apr2012_2_petrophysics for geomechanics & hydraulic fracturing_satya perumalla.pdf
TRANSCRIPT
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Influence of Petrophysical Calibrations on
Geomechanics and Resultant effects on
Hydraulic Fracturing: A Case Study.
SPWLA Kuwait Chapter – 17th April 2012
Satya Perumalla GMI Geomechanics Services,
Baker Hughes.
Introduction & Challenges
Geomechanical Setting of the Study Area in North Oman
Hydraulic Fracturing Experience from 2 Deep Gas wells
Geomechanical Investigations
Relation between Petrophysical & Geomechanical
Parameters
Conclusions, Learning & Recommendations
Outline
Introduction & Challenges (1)
• Active development of Paleozoic Amin Deep Tight Gas Sand
Reservoir in Oman.
• Reservoir depth > 5,000 m (16,000 ft) TVD.
• Low porosity: 3 – 7%, Low permeability: 0.001 – 1 mD
• High rock compressive strength: 110 – 250 MPa (15,000 –
35,000 psi).
• High Temperatures: 170° - 190°C
• Deepest well in the Middle East (> 7,000 m TVD).
Introduction & Challenges (2)
• High In-situ stresses High Fracture Gradient
• High Compressive Strength High Tensile Strength
• Structural Compartmentalization: Variable Reservoir
Pressure & Stress orientation in the region Faults.
• Presence of Natural Fractures & proven stress-sensitive
permeability.
• Production from natural fractures is not economical.
• Stimulation / Frakking is a must: for Economic viability.
Geomechanical Setting
• Strike-Slip Stress regime
SHmax >> Sv > Shmin
• Variable Pore Pressure
(Pp): Getting accurate Pp
is a challenge in this tight
formation.
• Extremely strong rocks
experiencing high
stresses.
30-35
kPa/m
~ 17 kPa/m
23-24
kPa/m
11-13
kPa/
m
110 – 250+
MPa
6
Maximum attempted
pressure is higher than
SHmax: Unsuccessful
fracturing attempt
Breakdown pressure:
Successful Fracturing
Pp
MW
Sh
min
Sv
SH
ma
x
Sh
min
+ T
en
sil
e s
tre
ng
th
Pe
rfo
rati
on
s
UCS Static Young’s
modulus (MPa)
Internal
Friction
Poisson’s
ratio
Hydraulic Fracturing Experience (Well # 1)
7
Geomechanical Investigation (Well # 1)
• Successfully Fractured Interval:
• Lowest UCS: 110 MPa possibly lowest Tensile strength.
• Consistent development of stress-induced wellbore failure
(borehole breakouts & tensile fractures)
• Slight over-pressure near the perforation interval
• Mud cooling by 10-15°C ( from 170 – 190°C) how
effective?
• Unsuccessful Interval:
• High UCS ~ 150 MPa possible high Tensile strength /
resistance.
• Inconsistent wellbore failure
• Hydrostatic pore-pressure.
• Same mud cooling as above.
8
Breakouts of ~ 60°
width at
successfully
fractured zone
Un-interpreted Image Interpreted Image
Geomechanical Investigation (Well # 1)
9
KCl attempt NaBr attempt
Attempt 1 Attempt 2
Pp
MW
Sh
min
Sv
SH
ma
x
Sh
min
+
Te
ns
ile
str
Pe
rfo
rati
on
s
UCS
Static Young’s
modulus (MPa)
Internal
Friction
Poisson’s
ratio
UCS
(MPa)
Hydraulic Fracturing Experience (Well # 2)
10
Geomechanical Investigation (Well # 2)
• No successful fracturing despite trying with various
fluids.
• Unsuccessful Intervals:
• Over all UCS of Amin is consistently stronger than Well # 1
More than 150 Mpa High Tensile strength.
• Very limited stress induced wellbore failure.
• Comparable stress magnitudes with Well # 1.
• Hydrostatic pore pressure; no over pressured zones.
11
Amin
Calipers
Geomechanical Investigation (Well # 2)
12
Petrophysics for Geomechanics
• Quality of log properties Borehole conditions, Temperature,
Mud, etc.
• Accuracy of parameter value: challenge in Tight gas
reservoirs.
• Good quality petrophysical parameters (density, porosity,
velocity, etc) Good quality rock mechanical parameters
(UCS, Poisson ratio, Young’s modulus, etc).
• Petrophysical uncertainty Rock mechanical uncertainty
Geomechanical Model uncertainty Decisions!!!
Petrophysics for Geomechanics Conventional
porosity
calibrated UCS
Perforations Perforations
UCS Calibrated
with porosity
derived from Core
Tests
14
Petrophysics for Geomechanics
• Calibration from log to rock mechanical property in multiple
stages:
• Log porosity Calibrate with core porosity & fluid type
Calibrated/Corrected porosity log curve.
• Laboratory UCS Laboratory porosity Corrected
porosity UCS curve.
• Uncertainty in corrected porosity Uncertainty in UCS.
• Uncertainty with UCS Uncertainty with Tensile Strength
• Impact on perforation decisions & Fraccability.
15
Conclusions (1)
Some observations relating Geomechanics to Hydraulic
Fracturing experience.
Well -1:
• The only successful fracturing job at a particular facies
indicated that this interval has significantly lower rock
strength of 110 MPa when compared to most of Amin
formation with average UCS of ~200 MPa.
• Significant presence of stress induced borehole
breakouts and tensile cracks supports that this interval
was a suitable candidate for hydraulic fracturing.
16
Conclusions (2)
Well -1 (continued):
• The Fracture Breakdown Pressure in this facies = Shmin +
Tensile strength: supporting sufficient pressure
contribution through perforation to initiate and propagate
the fracture.
• The other perforated intervals where hydraulic fracturing
was unsuccessful, have shown higher rock strengths of at
least 150-170 MPa, therefore possible higher tensile
strength to offer resistance for fracturing.
17
Conclusions (3)
Well -2:
• Amin in Well-2 is much tighter than in Well-1 with average
UCS of ~ 250 MPa (higher than in Well-1). As a result, the
formation appears to be having much higher tensile
strength to offer high resistance to initiate any hydraulic
fracture.
• Attempted fracturing the weakest interval of Lower Amin in
Well-2 was unsuccessful with similar operational setup as
Well-1.
18
Recommendations for Frac Jobs
• Creating tension in borehole to create tensile cracks with
higher mud cooling efforts. (BHT ~ 190°C)
• Horse Power Optimization using Geomechanics:
• Oriented perforations (sub)-parallel to SHmax (aligning
them with any present tensile cracks) would avoid loss
of pressures into unfavorably oriented perforations: to
enhance the fraccing success chances.
• Perforation interval selection based on lab calibrated
porosity/UCS and stress profiles.
• Placing horizontal wells parallel to Shmin in a weaker
zone and open hole multi-stage fraccing.
19
Recommendations: Field Development
• Mechanical characterization of Amin reservoir: Relationship
among Mechanical properties Mechanical Stratigraphy
Sedimentology. Use Scratch, Brazilian & Triaxial tests To
refine rock properties.
• Variable stress orientation & Pore pressure in North Oman
across fault blocks: Capture geomechanical variability
through: Regional Stress Maps & 3D Geomechanical Models.
• Stage wise coordination, data transfer and analyses are
important between exploration and geomechanics teams in
order to capture most important geomechanical aspects at
each stage of operation in order to provide decision support
for the next stage.
Thanks to You all!
Ministry of Oil & Gas, Sultanate of Oman.
Petroleum Development Oman &
Baker Hughes.
Acknowledgements