ARMA 20–1912
Laboratory Testing of Salt Deformation and Establishing Relationship with
Drilling Mechanics Data of Hybrid Drill Bit in Salt from Gulf of Mexico
Prasad, Umesh
Baker Hughes, Houston, TX, USA
Roy Chowdhury, Ashabikash
Baker Hughes, Damam, KSA
Rodrigue, Wayne
Baker Hughes, Houma, LA, USA
Copyright 2020 ARMA, American Rock Mechanics Association
This paper was prepared for presentation at the 54th US Rock Mechanics/Geomechanics Symposium held in Golden, Colorado, USA, 28 June-1 July
2020. This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and cri tical
review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its
officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent
of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. The
abstract must contain conspicuous acknowledgement of where and by whom the paper was presented.
ABSTRACT: Thick sequence of salt is regularly penetrated in Gulf of Mexico (GOM). Studies indicate that salt due to its viscoelastic-plastic properties do not fail efficiently by shearing action of conventional polycrystalline diamond compact (PDC) bits.
However, roller-cone bits with lower aggressiveness values when used to drill salt, provide lower penetration rate. By combining
shearing and gouging action of both PDC and roller-cone bits in a single bit frame, hybrid bit offers higher penetration rate due to
its dual cutting mechanics. Series of tests conducted in controlled laboratory environment confirms crystalline nature of salt with
very low density, porosity and permeability which matches with field observation. It has low strength but requires high strain or
energy at failure, which correlates with field data indicating higher weight-on-bit (WOB) and torque requirement. Paper attempts to
establish correlation between field drilling data and laboratory data and provides some interesting insight on drilling mechanics.
1. INTRODUCTION
Salt occurs worldwide both as a bedded deposit in land as
well as in various shapes and sizes offshore. Luann salt of
the Gulf of Mexico (GOM), Zechstein salt of Germany,
Hormuz salt of Iran, and the pre- and post-salts east of
Brazil are a few examples. Lessons learned from salt
solution-mining, drilling, underground-gas-storage or oil
and gas exploration are of interest to civil, mining and
petroleum industry professionals. The salt dome structure
encountered in GOM has special interest as most of the
deep water drilling penetrates these salt bodies.
Application of new tools, technology, and improved
operating practices are continuously applied to generate
time and cost savings in the petroleum industry.
A thick sequence of salt is regularly penetrated in the
central GOM to access deeper clastic reservoirs (Willson
and Fredrich, 2005; Israel et al, 2008; Crispin et al, 2010;
Prasad et al, 2019) in water depth of 1,800 – 2,400 m
(5,906 – 7,874 ft.). Thickness of the salt formation
typically ranges from 2,700 – 6,000 m (8,858 – 19,685
ft.). The mineralogical composition of GOM salt is
predominantly halite, with minor amounts of anhydrite,
gypsum or other varieties of salt. Salt types are inferred
using cutting samples and logs measurements consisting
of gamma, spontaneous-potential, neutron, density,
porosity, resistivity and acoustics.
Drilling salt in the GOM involves challenges such as:
tight hole due to creep, mud loss in sutures, tectonicinstability, drill-string dynamics, etc. (Willson and
Fredrich, 2005; Israel et al, 2008; Crispin et al, 2010;
Dykstra et al, 2018; Roy Chowdhury et al, 2016).
Geomechanical problems of wellbore instability, stress
distortion at sediment and salt interface, sudden change in
stress regimes from normal-faulting to strike-slip or
reverse-faulting from top to bottom salt sections, stress
rotations in a given stress regime, unpredictable pore
pressures, kicks, and losses are other areas of concern.
Development of new technology and application of
improved operating practices have not only improved the
confidence in early identification of drilling hazards but
also helped to efficiently drill safe and stable boreholes.
Lessons learned have been effective for choosing the right
tools, using the optimum drilling parameters, and
reducing the non-productive-time (NPT), and cost of
drilling. NPT has been continuously minimized by
adapting a holistic approach to well engineering involving
trajectory planning, mud design, casing-selection, proper
drilling assembly and drill bit selection. Use of real-time
drilling dynamics sensors and interactive monitoring of
well have also mitigated drilling dynamics dysfunction
problems (Dykstra et al, 2018; Roy Chowdhury 2019 et
al.).
2. OBJECTIVE
Recent successful applications of Hybrid bits in salt
drilling (Fig. 1) in the GOM (Roy Chowdhury et al. 2016,
2017) and their ability to drill with higher efficiency and
remain stable compared to PDC bits have motivated the
authors to: a) investigate more in detail the physicalmechanical and microstructural properties of analogue
salt measured in the laboratory, b) compare analogue salt
properties with the salt in the selected GOM wells, c)
analyze GOM field drilling data of hybrid bit runs in the
homogeneous salt sections, and d) explore potential
correlations between the failure mechanics of salt and the
drilling mechanics of a hybrid bit in salt.
3.1 Literature review
Review of the existing literature highlighted that a large
amount of information exists detailing the petrophysical
and mechanical behavior of salt. However, not much
information was available which attempted to correlate
salt failure mode and drilling mechanics. This acted as a
catalyst for the current work with an anticipation that
correlation of drilling mechanics and mechanical
behavior of rock may lead to identification of improved
cutting mechanics of the drill bit.
3.2 Application description and candidate salt run
selection
The field drilling data compiled and analyzed in the
present work is from Walker Ridge, Keathley Canyon,
Garden Banks and Green Canyon area of GOM. (Fig. 3).
Fig. 3. Location of the blocks in Gulf of Mexico represented
under current study (modified after Prasad et al., 2019).
Fig. 1. Recent successful runs of hybrid bits in GOM salt.
3. WORK FLOW
Work flow adopted for fulfillment of the objective in the
present work is summarized in Fig. 2.
Fig. 2. Work flow cycle.
These salt sections are generally allochthones, which, due
to it’s low density, have migrated and partially coalesced
with other salt bodies along the path of least resistance
assuming different shapes and sizes. Under the geologic
process, salt goes through minor compaction,
solidification, dissolution, and recrystallization, but
remains lowly compacted due to its mechanical
properties. The process of deposition and diagenesis in
salts has been described by several authors (Pettijohn,
1975; Selley, 1988; Willson and Fredrich, 2005; and
Prasad et al., 2019). Due to its low porosity and
permeability, salt bodies act as good seals for oil and gas
reservoirs.
Typical boreholes where salt is penetrated in the central
GOM are drilled with 26-in., 18⅛-in. and 16½-in. drill
bits. 18⅛-in. and 16½-in. drill bits are normally run with
a borehole enlargement tool to minimize the equivalent
circulating density (ECD). Having two rock cutting
devices placed approximately 30.5 – 45.72 m (100-150
ft.) apart in the bottom hole assembly (BHA) demands
careful selection of aggressiveness (μ) of each cutting
devices to avoid unsustainable drillstring dynamics (Roy
Chowdhury et al, 2016).
In order to understand the drilling mechanics, depth-based
drilling data from salt runs involving considerable footage
were selected. To discount torque and drag effects, data
from vertical runs were used for review. The lithology
column of the salt section is shown as pink color (Fig. 3)
and other clastic rocks types are shown in gray, yellow
and blue colors.
its strength, hydrostatic pressure in the borehole, and the
abrasive minerals in the rock. Fig. 5 shows the range of
drill bit options available for the given strength or
hardness range together with their typical μ values as a
range.
3.2.1 Rock strength and drill bit selection
The traditional method of drill bit selection uses
unconfined compressive strength (UCS) values of the
rocks to be drilled, calculated and calibrated using log
based estimations. This is followed by choosing a bit with
specific cutting elements to match the UCS values. A
frequency distribution chart of UCS helps to understand
the heterogeneity of the rock, indicating potential drilling
dynamics issues which may be triggered due to interbeddings (Fig. 4). The standard log of gamma is also
shown in the figure in the left hand side; the histogram on
the right shows the frequency distribution of UCS
indicating heterogeneity.
Fig. 5. Drill bit types and application (after Serrano et al. 2020)
3.2.3 Drilling parameters and their relevance
Drilling parameters are an important factor in drilling
performance. Many drill bit recommendations are based
on aggressiveness (μ) for the rock type to be drilled. PDC
bits have higher μ compared to roller cone bits. However,
stronger rocks can generate lower μ values based on the
cutting action of the drill bit. Teale, 1965 combined
drilling parameters such as weight-on-bit, torque, bit
revolutions, rate of penetration, and bit diameter to define
mechanical specific energy (MSE) Eq. (2). Pessier and
Fear, 1992, used MSE extensively in their study together
with defining bit-specific coefficient of friction or
aggressiveness (μ) for different bit types.
Fig. 4. Generic lithology column depicting depth based UCS of
rock with gamma ray (left) and a histogram (right).
3.2.2 Drill bit types and aggressiveness
Different bit types based on their design and on-bottom
cutting action generate varying amounts of torque for a
specific weight-on-bit (WOB). The ratio of bit torque to
WOB normalized to bit diameter is called aggressiveness
(μ) Eq. (1), which is an important parameter for drill bit
application analysis.
µ=
36. 𝑇
𝐷. 𝑊
(1)
Serrano et al, 2020, discussed the cutting action of
different types of drill bits and their application range
against rock hardness. Here, hardness is used loosely to
indicate rock strength which is represented as UCS (Fig.
5). However, wear potential of the formation is based on
𝑀𝑆𝐸 =
𝑊
𝐴
+
2. 𝜋 . 𝑅𝑃𝑀 . 𝑇
𝐴. 𝑅𝑂𝑃
(2)
Where W is WOB, and T is the torque is available from
the drilling rig through its available mechanical power.
The rate-of-penetration is ROP, bit revolutions per minute
is RPM and (A) is the cross sectional area. Since the axial
force (W/A) is extremely low, about 1% of the total MSE,
it is often ignored in the calculation.
For a range of WOB, RPM, and ROP data there is a
minimum MSE, which at 100 % efficiency equals
confined compressive strength, CCS [Dupriest et al.,
2005]. In terms of MSE observed, it’s in Eq. (3). Typical
efficiency in a GOM environment is about 12.5-35%. It
can be shown in Eq. (5) that ROP times MSE is a constant.
𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑀𝑆𝐸
Bit Efficiency = 𝑀𝑆𝐸 𝑜𝑏𝑡𝑎𝑖𝑛𝑒𝑑
𝐷𝑂𝐶 =
2. 𝜋. 𝑇
𝐴 . 𝑀𝑆𝐸
(3)
(4)
𝑅𝑂𝑃 ∗ 𝑀𝑆𝐸 =
2. 𝜋 𝑅𝑃𝑀. 𝑇
𝐴
= k * Power
(5)
3.3 Laboratory testing
For laboratory testing purposes, core plugs of 38.1 mm x
76.2 mm (1.5 inch x 3 inch) cylindrical shape were
obtained from a large block of quarried salt. Core plugs
of 25.4 mm x 50.8 mm (1 inch x 2 inch) cylindrical shape
were also obtained. A light oil/solvent was used as fluid
medium while coring, cutting, and surface grinding. The
samples were wrapped in plastic until the time of testing.
A test matrix was developed, Table 1.
using a radial extensometer. The limit to the tests was
maximum 5.08 mm (0.2 inch) displacement or 6.7%
strain for larger samples after which test was stopped.
Additionally, a few tests were run at different load frames
up to 20-25% strains being measured by load-frame cross
head displacement; no LVDT nor radial extensometer
were used for safety reasons.
3.3.1 Core analysis
Porosity and grain density of the cores were measured
while the bulk density was calculated. Fig. 6 shows the
results of porosimetry. The test results matched the
results available in the existing literature. Salt appears to
have a consistent grain density of 2.17 g/cc and very low
porosity.
Table 1: Test matrix for laboratory testing on salt plugs.
6
5
4
3
2
1
0
0.5
1
1.5
2
2.5
Bulk, g/cc
1
2.114
2
2.114
3
2.130
4
2.152
5
2.128
6
2.136
Por, %
2.45
2.34
1.52
0.63
2.24
1.75
GD, g/cc
2.167
2.165
2.163
2.166
2.177
2.174
Fig. 6. Results of routine core analysis, density porosity.
Standard practice as suggested by International Society of
Rock Mechanics (ISRM, 2007) were followed during the
sample preparations and tests. Details of the equipment
used are described in [Maharidge, R., 2012; Prasad,
2019b]. Tests performed included grain-density and
porosity, X-ray diffraction for mineralogy, ultrasonic tests
for compressional and shear wave velocity under
hydrostatic confining pressures of 3.45 – 34.5 MPa (5005,000 psi) and triaxial tests at six different confining
pressures of 0, 2.07, 20.7, 34.5, 68.9, 103.4 MPa (0, 300,
3,000, 5,000, 10,000, 15,000 psi) at the axial strain rate of
0.1%, 1%, and 10% per minute of loading rate at ambient
room temperature (74oF). Axial strain was measured
using two LVDT; whereas, radial strain was measured
Fig. 7. UCS and bulk density using cylindrical plugs.
3.3.2 Unconfined compressive strength (UCS)
The UCS obtained from three plugs carried over at strain
rates of loading of 0.1%, 1% and 10% strains/min,
respectively are shown in Fig. 7. The results obtained
from three tests are shown as a histogram. The bulk
density of the plugs (shown inside the bar chart as blue
color in secondary Y-axis) matched with that of routine
core analysis tests. The strengths obtained were 21.8-25.4
MPa (3,163-3,685 psi) consistent with other works as
reported by Willson and Fredrich, 2002. Fig 8 shows the
plot of axial stress and axial, radial and volumetric strain
on one of the core plugs from UCS testing.
The strain at failure is significantly higher in all of the
tests including unconfined tests, and is consistent with
literature. This means that the energy spent in failure is
very high. Fig. 9 shows how the core plug remained intact
even after extensive loading on triaxial samples. Post-test
thin sections showed micro-fractures resulting from axial
loading (Fig. 10) as hair-line fractures. The blue color
shows the extent of fracture-porosity resulting from the
test. The thin section micro-photograph also confirms the
crystalline nature of salt rocks.
Fig. 8. UCS test result showing axial stress against axial,
radial and volumetric strains. Energy spent till failure
3.3.3 Confined compressive strength (CCS)
Triaxial tests were performed on similar shaped and sized
samples as of UCS. However, a care was taken in safety
and stability of axial and radial strain gages. The unique
thing about salt is that it undergoes severe strain. In all
cases, the axial strain level reached the machine limit of
5.08 mm (0.2 inch) where tests were stopped. There was
very limited true elastic range in the axial stress-strain
curve, hence there was a problem calculating Young’s
modulus and Poisson’s ratio. Secant Young’s modulus
and corresponding Poisson’s ratio were calculated for
analysis. The deviatoric pressure considered for
evaluating secant modulus and Poisson’s ratio was
selected as 13.8 MPa (2,000 psi) for two reasons, a) the
deviatoric stress resided within the limits of elastic
behavior, and b) secant data existed for most confining
pressure testing at all three strain rates. Table 2 presents
the data; the secant Young’s modulus showed an
increasing trend calculated at higher confining pressure
tests. The corresponding Poisson’s ratio showed a
decreasing trend.
Table 2: Secant Young’s modulus and Poisson’s ratio
Fig. 9. Core plug after unconfined and confined strength
testing showing intactness of sample at the end of test.
Fig. 10. Petrographic micrograph from thin section of
failure sample together with loading axis (after Prasad et al,
2019).
3.3.4 Dynamic properties of salt using P & S wave
The dynamic elastic properties of Young’s modulus and
Poisson’s ratio were derived from measurements of the
compressional (P-wave) and shear (S-wave) velocities.
The wave velocity of P & S (the inverse of wave velocity
is termed as slowness in the unit of micro-second/ft.) were
measured on two one-inch diameter plugs using a
standard pulse through transmission method. The
confining pressure applied was hydrostatic in nature. Data
obtained is shown in Table 3. Both P & S waves and the
calculated Young’s modulus remain almost the same
except for a few data points at low confining pressure. The
weak behavior at low confining pressure shows the
compliant in nature behavior due to the presence of microcracks. Poisson’s ratio also shows nearly consistent
values all along the confining pressure. The results
obtained by dynamic means are consistent with that
observed by other works.
Table 3: Dynamic Young’s modulus and Poisson’s ratio
penetration (ROP) were analyzed. Fig. 12 shows the
surface and downhole torque against the depth of the well.
The upper bound values of torque (blue color) were
truncated due to top-drive setup. Furthermore, downhole
torque is much lower than the surface indicators (brown
color) showing losses due to drill string and borehole
interaction. Low spread values of the torque indicate the
ability of the hybrid bit to produce consistent torque due
to very consistent engagement of salt and cutting
elements. Gradual lowering of the downhole torque value
over the depth of the well is indication of gradual wear of
the cutting elements.
3.4 Computer simulation on Hybrid-bit cutters
Numerical modeling of roller-cone insert and PDC cutter
drilling under both atmospheric pressure as well as under
mud pressure has been investigated by Ledgerwood, 2007
using simulation software. Results obtained while
calibrating single cutter actions on salt and other rock
types have been discussed [Ledgerwood, 2007] and a
summary diagram is presented in Fig. 11 showing intact
grains (orange), broken bonds (blue), confining pressure
(black) and forces (blue-veins). Cutting action in highly
plastic rocks like salt under a heavy mud environment
clearly shows that the depth of indentation and the area of
damage in roller-cone tooth was much higher. Chip-holddown effect or no discrete chips were seen. Apparent high
strength consuming higher energy was also obvious.
Fig. 12. Surface and downhole torque variation with depth.
Fig. 13 shows the cross plot of WOB and torque for both
surface and downhole data. Upper bound truncation of the
torque represents top drive setup limit. However,
downhole data was much lower and proportionate. This is
expected, and is characterized by Eq. (1).
Fig. 13. Relationship of surface and downhole WOB and
torque.
Fig. 11. TCI and PDC cutting simulation.
4. HYBRID BIT FIELD RESULTS
To analyze the drilling mechanics of salt, drilling
parameters - WOB, Torque, and RPM obtained from
surface and downhole measurements together with rate of
The aggressiveness (μ) profile depending upon the
specific WOB and torque for both surface and downhole
data is shown in Fig. 14. The data spread is minimal for
the hybrid bit compared to PDC bits. This minimal
impairment in aggressiveness can be attributed to
efficient bit runs where roller cone teeth pre-fracture the
formations which is subsequently sheared off by PDC
blades. Another important observation is that the
aggressiveness (μ) decreases with increase in WOB (Fig.
15). This is interesting as higher WOB does not give a
proportionately higher torque as per Eq. (1). However,
this trend has been seen in several GOM bit runs. This
appears to be contrary to the ideal drilling lab
environment in which increase in WOB leads to increase
in aggressiveness due to increase in biting, chipping or
generation of torque [Morel et al, 2010; Lyons et al,
2018]. This unique observation could be due to top-drive
setup or to the plastic behavior of salt, and requires further
investigation. At shallow depth, WOB and torque have
the highest value, and the discrepancies between surface
and downhole data is minimum. However, as the depth
increases, surface and downhole data differ.
area under stress and strain under unconfined and at
various confining pressure tests.
Fig. 16. Typical axial stress and axial- and radial-strain
obtained from several triaxial tests on salt at 1% strain/min.
Fig. 14. Surface and downhole aggressiveness (μ) variation
with depth.
Fig. 15. Surface and downhole aggressiveness and WOB
5. CORRELATION OF FIELD & LAB DATA
Salt is a weak rock as evident from the laboratory testing
as the UCS values remained low 21.8-25.4 MPa (3,1633,685 psi) at strain rates of 0.1 % - 10% strain per minute.
However, it was confirmed that there is an extensive
amount of strain at failure in salt (see Fig. 8 as an example
under unconfined condition). The area under the stress
and strain curve (an indication of energy spent in
deforming or failing by drill bits) is very large as
compared to other brittle rocks. Fig. 16 also shows the
A similar plot was obtained for unconfined and confined
compressive testing performed at axial train rates of 0.1%
and 10% strain per minute of loading. The failure or drop
in peak stress was observed only in unconfined condition.
Under confined condition the testing had to be stopped as
soon as the actuator reached its limit. Under unconfined
condition and under various strain rates of loading, the
strain at peak stress ranged from 2.5% - 3.7%, beyond
which the test was stopped. The same results for other
common brittle rocks types are an order of magnitude
lower, ~ 0.3% - 0.5%. Further, the stress and strain
signatures for the salt show continuous increasing, the
strain-hardening or work hardening effect. As a result the
area under stress-strain keeps increasing which indicates
higher WOB and torque while drilling. This laboratory
observation coincided with field data which indicated that
salt formation required higher WOB to drill compared to
other clastic rocks of similar strengths. Willson and
Fredrich, 2005, recorded similar observation in their
study. Furthermore, the triaxial test indicated salt
behaving fully ductile or plastic. This posed a problem in
identifying elastic properties of one representative
Young’s modulus and one Poisson’s ratio. In the present
work, secant Young’s modulus and corresponding
Poisson’s ratio calculated at a fixed deviatoric stress was
used as discussed earlier (Table 2). However, for the UCS
or CCS estimate, the axial stress corresponding to 1%
axial strain was used to represent axial strength as
discussed in the next section.
Furthermore, the salt remained intact even after failure or
reaching the machine limit after undergoing strainhardening. To examine further, two additional tests were
conducted in another load-frame without use of axial and
radial sensors. The loading was carried out until 20-25%
axial strain but the salt remained still intact and supported
the load.
6. DISCUSSIONS
The plastic behavior of salt is not unique, as all rocks fail
under plastic or strain hardening mode. However, salt
shows this characteristic at room temperature and at low
confining pressure. Limestone shows similar plastic or
strain hardening characteristics at higher confining
pressure and temperature, whereas sandstone and other
igneous rocks fail under plastic or ductile mode at
extremely high temperature and confining pressure. Fig.
17 shows a plot of ductility in a typical carbonate
(Carthage limestone, UCS of 103 MPa, or 15,000 psi)
performed at confining pressures of 3.4, 13.8, 34.5, 68.9,
138, 207 and 276 MPa (500, 2,000, 5,000, 10,000, 20,000,
30,000 and 40,000 psi) as earlier reported and discussed
for other topics by Ledgerwood, 2007.
1% strain was assumed to represent CCS. However, the
key differences between this limestone and salt are a)
strain at failure at unconfined tests is ~ 0.5%, b) area
under axial stress-strain is comparatively less (indicating
low WOB and torque), and c) high confining pressure
tests (or deeper drilling) beyond 68.9 MPa (10,000 psi)
would indicate higher energy of drilling.
Fig. 18 further compares the differential stress (total axial
stress minus axisymmetric surrounding confining
pressure) cross-plotted with confining pressure for both
salt and Carthage limestone. Salt showed almost the same
value of differential stress all along the triaxial tests.
whereas limestone showed almost the same value of
differential axial stress once it behaved ductile or plastic.
This means that the effect of confining pressure in
reducing the rate of penetration at drilling beyond the
plastic behavior would be limited. This is consistent with
other works [Curry et al., 2015 and Dykstra et al., 2018].
Fig. 18. Comparing differential-stress from triaxial tests in
salt and limestone; differential stress remains nearly same.
7. CONCLUSIONS
Fig. 17. Typical axial stress and axial- and radial-strain
obtained from several triaxial tests performed on limestone.
The Fig. 17 shows the axial stress, axial strain and radial
strains at various confining pressures. There is a clear
brittle failure or drop in axial stress up to the confining
pressure of 68.9 MPa (10,000 psi) beyond which the
Carthage limestone behaves plastic or ductile. The
ductility, strain hardening, or work hardening in
carbonates, as discussed for salt, also posed a problem in
characterizing one specific value of peak stress at failure,
the Young’s modulus, and Poisson’s ratio. The CCS could
not be defined in case of salt, therefore, the axial stress at
Recent record runs of hybrid bits in the GOM have
motivated the authors to investigate the physicalmechanical and microstructural properties of analogue
salt, compare it with log based estimation in the field,
analyze field drilling data from hybrid bit runs, and
explore potential correlations between the failure
mechanics of salt and the drilling mechanics of a hybrid
bit drilling salt.
Salt porosity, grain density, bulk density and strength
(UCS) measured in the analogue samples matched well
with published works. X-Ray Diffraction mineralogy
showed pure halite with minor impurities. Further, the
thin section analysis confirmed the low porosity,
permeability, and crystalline nature of salt.
Dynamic salt properties obtained from ultrasonic wave
propagation indicated almost stiff behavior and there was
very limited compliant characteristics. The measured
properties were significantly higher than their static
properties.
Several UCS tests indicated salt to have low strength and
low stiffness but deforms to an excessive amount (3-10%)
compared to typical shale, limestone, sandstone or other
rock types.
Confined compressive strength tests on salt also show the
excessive amount of deformation or strain all along the
tests. There was no peak stress; the stress kept increasing
with increase in strain, also called strain-hardening.
The area under axial stress and strain shows the energy
spent in deformation, this indicates energy spent in
drilling salt in the field would consume more energy
(combination of WOB, torque and RPM) as compared to
other rocks of similar strength. This is consisted with
experience by other works.
Failure mechanics of salt indicates stress hardening,
requiring higher WOB while drilling. Conventional PDC
bits which fails the rock using shearing action is unable to
operate under high WOB as it generates very high torque
due to its increased aggressiveness. In contrast, hybrid bit
with its dual cutting mechanics uses two-step process to
fail the salt more efficiently. Roller cone elements by its
gouging and pre-indentation creates a volume of
weakened pre-stressed rock which is subsequently
sheared by following PDC cutters easily, yielding higher
penetration rate and efficiency. Lower aggressiveness of
the hybrid bit allows application of higher WOB to
counter stress hardening without generating high torque.
Unlike PDC bits, hybrid bit with lower aggressiveness
generates consistent and lower torque which helps to
avoid known drilling issues like over-torqued
connections, stick-slip and other coupled dynamic events.
8. REFERENCES
1.
Crispin C., et al., 2010. Overcoming a Difficult Salt
Drilling Environment in the Gulf of Mexico: A Case
Study. SPE # 28192. IADC/SPE Drilling Conference
and Exhibition held in New Orleans, Louisiana, USA 24 February 2010.
2.
Curry D.A. et al., 2015. The Effect of Borehole Pressure
on the Drilling Process in Salt.” Paper SPE/IADC
173023 presented at the 2015 SPE/IADC Drilling
Conference.
3.
Dupriest, F.E. and Koederitz, W.L. 2005. Maximizing
Drill Rates with Real-Time Surveillance of Mechanical
Specific Energy. SPE/IADC 92194 presented at the 2005
SPE/IADC Drilling Conference in Amsterdam, The
Netherland, 23-25 February 2005.
At the highest confining pressure tests, axial deformation
achieved was up to 25% axial strain. Axial and radial
strain sensors could not be used but the salt remained
intact and still able to absorb load and deformation, and
shape showed ‘barreling effect.’
4.
Dykstra M. W. et al., 2018. Converting Power to
Performance: Gulf of Mexico Examples of an
Optimization Workflow for Bit Selection, Drilling
System Design and Operation. OTC # 29065-MS.
Offshore Technology Conference held in Houston,
Texas, USA 30 April – May, 2018.
The authors believe that at high temperature and pressure
tests (similar to geothermal heat and borehole mud
pressure in GOM) an extensive amount of strain
hardening would take place which would necessitate
higher amounts of WOB, torque and mechanical specific
energy as compare to similar UCS of other rock types.
This is consistent with earlier works.
5.
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