08 fractured reservoir description
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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
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