Peng S., Zhang J., Engineering Geology for Underground Rocks
Подождите немного. Документ загружается.
3.6 Numerical modeling of in-situ stress 65
Fig. 3.13. Maximum principal horizontal stress contours calculated by numerical
method. The coordinates are distances in m, and contours are maximum horizontal
vertical stress changes in MPa. Negative value in this calculation means compres-
sive stress.
Fig. 3.14. Minimum principal horizontal stress contours calculated by numerical
method. The coordinates are distances in m, and contours are minimum horizontal
stress changes in MPa. Negative value in this calculation means compressive
stress.
Fig. 3.15. Maximum shear stress contours calculated by numerical method. The
coordinates are distances in m, and contours are shear stress changes in MPa.
Negative value in this calculation means compressive stress.
66 3 In-situ stress and pore pressure
3.7 In-situ stress measurements
In-situ stress is a non-uniform stress field and is one of the most important
aspects in engineering geology. In-situ stress measurement is one of the
most accurate methods to obtain the in-situ stress magnitudes and direc-
tions. Therefore, the measurement of in-situ stress magnitudes and direc-
tions is important for underground engineering, such as mine exploitation,
oil and gas exploration and development, geothermal development, and
civil engineering.
In-situ stress measurements can be carried out in deep boreholes from
the surface, using hydraulic fracturing or LOT techniques, as described in
Sec. 3.3 of this chapter. The in-situ stress measurement can also be con-
ducted from underground access using overcoring methods and acoustic
emission method (Peng and Meng 2002).
In-situ stress measurements are to measure the undisturbed three-
dimensional stress state in formations. The three-dimensional stress state at
any point inside the rock formation can be expressed by six independent
stress components (
V
x
,
V
y
,
V
z
,
W
xy
,
W
yz
,
W
zx
) as shown in Fig. 3.16. From the
six stress components, the magnitudes and directions of three principal
stresses can be calculated. The three principal stresses (
V
V
,
V
H
,
V
h
) are usu-
ally called the in-situ stress.
Fig. 3.16. Three-dimensional stress components in any point inside of rock forma-
tions.
Any system utilized for estimating the in-situ stress state must involve a
minimum of six independent measurements. There are methods of direct
3.7 In-situ stress measurements 67
stress measurement and methods of estimating the stresses via various in-
direct or indicator methods (Hudson and Harrison 1997).
The four direct methods recommended by the ISRM (Kim and Franklin
1987) are shown as follows:
1. The flatjack test.
2. The hydraulic fracturing test.
3. The United States Bureau of Mines (USBM) overcoring torpedo.
4. The Commonwealth Scientific and Industrial Research Origination
(CSIRO) overcoring gauge.
In this book the overcoring method is introduced.
3.7.1 Overcoring method
Overcoring stress measurement is conducted by drilling a borehole into the
formation, then a stress meter or deformation/strain gauge is installed to
measure stress or strain. The following procedures are normally used for
overcoring measurements.
1. A borehole of the 130-150 mm overcored is drilled into the forma-
tion to certain depth, where the rock is not disturbed, as shown in
Fig. 3.17 (1).
2. The bottom surface of the borehole is finished by using a tapered
bit. A 40-mm-diameter pilot borehole of about 50 cm length is
then drilled from the bottom center of this overcored tapered bore-
hole, as shown in Fig. 3.17 (2).
3. The stress meter or strain gauge is inserted into the 40 mm diame-
ter pilot borehole using the insertion rods fitted with insertion de-
vice and direction device (Fig. 3.17 (3)).
4. Overcoring operation using double core tube is started after the in-
sertion rod with the insertion device is extruded, as shown in Fig.
3.17 (4).
5. After the core is recovered with the stress meter or strain gauge in-
side, field experiment to evaluate the elastic properties of the rock
is carried out. Then the stress meter or strain gauge is taken out
from the core, data recorded by the data-logger in the stress meter
or strain gauge are downloaded for in-situ stress calculation.
68 3 In-situ stress and pore pressure
Fig. 3.17. Schematic drawing of the overcoring in-situ stress measurement.
3.7.2 Case application
In-situ stress measurement in Huainan Coalfield
Hydraulic fracturing and overcoring methods were applied to determine in-
situ stress in Huainan Coalfield. The measurement results of in-situ stress
are listed in Table 3.2 (Peng and Meng 2002). It shows that the ratio of the
maximum horizontal principal stress to vertical stress ranges from 0.82-
1.82 with an average of 1.23, when the depth of the cover is less than 600
m. When the depth is greater than 600 m, the ratio is 0.51-1.33 with an av-
erage of 0.91. Figure 3.18 presents the measurement results of the maxi-
mum horizontal principal stress to vertical stress ratio (Ȝ). Field measure-
ments also show that the two horizontal stresses are not equal, and the ratio
of two horizontal principal stresses ranges from 0.54 to 0.95 with an aver-
age of 0.74.
The maximum in-situ stress in Huainan Coalfield is the vertical stress,
thus this area belongs to normal faulting stress regime without the impact
of active tectonic activity. The direction of maximum horizontal principal
stress is EW, and changes to N49-N55q due to the influence of faults
(Peng
and Li 1996).
3.7 In-situ stress measurements 69
Table 3.2. In-situ stress measurements in Huainan Coalfield, China
Location Depth
(m)
Measure-
ment
method
Vertical
stress
(MPa)
Max. horizon-
tal stress
(MPa), direc-
tion
Min. horizon-
tal stress
(MPa), direc-
tion
Panji
Mine,
Huainan
550 Overcoring 16.47
13.43
N78.94q
7.22
N124.21q
Xin-
zhuangzi
Mine
650 Overcoring 19.50
15.70
N0q
12.80
N101q
Xinji
Borehole
# 201
350.7 9.47
14.20
N53q
9.50
373.6 10.09
10.10
N49q
7.72
460.1 12.42 14.90 10.20
553.46 14.94 26.63 16.53
632.63 17.08
8.63
7.93
666.6
Hydraulic
Fracturing
18.0
10.36
N52q
9.86
Xinji
Borehole
#1202
555.18 14.99 18.78 13.52
589.74 15.92 16.74 12.84
652.72 17.62
23.52
N51q
16.52
718.64 19.40 20.78 15.28
757.6 20.46
19.37
N55q
14.87
845.1
Hydraulic
Fracturing
22.82 25.97 18.84
70 3 In-situ stress and pore pressure
Fig. 3.18. The ratio of maximum horizontal stress to vertical stress versus the
depth obtained by in-situ stress measurement in Huainan Coalfield, China.
In-situ stress measurement results in China
According to the in-situ stress measurement in China, the maximum hori-
zontal and minimum horizontal principal stresses have the following linear
relationships with depth, when the burial depth is less than 500 m:
z
H
049.0)5.25.4( r
V
(3.32)
z
h
030.0)0.15.1( r
V
(3.33)
where
V
H
and
V
h
are the maximum and minimum horizontal stresses re-
spectively, in MPa; z is the depth in m.
The average horizontal stress (
V
a
), in MPa, and depth, in m, have the
following relationship:
z
a
041.072.0
V
(3.34)
Figure 3.19 shows the average horizontal to vertical stress ratio and
depth relationship. The vertical and horizontal stresses have the following
relationships:
3.7 In-situ stress measurements 71
4.1150/ z
vH
V
V
(3.35)
5.0128/ z
vh
V
V
(3.36)
where z is the depth in m.
Figure 3.20 presents the measured maximum stress and depth relation in
some coal mines in central and northern China. It shows that the maximum
stress increases as the depth increases. The maximum horizontal stress
magnitude in those coal mines is small compared to other regions.
Fig. 3.19. The average horizontal stress to vertical stress ratio versus depth ob-
tained by in-situ stress measurement in China.
72 3 In-situ stress and pore pressure
Fig. 3.20. The maximum horizontal stress and depth relationship measured in
coal mines in central and northern China.
References
Addis MA, Last N, Yassir N (1996) Estimation of horizontal stresses at depth in
faulted regions and their relationship to pore pressure variations. SPE forma-
tion evaluation 48 (3):11-18
Barton CA, Zoback MD, Burns KL (1988) In-situ stress orientation and magni-
tude at the Fenton geothermal site, New Mexico, determined from wellbore
breakouts. Geophys Res Lett 15(5):467-470
Breckel IM, von Eekelen HAM (1982) Relationship between horizontal stress and
depth in sedimentary basins. J Petro Tech, September, 2191-2199
References 73
Bredeoeft JD, Wolf RG, Keys WS, Shutter E (1976) Hydraulic fracturing to de-
termine the regional in situ stress field in the Piceance Basin, Colorado. Geol
Soc Am Bull 87:250-258
Eaton BA (1975) The equation for geopressure prediction from well logs. Paper
SPE5544
Edwards ST, Bratton TR, Standifird WB (2002) Accidental geomechanics – cap-
turing in-situ stress from mud losses encountered while drilling. SPE/ISRM
78205 presented at the SPE/ISRM rock mechanics conference held in Irving,
Texas
Fjær E, Holt RM, Horsrud P, Raaen AM (1992) Petroleum related rock mechan-
ics. Elsevier
Haimson BC, Fairhurst C (1970) In situ stress determination at great depth by
means of hydraulic fracturing. Rock Mechanics - Theory and Practice, Somer-
ton WH (ed). Am Inst Mining Eng pp559-584
Hoek E, Brown ET (1980) Underground excavations in rock. Inst Mining and
Metallurgy London
Hubbert MK, Willis DG (1957) Mechanics of hydraulic fracturing. Petro Trans
AIME 210:153-163
Hudson JA, Harrison JP (1997) Engineering rock mechanics, an introduction to
the principles. Pergamon
Ito T, Sato A, Hayashi K (1997) Two methods for hydraulic fracturing stress
measurements. Proc Int Sym Rock Mech, Kumamoto, Japan. AA Balkema
pp127-132
Kim K, Franklin JA (1987) Suggested methods for rock stress determination. Int J
Rock Mech Min Sci Geomech Abstr. 24:53-74
Knowledge Systems (2001) Best practice procedures for predicting pre-drill geo-
pressures in deep water Gulf of Mexico. DEA119 JIP report
Matthews MD, Standifird W (2006) Forward model wellbore pore pressures.
Hart's E & P 79(11):77-79
Madge C, Zhang J, Standifird WB (2006) Predict predrill pressure and stress.
E&P, May, pp81-83
Peng S (1995) Geological modeling techniques for longwall mining roof stability:
a case study. Proc Rock Mechanics and Strata Control in Mining and Geotech
Eng, Beijing
Peng S, Li Y (1996) Wedge-shaped sandstone roof strata and roof stability, exam-
ples from Seam 13-1 in Huainan Coalfield, China. Proc for Rock Mechanics
and Strata Control in mining and Geotech Eng Xi’an
Peng S (1998) Approach on mechanical properties of clastic rocks and rock micro-
structure. Proc. 5
th
Ann Conf of Young Scientists, China Coal Society. Coal
Industry Press (in Chinese)
Peng S (1999) The state-of-art and the future work of engineering disasters in-
duced by coal mining at deep depth in China. Symp Engineering and Disaster,
Dept Material Science and Engineering, China NSF, Mt Jiuhua, Jianxi (in
Chinese)
Peng S, Meng Z (1999) Numerical modeling on coal roof and floor stability in
longwall mining. J China Univ Min Tech 28(1):41-45 (in Chinese)
74 3 In-situ stress and pore pressure
Peng S, Meng Z (2002) Theory and practice of mining engineering geology. Geo-
logical Press (in Chinese)
Reinecker J, Heidbach O, Tingay M, Connolly P, Muller B (2006) The 2006 re-
lease of the world stress map
Rutqvist J, Tsang C-F, Stephansson O (2000) Uncertainty in the maximum princi-
pal stress estimated from hydraulic fracturing measurements due to the pres-
ence of the induced fracture. Int J Rock Mech Min Sci 37:107-120
Standifird WB, Matthews MD (2005) Real time basin modeling: improving geo-
pressure and earth stress prediction. Paper SPE 96464 presented at Offshore
Europe 2005 held in Aberdeen, Scotland, UK, 6–9 September
Standifird WB, Paine K, Matthews MD (2004) Improving drilling success requires
better technology and models. World oil 225(10):51-56
Williams K, Madatov AG (2005) Analysis of pore pressure compartments in ex-
tensional basin. GCSSEPM 25th Ann Research Conference: Petroleum Sys-
tems of Divergent Continental Margin Basins, pp862-909
Zhang J, Standifird WB, Adesina K, Keaney G (2006) Wellbore stability with
consideration of pore pressure and drilling fluid interactions. Paper ARMA
/USRMS 06-922 presented at 41
st
US Symp Rock Mechanics held in Golden,
Colorado
Zoback MD, Zoback ML (1989) Stress in the earth’s lithosphere. Encyclopedia of
Earth Sciences Series (Fairbridge RW, Series Ed), Van Nostrand Reinhold
Co, New York pp 1221-1232
Zoback MD et al. (2003) Determination of stress orientation and magnitude in
deep wells. Int J Rock Mech Min Sci 40:1049-1076