Post on 03-Apr-2018
Methods to determine insitu stress
A. Field techniques
B. Lab techniques
C. Calculate from elastic properties
Stimulation hydraulic fracturing
© Copyright, 2011
Idealized surface pressure during hydraulic fracture treatment (Allen & Roberts, 1982)
Net fracture pressure • pressure in fracture in excess of closure pressure
Dp = Pf - Pc
Pre
ssu
re
Time
Pad Volume Sand Placement in Fracture Frac Closure Time
Bre
akd
ow
n
Star
t Sa
nd
San
d t
o
per
fora
tio
ns
Shu
t d
ow
n
pu
mp
ing
Frac
ture
clo
sed
Tubing friction pressure loss
Fracture Closure Pressure-Hydrostatic
Reservoir Pressure-Hydrostatic
Constant pump rate, increasing sand concentration Pressure rise reflecting normal frac extension
Breakdown Pressure • the pressure required to initiate the fracture • Must exceed the minimum stress at the borehole and the tensile
strength of the rock.
Extension or propagation pressure • the pressure required to extend the existing fracture
Closure pressure • the pressure required to hold the fracture open • Equivalent and counteracts the minimum principal insitu stress; pc shmin
• Approximated by PISIP Pc.
Stimulation hydraulic fracturing
© Copyright, 2011
A. Field Techniques
Summary of Pre- and post-fracturing tests for determining extension and closure pressures SPE Monograph Vol 12(1989)
Stimulation hydraulic fracturing
© Copyright, 2011
A. Field Techniques
1. Hydraulic Fracture Stress-Test Procedure or Microhydraulic fracturing test
Objective: Method to measure insitu, minimum, horizontal stress Procedure: 5 to 10 gals injected at a constant rate in a packed off interval. Record p = f(time) for both pumping and falloff. Factors:
1. tested zone – uniform, thick formations 2. perforations – open, undamaged path to formation 3. pressure measurement system 4. type of fluid 5. flow rate, volume injected
6. interpretation – identification and reproducibility of ISIP
Microhydraulic fracture record Economides and Nolte (1980)
Stimulation hydraulic fracturing
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A. Field Techniques
1. Hydraulic Fracture Stress-Test Procedure or Microhydraulic fracturing test
Analysis: Assume one principal stress is parallel to borehole axis, i.e., sv. Must overcome the strength of the rock and the insitu stress concentrations upper bound due to no fluid penetration assumption. lower bound accounts for fluid seepage prior to breakdown
xyforT
pp
yx3
upperb
p ss
ss
ss
12
21
,where
12
Tp
p2yx
3
lowerb
p
rockofstrength
tensile
pressure
pore
stressborehole
induced
pressure
breakdown
Stimulation hydraulic fracturing
© Copyright, 2011
A. Field Techniques
1. Hydraulic Fracture Stress-Test Procedure or Microhydraulic fracturing test
Analysis: after pumping the pisip sx … slightly greater than minimum principal stress (assuming negligible borehole effect) Repeat a second cycle – difference is loss of tensile strength due to presence of fracture. Resulting in 3 equation and 3 unknowns (sx, sy, T)
Stimulation hydraulic fracturing
© Copyright, 2011
A. Field Techniques
1. Hydraulic Fracture Stress-Test Procedure or Microhydraulic fracturing test
Example:
“Ideal” stress test data with obvious ISIP SPE Monograph Vol 12(1989)
Stimulation hydraulic fracturing
© Copyright, 2011
A. Field Techniques
1. Hydraulic Fracture Stress-Test Procedure or Microhydraulic fracturing test
Example: From the ideal stress test the breakdown pressure, pb was observed to be 8620 psi and the minimum horizontal stress, shmin = sx was measured to be 8225 psi. Other parameters are: Pore pressure, pp = 6800 psi Vertical stress, sv = 8465 psi Biot’s constant, = 1 Poisson’s ratio, = 0.229 Tensile strength, T = 215 psi Calculate an upper and lower bound for the maximum horizontal stress, shmax = sy
psiyy
Tppyxupperbp
94752156800)8225*3(8620
3
ss
ss
Stimulation hydraulic fracturing
© Copyright, 2011
A. Field Techniques
1. Hydraulic Fracture Stress-Test Procedure or Microhydraulic fracturing test
Example: Calculate an upper and lower bound for the maximum horizontal stress, shmax = sy
psiyy
Tppyxupperbp
94752156800)8225*3(8620
3
ss
ss
35.012
21
,
8955)35.1(2
215)6800)(35(.28225*38620
12
23
ss
ss
where
psiyy
Tppyxlowerb
p
Stimulation hydraulic fracturing
© Copyright, 2011
A. Field Techniques
2. Steprate test
Objective: 1. finds upper bound for minimum stress (closure
pressure?), 2. determines range of pump rates for fracture extension
Procedure: 1. Fluid rate is progressively increased and a stabilized
pressure recorded. 2. Performed after first cycle to eliminate borehole
effects,e.g., breakdown pressure 3. Pressure measurement location: surface, downhole in
annulus
Analysis: 1. A change in slope identifies the fracture extension
pressure > closure pressure because of fluid friction in fracture and the fracture toughness.
2. Extrapolation to zero rate should coincide with reservoir pressure
Step rate injectivity test Earlougher (1977)
Stimulation hydraulic fracturing
© Copyright, 2011
A. Field Techniques
2. Steprate test
Example: Given a reservoir with the following properties: Bw = 1.0 RB/STB mw = 0.45 cp h = 270 ft f = 0. 186 ct = 1.5 x 10-5 psi-1 rw = 0.25 ft Depth = 7,260 ft Injected-fluid pressure gradient = 0.433 psi/ft
Determine the fracture gradient. The break in the data indicates a surface fracture pressure of about 1,000 psi. The fracture gradient is estimated by dividing the bottom-hole fracture pressure by the depth. The fracture gradient is: [(0.433)(7,260) + 1,000]/7,260 = 0.57 psi/ft
Step rate injectivity test Earlougher (1977)
Stimulation hydraulic fracturing
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A. Field Techniques
2. Steprate/Flowback test
Objective: • preferred for determining closure pressure….measures entire interval Procedure: • Inject fluid and create fracture • flowback at constant rate •Trial and error to find appropriate rate, 1/10 to ¼ of average injection rate Analysis: Pressure decline exhibits characteristic reversal in slope at closure pressure. Caused by flow restriction introduced when the fracture closes.
Application of step-rate and pumpin/flowback tests SPE Monograph Vol 12(1989)
Stimulation hydraulic fracturing
© Copyright, 2011
A. Field Techniques
2. Shutin/decline test
Objective: Closure pressure from slope change on plot….not easily identified or unique Procedure: Record pressure decline vs time function after injection. No flowback, hence shutin
Example of post-frac pressure decline to determine closure stress (Allen & Roberts, 1982)
Stimulation hydraulic fracturing
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B. Lab Techniques
1. Anelastic Strain Recovery (ASR) Objective: Obtains orientation of principal stress. Procedure: • Sensitive strain measurements are
obtained on retrieved oriented core. • Measures the volume change of core as
pulled from the surface. Analysis: • The strain orientation is assumed the
same as the principal axes of the insitu stresses.
• The time-dependent strain and total strain are directly proportional. (Economides & Nolte, 1980)
Stimulation hydraulic fracturing
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B. Lab Techniques
2. Differential Strain Curve Analysis (DSCA) Objective: Obtains orientation of principal stresses. Analysis: • Based on strain relaxation as an imprint
of the stress history • Relies on the assumption that the
resulting microfracturing is directly proportional to the stress reduction the core has sustained
(Economides & Nolte, 1980)
Stimulation hydraulic fracturing
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C. Calculate from elastic properties
Objective: Obtain minimum, insitu stress magnitude, stress profile Procedure: a. core triaxial tests under various confining pressures b. combine sonic and density log measurements Analysis: Obtain elastic properties, and E and calculate the minimum horizontal stress from the following equation
pp
pp
v1min,h
s
s