Naturally Fractured Reservoir Characterization

Know indication of fractures in a rock.1) Types of rocks

Know the basis of fractures generation: 1) Tectonic forces i.e. tension and compression.2) Dissolution process.3) Desiccation/dehydration process (temperature change). 4) Erosion process (excessive unequal pressure). 5) Drilling activity.

Know category of fractures:1) Crack: a partial (incomplete) fracture.2) Fissure: a surface of fracture along which there is a distinct separation. It is often filled with other

minerals.3) Joint: a surface of fracture without displacement. The surface is usually plane and occurs in parallel.4) Gash: a small scale fissure (centimeterdecimeter in length; and, millimeter-centimeter in width).5) Fault: a zone of fractures along which there has been displacement.

Know why fractures may be opened or may be cemented (i.e. by calcite, anhydrite, or clay materials):

1) Effect of tension (open fractures?) and compression (closed fractures?). 2) Chemical process (dissolution and precipitation).

Know the petrophysical analysis techniques in fractured reservoir.

Objectives - Fractured Reservoir Evaluation

INTRODUCTION OF FRACTURED RESERVOIR

Fracture is defined as all breaks or ruptures in a rock.

A reservoir fracture is a naturally occurring macroscopic planar discontinuity in rock due to deformation or physical digenesis. Natural reservoir fractures may have either a positive or negative effect on fluid flow within the rock(1).

Average fracture aperture is < 0.1 mm. Thus, porosity of fracture is generally negligible in the range of 0.25%, however the fracture related porosities due to solution porosity in granite or carbonate reservoir may attain much larger value(2).

In addition to collapse rock in carbonate karst, fractures appear predominantly in the brittle rocks, therefore in consolidated formations.

Dolomites and quartzites are the rocks in which fractures occur more frequently.

Very often fractures will disappear when entering formations which are more plastic such as clays, salts (halite), and/or friable-sands.

General Term of Fractures

(1) Geologic Analysis of Naturally Fractured Reservoirs, Ronald Nelson

(2) Crains Petrophysical Handbook

A fractured reservoirs is defined as a reservoir in which naturally occurring fractures either have, or are predicted to have , a significant effect on reservoirs fluid flow either in the form of increase reservoir permeability and/or reserves or increased permeability anisotropy(1).

The present of fractures may significantly enhance the drainage surface, and thereby the contribution of the matrix porosity to the production (type-II fracture).

Open fractures considerably increase the permeability. However, it may cut the potential output of a reservoir, if they are not taken into account during the secondary recovery phase.

In the case of fluid injection to maintain pressure, fractures act as preferred paths for the injected fluids, with the risk of isolating formation blocks which are still HC saturated, and of having early production of injected fluids.

Importance of Fractures

(1) Geologic Analysis of Naturally Fractured Reservoirs, Ronald Nelson

Frequency of fracture occurrences is a function of rock composition.

Fractures are common occurred in (in priority order):1. Igneous (granite) basement rock.2. Metamorphic quartzite basement

rock, depending on the stage of metamorphism.

3. Dolomites.4. And less occurrence in cemented

sandstones and limestones.

Fracture Occurrences in Rock Composition

A fractured reservoir is defined as a reservoir in which naturally occurring fractures should have considerable effect on reservoir properties i.e. increasing reservoir permeability and porosity.

For Geologists: A fractured reservoir is a reservoir with structural discontinuities resulting from a

paleo-stress history.

For Reservoir Engineers: A fractured reservoir is a reservoir with formation rock property discontinuities

affecting flows.

Fractured Reservoir - Definition

Notes from: R.A. Nelson, in Geologic Analysis of Naturally Fractured Reservoirs.

Fractures = matrix heterogeneity Impact fluid flow ( + or - )

Naturally Fractured Reservoirs

f

m

m

K

f FK F

f mKm

f FK F

f mKm

f FK F

f mKm

f FK F

f mKm

Matrix

fF

K F

Fractures

Fractures from outcrop Long Hai Southern Vietnam

Examples of hairline or micro fractures in granite rock basement.

White Tiger (Bach-Ho Field) Core VietsoPetro Core Warehouse

White Tiger (Bach-Ho) Field, Vietnam Fractured Granite Basement

Production History of the White Tiger Field (1986-1999)

Augila Field, Libya Basement Granite Rocks

Production History of the Augila Field (1965-1985)

West-East cross-section of the field (C-C).

Dongshengpu Field, China Basement Granite Rocks

Northwest-Southeast cross-section of the field (B-B).

Production History of the Dongshengpu Field (1983-1992)

Zeit Bay Field, Egypt basement Granite Rocks

Production History of the Zeit Bay Field (1983-1996)

Southwest-Northeastcross-section of the field (A-A).

Reserves in Fractured Reservoirs

Ultimate world-wide recovery from fractured reservoirs is > 40 Billion STBO? (McNaughton and Garb, 1975).

BP equivalent reserve on its various types of fractured reservoirs in the world is about 21 Billion STBO (Nelson, 2001).

Basements:

STOOIP of Vietnam White Tiger (Bach Ho) field is 1.59 Bbls, 95 % is in basement rocks (Geo review journal, 2010).

STOOIP of Yemen OMV Block S-2 is 1 Bbls (De Kok et.al., 2009).

Carbonates:

STOOIP of Ekofisk chalk Field, North Sea is 5.4 BBO.

STOOIP of Dukhan Jurassic carbonate field, Qatar is 2.4 BBO.

STOOIP of Miran carbonate Field is 744 MMBO.

Production Rates in Fractured Reservoirs

Vietnam:

White Tiger (Bach-Ho) Field produced:

~ 202,000 Bbl/d (PESGB report, Jan 2005).

~ 140,000 Bbl/d (Cuong and Warren, 2009).

Big Bear (Dai Hung) Field produced ~ 25,000 Bbl/d in mid 90s.

Yemen:

OMV block S-2 (Al-Uqlah field) produced ~ 1,000 Bbl/d from first well (Web news in 2006).

Fractured Basement Reservoirs for Petronas in Vietnam

Ruby Field

Current basement production:

Cumulative production:

Resources:

Peak production:

No of wells drilled:

Diamond Field

4 exploration and appraisal wells drilled.

DM-4X: Tested 2926bopd

DM-2X: Tested 226 bopd

Jade Field

3 exploration and appraisal wells drilled into basement.

Jade-2X: Tested 11Mscf/d

RUBY Basement Wells: Drilling Results and Production Performance

RB-4X

RBB

RB-2X

RBA-6P

RBA-

6PST

RB-1X

RBA-3P

RBA-4P

RBB-5P

RBA-10P

RBB-4P

RB-3X

RBA-

12PST4

RBB-4P

intermittent flow 150-

200 bopd. Well quit

after 1.5 months

production.

RBA-10P (barefoot)

Qi = 3,500 bopd

Np = 0.25 MMstb

P&A, high WCUT

Formation collapsed

RBB-3P (pre-drilled

liner)

WO twice.

Qi~ 180 bopd. 25% WC

Intermittent prod.

Currently shut-in

RBA-6P (barefoot)

Qi = 1,400 bopd

Np = 0.24 MMstb

P&A, high WCUT

Formation collapsed

RBA-12PST4

Flowed 727 bopd and

1697 bwpd @128/64 during DST clean-up

RBB-5P (pre-drilled

liner)

Good HC show and

mud loss while drilling.

Produced water in

testing (drilling fluids?)

RBA-4P (pre-drilled

liner)

Qi = 4,000 bopd

Np = 6.5 MMstb

Q ~ 1,100 bopd

stable rate, 8% WCUT.

RBA-3P (barefoot)

Qi = 1,000 bopd.

Np = 0.51 MMstb

Prod 1998-2004.

high WTC & sand

buildup from shallower

targets

RB-1X

Q=1,700-2,000 bopd

during DST

RBA-6PST1 (pre-drilled

liner)

Qi = 3000 bopd

Np = 0.39 MMstb

Flowing 80% WCUT.

Q~ 400 bopd

N

RBB-

3P

RBA

RBA-

10PST+

Fractured Field Performance Very Variable Production

In Vietnam White Tiger (Bach-Ho), each well produces 2,000 to 5,000 Bbl/d, best 15,000 bbl/d.

In Venezuela Mara basement field (in 1950s), each well produced 17,000 Bbl/d.

In Egypt Zeit Bay basement (in 1980s-1990s), each well produced 700 to 10,000 Bbl/d.

Fractured Field Performance Oil Recovery of White Tiger Basement Reservoir

BLOCK OIIP

(MMBBLS)

Prod

(MMBBLS)

RF

I 2261 1014 0.45

Ia 296 46 0.15

II 168 49 0.29

III 289 24 0.08

IV-1 25 0 0.00

IV-2 28 1 0.02

V 363 29 0.08

VI 227 0 0.00

TOTAL 3656 1162 0.32

*The actual unit in the Source is in MT and 7.33 conversion factor used to convert to MMBBLS

Source: PCVL-VSP Fractured Basement Workshop 30 Sep 2010

Typical Characteristics of Fractured Reservoirs

Very variable well performance.

High initial flow rates (production over estimates).

Rapid depletion (varies by well, mostly happen locally).

High reservoir column i.e. Al-Uqlah is 922 m, White Tiger (Bach-Ho) is 1,000 m, and Mara is 1,875 m.

Fractured porosity and fluid contacts hard to define. Thus, uncertain in reserve estimation.

Vary variable recovery factors ( 0 to 70 %, based on IFP report):

RF in oil-wet reservoir ranges between 5 and 25 %.

RF in water-wet reservoir ranges between 25 and 45 %.

FRACTURED BASEMENT RESERVOIRS

Landes et al (1960) - Basement rocks are considered as any metamorphic and/or

igneous rocks (regardless of age) which are unconformably overlain by a

sedimentary sequence.

PAn (1982) Defines basement rocks as metamorphic and igneous rocks which are

unconformably overlain by a younger oil-bearing formation (source rock). The oil,

which is generated from the overlying sediments, is stored in the older

metamorphic and igneous basement rocks.

North (1990) Considers basement rocks should include rocks of sedimentary

origin, providing these rocks have no or little matrix porosity. He also states that the

basement rock should not be compared with PreCambrian age rock, as long as it

has considerable fractures due to deformation, weathering, or both.

What is A Basement Rock ?

In Peninsula Malaysia basins, the basement rocks consist of metamorphic, igneous, meta-sediment, and clastic rocks.

The rocks are in Pre-Tertiary Jurassic age, ranging from 154 Ma (in northern areas) to 139 Ma (in southern areas).

The fractured and weathered rocks are abundantly occurred in the northeast margin of Peninsula Malaysia basin.

Multi stages of deformation creating fractured rocks, started in Early Cretaceous age.

Further tectonic activities, occurring in the basement rocks and developing fractures, were occurred at Middle Miocene age.

Fractured Basements in Peninsula Malaysia Basins

Malay Basin

Penyu

Basin

Basement in Peninsula Malaysia Basin Granite Rocks

No Map No Well Name Depth Remarks

1 8 North Lukut-2 1,945 m 5 m weathered granite (felsic) rock.

2 12 Abu Kecil-1 1,935 m 8 m weathered granite rock.

3 13 Abu-2 1,822 m 8 m granite (felsic) rock.

4 14 Bubu-1 1,609 m 28 m weathered granite (felsic) rock.

5 15 Lerek-1 1,096 m 28 m weathered granite (felsic) rock.

6 16 East Raya-1 2,410 m 16 m granite rock.

7 17 South Raya-1 2,261 m 1 m granite (felsic) rock.

8 18 West Belumut-2 1,455 m 1 m granite rock.

9 20 West Belumut-1 1,410 m 7 m granite rock.

10 19 Belumut-1 1,505 m 3 m granite rock.

11 25 Sotong-B4 2,635 m 72 m granite (felsic) rock.

12 29 Feri-1 2,918 m 22 m weathered granodiorite rock.

13 30 Malong 5G-17.2 1,583 m 11 m granite rock.

14 31 Malong 5G-17.1 1,580 m 50 m granite and metamorphic rocks.

15 32 Delah 5H-14.1 2,928 m 35 m weathered granite rock.

16 33 Jelutong 5G-23.1 1,627 m 13 m weathered granite rock.

17 34 Keledang 5G-24.1 1,726 m 29 m granite and metamorphic rocks.

18 6 Larut-1 2,686 m 87 m weathered volcanic (granite?) rocks.

Basement Rocks in Malay-Peninsula Basins Metamorphics

No Map No Well Name Depth Remarks

1 26 Anding-1 2,587 m

2 27 Anding-2 2,658 m 79 m phyllite and red-bed rocks

(Jurassic-Cretaceous age).

3 28 Anding

Barat-1

3,167 m 3 m phyllite and black-bed rocks

(Jurassic-Cretaceous age).

Notes:

Metamorphism is the process of changing the physical structure of rock due to high temperature and high pressure.

A metamorphic rock can be originated from igneous, clastic (sediment), and another metamorphic rocks, altering its original mineralogy, sedimentary structure, and chemistry.

In Peninsula Malaysia basins, the possible source rock is the M or pre-M shales(Oligocene age).

The top seal rock is the M or L shales, locally.

Reservoir basement rocks are varied in term of fractured and weathered significance, depending on the tectonic activities.

Fractured Basements in Peninsula Malaysia Basins Petroleum System Model for Anding and nearby Fields

SR

3,000 m

4,000 m

Granite is commonly occurred as an intrusive and felsic igneous rock.

Granites usually have a medium- to coarse-grained rock texture.

Granites color can be pink to gray, depending on their chemistry and mineralogy.

The average density of granite is between 2.65 and 2.75 g/cm3.

Its compressive strength (UCS) usually is above 200 Mpa.

Granitoid is a general term for light-colored, coarse-grained granite.

Granite Rock - Definition

Granite Rock Average Mineral Composition

Basement Rocks in Peninsula Malaysia Basins Clastics (sand, silt, claystones) and Limestones

No Map No Well Name Depth Remarks

1 1 Tok Bidan-1 2,118 m 534 m sandstone, siltstone, shale, and meta-sediments (Jurassic-Cretaceous age).

2 2 Badak-1 2,295 m 31 m lithic sandstone and conglomerates (Jurassic-Cretaceous age).

3 3 Bunga Orkid-1 3,671 m 174 m continental red-beds (Jurassic-Cretaceous age).

4 4 Bunga Pakma-1 3,522 m 59 m clastics and meta-sediments (Jurassic-Cretaceous age).

5 35 Kempas 5G-22.1 1,799 m 157 m sandstone (Jurassic-Cretaceous age).

6 36 Rhu-1 2,912 m 114 m siltstones, claystones, and volcanic rocks (Jurassic-Cretaceous age).

7 37 Pari-1 2,174 m 53 m calcareous siltstones.

8 5 Bunga raya-1 2,905 m 493 m limestones (Triassic age).

9 21 Sotong B-2 2,911 m 8 m limestones.

10 22 Sotong B-1 2,750 m 292 m limestone and argillite (clay-sediment of Jurassic-Cretaceous age).

11 23 Sotong B-3 3,016 m 38 m argillite (clay-sediment of Jurassic-Cretaceous age).

12 24 Sotong B-5 2,985 m 31 m argillite (clay-sediment of Jurassic-Cretaceous age).

13 7 Sumalayang-1 1,438 m 13 m un-differentiated rock.

14 9 Relau-1 2,177 m 9 m un-differentiated rock.

15 10 Penara-1 2,373 m 1 m un-differentiated rock.

16 11 Abu-1 1,910 m 1 m un-differentiated rock.

1. Tectonic fractures

Form by faults and folds

Typical on anticlinal folds

2. Regional fractures

Normal to bedding planes, show little offsets along fracture planes and may cut cross local structures

Mechanism is not well understood: maybe large scale vertical movements in the earth crusts

Excellent resevroirs when tectonic fractures cut the regional farctures

3. Contractional fractures

Extensional and tensional fractures formed as results of bulk volume reduction in volume rock

May be initiated by desiccation, mineral phase and temperature changes

Not as important as tectonic and regional fractures in hydrocarbon reservoirs

4. Surface related fractures

Created by unloading stress

Weathering for example may remove stabilizing masses and creates instability that leads to collapse of the reservoirs

Not considered to be important in hydrocarbon reservoirs.

5. Fractures can also be produced by differential compaction:

Compaction of strata above buried topography

Genetic Natural Fracture Classification (Nelson, 2001)

Type-I: Fractures provides essential porosity and permeability.

Type-II: Fractures provides the essential permeability but the matrix provides the storage.

Type-III: Fractures assist permeability in an already producible reservoir.

Type-IV: Fractures provides no additional porosity and permeability but create a significant barriers and anisotropy.

Fractured Reservoirs Types (Nelson, 2001)

Roles of Fractures in Reservoir (McNaughton and Garb, 1975)

Do fractures have roles i.e. conduits, seals, or baffles (barriers) ?

Or, do fractures have dual behavior at different times / pressures ?

How do I reduce risk in development decision ?

And, how do I optimize production ?

Fractured Reservoir types (McNaughton and Garb,1975)

Type-4 Type-3 Type-2 Type-1

Fra

ctu

re N

o

Typical Problems in Fractured Reservoirs

Economic failures because G&G is not fully understood or in-appropriate engineering

handling i.e. pressure maintenance, etc.

Heterogeneous fracture distributions.

Damage of fracture system during drilling.

Fail to intersect sweet-spot areas of open fractures.

Well completion in higher porosity but un-fractured intervals.

Rapid depletion (lack of connectivity).

High degree of inter-well interference.

Difficult to interpret fractured porosity, meaning incorrect volumetrics.

Early water breakthrough.

EOR: poor sweep efficiency due to early water breakthrough.

Petrophysic/RE Roles in Fractured Reservoir Key Issues

Define reservoir properties and mechanisms:

Block/field size, porosity, permeability, matrix vs. fracture for fluid flow,

wettability (especially in carbonates).

Best recovery mechanism.

Identify best modeling approach:

Single porosity-single permeability, dual porosity-dual permeability, and/or

dual porosity-single permeability fracture system model, etc?

Predict dynamic behavior during filed life (including EOR):

Water coning, water breakthrough, etc.

Identify best drilling and completion methods for draining the fractured reservoir:

Horizontal well scheme = higher production?

Geomechanics Role in Fractured Reservoir Key Issues

Couple stress effects:

Does present-day in-situ stress influence fracture?

Dominant open fracture orientation?

Is fluid flow controlled by stress?

Other:

Wellbore stability optimum drilling direction for production and stability?

Stress rotation and pressure boundaries at faults?

Fracture behavior during depletion and repressurisation?

Constraining the in-situ stress condition:

Orientation and magnitude of s Hmax, sHmin, and s v.

Variation across the field, or with faults or lithology.

Rock Property System in Fractured Reservoirs

Single porosity Single permeability fracture system:

Matrix makes no contribution.

Fractured intervals is the HC storage.

This is a type C (or Nelsons Type-I) of highly fractured formations, which behaves like a homogeneous system.

Dual porosity Single permeability fracture system:

Matrix is a storage medium but practically it has no permeability. This matrix feeds the fracture system which dominates the permeability (fluid flow).

This is a type C-B (or Nelsons Type-I and II/III) fractured reservoir.

Dual porosity Dual permeability fracture system:

Matrix and fracture system are a storage medium and both have significant permeability, which the 2 systems work together.

This is a type B (or Nelsons Type-II/III) fractured reservoir.

Scales of Observation

Detection of Fractures

Cores:

1) Recognition of open fracture is quite difficult because the core barrel is often

empty due to the rock is more less broken and falling down into the hole.

2) The detection is quite straightforward or easy if the fractures are cemented.

Well-logs:

1) Image log is the only logging tool that can see directly the fractures (open

and closed).

2) The other logging tools, most of the cases, the tool responses are indirectly

detected the presence of fractures, which are from the anomaly responses

created by fractures.

3) Each logging tool has different capacity/response for detecting fractures.