MSc Fractured Reservoirs

SEAL Basin Fractured Basement

Challenging the Exploration Paradigm

February, 1st, 2012

SEAL Basin Fractured Basement – Challenging the Exploration Paradigm2

Index

Basin presentation

Naturally Fractured Reservoirs Exploration Workflow

1

3

How it works

Fracture System Identification and Properties

3.1

3.2

2 Assessment of the Problem and Objectives

Technical Recommendations4

Volumes Calculations3.2

Final Considerations5

1. Basin Presentation


1. Basin Presentation

A narrow band of about 35 km wide, stretching for about 350 km in the NE-SW direction.

Area= 45,960 km2, Emerged Area= 13.200 km2

Formed during the opening of the SouthAtlantic in the late Jurassic and Cretaceous.

Late Aptian

2. Assessment of the Problem and Objectives



In 2005 GALP acquired a set of exploration blocks located in Sergipe-Alagoas Basin

Phase I: Four exploratory wells (Alpha, Beta, Charlie and Delta)

Objective: Ibura Member carbonates

First well demonstrated the potential of the fractured Basement play

Phase II: Three exploratory wells (Echo, Fox and Golf)

Confirmed the results of the previous campaign

Fractured metamorphic basement rock presented the best results in the cuttings and when Drill Stem Tested.

By the time of fourth well it had become the primary objective

Proved the presence of a Naturally Fractured Reservoir (NFR) in the Basement rock


GONDWANA

Congo-São Francisco Craton +

West African Craton

SEAL BASIN BASEMENT

IS

A FOLD BELT

STRUCTURALLY AND LITHOLOGICALLY VERY COMPLEX

2. Assessment of the Problem and Objectives – The Basement


2. Assessment of the Problem and Objectives – The Basement

Conditions for HC accumulation in Basement rock

3. Migration: laterally from sources to

fractured, topographically higher, basement

rock.

2. Source:

a) Overlying organic rock from which oil

is expelled downward during

compaction.

b) Lateral, off-the-basement, but

topographically lower, organic rock.

c) Lower, lateral reservoirs from which

earlier trapped oil is spilled.

4. Reservoir: fractured basement rock.

SEAL Fractured Basement Play

Maceió Fm. black shales, with typeII Organic Matter (OM) andaverage Total Organic Carbon(TOC) content of 3.5%.

Ibura Member anhydrite1. Seal: as in any other reservoir a

impermeable cap rock is necessary to retain

HC.

Through normal faults to theCarmópolis Mb. sandstones, andthen laterally to structural highs

Fractured basement rock.


Echo Alpha Fox Bravo

Riachuelo Fm.

Ibura Mb.

Basement

Cotinguiba Fm.

Calumbi Fm.

2. Assessment of the Problem and Objectives – Schematic Seismic Section



Complex Reservoir

Hard rock of low porosity and permeability

Fractures provide main flow path

Joint team

Universidade de Aveiro

Universidade Nova de Lisboa

Universidade do Algarve

Instituto Superior Técnico

THESIS

Integrated all the data and results

of the project

+

Analysis of the drilling and testing

procedures

FINAL OBJECTIVE

Provide technical recommendation

on how future exploration

campaigns should be run.

Discrete Fracture Network and simulate 3D permeability maps.

Structurally Characterize Basement Rock

Naturally Fractured Reservoirs Exploration Workflow

3. Naturally Fractured Reservoirs Exploration Workflow


Assess if the reservoir is fractured

NO

Seismic Acquisition

+Processing

Geomechanical Models

DFN Models

constrain

Volumetrics Well Design

YES?

Identify the Fracture System

Well data

Outcrop data

Regional geology

Seismic

AVAILABLE?

3. NFR Exploration Workflow – How it Works?

Fracture Properties Affecting Reservoir Performance

•Morphology

•Width/Permeability

•Spacing

+Fracture-Matrix interaction

derive

information

Lithology

Distribution of fracture patterns

Rock mechanical properties

YES

Seismic Interpretation

Fracture System Origin information


3. NFR Exploration Workflow - Fracture System Identification and Properties

Well data Outcrop data Regional geology Seismic

Fracture System Identification

Cuttings/Well Logs

Image Logs

Wireline Logs

Core Data

DST’s

Oil shows in Basement Cuttings.

Detected fractures at Basement level.

Did not detect fractures. Some did not cover entire Basement.

Not taken.

Erroneous procedures:

→ different wells tested

different stratigraphic

sections

→ standard procedures

were not followed

Does not allow to take

valid conclusions on the

provenance of the

sampled fluids and on

the pressure

characteristics of each

particular formation.

Oil and gas registered at surface. Burner lit in several wells. Even though..

Short build-up time

SP

IHP

FHP



Well data Outcrop data Regional geology Seismic

Fracture System Identification

Stress Direction

NS normal

E-W/ENE-WSW transfer faults

NW-SW normal

NNW-SSE transfer faults

Chagas

Faulting

E-W

NW-SE

Fracture System Origin

Faults and fractures were

considered to be formed by

tectonic stress, as a consequence

of both the collision during Pre-

Cambrian times, and the opening

of Atlantic Ocean.

Bad quality seismic was available

Horizons and faults were interpreted

+

Attributes were extracted

(Coherence + Dip Az.+ Dip Mag.)

+

U.Aveiro: new Methodology for Automatic

fault detection + seismic enhancement


More faults could be interpreted!



• Well data


4. FMI: Natural fractures, were observed inside the Basement when using image logs

Fracture planes measured by FMI were plotted into a Schmidt

Projection grid and statistically analysed by U.N.L team.

Method was extended to all the wells later.

Total ReadingsMean Planes

Alpha


Alpha Beta Delta Fox Golf

Alpha-2 Beta-1 Delta-2A Fox-1A Golf-1A A 104 N22ºW/60ºNE N30ºW/57ºNE

Alpha-1 Beta-2B - - - B2 16 N38ºE/63ºNW N43ºE/79ºNW

- Beta-2A - Fox-2 - B1 7 N30ºE/42ºSE N30ºE/35ºSE

- - Delta-1 - - - - - -

Delta-2B Fox-1B - D 28 N8ºE/56ºNW -

- - Delta-3 - Golf-2 E 25 N87ºE/35ºSE -

- - - - Golf-1B - - N20ºW/39ºSW -

General Trend

(U.N.L)

Correspondence of Fracture Families Per wellFinal

General Trend

(GALP)

Measurements

in Basement

Family A is the most common,

being present in every well with a

total of 104 measurements inside

the Basement.

Future Well should intersect Family A!

Five fracture families were found.




Properties of the Fracture System

Morphology Spacing Width/Permeabilty

Geomechanical Modelling (UALG): simulated the

density and orientation of fractures in the space

between the wells (the whole reservoir space).

Output

Mohr-Coulomb Shear Stress values and the predicted

conjugated fractures shear strikes and dips

By combining the Mohr-Coulomb Theory, with the

Anderson’s principles and the frictional fault theory,

regional tectonic stresses were simulated over the grid

with embedded faults


Properties of the Fracture System

Morphology Spacing Width/Permeabilty


DFN/Φ and K Modelling (UNL): Generated equivalent permeability histograms of each FMI

fracture family by using data from the FMI statistical analysis and the geomechanical

modelling.

1. The number of fractures by family and

correspondent distribution of areas that match

a specific Linear Fracture Density at vertical

direction (FMI direction) were estimated.

FTRIAN software:

a) Monte Carlo algorithm generates fracture

networks using a polygonal approximation

(squares).

b) Sampled with scan lines simulating a well.

…to do this…


#1 #2 #3

5000 0.072 0.079 0.064 0.072

10000 0.149 0.156 0.147 0.151

15000 0.214 0.211 0.214 0.213

20000 0.276 0.302 0.297 0.292

25000 0.360 0.366 0.369 0.365

30000 0.431 0.426 0.438 0.432

35000 0.531 0.514 0.522 0.522

40000 0.597 0.589 0.601 0.596

45000 0.657 0.633 0.695 0.662

50000 0.733 0.719 0.736 0.730

55000 0.793 0.817 0.804 0.805

60000 0.857 0.891 0.876 0.875

65000 0.966 0.974 0.961 0.967

70000 1.019 1.027 1.032 1.026

75000 1.115 1.111 1.088 1.104

80000 1.132 1.153 1.196 1.161

85000 1.222 1.264 1.230 1.239

90000 1.308 1.337 1.350 1.332

95000 1.370 1.403 1.420 1.397

100000 1.452 1.467 1.464 1.461

105000 1.485 1.557 1.527 1.523

110000 1.592 1.622 1.613 1.609

115000 1.645 1.705 1.679 1.677

120000 1.739 1.812 1.763 1.772

125000 1.788 1.870 1.832 1.830

130000 1.909 1.962 1.881 1.917

135000 1.971 2.019 1.959 1.983

140000 2.075 2.060 2.078 2.071

145000 2.132 2.149 2.133 2.138

150000 2.173 2.215 2.206 2.198

155000 2.287 2.293 2.297 2.292

160000 2.313 2.346 2.333 2.331

165000 2.405 2.439 2.472 2.439

170000 2.464 2.522 2.491 2.493

175000 2.562 2.583 2.583 2.576

Family A

Low

Intermediate

High

N Average LFD LFD classRealisations


This reads…

It is necessary to generate approximately175000 fractures of family A to reach a LFD indexof 2.6 (maximum observed).

Using the values from the table it was possible to

estimate permeability and porosity for each of

the fracture families using the Oda method.

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350 400 450

Fre

qu

en

cy

Permeability (Darcys)

Max equivalent permeability - T-412-429

Family A Family B1 Family B2

Block-A Max Equivalent Permeability

Interval Average Interval AverageA [0; 381.524] 171.526 [0; 0.0046] 0.0021

B1 [0; 238.733] 93.131 [0; 0.0029] 0.0011

B2 [0; 163.818] 60.454 [0; 0.0019] 0.00072A [0; 350.346] 147.504 [0; 0.0049] 0.0021

B1 [0; 221.449] 91.235 [0; 0.0026] 0.0011

B2 [0; 145.246] 57.182 [0; 0.0017] 0.00068

A

B

BlockFracture family

Permeability (Darcys) Porosity (%)

Values can now be used of Volumes calculations


3. NFR Exploration Workflow - Volumetrics

HIIP = GRV x NTG x Phi (Φ) x Shc x FVF

GRV = a common lowest closing contour was used at 1200m depth

1. Basement highs are narrow and may be interconnected

2. Oil shows below 1200m depth within the Basement were found in a well.

NTG = percentage of rock volume that is occupied by

fractures – Fracture Density.

DFN Modelling allowed fracture density to be

calculated for aperture values found in the FMI data:

Fracture

FamilyInterval NTG (%)

Min 0,0062

Max 0,3841

Min 0,0033

Max 0,2057

Min 0,002

Max 0,1313B2

A

B1

Porosity

1. Assess NTG and use Φ=1

2. Assess Φ and use NTG=1

When NTG is calculated it is already accounting for the pore

space that would be given by porosity, because what is

being calculated is the total volume of rock that is occupied

by open fractures

a) Φ = 0.001xWfxDfxKf1

b) Permeability modelling results

c) Average porosity values from literature (world analogues)


3. NFR Exploration Workflow - Volumetrics

Shc = 1; assuming that the fractures are saturated with HC

FVF = Min=1,04; Mode=1.14; Max=1,14

Reservoir Thickness: 455m for Block A and 610m for Block B

Maximum thickness between the lowest depth to which HC were found and the

crest of the structure.

Calculations were made in GeoX using the above parameters.

Scenario Based OnBlock A Total Mean

STOOIP (MMBO)

Block B Total Mean

STOOIP (MMBO)

0,38 0,51

44,7 60,2

NTG from FMI fracture density

Ф from empirical formula

Ф from modelling

Ф common range in analogues

30,2 40,4

38,7 52,1

1

2

3

4

Scenario 1 and 2 are the most acceptable

based on direct observations using the

FMI tool

no errors related to modelling assumptions

STOOIP is probably higher – Underestimated due to vertical wells.

calculated using the API gravity,formation temperature and pressuredata from DST’s.

4. Technical Recommendations



ProblemSeismic Data quality was bad

alternatively

Seismic Processing: Re-process the already available seismic with pre-stack techniques,

giving more attention to the picking of the initial stacking velocities particularly at Basement

depth.

Benefits

Better imaging allowing fracture patterns to be defined in seismic

Decide if a new seismic acquisition campaign is needed.

Seismic Acquisition: acquiring Wide-Azimuth seismic should be an option to consider as

the area to be covered is small.

Benefits

Cost increase relative to quality gain is small


Problemlack of data from other sources + erroneous procedures

alternatively

1. Drill-Stem Tests to be executed equally in all the wells and according to standards

2. Image logs to be run equally in all the wells and to TD

Benefits

Allow comparison between wells.

Obtain information on permeability of tested formations


1. Detailed geological field studies

2. Full-diameter cores to be taken from at least one well

Benefits

Direct information on fracture properties


Problemwells were vertical and overbalanced.

alternatively

Directional drilling: directional wells at an angle from vertical in a direction normal to

fracture planes and parallel to the minimum in situ stress. Intersect fracture Family A!!!

Benefits

Maximises Contact of well with fractures, increasing productivity.


Family A N22ºW/60ºNE

Hotel Well30º,240º

Well

W

N60ºE

N

E

S

N22ºW

Family A Fractures

Direction of Well

Well

W

N60ºE

N

E

S

N22ºW

Family A Fractures

Direction of Well

N60ºE

N

E

S

N22ºW

Family A Fractures

Direction of Well

X=180-(60+90)

X=30º

60º

90º

x

Inclination of Well

NE SW

X=180-(60+90)

X=30º

60º

90º

x

Inclination of Well

NE SW


BravoHotel

Ba

sem

en

tS

ed

ime

nta

ry co

ve

r

BravoHotel

Ba

sem

en

tS

ed

ime

nta

ry co

ve

r




Underballanced drilling: It is performed with a light-weight drilling mud that applies less

pressure than formation pressure preventing the mud to penetrate the formation

Benefits

Avoids formation damage.

Allows early production.

Reduces estimulation needs.

Maxmises HC recovery.

Leading Edge, 2002:

“reduction in well count could be as high as 25% as a result of the increased productivity”

Hotel-D Hotel-D+Ubd

DirectionalDirectional

Underbalanced

$ $ $

PLANNING 27.027,350 38.612,164 38.612,164

LOCATION 146.149,410 104.138,482 104.138,482

RIG 957.385,730 1.302.660,155 1.302.660,155

OUTSOURCING SERVICES 338.417,550 423.082,221 423.082,221

MATERIALS 414.456,130 433.687,246 494.837,246

HUMAN SUPERVISION 90.615,750 123.903,150 123.903,150

LOGISTIC SUPPORT 81.380,950 81.380,940 81.380,940

TOTAL 2.363.747,801 2.883.584,012 2.953.906,512

Δ to Fox 519.836,212 590.158,712

Δ to Hotel-D - 70.322,500

Fox

Directional well = 22% cost increase

Directional + UBD= 25% cost increase

With an increase of 3% in cost

relative to a directional well, drilling

Hotel underbalanced is an option to

be taken into account.

5. Final Considerations


5. Final Considerations

1. From the analysis of the references in the literature, seismic, well data (FMI and DST

data), it was possible to conclude that the Basement rock in the study area is a NFR.

2. Using FMI data, application of geomechanical principles, DFN modelling and Φ/k

simulations it was possible to identify fracture Family A as the best candidate to be

intersected with a well.

4. Fracture densities and apertures are underestimated due to the relation vertical-well

VS sub-vertical fractures, this means that results will be probably better than predicted.

5. In order to achieve better results this thesis suggests that a directional and

underbalanced well that intersects Family A perpendicularly should be drilled in the

future.

3. Using data from the models it was possible to calculate volumes for the Fractured

Basement – It had never been done despite the two exploration campaigns!!!


6. Naturally Fractured

Reservoirs Workflow as

proved to be efficient and will

be used by GALP to evaluate

other prospects were

fractured reservoirs may be

present.

FINAL CONCLUSION!!

Thank you for your attention!

Date post:	14-Apr-2017
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MSc Fractured Reservoirs

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