture in Arab C Member

Circumferential Core PhotoBorehole Image

N S N

N NS

Non-mineralized

open fractures

Partly cemented fractures

Stylofrac and subparallel

stylolitic bedding


Circumferential Core Photo

Thin-section photomicrographs

S N

Borehole Image

Stylolitic bedding and Stylo-Fracture in Arab D MemberN NS

Bedding Mudstone

Bedding

StylofracPartly cemented fractures

associated with stylofrac

Non-mineralized open fractures associated with

stylofrac

N







Porosity – Permeability Analysis in Arab C

A6-1VA2-1VA4-1VA5-1DA7-1VD1-1D

2

‘Normal’ Trend

‘Enhanced’ Trend

110143.30

1

Por:24.6%K: 41 mD

Por :23%K: 2.4 mD

2


Porosity – Permeability Analysis in Arab D

Mostly plugs with fractures &/or stylolites

+ some vugs

“Enhanced” K

A6-1VA2-1VA4-1VA5-1DA7-1VD1-1D

Por :21%K: 2.4 mD

1

Por :10%K: 1.3 mD

3

Por :8.7%K: 0. 1 mD

2

13

2







Layering Control on Fracturing in Arab Formation

Diffuse (layer-bound) fracturing

faults / fractures

Juba

ilaA

rab

DA

rab

CA

rab

AB

Ara

b Fo

rmat

ion

SW NE


Layering Control on Fracturing in Arab Formation

Diffuse (layer-bound) fracturing

faults / fractures

SW

Juba

ilaA

rab

DA

rab

CA

rab

AB

Ara

b Fo

rmat

ion

NE

Cementation ‘Halo’ Around

Faults / Fractures

Dissolution in more permeable layers &

along diffuse fractures/ stylolites


Conclusions & Implications

• The first development wells encountered significantly enhanced reservoir properties in both Arab C and D reservoirs.

• Highest fracture intensity in Arab D followed by Arab C and A/B members.

• Conductive fractures are observed mainly in Arab D, while the resistive fractures are interpreted in Arab C member. Conductive fractures show two predominant strikes WNW-ESE & NNW-SSE whilst resistive fractures show various orientations.

• Most fractures are layer-bound concentrating at mechanical contrasts, often associated with stylolites.

• Enhancement of reservoir properties is related to both dissolution along deep-rooted faults/fracture corridors and bedding-parallel where the stylolites occurred.

• Reservoir connectivity, well productivity and drainage area especially in the Arab D still carry high uncertainties which are being reduced by focused continued data acquisition, integration and modeling efforts.


ACKNOWLEDGEMENTS

Al Hosn Gas, ADNOC & Oxy Management for permission to present

“Badley Ashton & Associates” support and “Baker Hughes” for Borehole Image interpretation and support

Documents

Integrated Understanding of Enhanced Reservoir Properties through Fracturing and Dissolution