stimulation 3

8/10/2019 Stimulation 3

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Methods to determine insitu stress

A. Field techniques

B. Lab techniques

C. Calculate from elastic properties


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Stimulation hydraulic fracturing

Copyright, 2011

Idealized surfacepressure during hydraulic fracture treatment

(Allen & Roberts, 1982)

Net fracture pressure pressure in fracture in excess of closure pressure

Dp= Pf - Pc

Pressure

Time

Pad Volume Sand Placement in Fracture Frac

Closure Time

Breakdown

S

tartSand

Sandto

perforations

Sh

utdown

pumping

Fractureclosed

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 tensilestrength 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.


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A. Field Techniques

Summary of Pre- and post-fracturing tests for determining

extension and closure pressures SPE Monograph Vol 12(1989)


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A. Field Techniques

1. Hydraulic Fracture Stress-Test Procedure or Microhydraulic fracturing testAnalysis:

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

Tpp2yx3

lowerb

p

rockofstrength

tensile

pressure

pore

stressborehole

induced

pressure

breakdown


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Copyright, 2011

A. Field Techniques

1. Hydraulic Fracture Stress-Test Procedure or Microhydraulic fracturing testAnalysis:

after pumping the pisipsx slightly greater than minimum principal stress

(assuming negligible borehole effect)

Repeat a second cycledifference is loss of tensile strength due to presence

of fracture. Resulting in 3 equation and 3 unknowns (sx, sy, T)


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A. Field Techniques

1. Hydraulic Fracture Stress-Test Procedure or Microhydraulic fracturing testExample:

Ideal stress test data with obvious ISIP

SPE Monograph Vol 12(1989)


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A. Field Techniques


From the ideal stress test the breakdown pressure, pb

was observed to be 8620 psi and the minimum horizontal

stress, shmin= sxwas measured to be 8225 psi. Other

parameters are:

Pore pressure, pp = 6800 psi

Vertical stress, sv = 8465 psi

Biotsconstant, = 1

Poissons 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


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A. Field Techniques


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

s

s

ss

where

psiy

y

Tppyxlowerb

p


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A. Field Techniques

2. Steprate testExample:

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 ftInjected-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 thebottom-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)


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A. Field Techniques

2. Steprate/Flowback testObjective:

preferred for determining closure

pressure.measures entire interval

Procedure:

Inject fluid and create fractureflowback at constant rate

Trial and error to find appropriate

rate, 1/10 to 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 testsSPE Monograph Vol 12(1989)


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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)


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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 imprintof 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)


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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

ppppv1min,h s

s

stimulation 3

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