1.
Introduction
Graphene
materials have become a research hotspot in the field of materials because of
its unique two-dimensional structure and excellent properties [1-3]. The basic and application Research of Graphene
has become the current frontier and hot topic. At present, Graphene materials
have been widely used in energy storage and conversion[4-6],
catalysis[7,8], electronic devices[9,10], high strength materials[11], chemical and biosensor[12,13],
biotechnology[14,15] and other fields. Two
scientists at the University of Manchester, Andre Geim and Konstantin
Novoselow, successfully stripped Graphene from graphite by mechanical means in
2004, causing a worldwide sensation. Research on Graphene shows explosive
growth. By the end of May 8, 2017, the Web of Science showed 144925 related
reports after searching "Graphene". Graphene materials, the new
favorite of the battery industry, are expected to cause a revolution in the
battery industry. Graphene materials are considered to be the material with
largest strength unit mass. With the increasing energy consumption and the
failure of traditional energy sources, the development of unconventional energy
and complex wells such as, deep wells, ultra-deep wells, extended reach wells,
wells with long horizontal section has increased year by year [16-18]. With the continuous development of oil and
gas resources, the grade of conventional oil and gas resources is getting worse
and worse, and the difficulty of development is further increased[19-20]. The development of unconventional energy and
wells with complex conditions is limited by existing technology [21]. As the "Blood" of drilling industry,
wellbore fluids face great challenges. Graphene material has become the new
favorite of the material field after carbon nanotubes, and the continuous
innovation of Graphene material will greatly promote the development of the
oilfield.
According
to the development needs of petroleum industry, this paper discusses the key
technology of Graphene materials about applications and prospects on the basis
of extensive investigation of research status in oilfield. Four aspects, the
characteristics of Graphene materials, research status, key technology of
application and the prospect in oilfield are reviewed.
1.1
Characteristics
of Graphene material
Graphene
structural features
Graphene
is a monatomic layer of graphite that can exist independently in the external
environment [22]. Graphene is obtained by
stripping graphite in a physical or chemical manner. At present, the
preparation methods of Graphene mainly include epitaxial grown method [23], mechanical stripping method [24], chemical vapor deposition method [25], thermal or oxidation stripping method [26]. The improved Hummer method is adopted by many
scholars [27-28]. There are many different
groups such as carboxyl group, epoxy group and hydroxyl group on the surface of
Graphene oxide stripped by chemical method, as shown in Figure 1[29]. Graphene oxide Nano sheets are mainly composed of
hexagons which are not oxidized, and some parts are oxidized.
The
groups such as carboxyl groups are concentrated on the edges of Graphene oxide
nanosheets. There are a large number of hydroxyl groups and epoxy groups in the
middle zone of Graphene oxide nanosheets. The reduced Graphene is obtained when
the Graphene oxide is reduced. The hydroxyl and epoxy groups in the middle zone
of reduced Graphene nanosheets are reduced Hexagonal structure is destroyed and
Graphene nanosheets with structural defects are formed. A large number of
carboxyl groups, hydroxyl groups and epoxy groups are active groups, which are
easy to be functionalized. The structural characteristics of Graphene make it
easy to be functionalized. It laid the foundation for the extensive application
of Graphene.
Applications of Graphene
Graphene
is a new carbon material after carbon nanotubes. Graphene has maximum strength
per unit mass with unique electric conduction and is easy to be functionalized,
which all develop the application of grapheme. At present, Graphene composite
materials have attracted great attention in the fields of energy storage and
conversion, catalysis, electronic devices, high strength materials, chemical
and biological sensing, and biological technology and so on. Research and
concerns about Graphene continue to heat up.
1.2
Applications in
the oil industry
Some
scholars have carried out research on the application of Graphene in oilfield
because of its excellent performance. In June 2011, James M. Tour invented a
patent on the application of Graphene and modified Graphene in drilling fluid [30].The patent reports the preparation of chemical
modified grapheme, as in Figure 2.1.
The
benzene was grafted onto the surface of Graphene by using hydrazine hydrate as
reducing agent. Graphene and chemical modified grapheme are added to the
drilling fluid, and Graphenenanosheets are adsorbed onto the surface of
wellbore to reduce drilling fluid into the stratum, as shown in Figure 2.2.
The
cake with the addition of Graphene and modified Graphene is thin and compact.
Graphene and modified Graphene can effectively improve the quality of filter
cake and reduce drilling fluid into the formation. The same year in September,
James M. Tour publishes a paper about the research on Graphene oxide as
filtrate reducer of high performance [31] in ACS
Applied Materials & Interfaces. The paper describes the process of
preparation for Graphene oxide by improved Hummer method in detail. The
detailed study on property of Graphene oxide as filtrate reducer in drilling
fluid shows that the filtration is 6.1mL in 0.2 wt% low concentrations which is
less than that of conventional filtration reducer, and the average filtration
is 7.2 mL The thickness of the filter cake is about 20 μm,
far less than that of conventional filtration reducer which is 280 μm. Flexible Graphenenanosheets can enter pores
smaller than their size, and there is a large amount of Graphenenanosheets in
the filtration of the drilling fluid, as shown in Figure
2.3.The addition of methanol modified Graphene oxide into drilling fluid
results in lower filtration. Graphene oxide has excellent performance on
filtration control.
In
September 2013, Xuan Yang reported a paper about the preparation and evaluation
of nano-Graphene oxide as a high-performance fluid loss additive [32]. Filtration control properties of Graphene oxide
according to the API standards are evaluated and the results show that Graphene
oxide has excellent fluid loss performance. Compared with conventional filtrate
reducers, Graphene oxide has the advantages of low dosage and thin cake.
In
October 2013, Steve Young published a patent on Graphene material as shale
inhibitor [33]. Based on James M. Tour chemical
modified method, the patent evaluates rheological properties and rolling recovery
rate after modifying Graphene oxide, the inhibition is characterized by rolling
recovery rate.The research about application of Graphene as lubricant was
published by NorasazlyMohd Taha in IPTC in September 2015[34]. Graphene materials can form a dense film on the
surface of the metal under the pressure, temperature and mechanical force. This
Graphene film is formed under the synergistic action of physical adsorption and
chemical adsorption. Compared with traditional lubricants, Graphene materials
have much more excellent lubricity. In the aqueous polymer brine system,
laboratory tests show that the friction of Graphene materials is at 70%-80%
reduction, while it of conventional lubricant materials is generally at only
30%-40% reduction. At the same time, the Graphene material still has excellent
lubricity at 176℃. In May 2016,
our team studied the inhibition and the plugging performance for the nano pore
of shale of amino modified Graphene[35]. As
shown in Figure 2.4, we select sandstone with
high permeability and shale with ultra-low permeability as cores, downstream
pressure is respectively recorded in 4%NaCl brine system and 0.4% amino
modified Graphene solution. The longer time the downstream pressure reaches the
upstream pressure, the performance of plugging cores is better. The time which
the upstream pressure reaches the upstream pressure in brine system is about 5
minutes when sandstone is tested, while in modified Graphene system the time is
about 24 hours.
This
result shows that the amino modified Graphenehas the ability to seal pores in
sandstone. When natural shale is chosen as core, in brine system the time which
the upstream pressure reaches the upstream pressure is about 5 hours, while the
downstream pressure still does not increase after 66 hours in amino modified Graphene
system.
The
ability of Graphene to plug nano pores is further revealed. Amino modified Graphene
also has excellent ability to inhibit shale expansion. The mechanism of action
between amino Graphene and clay minerals is analyzed in detail in the paper.
Graphene is adsorbed on the surface of shale by hydrogen bonding and
hydrophobic interaction, and a compact film is formed. This film prevents
moisture entry and inhibits the hydration expansion of the clay. The experiment
data shows that the amino modified Graphene has excellent ability of plugging
the nano pore of shale and inhibiting the hydration expansion of clay.
1.3
Prospects of Graphene
in oilfield
Through
the domestic and foreign literature investigation, 4 related literatures and 2
patents are retrieved about the application of Graphene materials in oilfield.
Many scholars begin to explore the application of Graphene in the petroleum
industry. Although a lot of work can be carried out, it is still in laboratory
research now. Lots of work is just test on performance, not a thorough study of
mechanism. Graphene, as a new favorite in the field of materials, will have
broad application prospects in oilfield. Mainly in the following areas:
1.3.1
Drilling Fluid
Graphene
materials as filtrate reducer for drilling fluid, showing excellent fluid loss
agent performance, but the mechanism of Graphene materials and clay minerals
are not in-depth study, and not study compatibility with other additives for
drilling fluid. Graphene material, as a shale inhibitor of drilling fluid, is
only used for the determination of rolling recovery. The mechanism of action
was not studied. Graphene material is a kind of monolayer graphite with many
active groups. It has the following application prospects in drilling fluid.
1.3.2 Tackifier and
Shear Strength Agent
Intermolecular
weak interaction between active groups of Graphene materials is formed by
hydrogen bond, hydrophilic-hydrophobic interaction and π-π interaction.
Graphene materials have excellent stimuli-responsiveness. High Shear thinning
agents with high shear strength and low viscosity can be obtained by molecular
design. At high shear rate, the viscosity is low especially near the nozzle.
While at low shear rate, the viscosity is high, so it is easy to carry cuttings
and prevent the settlement of the weighting materials.
1.3.3 Fluid Loss Agent
Previous
work has shown that Graphene is an excellent filtrate reducer. However, the
mechanism of action has not been studied deeply. Temperature resistance needs
further improvement. Graphene with one single layer, when the salt molecules
are close to the edge of Graphene, Graphene will curl to some extent, but the
strong π-π interaction will limit its curling. So,Graphene
materials show excellent salt resistance, especially to divalent metal ions.
Graphene materials will be widely used in the fields of filtration control with
salt resistance and temperature resistance.
1.3.4 Shale Inhibitor
There
are many active groups such as carboxyl and hydroxyl groups on the edge of Graphene
materials, and adsorption groups such as amino groups are obtained after being
modified. The adsorption of Graphene materials on rocks is enhanced. Large
areas of high strength sheet materials are adsorbed onto the rock surface to
prevent moisture from entering. Graphene is thin but has high strength, so it
can effectively reduce costs with low dosage. Graphene materials are expected
to become the unique shale inhibitors through appropriate modification,
1.3.5 Nano-Pores
Plugging Agent
For
plugging nano-pores in shale, there are no reports of industrial products at
home and abroad. The permeability of shale is low, and the pores are rich. The
average size of nano-pores in shale is about 10nm. General rigid nanomaterials
are easy to agglomerate in horizontal solution and difficult to enter
nano-pores. Flexible organic materials, with large molecular weight and little
interaction with rock, are difficult to adsorb to the surface of shale. By
molecular design, strong adsorption groups are introduced to increase the
interaction between Graphene and rock. Graphene material is easy to be adsorbed
on the surface of rock. It is hopeful to solve the bottleneck of plugging
nano-pores in shale because of strong strength of Graphene materials and their
pressure-bearing capacity.
1.3.6
Lubricant
By
introducing groups which have interaction between metal ions, Graphene
materials can be adsorbed onto the surface of the drilling tools and form a flexible
film which reduces the friction between drilling tools and drilling fluids,
drilling tools and rocks. The lubrication performance of drilling fluids can be
effectively improved. The force acting between drilling tools and drilling
fluids can be reduced by introducing hydrophobic groups which can prevent bit
balling, sticking of tool and other accidents effectively.
1.3.7
Wet Ability
Reversal Agent
As
hydrophilic materials, the hydrophilicity and hydrophobicity of Graphene can be
changed by introducing hydrophobic groups. These modified Graphene can be
adsorbed onto the surface of the rocks and change the wettability of rocks.
Graphene materials are expected to be an excellent wettability reversal agent
1.4
Completion Fluid
and WorkOver Fluid
Completion
fluid and workover fluid generally use brine systems to protect reservoirs. The
adsorption of Graphene on the surface of rocks is beneficial to the stability
of the borehole. After the operation, the Graphene is desorbed from surface of
the rocks under the formation pressure and after encountering the oil layer,
then reverse to the ground. Graphene materials are environmentally friendly,
low-viscosity and easy to reverse.
1.5
Acidizing Fluid and
Fracturing Fluid
Diverting
acid is the core agent of acidizing fluid. Graphene is oxidized by strong acid
and can exist stably under acidic conditions. Calcium sensitive Graphene
materials can be prepared with molecular modification. It is expected to become
a steering acid with little amount, low cost and excellent performance.
Graphene and most polymers can form weak gel systems, which are environmentally
responsive by the weak forces through non-covalent bonds. It is expected to
become a new generation of fracturing fluid system.
1.6
Plugging Agent
There
are no mature technical countermeasures to severe leakage. The plugging
operation is carried out regardless of cost and times. Polymer gel has become
the new material for plugging because it is not limited by pore size, crosslinking
time and breaking time are controllable. Graphene can form the gel system with
polyacrylamide, polyacrylic acid, polyvinyl alcohol and Carboxymethyl
cellulose. Graphene can enhance the strength of gel, and also has excellent
adsorption ability. Therefore, Graphene and polymer gel complex are expected to
solve the problems of severe leakage.
2.
Conclusions
The
application of Graphene material in oilfield has been preliminarily explored.
Many experimental achievements are still in the laboratory research. The
research on the mechanism of action is not thorough, and the range of
application is still relatively narrow. The following aspects need to be
studied:
1.
Study the mechanism of action in depth and optimize the design of molecules.
2.
Expand the scope of research, conduct researches on drilling fluid, completion
fluid, workover fluid, acidizing fluid, fracturing fluid, plugging agent.
3.
Accelerate the industrialization of Graphene materials.
Graphene
materials with will excellent properties have broad application prospects in
oilfield. It is expected to become a new generation of working fluid system. It
is hopeful to play a unique role in the exploitation of unconventional oil and
gas resources such as shale gas, coalbed methane and tight gas.
3. Acknowledgements
We
would like to thank for the financial support from China Postdoctoral Fund
(H29216)for this work.
review article 
