Polymeric Nanoparticles in Drilling Fluid Technology

Abstract

New technologies are often sought to mitigate the problems associated with traditional drilling fluid formulations. Nanotechnology provides an alternative. A particle size of matter in the range of 1–100 nm in diameter (d.nm) is referred to as nanoparticle. Nanoparticles are broadly divided into various categories depending on their morphology, size and chemical properties. This size range lends their application in science and engineering. In rotary drilling operations where drilling fluid is at the center, performance and optimization issues have been observed. Use of polymer nanoparticles in mud formulations have been considered due to desirable properties such as wide specific surface area, high temperature stability and pollution resistance. Areas of application and advantages include improvement in mud rheology, fluid loss properties, improved lubricity, filter against hazard materials and cost effectiveness. Biodegradable polymeric nanoparticles possess the outlined properties and would continue to offer wider applications in drilling fluid technology now and in the nearest future due to their stable, film forming and gelatinization characteristics. To reliably estimate the quantity of polymeric nanoparticles to use, size and shape should be considered before concentration to apply to make prediction easier. Dispersion of different shapes, sizes and structures of polymeric nanoparticles might be a consideration to enhance polymer influence on fluid formulations.

1. Introduction

The drilling fluid is a general term for liquid-based, gas-based and combinations thereof that serves as the focus in a circulating system of a rotary drilling rig. Usually, it is designed to perform certain functions, possess the required properties to furnish the desired functions and satisfy recommended specifications when tests are conducted [1, 2]. Its design involves basic knowledge of chemistry, physics, mathematics and sound knowledge of petroleum engineering principles and oilfield practices.

2. Background

The drilling fluid provides the hydrostatic pressure necessary to balance the formation or pore pressure to control influx into the wellbore or prevent kick and blowout. This is achieved by using a densifier additive. The densifier also controls the buoyancy force for partial drillstring suspension in order to reduce the derrick load. Also, for effective wellbore cleaning and cleaning beneath the drill bit, the drilling fluid must possess viscosity as a property in addition to the required fluid hydraulics objective functions. Such objectives or combinations include hydraulic horsepower, jet velocity, Reynolds number and jet impact force. The fluid should also be able to suspend cuttings in the annulus when drilling is interrupted, and release the cuttings in the solids control equipment; the thixotropic property. Similarly, the fluid should have high heat capacity to be able to cool the drillstring and bit. It can also serve as a lubricant in the system. It is also designed to reduce or prevent formation damage and seal permeable formations by application of its fluid loss property. It must also control wellbore stability by preventing hydration of hydratable clays and retard other factors. Corrosion is also prevented by addition of agents that control the pH of the fluid. It must also be able to transmit hydraulic horsepower to the bit by maintaining its desired phase at every point. Formation evaluation must not also be interrupted. Nonetheless, in oil-based and water-based drilling fluids, it is the filtrate invasion that causes interruption of formation evaluation [3]. It is the fluid loss property that controls it. Zones of filtrate invasion and filter cake deposition are (i) the invaded zone into the formation (ii) the external cake on the wall of the wellbore and (iii) the internal cake that extends inches into the formation. An example is in the determination of petrophysical property known as resistivity. Low resistivity salty water-filled rocks are distinguishable from high resistivity hydrocarbon-filled rocks [4]. Even deep reading resistivity tools may only provide investigation data at depth not beyond the invaded zone. This leads to corrections to get the true formation resistivity that might introduce errors. This may be prevented by deposition of a suitable filter cake thickness with other qualities such as slickness, toughness and permeability. The high quality cake would prevent not only deep filtrate invasion but also formation damage.

Fluid systems have been developed to furnish and control the functions outlined. Based on the base fluid and materials deliberately added to the mud, such systems include; (i) air, gas, foam, mist (ii) calcium treated (iii) dispersed (iv) low lipids (v) non-dispersed (vi) oil/synthetic (vii) polymer (viii) saturated salt and (ix) workover fluid [5]. Cost, downhole conditions, type of well and environmental consideration are some factors that control the choice of drilling fluid type for application.

The traditional fluid systems have limitations that impede complete performance of the functions outlined. For instance, water based mud systems have the tendency to dissolve salts that might lead to density increase. Similarly, interference with oil and gas flow and clay dispersion has been potential problems. Moreso, oil based mud are not cost effective and pose environmental challenges. Gas based fluids have the tendency to cause explosion and cannot be used through water bearing formation. The stability observed under atmospheric conditions might not exist under high temperature and high pressure observed at increased depths both in marine and land environments. In deep offshore drilling, formation of hydrate also poses challenge. Though additives added to drilling fluids are expected to prevent or limit the adverse effects the conventional drilling fluids might experience, oilfield experience shows that problems still occur. New technologies are often sought to mitigate these problems. The field of nanoscience, where nanoparticles with desirable attributes are evaluated for application, provides an alternative.

3. Nanoparticles: classification and attributes in formulations

The science and technology of fine particles is known as micromeritics. Through understanding of their characteristics, thousands of nanoproducts exist, and they are mainly used in drug delivery [6, 7]. Introduction of nanoparticles in drilling fluid have been under investigation, and have been applied over a decade ago. They have the potential to create changes in size and composition that result into formulations that could be adopted for a wide range of operating conditions even in small concentrations. Nanoparticles can be organic or inorganic on the basis of molecular weight and durability. Similarly, organic nanoparticles can be natural or synthetic and involves organic or polymeric molecules. These biopolymer nanoparticles are biodegradable and highly stable in fluids and storage [8] and include cellulose nanoparticles (CNP) where cellulose nanofibers and cellulose nanocrystals are examples [9], chitosan nanoparticles, starch nanoparticles (SNP), lignin and pullulan nanoparticles, alginate and gliadin nanoparticles, polylactic acid (PLA) nanoparticles, and polycaprolactone (PCL) nanoparticles. Conventional carboxymethyl cellulose (CMC) and polyanionic cellulose (PAC) ground into smaller nanosize particles are also important substances. Cellulose is considered the most abundant biopolymer in the world.

The influence of size on the physic0-chemical properties of a substance cannot be overemphasized. Since the presentation made in 1959 by Richard P. Feynman, a Nobel laureate, on nanotechnology titled “There’s plenty of room at the bottom”, various strides in the field of nanotechnology have been recorded [10]. Nanotechnology enables evaluation of matter at nanoscale and enhances synthesis. Materials have been produced at nanoscale level. A particle size of matter in the range of 1 to 100 nanometers in diameter (d.nm) is referred to as nanoparticle, and could be 0D, 1D, 2D or 3D as reference dimension. Their surface to volume ratio is extremely high due to their submicroscopic size. It is an overlap of mesoscale, 1 to 1000 nm (polymeric nanoparticles), such as used in colloid science.

In drilling fluid formulations, with additives in the nanoparticle size range, significant improvement in drilling fluid properties have been documented [11]. Such fluids are referred to as nanofluids. This is further categorized as simple nanofluids with nanoparticles of single magnitude of particle distribution, and advanced nanofluids with diverse nanosize ranges of additives. They exhibit unique characteristics, hence, extraordinary potential for application in science and engineering [12], with particular interest in drilling fluid formulation. The base fluid could be water or oil. They have the potential to modify drilling fluid properties such as plastic viscosity, yield point, gel strength, barite sag, fluid loss volume, filter cake thickness as well as improve thermal and wellbore stability. They are also used as pollution filters for cadmium and hydrogen sulphide, for improved lubricity, heavy metal absorption [13] and reduction of torque and drag. They are applicable both in atmospheric and high-temperature high-pressure (HPHT) conditions.

They possess a variety of morphologies or shapes which help serve their objectives. Figure 1 shows a comparison of nanoparticles with smaller and larger sized particles such as atoms and cancer cells respectively. Structures of nanoparticles contain hundreds of atoms. In other words, nanoparticles are larger in size than simple molecules and atoms by hundredfold. By size, bacteria are larger in size than nanoparticles which can be used as barrier to prevent their activity. Their application in medicine has shown remarkable progress.

Figure 1.

Comparison of nanoparticles with other cells and atomic particles.

Nanoparticles exist in several shapes (Figure 2), and can further be classified based on crystallinity (amorphous and crystalline), nature of material (bimetallic, metallic chalcogenides, metallic oxides, organic, pure metals), origin and source (anthropogenic and natural), phase composition (multiphase solids, single phase solids), and shape and dimension (0D, 1D, 2D, 3D). Prism and helical shaped nanoparticles also exist.

Figure 2.

Diverse shapes of nanoparticles [2, 14].

For polymeric nanoparticles of organic origin such as cellulose and starch that are polysaccharides, the nanofluids formed exhibit unique properties and are called nanogels. They have been used both in water-based and oil-based drilling fluid formulations, where they served as viscosifier and fluid loss control additives. They are biodegradable, can act as antibacterial agents and are economically efficient since a relatively small quantity is required to create the nanogel. Generally, ceramic nanoparticles can be used for purification and pollution control activities. For instance, zinc oxide has been used to remove hydrogen sulphide in drilling muds. This is because hydrogen sulphide can dissolve in metal ions. At low concentrations, zinc oxide has been applied for antibacterial control due to high surface area to volume ratio in addition to distinctive physical and chemical characteristics. The metal nanoparticles have applications in catalysis, packaging, bio-engineering, cosmetics, water treatment, medicine and drug delivery, electronics, semiconductors, automobiles, paints, biosensors and soil pollutant removal. Carbon-based nanoparticles such as graphene can be combined with corrosion-protection chemicals to be an anti-corrosive material. In drilling fluid, it can act as a barrier to retard oxidation of hydrogen sulphide that can cause corrosion. In salty water based muds and situations where the mud becomes acidic due to intrusion of substances, graphene can serve as barrier against the chemical attack that causes corrosion. Categories of nanoparticles is shown in Figure 3, while their influences on drilling fluid properties and size ranges and concentrations applied are shown in Tables 1 and 2, respectively.

Figure 3.

Classification of nanoparticles.

Nanoparticle type	Reported drilling fluid property value
	Initial gel strength [Pa]	10 min. gel strength [Pa]	Fluid loss; [mL]
Silicon dioxide (SiO2)	3.5, 13, 7	6.5, 32, 8	4.8, 7.2, 10, 5.1
Silver	—	—	2
Copper (II) oxide (CuO)	16	35	12
Multi-walled carbon nanotube; (MWCNT)	4.5	7, 7	9, 5
Nanosilica	—	6	7
Sepiolite	—	—	8
Montmorillonite	—	21.5	7
Zinc oxide (ZnO)	15, 6	37, 9	14, 4.7
Graphene oxide	—	—	6.1
Aluminum oxide (Al2O3)	15, 11	39, 40	6
Yttrium oxide (Y2O3)	15	16	—
Calcium carbonate (CaCO3)	—	—	5.7
Bismuth ferrite (BiFeO3)	13	20	7.8

Table 1.

Nanoparticles used in drilling fluids and properties recorded [11].

Nanoparticle type	Nanoparticle size [nm]	Optimal concentration
Iron (III) oxide (Fe2O3)	3 and 30	0.5–2.0% wt.
Iron (III) oxide; (Fe2O3)-clay hybrid	3 and 30	0.5% wt.
Iron (II, III) oxide (Fe3O4)	10–20	0.05–0.5% wt.
Silicon dioxide (SiO2)	50	0.5% wt.
	10–20	5–10% wt.
	20	0.5% wt.
Titanium dioxide (TiO2)	10–15	0.5–10% wt.
	20	0.1–0.3% volume fraction
Yttrium (III) oxide (Y2O3)	20–30	0.5–3% wt.
Copper (II) oxide (CuO) and Zinc oxide (ZnO)	50	0.1–0.5% wt.
Graphene	2.71	0.5% wt.
	<3	0.1–0.75% wt.
Multi-walled carbon nanotube (MWCNT)	30	0.001–0.1 ppb
	8–40	0.001–0.01% wt.
	100	0.1 ppb
Carbon	<200	0–1% wt.
Aluminum oxide (Al2O3)	40	0–1.5 gr.
	20	0.05% wt.
Graphite-alumina	80–400	0–0.8% wt.
Copper (II) oxide (CuO)	3	Acrylamide monomer/CuO: 10/1

Table 2.

Reported nanoparticles used in drilling fluids, size and concentration [11].

Three types of gels are recognized in drilling fluid engineering. They include (i) zero-zero gels with gel strength too low and initial or 10 s and 10 min gel strengths close to zero. It is easier for cuttings to settle or barites to sag in this system, (ii) flat gels with initial and 10 min gel strengths having similar values. Gel strength would be maintained and mud will remain pumpable after left quiescent and (iii) progressive gels where there would be appreciable difference between the initial and 10 min gel strengths, with higher 10 min gel strength value [15]. This signifies rapid gelling of drilling fluid and excessive pump pressure would be required to pump the fluid with time. Multi-walled carbon nanotubes and Yttrium oxide showed flat gels, whereas copper (II) oxide, aluminum oxide and bismuth ferrite showed progressive gels (Table 1). However, Silicon dioxide and Zinc oxide, both synthetic polymeric nanoparticles exhibited both flat and progressive gels. No clear trend was observed. The issue of progressive gel might be addressed by application of those materials in smaller concentration if other properties are satisfactory. Similarly, it could be observed that the carbon-based nanoparticles such as graphene, carbon and multi-walled carbon nanotubes were applied at lower concentrations (Table 2). Multi-walled carbon nanotube yielded a flat gel. Also, Silicon dioxide did not show any clear trend in optimal concentration when nanoparticle size was considered. In summary, carbon-based nanoparticles where the natural polymers in particular belong required addition in small concentrations to provide good gels and fluid loss properties. All the materials presented provided relatively good fluid loss properties irrespective of the types of gels observed.

4. Polymeric nanoparticles: structures, gelation and preparation

These are also known as polymer nanoparticles (PNP). Natural and synthetic (anthropologic) polymeric materials can be used to prepare nanoparticles. Whereas synthetic polymers can be derived from petroleum raw materials and are understandable due to controllable chemical composition, natural polymers are derived from animal and plant based sources [16]. Biocompatibility, biodegradability, nontoxicity at extensive scope of concentrations and economic viability are the incentives to use natural polymers. They are extracted or synthesized from different natural sources and have varying compositions as well as ubiquitous. These processes can make control difficult during application. Proteins, polysaccharides and other biodegradable polymers are examples. It is these polymeric materials that are applicable in drilling fluids for improved performance. Most natural polymers are cellulosic and gellable. As such, they can act as viscosifiers. In other applications in drilling fluid, polymer nanoparticles can be used to prevent and control corrosion [17]. Structures of some polymers are shown in Figure 4. It is seen that they contain chains of carbon held by strong covalent bonds resulting in very long molecules. However, two types of polymers (i) cross-linked and (ii) linear are common in the oil and gas industry for various purposes [18]. In linear polymers the carbon-carbon bonds form continuous chain. The outstanding valence bonds link with hydrogen. Physical attractions keep the polymeric chains together. Conversely, cross-linked polymers are formed by short covalent or ionic bonds of polymer chains linked with one another. There could be natural or synthetic types of this kind of polymers. Cross-linked polymers are stronger and more stable polymer materials. It is seen that the natural polymeric structures (Figure 4) could be either linear or cross-linking formed. Cellulose is a linear polymer of glucose units connected by β-1,4-glycosidic links. Starch, a mixture of amylose and amylopectin which are linear and branched chain polysaccharides is abundant. It is the chain of carbon atoms that make up organic materials that form polymers. They are categorized as organic compounds because of the presence of carbon. Another common element found is hydrogen. Whereas chitosan is a polycationic linear polymer, alginate is linear polyuronic and lignin is a 3D cross-linked irregular polyphenolic polymer. Pullulan is a linear polysaccharide formed by α-1,6-glycosidic assembly linkages. Therefore, these structures can form gels that affect the rheological behavior of drilling fluid formulations by influencing viscosity property. Structural units known as monomers combine to form these polymers. Moreso the long molecules from chains of carbon results in greater viscosity since the attraction due to intermolecular forces increases with longer chains. The higher intermolecular forces in the long chains give the polymers high melting point.

Figure 4.

Structures of (a) cellulose (b) chitosan (c) alginate (d) gliadin (e) lignin and (f) pullulan polymeric materials [16].

Moreso, based on their chemical structures, natural polymers could be proteins, polysaccharides or polyesters. Chitosan, alginates and cellulose are polysaccharides. Cellulose is more crystalline than starch, and it is the most abundant organic compound on earth. Its chemical formula is [C₆H₁₀O₅]_n. Chitosan derived from chitin is the second most abundant. Recently, some nano-fillers which include graphene, graphene oxide, fullerene, nanodiamond, carbon black, carbon nanotube, nanoclay and inorganic nanoparticles have been developed for application in polymeric matrices to utilize corrosion prevention characteristics. By functionality, polymers can form hydrogels and encapsulate solid particles for effective interaction and control. Ultimately, they have the capacity to influence drilling fluid properties by their (i) film forming and (ii) gelatinization characteristics, adduced to their thermal, mechanical and barrier properties.

An example of natural polymer with nanoparticles is starch. Most plants store food in the form of starch. They are polysaccharides with animal or plant origins. Studies on mechanism of starch gelatinization are not new. According to a study, the summary of its application and gelation mechanism of starch was presented [19]. The researchers concluded that their applications include oil-drilling, coating, water-holding, viscous enhancing, gelling, emulsifying, protective and encapsulating agents. Also, they hinted that starch morphology is difficult to prove. Moreso, starch macroscopic properties and internal structure (supermolecular and molecular) were recognized to have an interconnection. They highlighted temperature and time-dependence of events in the gel forming process. It was pointed that shear conditions during preparation also affect viscoelastic behavior of gelatinized starch dispersions. “It has been observed that the gelation process due to intra and inter molecular associations that result in hydrogen bonding or van der Waals attractive forces is due to hydroxyl or methyl groups and hemiacetal oxygen of sugar residues”. Similarly, they projected that during the process of dissociation of amylopectin double helices coupled with increased shear; the swollen granules of a starch sample were disrupted and gave rise to amorphous gel with subsequent viscosity increase. Similarly, they highlighted the purpose of starch in food industry which was to control structure and rheology by addition of starch hydrocolloids, irrespective of whether plant, animal or microbial origin. They showed that starches swell in aqueous environments, and increase the viscosity of the system and that gels can be produced by altering the solvent’s pH. Gelling (rheological) characteristics were shown to be different.

Also, a study on a starch-hydrocolloids system with leguminous Mucuna sloanei reported the gelatinization temperature range of 29.52–98.0°C. It was concluded that starch was converted from a semi-crystalline to an amorphous form that involved initial hydration of the amorphous regions that facilitated mobility of the molecules in the amorphous regions that was followed by reversible swelling. The reversible swelling led to dissociation of the double helices within the regions of the crystal and subsequent granule expansion as the biopolymer hydrated.

Nanoparticles generally have a core surrounded by shell as an additional layer, and surface molecules covalently linked. Whereas the core controls some properties such as the electrical and magnetic properties, the surface layers of molecules control the binding affinity. It is the right combination of the core and surface molecules that afford design flexibility and preparation. Several methods exist for production of polymeric nanoparticles. They include ball milling and polymer nanoprecipitation. Whereas ball milling involves grinding by the balls under high energy depending on the size, number of balls, slipping velocity and residence time, polymer precipitation involves dissolution in an organic solvent, and introduction into a poor solvent where the polymer chains collapse, agglomerate and precipitate out of solution. Other methods include solvent evaporation, emulsification/reverse salting-out, emulsification/solvent diffusion, sulfuric acid hydrolysis [20], ultrasound, cold plasma, thermosonication and use of enzymes.

4.1 Polymeric nanoparticles: properties, shapes

Nanoparticles exhibit a wide variety of structures as a result of the significant physical properties which include; (i) much higher surface area to volume ratio when compared with micro and macro sized materials. That provides higher surface area for contact with surrounding substances for increased reactivity. This results in the use of smaller quantity and corresponding economic advantage (ii) high degree of mobility in free-state (iii) display of quantum effects (iv) electronic (super conductivity) and optical (light adsorption and emission) activity (v) very high mechanical strength (vi) magnetic (superparamagnetism) properties (vii) thermal (fast cooling) properties (viii) chemical (catalytic) reactivity and (ix) barrier properties [10].

Particle shape plays major roles in formulation and prediction of behavior of materials. Both end properties and processes are affected by particle shape. Though most polymeric nanoparticle shapes exist as nano-spheres, challenges of creation of other shapes has been reported in nanomedicine where nanotechnology has had widespread applications [21]. Other particle shapes can be dispersed with the nanospheres to yield desirable properties as reported when chitosan was dispersed in bentonite formulation to produce improvements in thixotropy, shear thinning tendencies and better yield stress [22]. The mechanical property of a polymer has been adjusted by this approach [23]. Similarly, it has reported that particle shape has significant effects on drilling fluid properties such as fluid loss and rheological behaviors [24]. Whereas rods and plate shaped sharp-edged particles by jamming and interlocking can cause quick thickening [25] in fluids, flow would be negatively affected. Nanospherical polymeric particles would build viscosity less aggressively, in addition to enhancement of rheology (Figure 5).

Figure 5.

Effect of particle shape on viscosity.

Both organic and inorganic nanoparticles exist in diverse shapes. Whereas cellulose nanocrystals (CNC) are needle shaped, starch nanoparticles (SNP) are spherical in shape and starch granules are angular shaped for maize, pentagonal and angular shaped for rice, disk-like or lenticular for wheat; which is also roughly spherical or polygonal in shape. Similarly, chitosan has an agglomeration and nano-aggregates of particles similar to what was observed by a research [19]. The nano-aggregates of Mucuna solannie biodegradable polymer were presented as responsible for its utilization and influence in mud formulations. Its formulations were stable and exhibited predictable characteristics in accordance with American Petroleum Institute (API) specifications [2]. Polycaprolactone nanoparticles have spherical shape and no aggregates occur, while alginate nanoparticles can be spherical. Nanocapsules and nanospheres differ in morphology and architecture and both are nanostructures. Synthetic, biodegradable and biocompatible polymers are used to prepare polymeric nanospheres. Most polymeric nanoparticles have nanosphere structure (Figure 6) and a particular biopolymer was observed to have nano-aggregate structure (Figure 7). Whereas nanocapsule is seen as a reservoir system, the nanospheres are seen as matrix systems. Various nanoparticle shapes can be combined to influence a desired property such as barrier property. These shapes are subunits of the shapes presented in Figure 2.

Figure 6.

Nanocapsule and nanosphere schematics.

Figure 7.

Mucuna solannie scanning electron micrograph showing nano-aggregate particle shapes [19].

5. Desirable changes by addition of polymeric nanoparticles in drilling fluids

1. Nanoparticles in drilling fluids can improve filter cake quality by both plugging and bridging. Improved, thin filter cake, with non-erodible and impermeable membrane with similar attributes as conventional filter cakes can be achieved. Formation damage is reduced or prevented as a result. Also, due to the thin filter cake, contact between the wellbore and the drillstring is reduced, with resultant reduction of the tendency to experience excessive torque and drag frictional forces while rotating the drillstring and pulling or running in hole respectively. Stuck pipe incidents are reduced due to adequate clearance between the drillstring and the wellbore as a result of reduction of friction.

2. Another function of the drilling fluid is to cool the drillstring and bit. To perform this function, it must have high heat capacity. Nanoparticles such as carbon-based types have high carbon content. Carbon is highly temperature stable with long covalent bonds, which gives carbon-based nanoparticles high stability and ability to conduct heat better. The high heat conductivity property is also due to high surface area that enables heat absorption.

3. Moreso, the concepts of spreading of simple liquids are not applicable to nanofluids. The theory of structural disjoining pressure (Figure 8), which explains the interaction between the particles in the nanofluid and the solid substrate is applicable [26, 27]. For lubricity, the nanoparticles, through other mechanisms, can adhere; spread and their particles ordered on the surfaces of solids such as drill pipes to lubricate. In other words, a polymeric film is deposited on the well wall and metal surfaces for lubrication. It is believed that it is the structural disjoining pressure gradient that drives the spreading of nanoparticles in nanofluids on surfaces. Nanoparticles layers with definite films spread on the solid surface with explanation of the association between the solid surface and the nanoparticles very complicated. It also yields desirable structural properties. Reduction in torque and drag are typical resultant effects.

4. As regards wellbore instability, causes are categorized under controllable and uncontrollable factors. One of the controllable factors is physico-chemical rock-fluid interactions. Some phenomena that are responsible include swelling, osmotic pressures, hydration, strength changes and rock softening. Magnitude of near wellbore damage; quality of filter cake; formation strength, stiffness, stress history, pore water composition, mineralogy, temperature; and properties of wellbore fluid all combine to determine the significance of these phenomena. However, a filter cake made of nanoparticles with desirable properties can fix potential problems. Similarly, selection of right size range of nanoparticles in a drilling fluid, typically not greater than one-third of the pore throat enables effective bridging and plugging. Natural polymeric nanoparticles function by plugging the pore spaces. Due to their cellulosic nature, the deformable structures enable them to squeeze into the throats. Since the traditional method of switching from water based to oil based to solve shale swelling problem due to water absorption, which can cause environmental problems is prevented by addition of nanoparticles, cost efficiency is achieved.

5. In cavernous, vugular formations and fractures in rocks, drilling fluid losses may be encountered. In that case, mud level in the wellbore will drop and hydrostatic pressure would be affected leading to influx, well kick or blowout. Also, there would be severe economic impact due to loss of mud. The traditional solution had been use of micro and macro sized particles for effective size distribution in drilling fluids as loss control materials. They include mica, paper, cellophane, nutshells, cottonseed hulls and sized calcium carbonate. Addition of nanoparticles would create more effective and stable particle size distribution for lower porosity and permeability seals for loss circulation control. Polymeric nanoparticles can plug into the tiny pores to provide a robust and stable lost circulation material.

6. Shear thinning (pseudoplastic) and non-Newtonian behavior is a desirable drilling fluid property. Flow behavior index in the range of 0.3 to 0.8 would yield drilling fluid of such attributes. The resulting velocity profile is then used to evaluate the carrying capacity. Cuttings transport at low shear rate is the best indicator of hole cleaning. Since viscosity of polymer systems is shear rate independent, inclusion of polymeric nanoparticles in drilling fluids can create improved cuttings carrying capacity at very low and adequately high flow rates.

7. Various forms of corrosion exist such as aqueous, atmospheric, galvanic and stray-current corrosion. Others are molten salt, liquid metal, high-temperature gaseous, pitting, crevice and filiform corrosion. Aside titanium, microbial corrosion affects all types of alloys. It is caused by biofilms. It is a kind of corrosion caused by microbes such as Acidithiobacillus thiooxidans, Thiobacillus thioparus, and Thiobacillus concretivorus. Some of the bacteria associated with microbiologically influenced corrosion (MIC) are sulphate reducing bacteria, iron-reducing bacteria and acid-producing bacteria [28, 29]. Similarly, hydrogen sulphide causes hydrogen embrittlement or sulphide stress cracking in pipes. To prevent pollution and corrosion from toxic gases such as hydrogen sulphide, nanoparticles can be effective. Zinc oxide has been used for the purpose [30]; because hydrogen sulphide can react with metal ions, and insoluble metal sulphides are formed. Similarly, concentration cells caused by these bacteria can cause and enhance galvanic corrosion, which may appear as pitting corrosion in pipelines during drilling referred to as microbial corrosion. This form of corrosion can be prevented by both organic and inorganic nanoparticles that are introduced in the drilling fluid (Figure 7). Another form of cost efficiency is achieved since the pipelines could be reused due to extended lifespan without rapid deterioration and degradation due to corrosion and associated wears. The polymeric nanoparticles of organic origin when used in drilling fluids would have biological significance in prevention and treatment of this form of microbial corrosion by preventing formation of biofilm on the metal surface [31]. Also, polylactic acid (PLA) has potential application for this purpose. In summary, it will be like from “biofilm formation” to “biofilm protection”. It is mainly the barrier properties of the nanoparticles, in association with other attributes that are utilized (Figure 9). This is a form of biological treatment.

8. Most factors are responsible for hole cleaning and cutting carrying capacity of drilling fluids. Considering the velocity profile of fluid with cuttings in the annulus, with the annulus modeled as a pipe, profiles for laminar flow are distinguishable from turbulent flow regimes. The flow behavior index as one of the determinants of velocity profile is preferred to have a value less than unity, or preferably in the range 0.3 to 0.8 for shear thinning fluids. It is desirable for drilling fluids to have flow behavior index within this rang; the closer to the lower boundary, the better. Nanoparticles have the potential to gel the drilling fluid to fall within that range since addition of nanoparticles increases friction within the fluid layers which results in increase in viscosity causing a nanogel to be developed. Flow behavior index of unity would define Newtonian fluids which are not considered good for drilling fluid behavior. Similarly, for values above unity, the fluid would behave dilatant. Consider Figure 10 for fluid flow dynamics.

9. It shows the velocity profile in the annulus of a wellbore modeled as a pipe. At the center where velocity is at its maximum, friction might be zero; whereas at the pipe wall or well wall, velocity might be zero and friction at its maximum. Velocity diminishes from the center to zero towards the well or pipe wall. To extend the velocity at the center outwards such that more cutting are subjected to that velocity for more efficient hole cleaning, it is the flow behavior index that might be lowered to achieve that as shown by the chain line. Polymer nanoparticles can provide a more effective nanofluid gel to give appropriate flow behavior index value.

10. Moreso, during transport of cuttings in the annulus, there is continuous shearing and the nanoparticles are agitated and encapsulate the cuttings to help carry and suspend them due to encapsulation of the solids; an excellent feature exhibited by polymers. Ability of drilling fluids to release these cuttings at the surface is a basic function. This is achieved by the shale shaker that agitates the returning drilling fluid. The nanoparticles are rearranged accordingly and detach from the cuttings. This causes shear thinning of the drilling fluid that frees and releases the cuttings, and they are separated from the drilling fluid. This is known as thixotropic property.

11. In particular, shale formations have permeability in nanodarcy scale; micro and macro sized fluid loss control additives may never be effective since formation of filter cake is hindered. These additives that are supposed to plug the pore spaces are larger and might not be able to. Mixture of particle sizes might yield a thick and ineffective filter cake. The reduced annulus size causes stuck pipe and high torque and drag. If clay hydration occurs, wellbore instability will result. Control of wellbore instability causes excess rig time and subsequent rise in investment cost. The challenge is tackled with the use of nanoparticles as fluid loss control additives. These particles can plug into or bridge the pore spaces.

12. Also, when drilling in deep water environment, conditions favorable for hydrate formation may arise. They include low temperature and high pressure conditions which could be altered by the use of inhibitors. With thermodynamic inhibitors for inhibition, the temperature for hydrate formation will be lesser or the pressure for hydrate formation will be greater. Operation status might be altered to prevent stable hydrate formation towards the left of the hydrate formation curve. Due to the capacity of nanoparticles to absorb heat because of the high surface area; the hydrate formation temperature is suppressed to a lower level. This discourages hydrate formation since a condition would be affected. Alteration of fluid properties due to elimination of water from the fluid as a result of hydrate formation and increase in fluid density are prevented.

13. Bit balling which could be caused by formation type, hydraulic factors, too high hydrostatic pressure in the wellbore, poor bit design and high weight on bit can result in low rate of penetration. When polymeric nanoparticles are added to the drilling fluid, films such as nanocellulose biopolymer–based biofilms and starch nanoparticles [20, 32] deposited on the drill bit can help retard development of bit balling for improved drilling efficiency due to their barrier properties. Prevention of clay hydration for hydratable clays in the cuttings would also prevent bit balling.

Figure 8.

Barrier creation process with nanofluid with polymeric nanoparticles.

Figure 9.

(a) Film formation with structural disjoining pressure and (b) barrier between the surfaces finally created.

Figure 10.

Velocity profiles; with n = 0.42 (highlighted in chain-line) in comparison with other profiles with higher flow behavior index, n.

6. Summary

New technologies are often sought to mitigate the problems associated with traditional drilling fluid formulations. The field of nanoscience, where nanoparticles with desirable attributes are evaluated for application, provides an alternative. Natural and synthetic (anthropologic) polymeric materials can be used to prepare nanoparticles. Two types of polymers (i) cross-linked and (ii) linear are common in the oil and gas industry for various purposes. Their shapes and sizes influence applications. They are also biodegradable, can act as antibacterial agents and are economically efficient since a relatively small quantity is required to create the nanogel. They have the potential to modify drilling fluid properties such as plastic viscosity, yield point, gel strength, barite sag, fluid loss volume, filter cake thickness as well as improve thermal and wellbore stabilities. They are also used as pollution filters, for improved lubricity, heavy metal absorption and reduction of torque and drag. They are applicable in diverse conditions both on land and marine environments.

7. Conclusion

Shortcomings of traditional drilling fluid additives can be overcome with nanoparticle additives. At deeper drilling depths with physical, chemical and thermal challenges, micro and micro sized additives might not be effective, leading to performance problems. Nanoparticles of which polymeric nanoparticles are the organic types have the potential to influence all the components in the circulating system where the drilling fluid is the main consideration. From the metals, the drilling fluid and its compositions and the surrounding wellbore, the impact of nanoparticles cannot be over emphasized. Conditions such as the pH of medium, nanoparticle concentration, particle size and shape are factors that determine performance of nanoparticles in nanofluids. Modification of rheological properties, reduction of fluid loss, deposition of thin and effective filter cake, thermal stability, friction reduction due to formation of polymeric film, improvement of wellbore stability, and hydrate inhibition are areas of application. However, addition of nanoparticles might have insignificant effect on mud weight. Due to their effectiveness and cost efficiency, more areas of influence and application will continue to emerge so long as drilling in harsh and remote environments in the frontier basins remain the option for increased production output in a world in need of petroleum and natural gas. Potential problem might be the issue of recovery of the materials and their influences on the fluid properties. Dispersion of different shapes and structures of polymeric nanoparticles might be considered to enhance their influences on fluid formulations.