FRACTURED RESERVOIR DESCRIPTION

Basic Geological Aspect of Naturally Fractured Reservoirs

Basic Geological Aspect of Naturally Fractured Reservoirs

Fracturing mechanics of geological formations.

Influence of stylolitization and jointing.

Fracture density and orientation.

Determination of block size.

Fracturing mechanics of geological formations.

Influence of stylolitization and jointing.

Fracture density and orientation.

Determination of block size.

Fracture

Fracturing Mechanics of Geological Formations

The processes which involves at least momentary loss of Cohesion, ability to resist differential stress, separation into two or more parts, and the release of stored elastic strain energy.

-Griggs and Handin, 1960

Fracture ClassificationExperimental fractures

• Shear fractures• Extension fractures• Tensile fractures

Naturally occurring fractures• Tectonic fractures

• Regional fractures• Contractional fractures• Surface-related fractures

Orientation of extension and shear fracturesrelative to principal stress direction

Fig.6 Saidi, p.10

Tectonic fracturesNelson

(Fault-related fractured systems)

Tectonic fracturesNelson

(Fault-related fractured systems)

Contractional fractures(Mud cracks)Kulander

Contractional fractures(Syneresis or chickenwire

fractures)Kulander

-From T.D Van Golf-Racht

Influence of Stylolitization and Jointing

Schematization of stylolitization with thinning

Overburden Pressure + Dissolution

Grain

porosity

Overburden Pressure + Dissolution

T= 0 T= t1 T= t2

-From T.D Van Golf-Racht, p.33

Classification of stylolites vs. beddingVan Golf-Racht

-From T.D Van Golf-Racht, p.35

Schematization of stylolitization and stylolithificationVan Golf-Racht

Fracture Density and Orientation

BFD V

SV

B

TFD S

LA or

B

fFD L

nL

LT = total length of fractureSB = bulk surface area in a cross flow sectionnf = number of fracturesLB = matrix bulk length

S = fracture-bulk surface VB = matrix-bulk volume

Volumetric fracture density

Areal or linear fracture density

aa

aVFD

663

2

cos

2cos22

aa

aAFD

Fluid flow direction

cos

2

aLFD

Flow around a cubic matrix block unit

a

aFluid flow direction

aa

aVFD

663

2

aa

aAFD

222

aLFD

2

Fracture density of a fracture system

S

lA

n

FD

1

cos

l

nLFD

cos

Fracture density of a fracture network

m

i

iFDFD

AV

1 cos

m

iFDFD iLA

1

cos

Fracture intensity

f

fINT Th

F

frequencyThickness

frequencyFractureF

If only one layer:

B

fFDINT L

nLF

Fracture intensity diagramVan Golf-Racht

Aguilera, pp38-39

Multidirectional permeability and fracture orientation plot

Determination of Block Size

Reiss, Fig.9 p.14Reiss, Fig.9 p.14

Outcrops studiesReiss

Shape Factor

222

1114

zyx LLL

For a rectangle with all matrix faces imbibing

n

i Ai

i

d

A

V 1

1V = the bulk volume of the matrixAi = the area open to imbibition at the ith directiondAi = the distance from Ai to the center of the matrixn = the total number of surfaces open to imbibition

For a cylinder with all matrix faces imbibing

22 2

114

rh

Fracture Detection and Evaluation from Cores

and Well Logs

Methods of Fracture Detection

Direct Detection• Direct observation and analysis of core• Downhole cameras• Visual logs

Indirect Detection• Well log analysis• Well testing• Manipulation of reservoir rock property data

Naturally Fractures Indicators from Core Analysis

1. Fully or partially mineralization2. Slickensides3. Pressure solution (stylolitic

gashes)4. Cataclastic zones (gouge)

Spraberry fractured core that are filled by secondary mineralization

Naturally fractures that are completely filled by

secondary mineralizationKulander

Slickensided natural fractures in coreKulander

-From Kulander et al., 1990, p.9

Slickensided induced fractures in coreKulander

Pressure solution (stylolitic tension gashes)Kulander

-From Kulander et al., 1990

Cataclastic zones (Gouge)Kulander

-From Kulander et al., 1990

Fractures-Induced Indicators

1. Drilling-induced fractures2. Coring-induced fractures3. Handling-induced fractures

Drilling-induced fracturesKulander

-From kulander et al., 1990

-From kulander, p.34

Drilling-induced fracturesKulander

-From kulander, p.54

Hammer-induced fracturesKulander

-From kulander, p.56

Handling-induced fractures Kulander

Wireline Log Responses• Borehole Geometry• Formation invansion by drilling fluids• Acoustical characteristics• other factors

Fracture Detection from Logs

Mud Logging Responses• Mud Losses, reflecting sudden and significant

increase of permeability• Increase of drilling penetration rate, linked to

decrease of rock cohesion in fractured formation

Reiss, Fig. A.1.1 p.48Reiss, Fig. A.1.1 p.48

Well Logging in a Fractured Reservoir Reiss

Anomalies in borehole geometrySaidi Formation invasion anomalies on caliper and density logsSaidi

-From Saidi, p.32

-From Saidi, p.32Formation invasion anomalies on caliper and resistivity logsSaidi

Formation invasion anomalies in Dipmeter ToolsSaidi

Acoustical Characteristics

1. Cycle skipping2. Waveform Analysis3. The Borehole Televiewer4. The Circumferential Microsonic

Tool

1. Cycle skipping2. Waveform Analysis3. The Borehole Televiewer4. The Circumferential Microsonic

Tool

Modification of Acoustic PropertiesSaidi

The Circumferential Microsonic ToolSaidi

Schematic Representation of Acoustic Borehole Televiewer ToolSaidi

matrix

porema V

V

fractotmatrix VVV

Matrix porosity

Fracture porosity

tot

fracfrac V

V

Total porosity

tot

fracporefrac V

VV

Matrix porosity estimate

maf

mama tt

tt

Fracture porosity estimate

Deep Laterolog:

w

nwf

m

w

nwf

mb

LLD R

S

R

S

R

fffb21

Shallow Laterolog:

mf

nxof

m

w

nwb

mb

LLS R

S

R

S

R

fbbb21

Assume Swf=0 and Sxof=1

fm

LLDLLSmf RRR

/1

2

11

Mf 3/2 (generally taken)

Fractured Evaluations from Logs

MATRIX AND FRACTURE CHARACTERIZATION IN FRACTURED SYSTEMS

Coring Operation and Description of Coring

Optimal analysis of NFRs requires:• Supervision of coring operations• On-site core processing• careful layout and marking of the core• detailed measurement of fracture

characteristics.

Natural fractures provide information on in-situ permeability system, while coring-induced fractures provide data on in-situ stresses.

Supervision of Coring OperationsObserve high or erratic torque. It is often used as:

1. Evidence of fractured formations, 2. Indicate points of correlation between rubbleized sections of

the core3. Indicate breaks in the orientation survey record.

Observe abrupt increases in pump pressure or weight-on-bit may induce fractures in the core.

• Obtain the exact depths of such increases to determine whether an otherwise ambiguous fracture is natural or induced.

Record the depth at which drill-pipe connections are made during the coring.

• Often associated with spinoffs in the core and abrupt scribe rotations

On-site Core Processing• Core Recovery

• Core damage can be avoided when using a conventional barrel by laying the barrel down on the pipe rack and pumping the core out hydraulically with a high-pressure, low-volume pump.

• A rubber plug should be inserted in the barrel to prevent water contacting the core.

• Layout, re assembly, and marking• Layout and as much reassembly of the core pieces as possible are

critical steps in orienting any core.• Marked with both a rotating PSL (Principal Scribe Line) and a

straight, blue MOL (Master Orientation Line). The MOL is most useful for comparing relative orientations of fractures within each continuous-fit interval and for absolute fracture orientation.

• Description and measurement• Packaging

Goniometer

Measurement of Fracture Dip and Strike

View is looking down on core

The strike () can be calculated:

21

121

coscos

sinsintan

Vertical Fracture

2

1

MOL

MOL : Master Orientation Line

L

1

L

d2

cos12

tan

1

1

L

d2

cos2

cos2

tan

12

1

L 1

2

Case 1: The dip is calculated with one strike measurement

Dip = 90 - where

d = core diameter and must be in the same unit as L

Case 2: The dip is calculated by measuring two angles and the vertical displacement between them

Dip = 90 - where

Schematic of core cross section and annular protractor

1. The principle scribe-line groove (PSL) is aligned on the protractor with its true orientation at that depth, provided by the orientation survey, of 65.

2. An imaginary line normal to the fracture is measured at 192.

3. True fracture strike is calculated as 90 from the imaginary line, at 102.

Lorenz and hill

Fracture parameters that should be measured in core Lorenz and Hill

• Host lithology (type and thickness, number and type of sedimentary heterogeneities).

• Total and remnant fracture width: character of remnant porosity.• Mineralization: type, character (crystal size, amorphous, slickencrysts, etc),

percent of fracture filled.• Vertical terminations: location and character, relationship to sedimentary

heterogeneity, relationship to core surface.• Strike and dip (dip azimuth): absolute if possible, relative to other fractures

and stress indicators if not.• Type of fracture: direction of separation/offset• Surface ornamentation beneath mineralization: slickensides, plumose

structure, etc:planarity: orientation of linear features, including orientation relative to fracture plane.

• Fracture height: note if this is a minimum height due to fracture exiting core, or due to missing core pieces, and termination(s) were therefore not observed.

• General fracture character: single, en echelon, anastamosed, etc.• Depth of fracture, and position within bed.• Spacing between fractures• Number of fracture sets, relationship to nearby fractures

Comparison of Types of Data Measurable in Core from Vertical and Deviated Wells Lorenz and Hill

Measurable Characteristic

SpacingTotal widthRemnant aperture/MineralizationFormation Fracture PorosityFracture PermeabilityStrike and DipNumber of Fracture SetsVertical Termination LocationsHeightPreferred Host RockSurface MorphologyFracture Type

1Qualitative estimate may be possible2May not require oriented core3Requires sufficient core

Vertical Core

-xx-1

xxxx3

x3

x3

xx

Deviated Core

xxxxxx2

x----x3