SPE 130413 Vital Role of Nanopolymers in Drilling and Stimulations Fluid Applications Subodh Singh and Ramadan Ahmed, University of Oklahoma
Copyright 2010, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in Florence, Italy, 19–22 September 2010. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.
Abstract Production of hydrocarbons from conventional, as well as unconventional resources, is becoming increasingly more difficult and expensive. Drilling fluid stability and performance in deeper, high-temperature high-pressure (HTHP) formations are still problematic, even for environmentally safer synthetic fluids. Recent research has demonstrated that nano fluids have attractive properties for applications where heat transfer, drag reduction, formation consolidation, gel formation, wettability alteration, and corrosive control are of interest. Nano fluids can be designed by adding nano-sized particles in low volumetric fractions to a fluid. The nano particles modify the fluid properties, and suspensions of nano-sized particles can provide numerous advantages. Nano-sized particles can impart sedimentary, thermal, optical, mechanical, electrical, rheological, and/or magnetic properties to a base material that can enhance its performance. This paper presents an extensive literature review to assess the application of nanotechnology in drilling and completion applications and evaluates the potential technical and economic benefits that this technology might provide to the industry. Introduction Nano materials have a great potential for a broad range of applications in the drilling industry (Zhang et al. 2009). Nanotechnology is not new, but its application in the oil industry is certainly in its infancy, including drilling applications. Leading oil companies and oilfield service companies have started their own research or are collaborating with academic universities and federal research labs to develop new fluids based on nano polymers to deal with problems of wellbore instability, emulsion instability, barite sag, lost circulation, and HTHP applications. Oil and gas well drilling involves a number of complex operations that often lead to operational problems. The success and cost of drilling operations highly depends on selection and application of appropriate techniques that minimize drilling problems, improve rate of penetration and productivity. Selection of suitable drilling fluids and maintenance of their properties within desirable ranges are critical aspects of successful oil well drilling. In some cases, conventional procedures that are established by the industry may not be sufficient to maintain acceptable mud properties, and new techniques based on nanotechnology may provide better solutions. Recent studies (Krishnamoorti 2006; Pourafshary et al. 2009) indicate that successful applications of nanotechnology in drilling are likely to occur with synthetic nanoparticles, where size, shape and chemical interactions are carefully controlled to achieve the desired fluid properties and drilling performance. Nanotechnology offers the promise of the transformation of oil and gas exploration and production. It could provide revolutionary solutions to problems related to upstream and downstream operations (Pourafshary et al. 2009). For example, nanotechnology can lead to a better understanding and control of rock/fluid interactions and their effects on wellbore stability, fluid loss and formation damage, thereby leading to improved drilling efficiency. The oil and gas industry relies on the strength and stability of its materials. The extreme precision of nano-scale manipulation offers geoscientists and engineers not only miniaturized devices, but also radically improved novel engineering materials. Nanomaterials can be made as light and elastic as silk yet as strong as steel. There are numerous robust and temperature/pressure-resistant nanotechnology applications already deployed or being developed for automotive, aerospace and military use that may offer a wide range of similar benefits for the petroleum industry. This article reviews important products developed and research conducted so far for the application of nanoproducts, specifically nanopolymers in drilling fluid applications. Properties that are thought to be accessible with nanotechnology include : i) ability to withstand harsh borehole conditions; ii) extension of the life of downhole equipment; iii) improved robustness, longevity and flexibility of tubulars and drill bits; iv) expandable tubulars for deeper wells without needing to telescope the well; v) improved cement integrity, hole quality and well placement; vi) innovative drill motors and downhole
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tools; vii) improved elastomers (e.g., clay-based polymer nano-composites); viii) miniaturized electronics, higher density batteries for small drill-holes (micro-holes) and monitoring methods to reduce drilling costs and environmental impact of drilling operation; and ix) improved drilling fluids that reduce mud invasion, formation damage and wellbore instability. Properties Nanoparticles have unique properties due to their small size and high surface area per unit volume. As a result, they are found useful in many applications including oil and gas exploration and production. The ability to measure and manipulate matter on the nanometer scale is making possible a new generation of materials with enhanced mechanical, optical, transport and magnetic properties. However, still much remains unknown about nanoparticles and why materials made from nanoparticles differ from those made using their larger counterparts. Nanomaterials appear to be stronger and more reactive than nonnanomaterials. It is also unclear why nano fluids conduct heat so effectively. Common speculation is that it may be related to the increased surface interaction. Since, for a given volume of material, there are a greater number of particles as their size decreases, perhaps there is more surface area for the nanoparticles to conduct the heat. The transition from micro- to nanoparticles leads to changes in physical as well as chemical properties of a material. Two of the major factors are the increase in the ratio of the surface area to volume, and the size of the particle. The increase in surface area-to-volume ratio, which increases as the particles get smaller, leads to an increasing dominance of the behavior of atoms on the surface area of particle over those in the interior of the particle (i.e., surface forces tend to dominate body forces); this affects the properties of the particles when they interact with other particles. Because of the higher surface area of the nanoparticles, the interaction with other particles within the mixture is greater, potentially leading to increased strength of the material, heat resistance and other properties of the mixture. Properties of nanomaterials depend highly on the shape, orientation and structure of individual nanoparticles. Descriptions and properties of commonly used nanomaterials that have the potential of revolutionary impact in drilling applications are presented here. Nanopolymers: Nanopolymers are nanostructured polymers (Zhao et al. 2004). The nanostructure determines important modifications in the intrinsic properties. Multi-scale nano structuring and stacking result in material properties that are very relevant for many engineering applications. One example of a nanopolymer is a silica nanosphere, which shows quite different characteristics and is much harder than bulk silica. The hardness of this nanosphere lies between that of sapphire and diamond. Figure 1 shows uniformly sized nanospheres (200 - 450 nm) of silica arranged in close packing fashion. The interest in nanopolymers lies in the observation that the dispersion of a relatively small number of these particles can provide significantly improved fluid properties. For example, in a nanopolymer-based drilling fluid, the nano-sized structures can accumulate within a filter cake, thereby diminishing its permeability and reducing fluid loss.
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Fig. 1 – Scanning Electron Microscopy image of close packed 415-nm uniform plain SiO2 nanospheres: a) square stacking; and b) triangular stacking (Míguez et al. 1997)
Polymer Nanocomposites (PNC): These materials are polymers or copolymers in which nanoparticles are dispersed (Sill et al. 2004; Soloman 2004). They may assume various shapes (e.g., platelets, fibers, spheroids); however, at least one dimension must be in the nano range 1 to 50 nm. Polymer nanocomposites are considered as multiphase systems, like foams and emulsions. Hence, they require controlled mixing, stabilization techniques and compounding strategies. Nanofluids: These fluids are engineered colloidal suspensions (Dyson 2008; Li and Xia 2003) of nanoparticles (one dimension must be 1-100 nm) in a base fluid. Common base fluids include water and organic liquids. Nanoparticles are typically made of chemically stable metals, metal oxides, or carbon in various forms. The size of the nanoparticles imparts some unique characteristics to these fluids, including greatly enhanced transfer of energy, momentum, and mass. Moreover, nanofluids reduce the rate of sedimentation of suspended particles and erosion of the containing surfaces. Nano fluids have potential for numerous applications including heat transfer operations, manufacturing and chemical processes. Experiments have shown that thermal conductivity of nano fluids is often much higher than expected. To explain this anomalous behavior,
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several theories have been proposed, one of which predicts increases in thermal conductivity of several orders of magnitude. When nanoparticles are properly selected and dispersed in a base fluid, main benefits that are seen include: • Better heat transfer efficiency, good fluid stability; • Improved transport through micro-channels; • Reduced erosion and lower pumping requirements; and • Controlled reactions with other chemicals present in the system. Nanofilms: These nanomaterials show considerable resistance when subjected to small shear rates, whereas at high deformation rates, the rheology approaches that of pure nanolatex. Solid state polymerization of molecularly oriented silica/monomer matrices lead to unique electrically conductive composites. The self assembly of conductive polymer-silica hybrids as films or fibers is suitable for integration into different kinds of devices.
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Fig. 2 Transmission electron micrograph of MnO: a) nanospheres; and b) nanorods (Park et al. 2004)
Nanosheres and Nanorods: Nanospheres and nanorods (Fig. 2) are interesting because they exhibit excellent optical, electrical, magnetic and mechanical properties. For instance, metallic nanorods exhibit high electrical capacitance. At the nanometer level, behavior of individual atoms and electrons becomes significant and atomic-scale effects come into play; these essentially alter the optical, electrical and magnetic behavior of nanorods. Carbon nanotubes have valuable electronic, magnetic and mechanical properties. Despite their low density, nanotubes can be stronger than steel. Hence, nanotube fibers could be used to improve the strength of downhole tools and tubulars. Moreover, nanotubes can conduct heat and electricity far better than copper. A recent study (Miller and Charles 2004) showed that the flow-induced alignment of nanotubes in a polymer matrix can lead to preferential orientation of the tubes to form ribbon fibers. Nanoribbons may be used in drilling fluids to control fluid loss, since the permeability of nanoribbon mats falls dramatically as the thickness of the mats increases (Tour et al. 2010). A newer study (Zhang et al. 2007) indicates that three-dimensional (3-D) nanostructured materials are quite different from one-dimensional nanotubes or nanowires. Full understanding of the formation mechanism of 3-D nanostructures could lead to significant progress in manufacturing nano-scale materials. Research Although the application of nanotechnology to oil and gas exploration and production is rapidly becoming an important area for research and development, a relatively small number of research studies have been carried out to explore the benefits of nanomaterials for drilling applications. The studies have indicated potential benefits of this technology in terms of improving wellbore stability, maintaining stability of drilling fluids and removal of toxic gases from drilling muds. Improvement of Wellbore Stability: Nanoparticle-containing fluids have the potential to minimize wellbore instability resulting from exposure of shale to drilling muds at downhole temperatures and pressures. A study conducted to investigate pore pressure transmission (PPT) in shales (Sensoy et al. 2009) revealed that significant reduction in pore pressure penetration (Fig. 3) and water influx into the shale occur when nanoparticles are added to the mud. It is anticipated that reduction of water invasion ultimately reduces wellbore instability problems. Tests carried out with two different nanoparticles (5 nm and 20 nm), showed that sealing of one type of shale is more effective with 20-nm nanoparticles than 5-nm nanoparticles. The mechanism for how the nanoparticles reduce the pore pressure transmission is not fully understood. However, pore plugging is believed to be one of the relevant mechanisms that controls fluid invasion and PPT phenomena in shales. For effective plugging of pore throats, the particle size needs to be within optimal range, which depends on pore size distribution. Studies (Abrams 1977; Suri & Sharma 2004) suggest that the particle size should not be larger than one third of the pore throat size to form a bridge and plug the pores. In addition to the particle size, the concentration of the nanoparticles influences the PPT. Increasing particle concentration substantially reduces the PPT, thus improving wellbore stability.
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Stability and Dispersibility of Colloidal Materials: Nanotechnology offers the opportunity to improve the stability of colloid-stabilized emulsions (Zhang et al. 2009). The creation of coated nanoparticles is beneficial in the areas of colloid and interface science (Jones and Lyon 2004; Zha et al. 2004). These particles can be utilized as model systems to investigate factors governing colloidal interactions and stabilization and to gain valuable information on the properties of concentrated dispersions. The synthesis of coated nanoparticles typically involves altering the surface properties of particles, which is often accomplished by coating or encapsulating them within a shell of a preferred material. The shell/coating can modify the charge, functionality, and reactivity of the surface, which can enhance the stability and dispersibility of the colloidal core. Encapsulating colloids in a shell of different composition may also protect the core from extraneous chemical and physical changes. Optimization of the surface characteristics of particles through coating processes is also of Fig. 3 Pore pressure vs. time (Sensoy et al. 2009) primary importance for the successful application of composite particles. Zhang et al. (2009) studied the effect of 5-nm and 20-nm coated silica nanospheres on the stability of colloidal emulsions. The nanosphere-treated emulsions maintained their stability for weeks at ambient and high-temperature conditions. This was achieved by encapsulating the spheres in a short-chain Polyethylene Oxide polymer. Stabilization of Foams and Emulsions: The use of foams (including gas-energized fluids) and invert-emulsions in drilling application is increasing. A number of studies have been conducted to investigate the stability of these fluids. The interaction between nanoparticles and surfactant molecules is important to develop stable foams and emulsions that can be used in many industrial applications. Surfactant and particle solids serve as emulsion and foam stabilizers because of their affinity for the interface between the two phases. In isolation, nanoparticles have proved to be extremely good interfacial stabilizers; however, despite the importance of understanding their behavior in multiphase systems such as foam and emulsions, the mechanisms by which particles interact with other species at interfaces are still not completely understood (Hunter et al. 2009). In multiphase systems, nanoparticles appear to generate interfacial synergy with surfactant molecules, resulting in emulsion or foam stability that is greater than that with either species alone. The potential explanations for the synergy differ markedly between systems and are complicated by the problems of separating individual particle or surfactant interfacial activity from particle-surfactant interactions either in bulk solution or at the interface. Hunter et al. (2009) studied the effects of mixing silica nanoparticles with a non-ionic surfactant (TX100) on the foamability, stability and elasticity of hydrophobic silica suspensions. Results indicated that at low-to-moderate concentrations of surfactant, the foam stability improves with the addition of the nanoparticles. The elasticity results (Fig. 4) show that in the absence of nanoparticles, most surfactant systems display a trend in which the elasticity rises to a maximum at a moderate concentration of surfactant, before dropping again as the concentration is increased. With the addition of nanoparticles, the peak shifts to higher surfactant concentrations, probably because of adsorption of surfactant onto the particles, reducing the bulk surfactant concentration. Foams are dispersions of a gas phase in a continuous liquid phase. Once these thermodynamically unstable mixtures are generated, the continuous phase irreversibly flows under gravity because of the large density contrast between the gas and the liquid phases. This process facilitates the drainage of foam. The drainage behavior of aqueous foams containing silicon oxide (silica) nanoparticles and cationic Fig. 4 Elasticity vs. TX100 concentration with (□) and surfactants has been studied by combining several approaches on both without (▲) nanoparticles (Hunter et al. 2009) at macroscopic and local level. The researchers (Carn et al. 2009) showed that predictions of foam drainage models do not agree with experimental data obtained via steady or free-drainage conditions with the presence of silica nanoparticles. Emulsions and foams exhibit very similar behaviors. Introduction of silica nanoparticles into oil-in-water emulsions has been shown to improve emulsion stability (Binks and Whitby 2005). There are two important criteria for particles to effectively stabilize emulsions: i) the particles must be wetted (at least partially) by both liquid phases; and ii) the particles need to be weakly flocculated. These properties can be controlled by controlling the surface chemistry of the particles.
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However, there are indications that emulsion stability also can be affected by changes the liquid phase. For example, invert emulsions of polar species in oil can be stabilized by montmorillonite particles. Furthermore, Binks and Whitby (2005) showed that emulsions can be stabilized by altering the particle charge through addition of cationic surfactants, changing pH, or addition of divalent electrolytes. Increasing the oil-phase polarity also can improve emulsion stability. The improvement is attributed to adsorption of polar solvent molecules to the silica surface changing the particle wettability. Emulsion stability is also improved by increasing the oil-phase polarity due to adsorption of polar oil species onto the surfaces of the silica particles. The average oil drop diameter of oil-in-water emulsions stabilized by hydrophilic silica particles (2% wt) increases (Fig 5) as the particle size increased from 5.5 nm to 34 nm. Stabilization of Immiscible Polymer Blends: Most polymer mixtures are thermodynamically immiscible (Hong et al. 2006). This means that they do not form homogeneous blends due to their unfavorable interaction and high molecular weight. The final properties of such a blend are strongly influenced by the interface and the size scale of the minor component and are determined by Fig. 5 Drop size distributions for emulsions stabilized by silica with different particle size: 5.5 nm (full line), 15 the relationship between the processing conditions and nanoparticles nm (dot–dashed line), 25 nm (dotted line) and 34 nm (dashed morphology development. The effect of organically modified line) (Binks & Whitby 2005) nanoclay (organoclay) on the morphology of immiscible polymer blends of polybutylene (PBT) and polyethylene (PE) has been investigated (Hong et al. 2006). When a small amount of organoclay is added to the blend, the 10-nm thick clay tactoids attach to the interface between the PBT and PE phases and change the interfacial tension, resulting in suppression of droplet coalescence. Moreover, the organoclay increases the viscosity of the suspension, which provides additional enhancement of the stability of the blend. Removal of Toxic Gases: Nanoparticles can be used to rid drilling fluids of toxic and corrosive gases, such as hydrogen sulfide. Such gases may be formed in-situ or can diffuse into the drilling fluid from formations during drilling of gas and oil wells. Hydrogen sulfide should be removed from the fluid to reduce the environmental pollution, protect the health of drilling workers and prevent corrosion of pipelines and equipments. Sayyadnejad et al. (2008) used 14 to 25-nm zinc oxide particles with 44-56 m2/g specific surface area to remove hydrogen sulfide from water-based drilling fluid. The hydrogen sulfide removal efficiency of the nanoparticles was evaluated and compared with that of bulk zinc oxide. Results showed that synthesized zinc oxide nanoparticles are able to remove hydrogen sulfide completely from water-based drilling mud in just 15 minutes, whereas bulk zinc oxide is able to remove only 2.5% of the hydrogen sulfide and required 90 min under the same operating conditions. Applications A number of commercial products or technologies have been developed recently using nanomaterials. Some of the case studies discussed here are based on new inventions and field application of nanomaterials for drilling and completion operations. Polymer-Free Fracturing Fluid: Nano-based polymer-free fluid enables high quality fracturing (OGE 2008). This is a viscoelastic surfactant (VES) fluid that performs as well as polymeric fracturing fluids, which provides superior formation and proppant pack cleanup capability. Nanoparticles stabilize the micelles and extend VES-based fluid capability to higher temperatures (300°F). In the presence of nanoparticles, VES micelles associate (pseudo-crosslink) to form stable structures that form a pseudo-filtercake composed of highly viscous VES fluid (micelles) and nano-particles. The fluid works synergistically with high-temperature stabilizers, viscosity enhancers, and fluid-loss-control agents. Seawater can be used in nanoparticle-based polymer-free fluids to generate practical fracturing fluids in offshore operations. Projects in the Adriatic Sea and the Gulf of Mexico (Pitoni et al. 2000) have demonstrated that seawater can be mixed successfully with these fluids to enhance production while simplifying the process, saving time and reducing cost. In addition to achieving treatment design objectives, each project saved rig and process time, streamlined logistics, and reduced material and equipment requirements. The results were enhanced production at significantly lower stimulation costs. Improving Spacer Fluid Performance: Development of an effective displacement spacer fluid to remove oil-based drilling fluid residue is critical in creating an effective cement bond between casing and formation. This leads to: i) proper zonal isolation; ii) improved casing integrity; and iii) reduced annular gas migration. A recent study (Zanten et al. 2010) shows the application of nano-structured surfactant to create a micro-emulsion that solubilizes large volumes of oil while simultaneously
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water wetting the casing and formation. The environmentally friendly surfactant was field-tested in the Rocky Mountains. It showed improved cement bond logs and eliminated the need for solvents that might pose environmental problems. Sloughing Control: The Jie-207 well is the first long openhole evaluation well drilled in the fractured Yingijie formation in the Jinyang depression in China (Xu et al. 2006). The aim of the drilling operation was to determine the oil and gas content of the shale section. Due to extensive sloughing in a neighboring well, low doses of an anti-sloughing lubricating agent (RS-2) and a binary nanoemulsion (SDJ-2) were introduced to prevent hole collapse. As a result, the rate of penetration improved and the residence time of the drilling fluid was reduced, thereby avoiding wellbore instability and loss of the well. Clay Stabilization: Coveney et al. (2004) presented a new method for strengthening and stabilizing shale formations through a process of in-situ polymerization. The method involves using additives (both monomers and polymers) that react in the wellbore to form nanocomposite materials, which dramatically enhance the mechanical properties of the shale formation. Fluid Loss Control: Drilling fluid additive compositions containing thermoset nanocomposite particles might be used as fluid loss control and wellbore strengthening agents. A recent invention (Bicerano 2009) relates to the use of thermoset nanocomposite particles as components of drilling, completion, and workover fluid additive packages to reduce fluid loss and/or enhance wellbore strength. The invention uses spherical thermoset nanocomposite particles (specific gravity ranging from 1.0 to 1.15), wherein the matrix consists of carbon black particles incorporated as a nanofiller. Viscosity Enhancement: Nano-sized clay minerals can enhance the viscosity of aqueous fracturing fluids containing a VES (Crews 2008). Some non-limiting theories suggest that the nano-sized clay particles associate and interact with the VES micelles, thereby increasing the viscosity of the fluid; it is hypothesized that this occurs via mechanisms involving chemisorption or surface charge attractions. The higher fluid viscosity increases efficiency of fracturing operations, reduces fluid leak-off, and improves carrying capacity of proppants, thus maintaining high fracture conductivity. Discussion Nanomaterials are expected to play a large role in solving drilling and completion-related problems. These include emulsion and foam instability, high fluid loss, microbiological degradation, wellbore instability, wettability issues and toxic gas contamination. Future drilling operations are likely to face even greater technical challenges due to the exploitation of economically and technically challenging oil and gas resources such as unconventional reservoirs and deepwater resources. Unconventional gas reservoirs (gas shales and tight gas sands) contain the largest reserves of remaining natural gas resources in the United States. In recent years, gas production from these resources has been significantly increased due to high demand for natural gas and development of new technologies that improve productivity. However, there are many areas that need to be improved to make these resources more economically competitive. Nanotechnology could provide solutions that are economically and technically sound. Construction of deepwater wells is difficult and expensive, and the narrowness of the operating pressure window poses many potential problems, including lost circulation, poor hole cleaning, stuck pipe, wellbore instability and tough environmental constraints. Thus, deepwater drilling requires the development of new technologies to reduce drilling cost, improve safety and minimize environmental impact. Moreover, extended reach drilling is becoming routine and itself involves complex operations that present great technical difficulties. Future drilling operations require new materials that satisfy the specific needs of these new drilling technologies. Such materials are expected to have unique characteristics in terms of size, mechanical strength, physical and chemical stability, biological degradability and environmental friendliness. Many nanomaterials exhibit at least some of these features and/or generate unique characteristics when they interact with conventional materials. 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