Progress Reports
Estimation of Minimum Principal Stress from an Extended
Leak-off Test Onboard the Chikyu Drilling Vessel and
Suggestions for Future Test Procedures
by Weiren Lin, Koji Yamamoto, Hisao Ito, Hideki Masago, and Yoshihisa Kawamura
doi:10.04/iodp.sd.6.06.008
Introduction expeditions, LOT or XLOT (which are sometimes used to
determine drilling parameters such as optimal mud density)
To understand the physics of faulting and rupture propa- have not been conducted because the borehole was open to
gation for the great M8-class Nankai earthquakes that recur the seafloor. Thus, it has been impossible to pressurize a
approximately every 100 years, a comprehensive drilling short interval of open hole below the casing as needed to
project is underway: the Nankai Trough Seismogenic Zone conduct a LOT or XLOT (see below) without utiliz-
Experiment (NanTroSEIZE; Tobin and Kinoshita, 2007),, ing time-consuming and frequently unreliable
which is part of the Integrated Ocean Drilling Program drill-pipe-deployed packers. In contrast, the new drilling
(IODP). Stress levels along seismogenic
eismogenic
ismogenic faults must be vessel Chikyu provides a riser-drilling capability that allows
known in order to understand processes controlling the pressuring the entire casing string with drilling mud
timing, energetics,, and extent of earthquake ruptures. For immediately after the casing is cemented in place. Therefore,
scientific drilling projects such as NanTroSEIZE,, it is very NanTroSEIZE Stage 2 will present the first opportunity for a
important to determine the in situ stress state at the scientific ocean drilling program to use LOT or XLOT
decollement and the mega splay fault in the Nankai Trough. procedures without using a packer, providing direct
information on the in situ magnitude of the minimum princi-
Preliminary experiments to determine the orientations pal stress at minimal cost and risk.
and magnitudes of principal stresses in the Nankai Trough
were undertaken during the NanTroSEIZE Stage 1 expedi- In this study we will demonstrate the feasibility of using
tions using borehole image analysis (stress-induced LOT and XLOT data acquired during the new riser-drilling
breakouts and tensile fractures; Kinoshita et al., 2008) and program to determine stress magnitude. We will first
indirect, core-based methods such as anelastic strain describe LOT and XLOT procedures, and then use an XLOT
recovery (ASR; Lin et al. 2006). These experiments will pro- data set that was acquired during the 2006 Shimokita
vide necessary and important information about in situ shakedown cruise of the Chikyu drilling vessel to estimate
stress. However, to improve reliability and reduce experi- the magnitude of minimum principal stress. We then
mental uncertainties in these stress determinations, it is recommend what we believe to be the optimum procedures
necessary to have direct in situ measurements of stress for implementation of LOT–XLOT for determination of stress
magnitudes—in particular,, the minimum principal stress— magnitude during future Chikyu riser-drilling programs.
at depth. These direct measurements are best obtained
using methods involving the initiation and propagation of Description of the Tests
hydraulic fractures at depth, such as the traditional hydraulic
fracturing test, a leak-off test (LOT), or an extended leak-off A LOT is a pumping pressure test carried out immediately
test (XLOT, sometimes ELOT) (Zoback et al., 2003). In the below newly set casing in a borehole (Fig. 1). It is similar to
present paper, we other pumping pressure tests known as the pressure integrity
Pressure and Valve aim to show that test, formation integrity test, or casing-shoe integrity test.
flow meter
with the advent Each of these tests has a different target pumping pressure.
Blow-out Cementing Fluid
preventer
Rig floor
pump tank
of the riser drill- The LOT technique was originally developed in the oil
ing vessel Chikyu, industry to assess the “fracture gradient” of the formation
the XLOT is (i.e., the maximum borehole pressure that can be applied
Casing
pipe applicable and without mud loss) and to determine optimal drilling
effective in deep parameters such as mud density (Kunze and Steiger, 1991).
Cement scientific ocean The LOT procedures are relatively simple. An XLOT is a
drilling projects. more complex test with extended pressurizing procedures,
Created Drill pipe
fracture as described in detail below. In future riser-drilling by
Open hole (e .g, 3 m -length)
During previ- Chikyu, it may be possible to regularly implement LOT or
Figure 1. Schematic borehole configuration ous ODP expedi- XLOT at each casing shoe immediately after casing has been
during a leak-off test (LOT) or extended leak- tions and run and cemented.
off test (XLOT; after Yamamoto, 2003)
non-riser IODP
Scientific Drilling, No. 6, July 2008 4
, Progress Reports
ceases (known as “shut-in”). The instantaneous shut-in pres-
Formation Breakdown
Pressure (FBP)
Residual tensile
strength component
sure (ISIP) is defined as the point where the steep pressure
Pumping pressure
Instantaneous Shut-In
Pressure (ISIP)=initial decreasess after shut-in
-in
in deviates from a straight line. From
pressure decline after
hole
Pumping ceases pump turned off
Second shut-in our perspective, the most important pressure parameter is
bore
Fracture propagation pressure
the fracture closure pressure (FCP), which occurs when the
into
Fracture
Leak-Off Propagation Bleed-off
mud
Pressure (LOP) Pressure (FPP) Fracture Re-opening newly created fractures closes again. FCP is determined by
ping
Pressure (Pr)=re-opening
of fractures therefore no the intersection of two tangents to the pressure versus mud
Pum
Formation tensile strength or stress
Integrity Test Fracture Closure
(FIT) Pressure (FCP)=
perturbation components volume curve (Fig. 2). The value of FCP represents the
picked using a
double tangent minimum principal stress (Yamamoto, 2003), because the
1st cycle 2nd cycle
Time (Volume of mud pumped in borehole) stress in the formation and the pressure of fluid that remains
in the fractures have reached a state of mechanical
Figure 2. Idealized relationship between pumping pressure and time equilibrium. White et al. (2002) collected high-quality XLOT
or volume of injected fluid during an XLOT (after White et al., 2002).
data and showed that both FCP and ISIP provide better
estimates of minimum principal stress than LOP, although
LOT and, in particular, XLOT procedures have been
the difference in the values of LOP and ISIP was small in
successfully and widely used to estimate the magnitude of
their study. In addition, ISIP is visually easier to determine
minimum in situ horizontal stress (Addis et al., 1998; White
than FCP. To end the test, the valve in rig floor is opened,, and
et al., 2002; Yamamoto, 2003), mainly for the practical
some of the fluid in the borehole flows back into the fluid
purpose of determining borehole stability during drilling
tank (known as “bleed-off”).
operations. These data can be used for another important
application—that is, to obtain in situ stress information that
To confirm the pressure values obtained from the initial
can be used in scientific objectives. In a similar case in which
XLOT, a second pressurization cycle is warranted (Fig. 2).
high borehole temperatures precluded use of a packer
Because a fracture has been created by the first execution of
Hickman et al. (1998) conducted this kind of test to obtain
XLOT, in the second cycle the pressure at the time of
in situ stress magnitude.
re-opening of the fracture corresponds approximately to the
FPP of the first cycle. In general, it is advisable to conduct
To carry out LOT or XLOT after setting casing and
additional pressurization cycles beyond the second cycle in
cementing, a short length (several meters) of extra open hole
order to confirm that stable values of FCP and ISIP have
is drilled below the casing shoe. The casing shoe is then
been obtained.
pressurized by drilling fluid delivered through drill pipe
from a cementing pump set on the rig floor of the drilling
vessel. The pressure at the casing shoe is equal to the sum of
An Extended Leak-off Onboard the Chikyu
the hydrostatic pressure of the drilling fluid column and the
During the Shimokita shakedown cruise (6 August to 26
ship-board pumping pressure. Figure ure 2 shows an idealized
October 2006), an XLOT was conducted onboard the Chikyu.
pumping pressure curve for XLOT (White et al., 2002).
The test was carried out at a depth of 525 meters below sea-
floor (mbsf) in 1180 m water depth; fluid density (seawater)
Initially, pumping fluid into the borehole results in volu-
was 1.030 g·cm -3 , and the injection flow rate was 0.5 bbl·min -1
metric compression of the drilling mud column and elastic
·min -1). Pressure and flow rate were recorded at
(about 80 L·min
expansion of the casing string plus rock around the borehole.
the surface, using a sample rate of 5 min -1. The resolution of
As the pressure in the borehole increases, the leak-off
the pressure measurements was 1 psi (about 7kPa) its accu-
pressure (LOP) is reached when the relationship between
racy is less than ±37 psi (about ±259 kPa). Because the main
pressure increase and volume of fluid pumped deviates from
objectives of the first drilling operation test of the Chikyu
linear. This occurs when fluid begins to diffuse into the
during the Shimokita shakedown cruise were re confirming
formation at a more rapid rate as the rock begins to dilate
basic drilling procedures, pure sea water was used,, and
(Fig. 2). Generally, a LOT is a test that finishes immediately
rough measurement conditions were adopted for the prelimi-
after LOP is reached.
nary XLOT. At the Shimokita site, core samples were
retrieved only to a depth of 365 mbsf. However, the lithology
An XLOT is an extended version of a LOT, but it is also
at the XLOT depth was identified from cuttings analysis as
similar to the hydraulic fracturing test used for stress
volcanic tuff.
measurement. During an XLOT, pumping continues beyond
the LOP point until the pressure peaks at formation break-
The fluid pumping rate was constant, and pumping was
down pressure (FBP). This creates a new fracture in the
stopped immediately after formation breakdown (Fig. 3).
borehole wall. Pumping is then continued for a few more
About 400 liters
iters (2.5 bbl) of seawater was injected into a
minutes, or until several hundred liters of fluid have
ve been
length of about 3 m of uncased borehole for about 6 min, thus
injected, to ensure stable fracture propagation into the
creating a fracture in the borehole wall. After shut-in, pres-
undisturbed rock formation. The pumping pressure then
sure was monitored for about 14 min and then released
stabilizes to an approximately constant level, which is called
the fracture propagation pressure (FPP). Pumping then
44 Scientific Drilling, No. 6, July 2008
Estimation of Minimum Principal Stress from an Extended
Leak-off Test Onboard the Chikyu Drilling Vessel and
Suggestions for Future Test Procedures
by Weiren Lin, Koji Yamamoto, Hisao Ito, Hideki Masago, and Yoshihisa Kawamura
doi:10.04/iodp.sd.6.06.008
Introduction expeditions, LOT or XLOT (which are sometimes used to
determine drilling parameters such as optimal mud density)
To understand the physics of faulting and rupture propa- have not been conducted because the borehole was open to
gation for the great M8-class Nankai earthquakes that recur the seafloor. Thus, it has been impossible to pressurize a
approximately every 100 years, a comprehensive drilling short interval of open hole below the casing as needed to
project is underway: the Nankai Trough Seismogenic Zone conduct a LOT or XLOT (see below) without utiliz-
Experiment (NanTroSEIZE; Tobin and Kinoshita, 2007),, ing time-consuming and frequently unreliable
which is part of the Integrated Ocean Drilling Program drill-pipe-deployed packers. In contrast, the new drilling
(IODP). Stress levels along seismogenic
eismogenic
ismogenic faults must be vessel Chikyu provides a riser-drilling capability that allows
known in order to understand processes controlling the pressuring the entire casing string with drilling mud
timing, energetics,, and extent of earthquake ruptures. For immediately after the casing is cemented in place. Therefore,
scientific drilling projects such as NanTroSEIZE,, it is very NanTroSEIZE Stage 2 will present the first opportunity for a
important to determine the in situ stress state at the scientific ocean drilling program to use LOT or XLOT
decollement and the mega splay fault in the Nankai Trough. procedures without using a packer, providing direct
information on the in situ magnitude of the minimum princi-
Preliminary experiments to determine the orientations pal stress at minimal cost and risk.
and magnitudes of principal stresses in the Nankai Trough
were undertaken during the NanTroSEIZE Stage 1 expedi- In this study we will demonstrate the feasibility of using
tions using borehole image analysis (stress-induced LOT and XLOT data acquired during the new riser-drilling
breakouts and tensile fractures; Kinoshita et al., 2008) and program to determine stress magnitude. We will first
indirect, core-based methods such as anelastic strain describe LOT and XLOT procedures, and then use an XLOT
recovery (ASR; Lin et al. 2006). These experiments will pro- data set that was acquired during the 2006 Shimokita
vide necessary and important information about in situ shakedown cruise of the Chikyu drilling vessel to estimate
stress. However, to improve reliability and reduce experi- the magnitude of minimum principal stress. We then
mental uncertainties in these stress determinations, it is recommend what we believe to be the optimum procedures
necessary to have direct in situ measurements of stress for implementation of LOT–XLOT for determination of stress
magnitudes—in particular,, the minimum principal stress— magnitude during future Chikyu riser-drilling programs.
at depth. These direct measurements are best obtained
using methods involving the initiation and propagation of Description of the Tests
hydraulic fractures at depth, such as the traditional hydraulic
fracturing test, a leak-off test (LOT), or an extended leak-off A LOT is a pumping pressure test carried out immediately
test (XLOT, sometimes ELOT) (Zoback et al., 2003). In the below newly set casing in a borehole (Fig. 1). It is similar to
present paper, we other pumping pressure tests known as the pressure integrity
Pressure and Valve aim to show that test, formation integrity test, or casing-shoe integrity test.
flow meter
with the advent Each of these tests has a different target pumping pressure.
Blow-out Cementing Fluid
preventer
Rig floor
pump tank
of the riser drill- The LOT technique was originally developed in the oil
ing vessel Chikyu, industry to assess the “fracture gradient” of the formation
the XLOT is (i.e., the maximum borehole pressure that can be applied
Casing
pipe applicable and without mud loss) and to determine optimal drilling
effective in deep parameters such as mud density (Kunze and Steiger, 1991).
Cement scientific ocean The LOT procedures are relatively simple. An XLOT is a
drilling projects. more complex test with extended pressurizing procedures,
Created Drill pipe
fracture as described in detail below. In future riser-drilling by
Open hole (e .g, 3 m -length)
During previ- Chikyu, it may be possible to regularly implement LOT or
Figure 1. Schematic borehole configuration ous ODP expedi- XLOT at each casing shoe immediately after casing has been
during a leak-off test (LOT) or extended leak- tions and run and cemented.
off test (XLOT; after Yamamoto, 2003)
non-riser IODP
Scientific Drilling, No. 6, July 2008 4
, Progress Reports
ceases (known as “shut-in”). The instantaneous shut-in pres-
Formation Breakdown
Pressure (FBP)
Residual tensile
strength component
sure (ISIP) is defined as the point where the steep pressure
Pumping pressure
Instantaneous Shut-In
Pressure (ISIP)=initial decreasess after shut-in
-in
in deviates from a straight line. From
pressure decline after
hole
Pumping ceases pump turned off
Second shut-in our perspective, the most important pressure parameter is
bore
Fracture propagation pressure
the fracture closure pressure (FCP), which occurs when the
into
Fracture
Leak-Off Propagation Bleed-off
mud
Pressure (LOP) Pressure (FPP) Fracture Re-opening newly created fractures closes again. FCP is determined by
ping
Pressure (Pr)=re-opening
of fractures therefore no the intersection of two tangents to the pressure versus mud
Pum
Formation tensile strength or stress
Integrity Test Fracture Closure
(FIT) Pressure (FCP)=
perturbation components volume curve (Fig. 2). The value of FCP represents the
picked using a
double tangent minimum principal stress (Yamamoto, 2003), because the
1st cycle 2nd cycle
Time (Volume of mud pumped in borehole) stress in the formation and the pressure of fluid that remains
in the fractures have reached a state of mechanical
Figure 2. Idealized relationship between pumping pressure and time equilibrium. White et al. (2002) collected high-quality XLOT
or volume of injected fluid during an XLOT (after White et al., 2002).
data and showed that both FCP and ISIP provide better
estimates of minimum principal stress than LOP, although
LOT and, in particular, XLOT procedures have been
the difference in the values of LOP and ISIP was small in
successfully and widely used to estimate the magnitude of
their study. In addition, ISIP is visually easier to determine
minimum in situ horizontal stress (Addis et al., 1998; White
than FCP. To end the test, the valve in rig floor is opened,, and
et al., 2002; Yamamoto, 2003), mainly for the practical
some of the fluid in the borehole flows back into the fluid
purpose of determining borehole stability during drilling
tank (known as “bleed-off”).
operations. These data can be used for another important
application—that is, to obtain in situ stress information that
To confirm the pressure values obtained from the initial
can be used in scientific objectives. In a similar case in which
XLOT, a second pressurization cycle is warranted (Fig. 2).
high borehole temperatures precluded use of a packer
Because a fracture has been created by the first execution of
Hickman et al. (1998) conducted this kind of test to obtain
XLOT, in the second cycle the pressure at the time of
in situ stress magnitude.
re-opening of the fracture corresponds approximately to the
FPP of the first cycle. In general, it is advisable to conduct
To carry out LOT or XLOT after setting casing and
additional pressurization cycles beyond the second cycle in
cementing, a short length (several meters) of extra open hole
order to confirm that stable values of FCP and ISIP have
is drilled below the casing shoe. The casing shoe is then
been obtained.
pressurized by drilling fluid delivered through drill pipe
from a cementing pump set on the rig floor of the drilling
vessel. The pressure at the casing shoe is equal to the sum of
An Extended Leak-off Onboard the Chikyu
the hydrostatic pressure of the drilling fluid column and the
During the Shimokita shakedown cruise (6 August to 26
ship-board pumping pressure. Figure ure 2 shows an idealized
October 2006), an XLOT was conducted onboard the Chikyu.
pumping pressure curve for XLOT (White et al., 2002).
The test was carried out at a depth of 525 meters below sea-
floor (mbsf) in 1180 m water depth; fluid density (seawater)
Initially, pumping fluid into the borehole results in volu-
was 1.030 g·cm -3 , and the injection flow rate was 0.5 bbl·min -1
metric compression of the drilling mud column and elastic
·min -1). Pressure and flow rate were recorded at
(about 80 L·min
expansion of the casing string plus rock around the borehole.
the surface, using a sample rate of 5 min -1. The resolution of
As the pressure in the borehole increases, the leak-off
the pressure measurements was 1 psi (about 7kPa) its accu-
pressure (LOP) is reached when the relationship between
racy is less than ±37 psi (about ±259 kPa). Because the main
pressure increase and volume of fluid pumped deviates from
objectives of the first drilling operation test of the Chikyu
linear. This occurs when fluid begins to diffuse into the
during the Shimokita shakedown cruise were re confirming
formation at a more rapid rate as the rock begins to dilate
basic drilling procedures, pure sea water was used,, and
(Fig. 2). Generally, a LOT is a test that finishes immediately
rough measurement conditions were adopted for the prelimi-
after LOP is reached.
nary XLOT. At the Shimokita site, core samples were
retrieved only to a depth of 365 mbsf. However, the lithology
An XLOT is an extended version of a LOT, but it is also
at the XLOT depth was identified from cuttings analysis as
similar to the hydraulic fracturing test used for stress
volcanic tuff.
measurement. During an XLOT, pumping continues beyond
the LOP point until the pressure peaks at formation break-
The fluid pumping rate was constant, and pumping was
down pressure (FBP). This creates a new fracture in the
stopped immediately after formation breakdown (Fig. 3).
borehole wall. Pumping is then continued for a few more
About 400 liters
iters (2.5 bbl) of seawater was injected into a
minutes, or until several hundred liters of fluid have
ve been
length of about 3 m of uncased borehole for about 6 min, thus
injected, to ensure stable fracture propagation into the
creating a fracture in the borehole wall. After shut-in, pres-
undisturbed rock formation. The pumping pressure then
sure was monitored for about 14 min and then released
stabilizes to an approximately constant level, which is called
the fracture propagation pressure (FPP). Pumping then
44 Scientific Drilling, No. 6, July 2008
