depth conductor

8/18/2019 Depth Conductor

1/10


2/10

2

ASSESSMENT OF CONDUCTOR SETTING DEPTN

OTC 671:

3.

The packer is inflated to seal the test sec-

tion, and a wireline dart is lowered to the bit

to measure pressure during the test.

4.

The test section is

pressurised by pumping

fluid into the drillstring at a given flow rate

and the test performed as outlined on Fig. 2.

The test is therefore generally performed in a

pre-drilled section, and is flow controlled. The

measurement made during the test include the fol–

lowing:

1.

The initial fracture pressure.

2.

The steady state pressure

rate.

3. The close up pressure after

4.

The re-fracture pressure.

under a given flow

initial fracture.

Whilst all stages of the HFT

test may be used

to infer geotechnical soil parameters, this paper

concerns

the pressure required to cause

initial

fracture, which is generally adopted as the limit of

allowable

excess fluid pressure during drilling

operations.

Initial fracture pressures from such

tests have been used to check the theoretical method

presented in the following.

THEORETICAL BACKGROUND

Minor Principal Stress Approach

The hydrostatic,

drilling

fluid and in situ

soil pressures during drilling for the first casing

string

are shown

schematically on Fig. 3.

The

drilling fluid pressure which may be expected to

fracture the soil formation has been the subject of

analysis by Bjerrum et al.

(1972).

This approach

was developed following obaervationa of fracture

occurring

whilst

installing push-in piezometers.

The excess pressure, Au, required to cause a ver-

tical crack in the soil as derived by Bierrum et al.

is given by:

Au =

or Au =

where v =

CY=

Po’ =

k=

o

.-

Po’(l/v-l) [(l-~)ko + Pt’/po’] ... (1)

po’(l-v) [(2+&c@ko + pt’/pol] ... (2)

Poisson’s Ratio of the soil

effect of installation on circumferen-

tial stress

effect of installation on radial stress

vertical effective stress in-situ

coefficient of lateral earth pressure

at

tensi

est

e stress sustainable by the soil

Equation 1 represents the case of fracture

occurring prior to blow off, and equation 2 repre-

sents fracture occurring following blow off of the

soil from the piezometer.

This assumption of a “perfect” installation and

a Poisson’s ratio of 0.5 (undrained response) re-

duces equations 1 and 2, respectively, to:

Au = ko.p ‘ + pt’

o

... (3)

or Au = ko.p ‘ + pt’/2

... (4)

o

Bjerrum also notes that there is a possibility

of a horizontal crack forming if the excess head

exceeds p ‘.

In his recommendations on allowable

pressures,”derived from theory and field and labora-

tory observations,

the tensile stress ptt was con-

servatively ignored.

These results then reduce to

the assumption adopted by many oil companiea in

estimating required conductor setting depth, that an

excess pressure equal to the lower of the principal

stresses in the ground should be assumed to cause

hydrofracture, i.e.:

Au=p’

o

(ko>l)

... (5)

Au = ko.p ‘

o

(ko


3/10

WC 6713

ALDRIDGE AND HAIMD

?

tial stress falls to zero or to any given value of

tensile stress,

pt’, as follows:

Au = 2,ko.p ‘ + uh + ptt

o

... (7)

where

‘h

= hydrostatic pressure at the given depth

Equation 7 is consistent with the results given

by Jaeger (1969) for a porous elastic medium, if the

permeability of the medium is set to zero.

It may

therefore be expected that hydrofracture will occur

at the preseure given by equation 7; unless a gen-

eral shear failure of the clay at the wall of the

borehole occurs at a lower pressure.

Examination of

the three principal stresses given on Fig. 4 may be

used to calculate the maximum deviator streea in the

clay

material.

If the maximum deviator stress

exceeds twice the undrained shear strength of the

clay, then a plastic failure of the borehole wall

may be expected to occur.

The deviator stresses

derived from the vertical (v), radial (r) and cir-

cumferential (c) principal etresses are as followe:

or-u

= Au- p ,

v

o

.,. (8)

Ur - IJC

= 2.Au -

ko.po’

... (9)

Uv-u

‘Au+p ’-2,ko.p ’

c

o

0

... l o

Shear

failure will occur when any of these

deviator stresses exceeds twice the undrained com-

preaaive ahear strength of the soil.

Using equa-

tions 81 to 10 it is therefore possible to derive

eqUatiOnS 11 to 13,

respe tively

defining the

eXCeSS

fluid pressure which would cause a shear

failure in the borehole wall:

Au = 2.s

u + Po’ ... (11)

Au= S + k

u

O.PO’ . . . (12)

Au = 2.au + po’(2ko-1)

... (13)

where s

= undrained ehear strength in compression

u

An alternative possible mode of failure to that

considered above is a uniform cavity expansion,

for

which the excees pressure to cause failure is of the

order of

5.7 to 6.3 times

the undrained shear

strength, depending on the overconsolidation ratio

of the clay (Randolph et al. (1979)).

It ia poaaible that a ahear failure, as given

by the lowest of equations 11 to 13, will occur

prior to the tensile failure given by equation 7.

There are assumptions inherent in both theoretical

approaches, however. Observations made during

drilling for the first casing string are therefore

reviewed below to assess the validity of each ap-

proach.

The results of these approaches are also

compared

with

the method traditionally used, as

given by equations 5 and 6.

method.

RESULTS OF FIELD TESTS

A review has been made of the reeults of 34

hydraulic fracture tests ‘(HFT’s) performed in pre-

drilled sections in geotechnical boreholes performed

during platform

site-investigations in

the North

Sea,

The teste were performed at aix sites, at

depths of between 40 and 140 metres below mudline,

in hard clay strata. Of the 34 tests, three resul-

ted in very high fracture pressures close to

those

expected from cavity expaneion theory, as previously

reported by Overy and Dean (1986).

Three resulted

in anomalously low pressures, believed to have been

due to leakage around the packer.

Data from the

remaining 29 tests are reviewed below.

The predicted and meaaured test results are

compared for the six test sites on Fig. 5.

The

dashed line representa the calculated minor princi-

pal stress and the solid line is the lowest of the

preseures

derived from equations 11 to 13.

The

results chow graphically that the “shear failure

approach gives a closer fit to the HFT test data

than the traditional “minor principal stress method

at these sites.

The ratio of measured to calculated fracture

pressure has been plotted for the traditional ap-

proach, i.e.

equations 5 and 6,

on Fig, 6, and for

the pressures given by the lowest of equations 11 to

13 on Fig.

7, for all 29 sites.

The valuee given by

equation 7 are not plotted, since they are alwaYs

higher than the values given by equations 11 to 13,

and the

“shear failure”

mechanism may therefore be

assumed to control.

It may again be seen from Figs.

6 and 7 that the “shear failure ’approach repre-

sented by equations 11 to 13 gives a significantly

better overall fit to the data than the “minor

principal stress” method.

Observations of drilling mud pressures and

returns or lack of returns during offshore drilling

operations are not generally available to the geotech-

nical consultant.

Records obtained during drilling

from a semi-submersible drilling rig at the Draugen

site in the Norwegian Sea are presented on Fig. 8.

This figure shows the mud preaeure actually applied

during drilling and the estimated fracture pressure

based on the

“minor principal stresstt

and

“shear

failure” methods.

Again this data confirms that the

“ahear failure”

approach gives

results which are

more coneiatent with observations,

since although

excess drilling fluid pressures exceeded those given

by the “minor principal stress” approach, no IOS5 of

returns waa encountered.

The results of a statistical analysis of the

data are also presented on Figa. 6 and 7, and show

that the measured test pressures

are on

average

almost

exactly double those given by the “minor

principal strees”

method but only 34 per cent higher

than given by the “shear failure” method.

The

statistical correlation,

as measured by the standard

deviation,

is also better for the ‘rehearfailure”

169


4/10


5/10

)TC 6713

A.LDRII GENI

10

2.

3.

4.

5.

6.

Bjerrum, L.,

Nash, J.K.T.L., Kennard, R.M.

and Gibson, R.E. (1972), “HydraulicFractur-

ing in Field Permeability Testing , Geotech-

nique Vol. 22, No. 2, pp. 319-322.

Den Hartog, J.P. (1952), “AdvancedStrength

of Materialsit,McGraw-Hill.

Jaeger, J.C. (1969), “Elasticity, Fracture

and Flow”, Halsted Press.

Randolph, M.F., Carter, J.P. and Wroth,

C.P. (1979),

llDri~enpilee in Clay - ‘he

Effects of

Installation and

Subsequent

Consolidation”,

Geotechnique Vol. 29, No.

4, pp. 361-393.

Overy, R.F. and Dean, A.R. (1986), “Hydrau-

lic Fracture Testing of Cohesive soil”.

Proc.

Offshore

Technolozv

Conference.

Paper No. OTC 5226.

Poulos, H.G.

and

“Elastic

Solutions

Mechanicst .

Series

John Wiley and Sons.

-.

Davis, E.H.

(1974),

for

Soil and

Rock

in Soil Engineering,

.

—SIGNAL CABLE

—~ F T DART

_SLllJING VALVE

FOR PACKER

—PRESSURE DROP VALVE

—PRESSURE SENSOR

—ROUGH HOLE PACKER

-OPEN BIT

iil

,.

Fig.1 HydraulicFractureTestEquipment

I

I

1

I I

I

I I

I I

I I

I

,,

TJUF

II

RF

STEAOY STATE

PREsSUBE

1

I

1’

CLOSE UP

PRESSURE

800

I ‘i

k

I

/ “

4

600

I

\

? w P

‘

\ <

400

T

200

PUMP OFF -

+

PUMP O J

b +

PUMP OFF

o

0

4

a

12

16

20

24

28

32

36

40

TIME AFTER START OF TEST (rein)

Fig. 2 Hydraulic Fracture Test Procedure

171


6/10

.

-1

IO

200

0

-200

-a

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

‘, ‘, ‘, ‘1 ‘1 ‘1

I

I I

I I

I

I

I I I

1 1 l,i, i l~ll

l,l,

,, 1 :’ “’”

II

,1,1, ,,11, ,

II

11,,,,1,,1’~ ~~

II

II II II

,’,,,,

I

I I

I I

I

I

I I I

1, 1, 1, 1, 1, 1,i,1,1; 1 1,1 ,1 ,

,, 1 :

““

,, II

II J ,1

II

II II II

I I

I I

I I

I

I I I

I

I

1 1 1,1, i lll 1 1 l,

,, : “’’”

II

II

II

1, i II II

,, ~ ~ ~ ~

II

II

II II II II II

I I

I I

I I

I

I

I I

I 1, 1, 1, i, i , 1 1 1 , 1 , I

I

,; :” ““

II

II II II

1,1,II II ,,

~ ~ ~ ~

II II II II II II

,,

I

I I

I I

I I I

I I I

I

I

1,1, i, , i lll

l,l,

,, 1 : “’’”

II

II II II II II

II

I I I I

II II II 1’

I I

I I

I I I

I I I I

1 1 l,i, i l ill l,l,

,, 1 ’ “’”

II

II II II II II II II

~ ~ ~

II II II II II II

I

I I

I I

I

I

I I I I I

l,l,l,i,i,l~l~,~,~,

,,, 1 : “’’”

II

II II

) ;

II

II

II II II II

I

I I

I I

I

I

I I I

1, 1, 1, 1, i,

1 , 1, 1

1

1,1,

,, 1 : “’’”

II II II ‘1

II II II II

1, 1, 1, 1, 1,i,1,1; 1,1,1 ,1 ,

;“’’’’l lllllllllll l

““

“““““““““

“““

1 l,i, i lll 1 1 l,

,,

: “’’”

II

II II II II II ~

II

II

II II II II II

I

I

I I

I I

I

I I I

I

I

1, 1, 1, i, i, 1 ~

1

1 , 1 , I

,, 1 : “’’”

II

II II II II II

II II II II II II

‘1

I

I I

I I

I I I

I I I

I

I

1,1, i, , 1 111

l,l,

,, 1 “’’”

II

II II II II II

II II II II II II

I I

I I

I I I

I I I I

1 1 l,i, i l ill l,l,

,, 1 1’ “’”

II

II II II II II II II

~

II

~

II II II II II II

I

I I

I I

I

I I I I

1 1 l,i, i lll 1 1

l,

,,,

1 : “’’”

II

II II

‘1

II

II II II

I

I I

I I

I

I

I I I I

1 1 1,1, i llll l,l,

,, 1 : “’’”

II

II II

III II II

I I

I I

I I

I

I I I

I

I I

1 1

1,1,

i lll 1 1 l

1 : “’’”

II

I

, ,1

,,,,,,,

11111, 1111

,’,,,

I I

I I

I I

I

I

I I

I 1, 1, 1, i, i ,

I ~

1,

1 , 1, I

I

,; :” ““

II

II II II

1,1, l’,II ~

II II II II II II

‘1

I

I I

I I

I I I

I I I

I

I

1, 1, i, , i lll

l,l,

,, 1 : “’’”

II

II II II II II

)

I

11111111111111

‘1

‘1

I I

I I

I I I

I I I I

1 1 l,i, i l ill

l,l,

,, 1 ’ “’”

II

II II II II II

II II II II II II

I

I

I I

I I I I

I I I I I

1, I, I, i, i, 1, ~ 1

I 11

,, 1 ’ “’”

,, 11

-ma

-ma

-1000

I

1

1 I

i

/l

I

TOTAL STRESSESIN-SITU

koPo’+ Uh

TOTAL STRESSES DURING ORILL ING

UhdJ

Fig.3 Hydrostatic,Mud-umand

InsituSoilStresses

Fig.4 Changes inTotalSoilStresses

DuetoDriUingOperations


7/10

EXCESS FRACTURE PRESSURE (MPal

40

60

80

100

120

140

0

1

z

‘Y\

\

1

\

\

\

o

i

2

50

I 1

I

70

I

I

I

‘*

I

90

1 ,.

1

x

110

0

i

2

00

\

\

100

\

\

\

120

\

w

\

\

D

140

EXCESS FRACTURE PRESSURE (MPa)

o

2

40

60

80

100

120

140

1-

1

I

I

I

)

1

I /

I

I

1

I

I

I

I

I

I

I

m

60

80

100

120

0

2 4

\

\

t

I

I

I

1

I

o

2

A

80

1

\

100

\

\

\

o

I

\

120

\

\ o

.

(B

140

HFT RESULTS

‘-’-- MINOR PRINCIPAL STRESS PREDICTION

— SHEAR FAILURE PREDICTION

Fig.5 Comparison ofHFI’Reaults withPredictions

173


8/10

40

60

100

120

140

RATIO MEASURED/CALCULATED

o

1 2 3 4

5 6

-1 SD MEAN

+1 SD

I

I

[

I MEAN =

1.99

I

I

I

S.D. =

4

0.52

I

I

1

b

I

1: ~

10

I

I

I*

1

I

I

I I

1°

‘1

P

q

I

I

I

‘1

I

I

Fig. 6 Ratio of Measured to Calculated Fracture Presaure-

Minor Principal Stress Method

RATIO MEASURED/CALCULATED

o i

2

3

4

5 6

I

-1

SD MEAN

+1 SD

I

I

I

1

I

L

I

MEAN =

1.34

-1

I

S.D. = 0.32

60

.1

I

1°

m

I

I I

I

I I

ao

I

1010’

dw I I

D

I

{

I

a

I

100

18

I

-0

I

120

1-

1 I

I

I

I

I

f 10

Ill

140

J

r

I I

I I

I

1

Fig. 7 Ratio of Measuxwl to Calculated Fracture Pres.sure-

Shear-Failure Method


9/10

I

(-ah

&—._

1H--l Hl---

r Y

—.

n

.“

‘H

Ill .—

—. —.

g

2? –

c ”

. : ; -

1/:

::--H-I-W+H47H:H4

-.-—._. ,._.

..

4-’”:F

“l’’t-+’=t’;--’l:+:F+l


10/10

.

.

.

—.—

.. ..—

—...—.

=n

g“

~ ..-_._ . . —.“-

~.-—-–——

—..——

z

8

:. . .._ .

“

8

$

—.

s

s.

;

)

,/’

-

-= .>=

-—

-.. — . . .- -

“8

s

W 11MV4 lk01 30 H1d30

176

gl

e

r%

1+

.

.-

2

depth conductor

Documents