The poor lubrication performance of water-based drilling mud hinders its application in the drilling process of extended, reach horizontal wells. To overcome this shortcoming, polyaspartate (PA) and poly(ethylene glycol) (PEG) were used to improve the lubrication performance of the water-based drilling mud. The conventional performances, lubrication performance, and micro-image of antiwear steel balls of the modified water-based drilling mud were analyzed. The results show that the coefficient of friction (COF) of the water-based drilling mud mixed with 10% PA and 5% PEG was the lowest, reaching 0.094, the reduction rate of COF was 63.1%, and the drilling mud cake stuck factor was also the lowest. The addition of PA and PEG had no effect on the rheological properties of water-based drilling mud and also can significantly reduce the filtrate volume; the reduction rate of the filtrate volume reached 43.5%. All of these result from the synergistic effect of PA and PEG; they are adsorbed on the metal surface and the mud cake to form a lubricating film. At the same time, the lubricants also changed the appearance of the solid particles in the mud cake, which reduced the friction between the mud cake and the drilling tool. Moreover, effects of the influence of drilling mud properties on lubricants (alkali metal ions, pH, temperature, and drilling mud density) were examined. The water-based drilling fluid with the synergistic effect of PEG and PA shows similar lubrication performance as the oil-based drilling fluid and can meet the technical requirements of corresponding horizontal wells.
The poor lubrication performance of water-based drilling mud hinders its application in the drilling process of extended, reach horizontal wells. To overcome this shortcoming, polyaspartate (PA) and poly(ethylene glycol) (PEG) were used to improve the lubrication performance of the water-based drilling mud. The conventional performances, lubrication performance, and micro-image of antiwear steel balls of the modified water-based drilling mud were analyzed. The results show that the coefficient of friction (COF) of the water-based drilling mud mixed with 10% PA and 5% PEG was the lowest, reaching 0.094, the reduction rate of COF was 63.1%, and the drilling mud cake stuck factor was also the lowest. The addition of PA and PEG had no effect on the rheological properties of water-based drilling mud and also can significantly reduce the filtrate volume; the reduction rate of the filtrate volume reached 43.5%. All of these result from the synergistic effect of PA and PEG; they are adsorbed on the metal surface and the mud cake to form a lubricating film. At the same time, the lubricants also changed the appearance of the solid particles in the mud cake, which reduced the friction between the mud cake and the drilling tool. Moreover, effects of the influence of drilling mud properties on lubricants (alkali metal ions, pH, temperature, and drilling mud density) were examined. The water-based drilling fluid with the synergistic effect of PEG and PA shows similar lubrication performance as the oil-based drilling fluid and can meet the technical requirements of corresponding horizontal wells.
In recent years, drilling engineering has gradually developed to
directional wells, extended-reach wells, cluster wells, deep wells,
and ultra-deep wells. These different wellbore structures increase
the torque and friction force in the drilling process, which is easy
to accelerate the wear of drilling tools, increase the power consumption
of the drilling equipment, and even lead to drilling safety accidents
such as broken drilling and sticking.[1,2] The oil-based
drilling mud has strong inhibition and high lubricity, which can effectively
prevent sloughing and sticking. However, environmental pollution,
affecting logging quality, and high cost are also its disadvantages.
The water-based drilling mud does not have these problems, but the
poor lubrication performance and high friction have restricted its
development. Therefore, improving the lubrication performance of the
water drilling mud has great significance in solving the above problems.[3−5] Adding high-quality lubricating oil into water-based drilling mud
to reduce downhole friction is one of the main technical means to
prevent and solve drilling safety problems at present.[6,7] The research and development of water-based drilling mud systems
and corresponding lubrication treatment agents are of great significance
to the wide application of water-based drilling muds in the future.PEG is a polyol, and the diversity of its molecular structure determines
the diversity of products. It not only has good shale inhibition performance
but also has a good lubricating effect, making it widely useful in
petroleum drilling.[8−11] In addition, it has been widely used in artificial joint materials,
biopharmaceuticals, lubricating oil, metal cutting fluid, and so on.[12−14] Luo and Xu had synthesized a variety of water-based antisloughing
lubricants with good lubricating properties according to the polymerization
principle.[15,16] Abdullah developed a lubricant
composed of triethanolamine and PEG, which not only possesses lubrication
and antifriction functions but also provides basic buffer capacity
for drilling mud.[17] A lubricant was prepared
by Alonso-debolt by reaction of 2-ethylhexanol with 1-hexadecene epoxide,
which does not need traditional oil-based components and can be biodegraded.[18] However, domestic and foreign scholars have
made a comprehensive study on the inhibition of polyalcohol in water-based
drilling mud, but the research on the improvement of water-based drilling
mud lubrication is relatively less at present.Polyaspartate
is a new type of biodegradable and biocompatible
green polymer, which has many functions such as scale inhibition,
corrosion inhibition, dispersion, chelation, and moisture preservation.
It has been widely used in chemical fertilizer, pesticide, industrial
water treatment, metal cutting fluid, surfactant, daily chemicals,
and other fields.[19,20] PA is a novel metal working fluid,
which is used as a lubricant in the process of cutting, bending, grinding,
and forming ferrous and nonferrous metals. At the same time, the “extreme
pressure four-ball test” was carried out to test the relative
bearing capacity of the lubricant under constant conditions, and the
result shows that it has good lubrication performance.[21] Based on the good lubrication performance of
PA in metal cutting fluids, it also can be used in water-based drilling
mud.Both PA and PEG are environmentally friendly and degradable
treatment
agents, which fully conform to the development concept of water-based
drilling mud. Many studies on improving the lubricity of water-based
drilling mud with a single material have been well proved, but the
application of composite materials in water-based drilling mud has
not been well reported. To develop high-quality water-based drilling
mud suitable for drilling technology, PEG and PA composite materials
are used in the water-based drilling mud, and the relationship between
their synergistic effect and their representative functions is studied.
The research results provide a reference for the development of high-performance
water-based drilling mud.
Results and Discussion
To understand the influence of various additives on the performances
of water-based drilling mud more intuitively, effects of PA and PEG
on the rheological properties, filtrate volume at high temperature
and high pressure, coefficient of friction, and the mud cake stuck
factor were studied. The control sample (excluding PEG and PA) was
plotted as “WBM”.
Effect of PA on the Performance
of Water-Based
Drilling Mud
Effect of PA on the Rheological
Property
of Water-Based Drilling Mud
The rheological property is one
of the basic properties of drilling mud, which plays an important
role in solving the following drilling problems: (1) carrying cuttings
to ensure the cleanness of bottom hole and well suit; (2) suspending
cuttings and barite; (3) improving the mechanical drilling speed;
and (4) maintaining borehole stability and ensuring downhole safety.
Therefore, the rheology of drilling mud directly affects the performance
of these functions.[22]Figure shows the effect of PA content
on the plastic viscosity (PV) and yield point (YP) of water-based
drilling mud. Data were collected at a standard temperature of 50
°C. With the increase of the PA content, the PV and YP of the
water-based drilling mud, respectively, increased by 2 cP and 2.1
lb/100 ft2, but the changes in these values are almost
negligible. Therefore, PA has no effect on the rheological property
of water-based drilling mud.
Figure 1
Dependence of PA on PV and YP of water-based
drilling mud.
Dependence of PA on PV and YP of water-based
drilling mud.Figure shows the
effect of PA content on the gel strength of water-based drilling mud.
It exhibits an increase of gel strength at an increased PA concentration,
following the trend for YP. Moreover, the change at 10 s and 10 min
is not obvious at the same concentration. This phenomenon shows that
PA can slightly improve the suspending ability of cuttings under static
conditions.
Figure 2
Dependence of PA on the gel strength of water-based drilling mud.
Dependence of PA on the gel strength of water-based drilling mud.
Effect of PA on the Lubrication
Performance
of Water-Based Drilling Mud
The lubrication performance of
drilling mud usually includes two aspects: the lubrication performance
of drilling mud and the mud cake stuck factor. These two indexes are
the main technical indicators for evaluating the lubrication performance
of drilling muds. PA has good lubricity and antiwear property in metal
cutting fluids, so it can be used as the base fluid of water-based
drilling mud to improve the lubrication performance of water-based
drilling mud.[23]The relationship
among the content of PA, the coefficient of friction of water-based
drilling mud, and the mud cake stuck factor was studied, and the results
are clearly shown in Figure . With the increase of the PA content, the coefficient of
friction of the water-based drilling mud shows a decreasing trend,
but it is not obvious. After 20% PA treatment, the coefficient of
friction of water-based drilling mud decreases to 0.175, merely.
Figure 3
Dependence
of PA on the lubrication performance of water-based
drilling mud.
Dependence
of PA on the lubrication performance of water-based
drilling mud.PA had a very low coefficient
of friction, but when it was added
to water-based drilling mud, it did not show a positive effect on
the lubrication performance. Moreover, it was found that when sodium
hydroxide was added into the PA pure agent, it was difficult to dissolve
sodium hydroxide in PA due to the low water activity and poor solubility
of PA, which made PA molecules unable to fully diffuse in solution.The mud cake stuck factor increased with the growth of PA, which
was not favorable. This is because PA has high adhesion, forms chelates
with various ions (such as calcium, magnesium, copper, and iron),
and adheres to the metal surface.[24] Therefore,
the metal block was blocked when it started to slide.
Effect of PA on the Filtrate Volume of Water-Based
Drilling Mud
Filtration and wall building performance are
important performances of drilling mud, which have very important
impact on the stability of the wellbores of loose, broken, and unstable
formation. The filtrate volume of drilling mud is the most intuitive
response to its filtration and wall building performance.[25]As shown in Figure , with the increase of PA content, the volume
of the filtrate of water-based drilling mud at high temperature and
pressure gradually decreases. When the PA content reaches 10%, the
volume of the filtrate decreases to a peak value of 1.6 mL. After
that, the PA content continues to increase, and the volume of the
filtrate no longer decreases, but increases again. The reason for
the decrease in the filtration volume of drilling mud is that PA is
a long-chain polymer molecule with polar groups (such as amino and
carboxyl groups) at the end of the chain. So, the PA molecules have
adsorption and cross-linking with mud cake, which can plug pores and
achieve a certain filtration reduction effect.[19]
Figure 4
Dependence of PA on the filtrate volume of water-based drilling
mud.
Dependence of PA on the filtrate volume of water-based drilling
mud.In summary, PA had little effect
on the rheological property of
water-based drilling mud and could reduce the filtrate volume, but
its performance as a lubricant was not outstanding, which needs to
be studied.
Effect of PEG on the Performance
of Water-Based
Drilling Mud
Effect of PEG Content
on Rheological Properties
of Water-Based Drilling Mud
It is found that the rheological
properties of drilling mud do not change with the molecular weight
of PEG. Therefore, we chose one of the molecular weight PEG to study
the effect of its content on the rheological properties of drilling
fluid.Figure illustrates the effect of PEG content on the rheological properties.
With the increase in the content of PEG, the PV and YP values of the
water-based drilling mud slightly increased, but it is not obvious.
The PV increased by 3 cP and the YP increased by 2.1 lb/100 ft2. These changes can be ignored.
Figure 5
Dependence of PEG on
rheological properties of water-based drilling
mud.
Dependence of PEG on
rheological properties of water-based drilling
mud.Figure shows the
effect of PEG content on the gel strength of water-based drilling
mud. As the PA concentration increases, the gel strength increases.
Under the same concentration, the changes at 10 s and 10 min are not
obvious. This also shows that PEG can slightly increase the slurry’s
ability to suspend rock cuttings under static conditions. It can be
seen that the effect of PEG on the rheological properties of water-based
drilling mud is not very significant.
Figure 6
Dependence of PEG on the gel strength
of water-based drilling mud.
Dependence of PEG on the gel strength
of water-based drilling mud.
Effect of Molecular Weight of PEGs on the
Lubricating Performance of Drilling Muds
PEG has the advantages
of strong inhibition, high lubricity, and miscibility with water in
any ratio, which is suitable for the development of water-based drilling
mud suitable for strong water-sensitive formation.[26] Therefore, the influence of PEGs on the lubrication performance
of water-based drilling mud was studied.The influence of PEGs
with different molecular weights on the lubrication performance of
water-based drilling mud is shown in Figure . With the increase of the content, the COF
of the drilling mud shows a decreasing trend, with the highest reduction
rate being 19.6% and the lowest being 13.0%. PEG can improve the lubrication
performance of water-based drilling mud, but it is not as expected.
When drilling extended, reach horizontal wells, the formation can
be drilled smoothly only when the COF of drilling mud is generally
lower than 0.10.[27] Obviously, PEG does
not meet the requirements.
Figure 7
Dependence of PEG types and contents on COF
of water-based drilling
mud.
Dependence of PEG types and contents on COF
of water-based drilling
mud.
Effect
of PEG Content on Filtrate Volume
of Water-Based Drilling Mud
Figure exhibits the change in the fluid loss volume
of water-based drilling mud with different molecular weights and different
concentrations of PEG. As the concentration of PEG increases, the
fluid loss volume shows a decreasing trend. Also, as the molecular
weight of PEG increases, the fluid loss volume gradually decreases.
This result shows that PEG can reduce the filtrate volume of water-based
drilling mud to a certain extent. This is because PEG is a long-chain
molecule with polar groups (hydroxyl group) at the end chain. The
hydroxyl groups were adsorbed on the surface of the clay and barite
of mud cake, which can effectively block the micropores and achieve
the effect of reducing the filtrate volume. Small molecular-weight
PEG can only block smaller pores, while larger molecular-weight PEG
can also block larger micropores through cross-linking adsorption.
Figure 8
Dependence
of PEG types and contents on filtrate volume of water-based
drilling mud.
Dependence
of PEG types and contents on filtrate volume of water-based
drilling mud.
Synergistic
Effect of PA and PEG on the Performances
of Water-Based Drilling Mud
In view of the limitations of
PA and PEG in improving the lubrication performance of water-based
drilling mud, PA and PEG were considered to be mixed into the water-based
drilling mud, with an expectation that the synergistic effect could
exhibit good lubrication performance.However, when PEG and
PA were mixed into the water-based drilling mud, and the amount of
PA reached 15%, insoluble particles appeared in the drilling mud,
as shown in Figure b. Even when the amount of PA reached 20%, the insoluble particles
in the drilling mud increased and became more obvious, as shown in Figure c. In addition, as
shown in Figure , with the increase of PA content, the filtration of water-based
drilling mud is significantly reduced, but when the amount of PA is
more than 10%, the filtration gradually increases. Therefore, the
black particles in the drilling mud should be the water-loss additive
SPNH or LSF. With the increase of the PA content, the solubility of
these solid particles in water-based drilling mud also decreased.
This also explains why the filtrate volume of water-based drilling
mud tended to increase with the increase of PA content. Therefore,
the final dosage of PA was chosen as 10%.
Figure 9
Appearance of water-based
drilling mud with different contents
of PA.
Figure 10
Dependence of PA content and filtrate
volume of water-based drilling
mud.
Appearance of water-based
drilling mud with different contents
of PA.Dependence of PA content and filtrate
volume of water-based drilling
mud.
Synergistic Effect of
PA and PEG on Lubrication
Performance of Water-Based Drilling Mud
In view of the facts
discussed in Sections and 2.2, the performances in improving
the lubricity of water-based drilling mud were not very satisfactory
when PA and PEGs were used alone. The most suitable additive and proportion
can be selected by adding PEG with different molecular weights and
contents in the water-based drilling mud and using it in combination
with PA.The effect of different molecular weights and contents
of PEGs on the COF of WBM + 10% PA is illustrated in Figure . With the increase of molecular
weight and content of PEGs, the COF decreases; when the molecular
weight of PEG is 6000, the COF reaches the lowest value. Compared
with 0.23 of WBM + 10% PA, the COF of WBM + 10% PA + 5% PEG-6000 decreases
to 0.094, and the reduction rate of COF reached 59.1%. However, when
the molecular weight of PEG was 8000, the COF increased.
Figure 11
Dependence
of PEG types and contents on COF of WBM + 10% PA.
Dependence
of PEG types and contents on COF of WBM + 10% PA.It can be seen from Figure that when the concentration of PEG reaches 5%, the
COF reaches the lowest value, and the COF of PEG-6000 is the lowest.
Therefore, PEG-6000 is selected as the additive of water-based drilling
mud, and its addition amount is 5%, which can improve the lubricity
of drilling mud.PEG is a nonionic polymer with hydroxyl groups
at both ends. Due
to the cloud point effect, poly(ethylene glycol) in drilling mud had
“phase separation” and became an oil-soluble substance.[28] The hydroxyl groups on the molecular chain adsorbed
on the surface of the mud cake and metal to form a lubricating film.
Moreover, with the increase of the molecular weight of PEG, the lubricating
film gradually thickens and the lubricating performance of the drilling
mud decreases. However, with the increase of the molecular weight
of PEG, the hydroxyl value also decreased.[29] PEGs with different molecular weights have different hydroxyl values,
i.e., the amount of hydroxyl groups in unit mass sample, as shown
in Table . When the
molecular weight reached 8000, the hydroxyl groups on the molecular
chain were relatively less to be effectively adsorbed on the mud cake
and metal surface, thus increasing the COF.
Table 6
Basic Properties of PEG
specs
appearance
molecular weight
hydroxyl
value
PEG-600
clear liquid
540–660
170–208
PEG-2000
white flaky solid
1800–2200
51–63
PEG-4000
white powdery solid
3500–4400
26–32
PEG-6000
white powdery solid
5500–7000
17.5–20
PEG-8000
white powdery solid
7200–8800
12–16
The control sample
(excluding PEG and PA) was plotted as WBM. Figure shows the comparison
of COF, which includes WBM, WBM + 10% PA, WBM + 5% PEG, and WBM +
5% PEG + 10% PA; 10% PA and 5% PEG reduce the COF of water-based drilling
mud by 12.2 and 19.6%, respectively. These two ratios are selected
because they have the most significant effect on water-based drilling
mud. However, when they all were added to the water-based drilling
mud together, the COF of the water-based drilling mud is only 0.094
and the COF decreases by 63.1%. The oil-based mud is equipped indoors
for lubrication performance test, and the composition is presented
in Table . The results
show that the COF of oil-based mud is generally between 0.070 and
0.080. Therefore, it can be considered that the lubricating performance
of WBM + 5% PEG + 10% PA is close to that of the oil-based mud.
Figure 12
Effect of
additives on COF of water-based drilling mud.
Table 1
Oil-Based Mud Composition
components
function
concentration (g)
water
base fluid
80
diesel oil
base fluid
270
HIEMUL-B
emulsifier
12
HICOAT-B
emulsifier
2.4
CaO
adjuvant of
emulsifier
6.0
MoGel
viscosifier
10.0
HIFLO
filtration control
12.0
HIFLO-L
filtration control
8.0
barite
weighing agent
460
Effect of
additives on COF of water-based drilling mud.The reasons
for the low COF of 10% PA + 5% PEGwater-based drilling
mud are as follows: the polar group formed by hydrolysis of PA adsorbed
on the metal through chelation and formed firm lubrication on the
metal surface.[24] Then, the hydrophilic
group of the ether bond and the terminal hydroxyl group of PEG molecules
through hydrogen bond form a second layer of lubricating film outside
the PA lubricating film.[26] The synergistic
effect of the two layers of lubricating films reduced the contact
friction between friction surfaces, improved the extreme pressure
and antiwear property of drilling mud, and enhanced the lubricity
of water-based drilling mud.Figure shows
the comparison of the mud cake stuck factor of the four water-based
drilling muds. Obviously, the synergistic effect of PA and PEG is
more effective than that of individual use. Because PA has high adhesion,
it can form chelates with various ions (such as calcium, magnesium,
copper, and iron) and adhere to the metal surface, so it shows a higher
mud cake adhesion coefficient. However, PEG chains combine with water
molecules through hydrogen bonding and polarization, and the hydrated
PEG chains become more lubricated, more fluid, and thus have lower
friction.[30] The lubricating film formed
by PEG molecules adsorbed on PA, which blocks the direct contact between
the metal slider and mud cake, so PEG and its synergistic effect with
PA show a lower mud cake stuck factor.
Figure 13
Effect of additives
on the mud cake stuck factor of water-based
drilling mud.
Effect of additives
on the mud cake stuck factor of water-based
drilling mud.
Synergistic
Effect of PA and PEG on Rheological
Properties of Water-Based Drilling Mud
In Sections 2.1 and
2.2, it was found that there was little effect on the rheological
properties of water-based drilling mud when it was used alone. However,
the rheological properties of water-based drilling mud changed after
it was treated with PA and PEG. PEG was added into the water-based
drilling mud containing 10% PA, and the synergistic effect of different
amounts of PEG and PA on the rheological properties of the water-based
drilling mud was observed. The data related to the synergistic effect
of PA and PEG on viscosity profiles are presented in Table .
Table 2
Data Related
to the Synergistic Effect
of PA and PEG on Viscosity Profiles
viscosity
of different drilling muds/cP
shear rate/s–1
WBM
WBM + 10% PA
WBM + 10% PA + 1% PEG
WBM + 10% PA + 3% PEG
WBM + 10% PA + 5% PEG
WBM + 10% PA + 7% PEG
5.11
0.77
1.02
1.02
1.02
1.28
1.53
10.22
1.28
1.53
1.53
1.53
2.04
2.04
170.3
7.67
8.69
8.18
9.20
10.73
10.73
340.6
11.75
13.80
13.29
14.31
15.84
16.35
510.9
15.33
17.89
17.37
18.91
21.46
21.97
1021.8
24.53
28.10
27.08
30.66
34.75
35.77
The measured shear stress of WBM
containing PA and PEG as a function
of shear rate and PA or PEG concentration after hot rolling is presented
in Figure . The
result shows that the shear stress of WBM with PA and PEG is higher
than that of WBM, and the shear stress increases with the increase
of the PEG content. At low shear rates, the increase of shear stress
of drilling mud is small. At a higher shear rate, with the increase
of PEG content, the increase of shear stress is more obvious. Especially
at a shear rate of 1021.8 s–1, the shear stress
increases by 45.8%. Therefore, the addition of PEG and PA significantly
improved the viscosity of water-based drilling mud and increased the
pump pressure in drilling engineering.
Figure 14
Synergistic effect of
PA and PEG on the shear stress of water-based
drilling mud.
Synergistic effect of
PA and PEG on the shear stress of water-based
drilling mud.Figure illustrates
the change in the rheological parameters, YP and PV, of WBM + 10%
PA for PEG-6000. Theoretically, YP measures the electrochemical force
or attraction of a fluid under flowing conditions. YP depends on the
surface properties of drilling mud solids, the volume concentration
of solids, and the electrochemical environment of these solids.[31] The result shows that with the increase of PEG
concentration, YP value only increases from 13.65 to 16.8 lb/100 ft2, which can be ignored. Therefore, the addition of PEG has
little effect on YP value.
Figure 15
Synergistic effect of PA and PEG on PV and
YP of water-based drilling
mud.
Synergistic effect of PA and PEG on PV and
YP of water-based drilling
mud.The variation of the rheological
parameter PV of WBM + 10% PA as
a function of PEG concentration is also shown in Figure ; the addition of PEG has
no effect on PV. The calculated PV range is 20 to 27 cP. In most practical
applications, the value of PV is determined by the concentration of
drilling mud solids. Moreover, low and stable PV is desirable as excessive
PV will increase the pump pressure required to pump the fluid, resulting
in an excessive equivalent circulating density (ECD).Figure shows
the effect of PEG content on the gel strength of WBM + 10% PA. It
shows that the gel strengths of drilling muds increase with the increase
of PEG concentration, and the 10 s and 10 min reached 7.35 lb/100
ft2 finally. Generally, this phenomenon suggests that the
WBM containing PEG and PA can better suspend cuttings under static
conditions.
Figure 16
Synergistic effect of PA and PEG on the gel strength of
water-based
drilling mud.
Synergistic effect of PA and PEG on the gel strength of
water-based
drilling mud.
Synergistic
Effect of PA and PEG on Filtrate
Volume of Water-Based Drilling Mud
Figure depicts the synergistic effect of PA and
PEG on the filtration of water-based drilling mud. When 1% PEG is
added to the drilling mud, the filtrate volume decreases significantly.
However, with the addition of PEG, the filtrate volume showed a decreasing
trend, but it was not particularly obvious. Obviously, PEG and PA
can significantly reduce the filtrate volume of water-based drilling
mud, which is good for site construction.
Figure 17
Synergistic effect of
PA and PEG on the filtrate volume of water-based
drilling mud.
Synergistic effect of
PA and PEG on the filtrate volume of water-based
drilling mud.Figure shows
the comparison of HTHP filtration of WBM, WBM + 10% PA, WBM + 5% PEG,
and WBM + 10% PA + 5% PEG. The addition of 10% PA or 5% PEG can reduce
the filtrate volume of water-based drilling mud by 12.9 or 21.0%,
but once they are all mixed into the water-based drilling mud together,
the reduction rate of the filtrate volume reaches 43.5%. Because PA
and PEG can be adsorbed on the surface of clay and barite through
chelation and hydrogen bonding, microcracks and micropores are prevented,
and the volume of the filtrate is reduced.[26]
Figure 18
Comparison of filtrate volumes of water-based drilling mud with
PA, PEG, or their synergistic action.
Comparison of filtrate volumes of water-based drilling mud with
PA, PEG, or their synergistic action.
Surface Characteristics
Micromorphology
of Wear Scars on Wear-Resistant
Steel Balls
In water-based drilling mud, bentonite particles
are hard and can be regarded as abrasives. When the antiwear test
was started, these bentonite particles were pressed against the friction
pair, which would produce plowing and wear. However, with the addition
of lubricants, the lubricant molecules would adsorb on the surface
of the friction pair to form a film, which effectively protects the
steel surface from the influence of abrasive particles, and the worn
surface becomes smooth.[32]The wear
scar morphology of the wear-resistant steel ball was observed by scanning
electron microscopy (SEM). Figure shows a comparison of wear caused by the WBM (excluding
PA and PEG), WBM + 10% PA, WBM + 5% PEG, and WBM + 10% PA + 5% PEG.
Figure 19
Micromorphology
of the steel balls of (a) WBM, (b) WBM + PA, (c)
WBM + PEG, and (d) WBM + PA + PEG.
Micromorphology
of the steel balls of (a) WBM, (b) WBM + PA, (c)
WBM + PEG, and (d) WBM + PA + PEG.As shown in Figure a, the wear scar of WBM reveals considerable rough river-vein patterned
layers, mainly in the direction of sliding. In WBM containing PA or
PEG, as shown in Figure b,c, there are multiple relatively regular plowed tracks,
but there is still polished wear. Figure d shows the steel ball wear mark of WBM
+ 10% PA + 5% PEG. From the low-power observation, it can be found
that the surface of the grinding mark is smooth, with no deep plow
path. The high-power observation shows that the surface of the steel
ball is smooth with some friction marks in the longitudinal direction,
but no obvious polishing wear and plowing path. It has the best ability
to protect metal surfaces from wear. Compared with the above three
kinds of drilling muds, the SEM image of WBM + 10% PA + 5% PEG is
more clearer and smoother. PA or PEG was adsorbed on the metal surface
by chelation, forming a weak film; then, PEG was connected with PA
through the hydrogen bond of polar groups, forming a more solid and
thick lubricating film on the surface of steel ball, reducing the
direct contact between friction pairs, so as to achieve the effect
of reducing friction and drag.Apart from the load applied to
the friction testing machine, solids
contained in a drilling mud evidently contribute to friction and scar.
In this study, each drilling mud consisted of 50% (w/w) solids that
might lead to high friction.[33]
Micromorphology of Mud Cakes
The
solid particles in the mud cake structure without a lubricant (Figure a) are scattered
and full of edges and corners, while in the mud cake with lubricants
(Figure b), the
solid particles are tightly wrapped and round. By observing the microstructure
of the mud cakes, it can be found that the lubricants changed the
appearance of the solid particles by wrapping the solid particles
in the mud cake, transforming the irregular shape into a regular spherical
shape. PA and PEG can change the friction between the drilling tool
and the mud cake, forming a “ball bearing effect” on
the mud cake surface, effectively reducing the friction resistance
between the drilling tool and the mud cake.[34]
Figure 20
Micromorphology of the mud cakes of (a) WBM and (b) WBM + PA +
PEG.
Micromorphology of the mud cakes of (a) WBM and (b) WBM + PA +
PEG.
Analysis
of Influence of Drilling Mud Properties
on Lubricity
Influence of pH
In drilling operation,
salt alkali formation may be encountered, which may change the acid–base
property of the drilling mud. Therefore, it is necessary to study
the effect of pH on the lubricant and the drilling mud. The influence
of pH is tested with the addition of NaOH to the base drilling mud;
10% PA and 5% PEG are used as lubricious additives. The performance
of the lubricants and the lubricity of the drilling mud do not change
with pH increasing up to 11.5.
Influence
of Alkali Metal Ions
A rock salt layer or a salt gypsum layer
is often encountered in
drilling operation. When salts dissolve into the water-based drilling
mud, the performance of the water-based drilling mud becomes unstable.
Sodium, potassium, and calciumsalts were used to evaluate the salt
resistance of the system.[35] The influence
of alkali metal ions is tested with the addition of NaCl, KCl, and
CaCl2 to the base drilling mud; 10% PA and 5% PEG were
used as lubricious additives. It can be seen from Figure that alkali metal ions with
different qualities have different effects on the lubrication effect
of water-based drilling mud. The effect of K+ is higher
than that of Na+ in the case that both can reduce the coefficient
of friction of the drilling mud. The performance of lubricants and
the lubricity of the drilling mud are affected negatively by the increase
of Ca2+ content.
Figure 21
Effect of different alkali metal ions on the
lubrication performance
of the system.
Effect of different alkali metal ions on the
lubrication performance
of the system.
Influence
of Temperature
The temperature
of drilling mud varies with geological conditions and the well depth.
In the process of drilling, the circulating drilling mud system is
always in an environment of constant temperature change. Different
drilling mud systems have their own applicable temperature ranges.
To observe the change in system performance with temperature and obtain
the temperature range, the temperature resistance of the lubricant
was evaluated in a laboratory. The temperature resistance performance
was evaluated at 70–130 °C, as shown in Table , which showed that the lubrication
performance was basically unchanged and the temperature resistance
was good. The COF was determined with an FANN extreme pressure lubricator.
Table 3
Effect of Temperature and Density
on the Lubrication Performance of Drilling Mud
factor
COF
T (°C)
70
0.097
90
0.094
110
0.097
130
0.093
ρ (g·cm–3)
1.4
0.094
1.6
0.096
1.8
0.103
Influence of Drilling Mud Weight
Mud with different
densities is needed for different formation conditions
and reservoir depth. Barite is added to the basic drilling mud and
weighed to the densities of 1.6 and 1.8 g/cm3, respectively.
The influence of specific gravity of the drilling mud on its quality
is tested, as shown in Table . However, increasing the drilling mud weight with the addition
of 10% PA and 5% PEG lubricants has no effect on lubricity increasing
with the barite content. The COF was determined with an FANN extreme
pressure lubricator.
Table 5
Basic Properties of PA
appearance
COF
pH
molecular weight
ρ (g·cm–3)
amber clear liquid
3
9.7
1000–5000
1.15
Conclusions
In this paper, the conventional and lubrication performances of
water-based drilling mud including PA or PEG and their hybrid combination
were comparatively examined and the following conclusions were drawn:The COF of the water-based
drilling
mud mixed with 10% PA and 5% PEG was the lowest, reaching 0.094, the
reduction rate of COF was 63.1%, and the mud cake stuck factor was
also the lowest.The
addition of PA and PEG had no
significant effect on the PV of the water-based drilling mud, but
they increased the YP and gel strength of the drilling mud, which
was advantageous. At the same time, they significantly reduced the
filtrate volume, the reduction rate of filtrate volume reaching 43.5%.Through chelation and
hydrogen bonding,
PA and PEG formed a tough and thick lubricating film on the surface
of the friction pair, which also changed the appearance of solid particles
in the mud cake, thereby achieving the purpose of reducing friction.
Materials and Methods
Materials
Sodium hydroxide pellets
(NaOH, 99.9%) were obtained from Shanxi Sun Sheng Chemical Co. Ltd.
Wyoming sodium bentonite (Aquagel Gold Seal) was obtained from Shanghai
THIOE Mining and Construction Machinery Co., Ltd, China. Polyanionic
cellulose (PAC) was obtained from Nanjing Landoil Chemical Co., Ltd.
Sodium chloride (NaCl), potassium chloride (KCl), calcium chloride
(CaCl2), xanthan gum, lignite resin (SPNH), modified starch
(FLOTROL), asphalt resin (LSF), and PA (l) were supplied by Jingzhou
Jiahua Technology Co., Ltd. PEG(s) were supplied by Jiangxi Yipu Pharmaceutical
Co., Ltd.
Experimental Procedure
Drilling
Mud Preparation
The drilling
mud was prepared by mixing water, PA, NaOH, sodium bentonite, polyanionic
cellulose, xanthan gum, modified starch, lignite resin, asphalt resin,
and barite, using a high-speed agitator. First, prehydrated bentonite
slurry and water were mixed and stirred in proportion for 10 min.
Second, xanthan gum was added and stirred for 5 min to increase the
viscosity. Third, the filtrate reducers polyanionic cellulose, modified
starch, lignite resin, and asphalt resin were added, in turn, and
stirred for 20 min. Finally, barite was added to improve the density
of the slurry and stirred for 20 min. The drilling mud’s weight
was 1.40 g/cm3. Table shows the formulation of WBM used in this study. The
basic properties of PA and PEG are shown in Tables and6, respectively.
To simulate the circulation of the drilling mud in the borehole under
the actual drilling conditions, all drilling mud samples were aged
in a hot rolling oven at 90 °C for 16 h.
Table 4
Drilling
Mud Composition
components
function
concentration (g)
water
base fluid
320
prehydrated bentonite slurry
viscosifier,
filtration control
88
NaOH
regulate alkalinity
1.2
xanthan gum
viscosifier
0.8
polyanionic cellulose
filtration control
1.2
modified starch
filtration control
8
lignite
resin
plugging agent
8
asphalt resin
plugging agent
8
barite
weighing agent
205
Rheological Tests
The rheological
properties of the samples were tested using a ZNND 6 rotational viscometer.
The hot-rolled drilling mud was stirred at a high speed for 20 min
and then heated to 50 °C to determine its rheological properties.
The rheological properties including apparent viscosity (AV), plastic
viscosity (PV), and yield point (YP) were calculated from the readings
of the 600 and 300 rpm dials according to the following formulas obtained
by the American Petroleum Institute’s (API) drilling mud evaluation
standard procedure.Gel strength is the shear stress
of the drilling
mud measured at a low shear rate (3 rpm) after a static period of
10 s and 10 min.
Filtration Testing
API Fluid Loss
The drilling mud
was poured into a filter cup, a filter paper was placed on it, and
it was covered with a filter cover and pressed tightly. The pressure
was quickly pressurized to 100 psi and released into the cup. When
the first drop of the filtrate began to appear, the volume of the
filtrate was recorded using a stopwatch for 30 min.
HTHP Fluid Loss
First, drilling
mud was injected into the filter cup, the upper and lower valve stems
were closed, and then put into a heating sleeve. Second, the top and
bottom regulators were adjusted to 100 psi, respectively. Third, the
upper valve stem was opened, the pressure was released into the filter
cup, and the pressure was maintained until the required temperature
was reached. Finally, when the pressure reached 600 psi, the pressure
at the top was increased to 600 psi and the bottom valve was opened
simultaneously to start collecting the filtrate for 30 min.
Lubrication Test
The coefficients
of friction of drilling mud and mud cake are technical indexes to
evaluate the lubrication performance of drilling mud.[36] Friction is related not only to the lubricity of drilling
mud and the roughness of the contact surface between the drilling
tool and formation but also to the plastic deformation of the contact
surface, the size and distribution of the lateral force of the drill
string, and the size and rotation speed of the drill string.The basic principle of most lubricity testers is to measure the sliding
coefficient of friction, such as a mud cake stuck factor tester, which
is used to measure the “stuck pipe tendency” of the
mud cake. Or by measuring the torque between the rotating surface
and the stationary surface, such as EP extreme pressure lubricator,
it can simulate the torque produced by the wellbore to the rotation
of the drill pipe and the resistance to the running in. The lubrication
performance test was carried out according to the Chinese standard .Lubricity coefficient: The lubricity coefficient
was determined
with an FANN extreme pressure lubricator. The friction part of the
instrument was known as the lubrication ring and slider. During the
dynamic friction test, the slider was pressed on a rotating steel
ring, and the reading of the coefficient of friction was taken at
150 pounds after 5 min. Since each measurement result did not have
the same value, a correction factor of 34 divided by the water reading
was used, as shown in eq .where k0 is the
lubricity coefficient, kd is the torque
reading of lubricity ring–block pairs in the sample, kw is the torque reading in deionized water,
and 34 is the correction coefficient.To further indicate the
lubricant effect, the percentage reduction
in the lubricity coefficient was used to quantify the lubrication
performance, as shown in eq .where Rk is the
reduction percentage of the lubricity coefficient, k0 is the lubricity coefficient of water-based mud, and k1 is the lubricity coefficient of water-based
mud with lubricant.All tests were performed at room temperature.
Mud Cake Stuck Factor
When the
working slide plate was inclined, the slide plate placed on the mud
cake was affected by the downward gravity and started to slide after
overcoming the viscous force, as shown in Figure .
Figure 22
Schematic diagram for the measurement of the
mud cake stuck factor.
Schematic diagram for the measurement of the
mud cake stuck factor.Suppose the weight of
the slider is W, its component
in the inclined plane is F, that is friction. Its
vertical component is P, that is positive pressure.
According to Newton’s law of frictionIt can be seen from the similar
triangle relationship
that∠β is measured
by an instrument,
and tan β is the coefficient of friction of the mud cakes;
that is, the mud cake stuck factor is measured by the instrument.
Antiwear Performances
First,
the oil box was filled with water-based drilling mud, and the motor
was started to make the bearing rotate and rub against the steel ball.
Second, a weight was added every 3 s until the film broke and the
bearing locked up. Finally, the motor was turned off, and the steel
ball was taken out and kept for the follow-up observation of the wear
trace. The antiwear machine is shown in Figure .
Figure 23
Antiwear machine.
Antiwear machine.After the antiwear test, the steel ball was taken off. The wear
scar developed on the ball was determined using a microscope to assess
the antiwear behavior of the lubricant in the drilling mud.
Microstructural Observations
The
wear scar formed on the antiwear steel ball was observed using a scanning
electron microscope (SEM) (SU8010, Hitachi, Japan) to evaluate the
antiwear performance of the lubricants in the drilling mud. At the
same time, mud cakes were characterized using the SEM, which was used
to describe the geometry change of solid particles in mud cakes after
adding lubricants into the drilling mud, so as to evaluate the effect
of lubricants on the lubrication performance of mud cakes.
Drilling Mud Properties
To determine
the influence of various property changes of drilling mud on its lubrication
performance, density variables, temperature variables, and various
alkali metal ion contamination experiments were carried out.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Figure 17
Figure 18
Figure 19
Figure 20
Figure 21
Figure 22
Figure 23