Modified Rectorite Provides Reliable Rheology and Suspendability in Biodiesel-Based Fluids
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The use of organophilic clays as additives to provide proper rheological and filtration properties in nonaqueous drilling fluids (NADFs) has long been a topic of study. Currently, most of these clays are based on the modification of bentonite with quaternary ammonium salts. As new NADF systems emerge, novel clay-modification technologies are needed to obtain more-effective organophilic clays for specific drilling fluids. This paper introduced a modified rectorite designed for biodiesel-based drilling fluid (BBDF).
Biodiesel has drawn a great deal of attention from petroleum scientists and drilling engineers who are seeking environmentally friendly, high-performance, low-cost drilling fluids. The advantageous properties of biodiesel—such as a high flash point (for fire safety), sufficient viscosity (to carry cuttings to the surface during drilling), low eco-toxicity, excellent biodegradability, and good lubricity—make it suitable as a base oil for drilling fluid. Biodiesel also is relatively inexpensive in many situations. Many investigations and trials are being conducted on BBDF, suggesting some exciting results.
A general necessity for new drilling-fluid techniques is to customize a series of corresponding chemical additives; BBDF is no exception. Preliminary experience has shown that an additive used in traditional oil-based drilling fluids (OBDFs) may be unable to perform well in a BBDF system. Consequently, new additives should be developed for BBDF in order to obtain the optimal operational and environmental properties.
Traditionally, most organophilic clays for drilling fluids are based on the intercalating reaction between bentonite and quaternary ammonium salts, which are classified as cationic surfactants. However, this type of organobentonite (OB) still has some disadvantages, such as eco-toxicity and high resistance to degradation when discarded in the environment.
Inexpensive rectorite has been selected to replace the commonly used bentonite as the raw clay to produce organophilic clay through a modification reaction with nonionic surfactants. Crucial properties, including swelling index, viscosity, yield point, gel strength, and suspendability, were evaluated.
Like bentonite, rectorite has excellent colloid properties (i.e., swelling and gelation in water), which implies that it has the potential to be organophilized. The bonding between rectorite crystal layers is weak, and numerous exchangeable cations exist on the rectorite surface. This, coupled with a large specific surface area, facilitates surfactant intercalation into rectorite. A previous market survey has shown that commercial rectorite is generally cheaper than its bentonite counterpart by 10–20% (based on the price of bentonite) in China. For this reason, developing an eligible organorectorite (OR) for BBDF will help reduce costs for drilling operations.
Nonionic surfactants were selected to alter the wetting characteristic of the rectorite surface through intercalation. Preparation of OR is based on a suspension production technique with nonionic surfactants. Raw rectorite experiences three modification steps—pretreatment, reaction, and purification—before OR can be obtained. This process is relatively simple and produces uniform products. For BBDF, experience suggests that the surfactants with some unsaturated carbon bonds can produce ORs that have good swelling behavior in a biodiesel emulsion. Fig. 1 shows rectorite powder and OR. X-ray diffraction analysis revealed that the basal spacing of rectorite increased from 1.097 to 1.619 nm after organophilization, which reflects a high degree of intercalation.
Basic Property Evaluation
Structure and Composition. Laboratory tests were conducted to characterize the OR through chemical analysis. A commercial OB was also tested for comparison. OR and OB have comparable densities and particle sizes, but OR has a much larger basal spacing than OB, which reflects its looser crystal structure. This loose structure may allow biodiesel to enter the interlayer space of OR and form the thixotropic biodiesel-based gel required by drilling-fluid rheology.
Swelling Test. The swelling abilities of OR and OB in base fluid were tested with procedures following the speciﬁcation deﬁned in ASTM Standard D5890, which is for water-swell clay minerals. In these tests, water has been replaced with biodiesel, diesel, white oil, and the invert emulsions of these three oils.
Analyzing the swelling indices of OR and OB in different organic liquids reveals that OR swells better than OB both in biodiesel and in a biodiesel emulsion, while results for white oil and the white-oil emulsion revealed the opposite. In diesel and the diesel emulsion, these two clays have comparable swelling ability. This indicates that OR is particularly suitable for BBDF and can be used also in diesel-based drilling fluid. In general, the swelling behavior of the organophilic clays in emulsions is greater than that in pure oil. Emulsified water droplets probably are helpful for swelling, especially in the presence of several surfactants. Therefore, a biodiesel emulsion was used instead of pure biodiesel for subsequent studies.
Emulsion Rheology Test. First, studies examined the viscosity growth rates of the biodiesel emulsion with different organoclays (1.2 wt%) under high-speed agitation. Both OR and OB enhance the apparent viscosity and yield stress of biodiesel emulsion significantly with intense agitation. The times required for reaching the apparent viscosity plateau for the two organophilic clays were similar, while OR reaches the yield stress plateau more quickly than OB. This means that OR can disperse and swell in biodiesel emulsion quickly. OR exhibits a greater viscosifying ability than OB with the same dosage, suggesting high efficiency in a biodiesel emulsion.
Electrical Stability Test. The addition of organoclay influences the stability of invert emulsions and, consequently, the corresponding drilling fluids. OR is able to enhance the electrical stability by approximately 200 V, while OB only influences the electrical stability to a marginal degree. Some surface-active organoclays can move to the oil/water interface and strengthen the film formed by emulsifiers, generating a Pickering emulsion. As a consequence, the droplets have more difficulty agglomerating after they collide with one another, ensuring emulsion stability.
Near-Infrared Spectroscopy Test. A near-infrared spectroscopy stability test based on the multiple-light-scattering technique was used to study the stability of the biodiesel emulsion containing organoclays. The stability indices of the biodiesel emulsions containing organoclays were calculated according to the literature; a lower stability index signifies greater emulsion stability. Results show that the addition of OR or OB can decrease the stability index remarkably, reflecting a formation with a more-stable emulsion. Moreover, the stabilization effect of OR is greater compared with that of OB.
Evaluation of Formulated BBDF With OR
Rheology and Filtration. A fully formulated BBDF was prepared to evaluate the eligibility of OR. The investigated properties include electrical stability, basic rheological properties, and high-temperature/high-pressure fluid loss volume (HT/HP FL). The rheological profile of BBDF is relatively flat, which helps obtain a smooth equivalent circulation density and benefits well control during drilling. The 6-rev/min reading and yield stress change slightly with temperature regardless of whether the BBDF is weighted. Plastic viscosity, 10-minute gel strength, and HT/HP FL results also are acceptable. These results reveal that the rheological and filtration behavior of BBDF can be controlled by using the appropriate combination of OR and rheological modifiers.
Suspendability. Static barite-sag tests were conducted to evaluate the suspendability of BBDF. All the studied BBDFs have satisfactory suspendability.
Environmental Acceptability. The formulated BBDF containing OR conforms to Chinese standards and is practically nontoxic. In an aerobic biodegradability test, BBDF biodegraded by more than 75% in 28 days, while the OBDF (based on white oil for comparison) only degraded by 38.5%. Also, an anaerobic biodegradability test indicated that 62.2% of the BBDF biodegraded after 60 days, contrasted with 32.9% of the OBDF. These results reveal excellent environmental acceptability of BBDF and confirm the acceptability of the additives (including OR) in environmentally sensitive drilling operations (e.g., marine drilling).
- Nonionic surfactants can be intercalated into the interlayer space of rectorite, altering its surface wettability and producing OR.
- OR has excellent swelling behavior in a biodiesel invert emulsion, which leads to the improvement of rheological properties.
- OR can enhance the stability of invert emulsion biodiesel to some degree.
- OR can be used in BBDF as a high-efficiency viscosifier that provides reliable rheology and suspendability at high temperatures (up to 120°C).
- OR, coupled with a rheological modifier, offers BBDF a flat rheological profile to ensure smooth drilling operations.
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Modified Rectorite Provides Reliable Rheology and Suspendability in Biodiesel-Based Fluids
01 November 2018
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