a. field techniques b. lab techniques c. calculate from...

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.


© Copyright, 2011

A. Field Techniques

Summary of Pre- and post-fracturing tests for determining extension and closure pressures SPE Monograph Vol 12(1989)


© 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)


© Copyright, 2011

A. Field Techniques


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


© Copyright, 2011

A. Field Techniques


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)


© Copyright, 2011

A. Field Techniques


Example:

“Ideal” stress test data with obvious ISIP SPE Monograph Vol 12(1989)


© Copyright, 2011

A. Field Techniques


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


© Copyright, 2011

A. Field Techniques


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


© 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)


© 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)


© Copyright, 2011

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)


© 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)


© Copyright, 2011

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)


© Copyright, 2011

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)


© Copyright, 2011

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

a. field techniques b. lab techniques c. calculate from...

Documents