Download Li-Ming Zhang Institute of Polymer Science, School of Chemistry and

Li-Ming Zhang
Institute of Polymer Science, School of Chemistry and Chemical Engineering and Key Laboratory
for Polymeric Composite and Functional Materials of Ministry of Education, Zhongshan
University, Guangzhou 510275, China
A Review of Starches and Their Derivatives for Oilfield Applications in China
As a class of important polymeric additives, numerous starches and their derivatives have been
prepared and their suitability studied for various oilfield applications such as filtrate-loss control,
mud-rheology modification, shale stabilization, enhanced oil recovery (EOR). drag reduction, and
water shutoff, and some of these starches have been widely used in this area. These materials
include mainly pregelatinized starches, ether-ified starches, and grafted starches. This article
provides a review of their research and development in view of the special situation in China. Some
important features of their properties are also presented.
Keywords: Starch derivatives; Pregelatinized starches; Etherified starches; Grafted
starches; Oilfield applications
1 Introduction
Over the past fifty years, starch has been used as the raw material for developing polymeric
additives in oilfield industry. Starch's unique structure and reactivity opens up the possibility for the
design of various oilfield chemicals based on this biomacromolecule. Nowadays, numerous
modified starch derivatives have been prepared and studied for the oilfield applications, and some
are commercially available. They mainly include pregelatinized starches, water-soluble starch
ethers, water-soluble starch graft copolymers, crosslinked and blended starches [1-6].
In China, the choice of starch as the raw material for the production of oilfield polymeric additives
began in the early 1980s [7]. In the first decade of their application, the resulting modified starches
were used mainly as fluid-loss control agent, an important drilling mud additive. Tab. 1
Correspondence: Li-Ming Zhang. Department of Chemistry, State University of New York at
Stony Brook, Stony Brook, NY 11794-34001; E-mail: Imzhang® [email protected].
shows the amounts of starch-based oilfield chemicals and other polymeric additives used in the
filtration control in China from 1983 to 1993 [8]. These products involve mainly pregelatinized and
etherified starches [9]. It should be noted that a small tonnage of starch and its derivatives were
used before 1986, and then the amounts have obviously increased. But in comparison with other
filtration control agents listed in Tab. 1, the use of starch and derivatives was relatively limited. In
recent years, the design and synthesis of new kinds of starch derivatives including cationic and
amphoteric starch graft copolymers has become an active area of research. This is due in part to
their potential applications as water-sensitive shale- or clay-hydration suppressants and
multifunctional drilling-fluid additives. This paper reviews briefly the research and development of
starch-based oilfield chemicals in China.
2 Pregelatinized Starches
Due to low cost, pregelatinized starches were used for oilfield treatments in China [7, 9]. To obtain
pregelatinized
Tab. 1. Filtrate control agents used in China oilfields from 1983 to 1993 (based on the data by Xu et
al. [8]).
Tab. 2. Gelatinization temperature and solubility of some native starches (based on the data by
Williamson and Allenson [6]).
starches, the gelatinization of native starches was usually carried out in acid and alkali media.
Elevated temperatures are needed to cleave the hydrogen bonds between the starch granules. The
temperature at which gelatinization occurs depends on the source of the starch. Tab. 2 gives the
gelatinization temperatures and solubility for various native starches [6].
Zhang [10, 11] was the first to describe the preparation and properties of pregelatinized starches as
drilling mud additives. Two methods, namely sulfuric acid hydrolysis and sodium hydroxide
hydrolysis, were used. Corn starch was first reacted with suitable concentration of aqueous sulfuric
acid or sodium hydroxide for a certain time. The resulting starch paste was then hydrolyzed by
H2SO4 or NaOH, neutralized, precipitated with alcohol, filtrated, purified, dried under vacuum, and
powdered, producing the pregelatinized starch. It was reported [11, 12] that this product could be
used as a polymeric additive for saline drilling fluid systems, in particular as fluid-loss control
agent. As shown in Tab. 3, application of pregelatinized starch leads not only to high apparent and
plastic viscosity of the treated mud, which makes the mud stable and viscous, but also reduces
effectively the filtration volume of the saturated saline mud from 178 ml to 12 ml. measured according to the American Petroleum Institute (API) specifications.
Compared with carboxymethyl cellulose (CMC), a commonly used fluid-loss control agent,
pregelatinized starch shows matched filtration-control ability and enhanced viscosification (Tab. 3)
at a lower price. However, this kind of starch can not be used effectively in freshwater drilling fluid
systems because of its relatively low tolerance to thermal and bacterial actions, the mechanism of
which was investigated previously by some researchers [13-15].
Another possible application of pregelatinized starches is its use as a non-damaging filtrate reducer
for water-based fracturing fluids. This is especially interesting because of the increasing concern of
the damage that conventional, non-degradable fluid-loss reducers such as silica flour and clay cause
to the producing formation. In contrast, pregelatinized starches are completely degradable and
residue free. Williamson and Allenson [6] examined the effectiveness of pregelatinized potato starch
and pregelatinized corn starch as fracturing fluid loss reducers. The results show that such material
can lower the spurt loss and control the leak off of fracturing fluid in some extent. To date,
however, there is no similar work reported in China.
3 Etherified Starches
In etherified starches part of hydroxyl groups are substituted by ether functions [16]. The degree of
chemical modification may be expressed as the degree of substitution (DS), i.e. the average number
of substituents per o-glucose unit. Compared with native starches and pregelatinized starches, these
products possess good solubility and more functional groups. Tab. 4 summarizes important starch
ethers studied for the oilfield applications in China. They include anionic sodium carboxymethyl
starch (CMS-Na) [17-20], potassium carboxymethyl
Tab. 3. Rheological and filtrate-control properties of pregelatinized starch in saturated saline mud
(based on the data by Wang [12]).
Tab. 4. Summary of etherified starches for the oilfield applications.
starch (CMS-K) [21], 2-hydroxy-3-sulfopropyl starch (HSPS) [22], carboxymethyl hydroxyethyl
starch (CMH-ES) [23], nonionic hydroxyethyl starch (HES) [24]. hy-droxypropyl starch (HPS)
[25], and cationic 2-hydroxy-3-(triethylammonium)propyl starch (HTPS) [26]. Among them, CMSNa prepared by etherification of alkalized starch with chloroacetic acid or sodium chloroacetate is
most commonly investigated and has been used as polymeric anion for controlling the filtrate-loss
of drilling muds.
There are many studies dealing with the preparation of starch ethers and their properties as oilfield
chemicals, in particular preparation of higher-substituted derivatives. Due to the swelling of starch
granules, it is usually difficult to prepare higher-substituted starch ethers in aqueous
suspensions. In order to obtain higher DS, it is necessary to limit the amount of water during the
etherification. Thus, the reaction is usually conducted in a water-misci-ble solvent containing minor
amounts of water. Iso-propanol is probably the solvent of choice, although both methanol and
ethanol have been widely used. This technique has been successfully applied by Sun et al. [19] to
the preparation of CMS-Na with a DS of 1.1, and by Wue\ al. [20] to the preparation of CMS-Na
with a DS of 1.7. It was found [19, 20] that increasing the DS to some extent could lead to enhanced
filtrate-loss control, salt tolerance and thermal stability of the resulting starch ether. The data listed
in Tab. 5 show the effectiveness of the CMS-Na with high DS of 1.7 as drilling mud additives.
Tab. 5. Property evaluation of high-substituted CMS-Na (DS = 1.7) in various types of drilling
muds (based on the data by Wu and Zhang[20]).
a)
Measured after aging at room temperature for 24 h according to the method listed in Tab. 2.
Measured after aging at room temperature for 7 days according to the method listed in Tab. 2.
c)
Measured after aging at 95 °C for 24 h according to the method listed in Tab. 2.
b)
Besides the DS, the kind and nature of substitutents is another major factor affecting the properties
and functions of etherified starches as oilfield additives. In spite of good filtrate-control and
viscosity-building properties in some conditions, anionic CMS-Na [17, 18] and CMS-K [21] are
relatively readily influenced by oil reservoir temperature and salinity when compared with nonionic
HES and HPS [24, 25] due to the presence of anionic carboxymethyl groups. To combine the
advantages of anionic and non-ionic starches, Yao [23] synthesized CMHES containing two
different types of substituents, which show a better behavior in drilling muds. In the case of HSPS,
Wang [22] found that the introduction of sulfonate-containing groups both increases the tolerance of
the modified starch towards divalent ions and extends its thermal stability to high reservoir
temperature.
In addition, Wang and Luo [26] observed that cationic HTPS is characterized by a poor filtrate-loss
control ability but a good inhibition to water-sensitive clay and shale in oilfields, which is caused by
the affinity of quaternary ammonium substituent groups to negatively charged clay surfaces.
Nowadays, the oilfield industry shows a growing interest in the use of sodium-free, potassiumbased drilling fluids with inhibitive properties. For this reason, Liu [21] prepared CMS-K. CMS-K
has proved to be effective in controlling troublesome shales, which offers the possibility to prepare
sodium-free drilling fluids [21].
4 Grafted Starches
In order to promote the utilization of starch in oilfields, the grafting of various vinyl monomers onto
starch substrate has been another subject of extensive investigation in China [27-36]. The possibility
of using water-soluble grafted starches as filtrate-loss controllers, thickeners and shale stabilizing
polymers for aqueous drilling fluids, as viscosity modifiers in displacement fluids for enhanced oil
recovery (EOR), and as oilfield drag-reduction agents have generated considerable interest in
academic and industrial research. Tab. 6 summarizes important grafted starches studied for oilfield
applications. In comparison with pregelatinized starches or etherified starches, these grafted
starches show some advantages in optimizing the
Tab. 6. Summary of grafted starches for the oilfield applications.
chemical structure I/M it is, e.g. easier to increase the molecular weight and introduce various
groups I/M and functions of the starch derivatives as oilfield additives.
Among a number of monomers available for grafting, acrylamide (AM) has been widely used. Yao
and Wang [27] reported the synthesis of polyacrylamide-grafted potato starches, and tested their
properties in the drilling muds; Huangand Shen [28] prepared three series of graft copolymers of
oxidized starch, soluble starch, and potato starch with AM using a ceric-ion-initiated solution polymerization technique and their corresponding hydrolyzed products. They investigated the viscosity
properties of these derivatives at various polymer and salt concentrations, shear rates and
temperatures. Synthetic linear polyacrylamide (PAM) or its hydrolyzed product (HPAM) and
ungrafted starch have several disadvantages. For example. PAM and HPAM suffer from poor
mechanical and chemical stability, which increases with the molecular weight [37]. Ungrafted
starch has poor solubility, limited functionality and less resistance to bacterial attack [7]. One of the
great advantages of grafting is the reduced biodegradability of the grafted products due to the
drastic change in the original structure of the natural polymer as well as the synthetic polymeric
content in the product that can not be digested by bacteria. Moreover, it has been observed that
grafting of shear degradable PAM onto the rigid starch backbone provides fairly shear stable systems [28]. In addition, by varying the number and length of grafted PAM chains onto the backbone,
it has been found that the grafted starches with fewer but longer PAM chains are more effective as
drilling-mud additives or viscosity modifiers for EOR [27, 28]. Similar studies were reported by Deshmukh and Singh et al., who used polyacrylamide-grafted amylose or amylopectin as
oilfield drag-reduction agents [38, 39].
T
o enhance the tolerance of grafted starches to higher salt content and temperature, sulfonatecontaining monomers, including sulfomethylated AM and 2-acrylami-do-2-methyl-1propanesulfonic acid (AMPS), have been grafted on starch substrates. Tan and Yao [29] prepared
sulfomethylated polyacrylamide-grafted potato starches; Quo et al. [31] prepared graft copolymers
of corn starch with AMPS; Wang [34] prepared the grafted polymers of corn starch with AMPS and
AM. These grafted starches were found to be effective in freshwater, saline, saturated saline and
highly mineralized drilling fluids, being resistant to high temperature and tolerant to salt, especially
the resulting AM/AM PS/starch graft terpolymers. Tab. 7 gives the properties of the muds treated
with the grafted polymer of corn starch with AM and AMPS before and after hot rolling at 150 °C
for 16 h.
Recently, some cationic vinyl monomers and their combination with a neutral monomer have been
grafted onto starch [32, 35]. It was found that such grafted starches could provide more effective
shale inhibition than anionic or nonionic modified starches, showing the potential as a shale- or
clay-hydration inhibitor for oilfields. This fact is due mainly to two reasons. Firstly, the cationic
groups along the grafted chains can neutralize the negative charges on the shale or clay surfaces,
thus reducing the hydration ability of the shale or clay. Secondly, the cationic groups can reinforce
the adsorption of the grafted starch and form a film on the shale or clay, thus hindering
Tab. 7. Properties of the muds treated with the grafted polymer of corn starch with AM and AMPS
before and after hot rolling at 150°C for 16 h (based on the data by Wang [36]).
Note: The mud rheological parameters and API filtration volume were determined according to the
methods listed in Tab. 2.
Tab. 8. Properties of amphoteric grafted starch (AGC) as drilling fluid additive a) (based on the data by Wang [31]).
a)
The mud Theological parameters and API filtration volume were determined according to the
methods listed in Tab. 2. b) Determined after hot rolling at 120°C for 16h according to the equation:
Shale recovery (%) = (IV/W0) x 100, where Wand WQ denote the weight after hot-rolling and the
weight before hot-rolling, respectively.
water to enter the shale or clay. Obviously, this is an advantage when compared with anionic or
nonionic modified starches. However, formulating practical drilling fluids using positively charged
grafted starches is usually difficult because these starches tend to react with bentonite, anionic
polymers, and weighting agents causing severe flocculation and precipitation. A possible pathway
to solve these problems is to develop water-soluble amphoteric grafted starches containing both
cationic and anionic groups. In this respect, Wang [36] reported the properties of amphoteric graft
starches, which were prepared by grafting potassium acrylate and 2-hydroxy-3-methylacrylamidopropyltrimethylammonium chloride onto corn starch, in aqueous drilling muds. The results
shown in Tab. 8 demonstrate that this class of grafted starches is characterized both by good shale
inhibition, filtrate-loss control and mud rheology. and may overcome the limitations inherent in
anionic and cationic starch-based additives.
Other monomers such as acrylonitrile (AN) and vinyl alcohol (VA) [30, 33] have been also grafted
onto starch substrate, resulting in the nonionic grafted starches having the possible use as drilling
mud additives.
5 Conclusion
Oilfield applications of starches and their derivatives are receiving increased attention because of
the low cost and availability of these starches and their environmental acceptability. It is known that
the specific applications of starches and their derivatives in oilfields depend on their solubility,
molecular weight, type and number of the functional groups on the polymer chain, and chemical
nature of the backbone structure. Meanwhile, the reservoir conditions such as temperature, salinity,
hardness and the pH of formation water influence the use effectiveness of these starches. This paper
reviews briefly the research and development of starch-based oilfield chemicals in China involving
mainly pregelatinized starches, etherified starches and grafted starches. Further research will focus
on the property improvement of conventional starch products, the preparation of new kinds of
starch derivatives and their novel oilfield applications.
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(Received: December 5, 2000)
(Revision received: April 11, 2001)