Patent Application
20150233373
1. Field of the Invention
The present invention relates to improved elastomeric stator assemblies.
2. Description of the Related Art
Progressive cavity pumps having elastomeric stator assemblies have been used in drilling operations for some time. In downhole drilling operations, progressive cavity motors pump fluids by spinning a rotor within a stator assembly. In particular, power sections for mud motors have seen extensive use of progressive cavity pumps in the inverse application where drilling fluid is used to turn a rotor within a stator. Often, progressive cavity motors are configured with helical metal lobed rotors that turn within elastomeric stators consisting of rubber with high carbon black filler content. The high carbon black rubber provides a suitable yet cost efficient material having some compressive modulus and abrasion resistance properties. As the metal lobes of the rotors press against the elastomeric stator inner walls, a sealing line is formed and fluid is thus pumped through the cavities as they are formed between the metal lobes of the rotors and the elastomeric stator inner walls.
A challenge to producing a high power, high torque, and high speed power section stator, is that manufacturing equipment and cost effective tooling materials require a low viscosity uncured elastomer compound that is capable of flowing through a tight mould cavity over a long distance while maintaining its uncured state. If a compound is too viscous it cannot flow the appropriate distance to fill the mould. If a compound begins the vulcanization reaction before the mould is filled, the compound will increase in viscosity exponentially, possibly resulting with a mould that is not filled, or a mould that will fill with cross-links that form in separate matrices. Separately formed matrices create undetectable grain boundaries in the elastomer product which will often fail prematurely due to significant losses in tear resistance, losses in modulus, and or friction points internally that facilitate rapid physical deterioration of the surrounding elastomer matrix. Traditionally, designers of power section elastomers have sought to address these issues using reinforcing and semi-reinforcing carbon blacks, low viscosity low molecular weight base NBR and HNBR polymers, and process aids in the recipe. Although such combinations are favorable for manufacturability, the resulting recipe negatively impacts the final cured state properties of the elastomer, often making the formulation softer and less dynamically stable. For example, plasticizer oil may be used to decrease viscosity during manufacturing but, in the finished product, it has a tendency to leach out of the elastomer at high temperatures when exposed to diverse drilling fluids, which can cause shrinkage in the product or de-bonding of the rubber to metal bonding agents, and also facilitate the absorption of chemicals from drilling fluid. Plasticizers are used to reduce the viscosity of an uncured rubber compound by lubricating between the polymer chains and aiding in the dispersion of carbon blacks. Once in cured state, plasticizers continue to lubricate the polymer chains creating an effect of lowered modulus. Additionally, plasticizers, being significantly lower molecular weight than polymers, can migrate out of a compound. Controlling the migration of plasticizers is a function of choosing a plasticizer with the right molecular mass/branching and carbon-to-oxygen ratio for a particular compound. The more branching a plasticizer has, the more resistant the plasticizer is to fluid extraction in oils. The potential to react an ester-based plasticizer into the polymer matrix will substantially increase the resistance to extraction. If plasticizers can be peroxide cross-linked to a polymer, then the same plasticizer can be cross-linked to a functionalized graphene particle, permanently locking the plasticizer into the polymeric matrix. By reacting a plasticizer into the polymers and graphene particles, higher than traditional loadings of plasticizer can be compounded into a recipe without the drawback of having plasticizer leaching out of the compound to the rubber to metal bond of the stator or out of the inner diameter surface. Higher loadings of plasticizer offset the high viscosity effect that nano-particles induce in a compound. Higher loadings of plasticizer also reduce the mixing times of uncured compound by aiding the peptization and dispersion of filler materials like carbon blacks and nanoparticles. By reducing the mixing times and shear rates, less polymer is broken in the mixing process, which will improve physical and dynamic properties such as: modulus, tensile strength, tear resistance, tan delta, shear modulus, compressive modulus and hardness.
Elastomeric compounds have seen the incorporation of phenolic resins as referred to in U.S. Pat. App. Pub. No. US 2008/0050259, which reduces the uncured viscosity of the compound and increases the hardness of the cured state product at the cost of reduced tear resistance.
Elastomeric compounds have also seen some use of nanoparticles; however, due to the extraordinary surface area to particle volume (i.e. aspect ratio), these compounds can greatly increase the viscosity of the elastomer with only small amounts of additive nanoparticles. This means their potential in power sections stator compounds requires such low loadings (to maintain manufacturability) that the cured state physical properties are not attainable at an affordable, reproducible level of satisfaction.
Further, though the helical metal rotors of progressive cavity motors are heat tolerant, abrasion resistant, and have generally long useful lives, the stators of progressive cavity motors are far less reliable and often fail, need servicing, or replacement before their rotor counterparts. The carbon black reinforced liner of stators tends to wear down when exposed to abrasive materials, can develop leaks between cavities. When exposed to harsh temperatures a rubber compound will soften and can result in seal lines being less capable of handling high differential pressure which can result in a loss in torque. High temperatures can also cause the rubber in a liner to thermally expand, which can lead to overheating. Long term exposure to such conditions can cause the rubber to become brittle and lead to low tear resistance. Failures can occur in the form of a section worn down by abrasion leaking and not providing proper sealing pressure against the metal rotor lobes; a physical tear of the inner lining can also occur and cause an immediate shutdown of the entire system. For example, when the stator fails, the rotor can pump torn rubber pieces through cavities and damage other components of the downhole assembly or stop rotating all together. Exposure to certain chemicals or downhole fluids can additionally cause degradation of the stator inner walls. Harsh drilling fluids can be absorbed into the rubber liner, causing swelling which leads to the rubber liner overheating in operation. Fluids can also extract chemicals from the rubber, thereby degrading it.
It would thus be desirable to have a more robust elastomeric stator assembly with increased heat tolerance, abrasion resistance, tear resistance, and other beneficial properties. Further, it would be desirable to provide increased meantime between failures, increased reliability, and an expectation of extended runtime for operations running elastomeric stator assemblies downhole. This would allow greater drilling time and decreased time spent installing, retrieving, and servicing elastomeric stator assemblies and other components of the associated downhole assemblies that can fail as the result of a stator failure. It would further be desirable to increase the predicted time interval between required servicing of elastomeric stator assemblies.
The present invention involves a graphene enhanced elastomeric stator assembly and method of making the same. The elastomeric stator can be structurally and thermally enhanced through the incorporation of graphene particles, cross-linkable polymers, coupling agents that extend cross-links, and by the reduction of filler material. The graphene particles can be incorporated in functionalized or non-functionalized form or in a combination thereof. The graphene particles can increase polymer to filler interactions in the overall structure of the elastomeric stator, thereby enhancing the structure of the stator and increasing the stator's useful life by providing additional structural and thermal dissipation benefits. The incorporation of graphene particles can also provide a lower coefficient of thermal expansion for the rubber compound, which can reduce damaging heat build-up and extend the useful life of a power section.
Cross-linkable plasticizers can be integrated into graphene enhanced elastomeric stator formulations to additionally enhance the structural rigidity and thermal characteristics of the compound and thus the elastomeric stator. The structural rigidity of the compound is generally increased by having more of the materials in the compound cross-linked. By combining cross-linkable plasticizers and functionalized graphene, such as in the preferred embodiment, the density of cross-links between compounds is further increased over a given area. Further, graphene has conductivity properties not found in materials traditionally used in stators, properties which can transfer, disperse, and dissipate heat through the stator in a quick and efficient manner. Additionally, in an embodiment, the graphene enhanced elastomeric stator compound can be formulated to have a relatively low viscosity, through the use of a cross-linkable plasticizer, and also to have other characteristics that allow the material to flow through a mould cavity over a long distance, similar to how current elastomeric stators are made. In a formulation, a cross-linkable plasticizer can covalently bond to rubber polymer chains, functionalized graphene particles, co-agents, or a combination thereof, which will combat the extraction of the plasticizer in drilling fluids.
The below drawings and figures serve to illustrate various embodiments of the invention but were not meant to be limiting.
a-7c depict a step-by-step illustration of how peroxide cross-links are formed in a graphene enhanced stator compound.
a-13d depict a step-by-step illustration of the sulfur vulcanization of polymers.
Elastomeric stators in progressive cavity motors are often configured with hard metal tubular exterior sleeves and inner elastomeric stator liners that are typically formed by injection molding the liner over a mould mandrel.
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Elastomers can be cross-linked through a chemical vulcanization reaction that ties polymers to other polymers through the use of sulfur and/or organic peroxides. Traditionally, sulfur is used in an application where modulus and high temperature resistance are lower priorities. On the other hand, organic peroxides have the advantage of being shorter and having more electronegative covalent bonds. These “harder” peroxide induced carbon-to-carbon cross-links have a few disadvantages as well: the resulting rubber compounds can be more brittle with lower tear resistance despite exceptional modulus; the vulcanization reaction can be higher energy and difficult to slow down; and cross-linking of elastomer polymers to bonding agent polymer coatings (adhesives) is more challenging to consistently reproduce.
The addition of nano-particles, including carbon nanotubes, graphene particles, nano-clays, bucky balls and other three dimensional engineered carbon structures (reinforcing fillers), that offer large surface-area to weight ratios can be beneficial to reinforcing elastomeric polymers by utilizing the high surface area particles to create an increase in van der Waals attractive forces between the polymer and filler particles. Platelet shaped particles can also influence the chemical resistance of an elastomer, by creating inert barriers that stop the progress of permeating drilling fluid chemicals.
Power sections for mud motors are continually pushed for performance and downhole drilling reliability. One advantage of graphene enhanced elastomeric stator compounds, is that higher modulus materials are capable of sustaining more differential pressure across an elastomer lobe sealed by a metal rotor lobe. The cavity between a rotor and stator can only maintain differential pressure and imparted torque efficiently if the stator elastomer is of high enough modulus to not deflect, thus preventing fluid from progressing forward to the subsequent cavity. Fluid slippage between the rotor and stator interface can cause a loss in the volumetric fluid pressure to torque efficiency. A further benefit of the higher modulus material, is that the more differential pressure the elastomer lobe can sustain, the more torque will be imparted to the metal rotor. In power sections, the flow through is proportional to the eccentric rotating speed of the rotor for any given standard geometry and power section stators can function as a dynamic sealed interface with which the rotor interacts. Not only must an elastomer compound maintain modulus to make the seal, but the visco-elastic dynamic properties must maintain a mostly elastic response over high frequencies in high temperature drilling environments. The ability for a lobe to “snap” back is a function of the elastic dynamic decay of the modulus around the frequency of the power section's maximum rated flowrate and differential. The less decay in the elastic response the more differential pressure a power section stator can handle at higher flowrates and the more powerful and reliable the power section is likely to be in challenging drilling environments.
Graphene particles and other nano scale sheets of carbon are not bound together or to one another by the strong interfacial van der Waals forces that are common among graphitic materials. Other nano scale sheets can be substitutes for graphene for certain formulations. Further, and as referenced previously, graphene particles can be chemically altered, with a reactive functional group covalently bonded to the particles. Functional groups may include phenolic ring structures, sulfur atoms or sulfur chains, organic peroxide groups, formaldehyde functional groups, isocyanates, isocyanurates, tetramethylmethylamine (TMTM), hexamethylmethylamine (“hexa,” HMT), and/or fatty acid groups/hydroxl groups.
Table 1 below lists embodiments of graphene enhanced elastomeric stator compounds having functionalized or non-functionalized graphene particles that are dispersed in a polymer matrix of NBR, HNBR, XNBR, XHNBR, or FKM (fluoro/perfluoro elastomer) for use in power section drilling stators that require exceptional cured state tensile modulus, tear resistance, shear modulus, compressive modulus, elastic dynamic stability, high temperature resistance to polymer chain scission, surface abrasion resistance to the drilling fluid solids and/or rotor metal finish and fluid swelling resistance (when exposed to various water based, oil based, or synthetic oil based drilling fluids, as well as other similar fluids).
Table 2 below lists example embodiments non-functional and functional group that include a reactive group or groups common to rubber chemistry and require little or no special processing to react into the filler-polymer-crosslink network in an embodiment of a graphene enhanced elastomeric stator compound.
A graphene enhanced elastomeric stator can begin with the dispersion of graphene particles or sheets into an uncured rubber compound. In an embodiment, before dispersion the graphene is configured with graphene particles or sheets of optimal size for a given formulation. The sizing of the graphene particles can be optimized while keeping in mind the later steps of the process that can further break apart or break down some of the graphene particles. Alternatively, optimizing the tear resistance of a group of compounds with the same graphene concentration and variable graphene particle size can be more cost effective. Chemically etching fracture surfaces of graphene enhanced elastomers can be viewed under an electron microscope to determine particle sizes, particle density, and the level of optimization achieved. Further, in an embodiment, the graphene can be functionalized before dispersion to increase the cross-link density of what will become the graphene enhanced elastomeric stator.
In an embodiment, semi-reinforcing and highly reinforcing carbon black can be used to aid in the graphene and polymer dispersion of graphene into the polymer matrix.
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Due in part to increased heat resistance, faster thermal dissipation, and enhanced structural rigidity, a graphene enhanced elastomeric stator allows for longer runtime between failures downhole than prior designs. More specifically and in addition, a graphene enhanced stator can have exceptional cured state tensile modules, tear resistance, shear modulus, compressive modulus, elastic dynamic stability, high temperature resistivity to polymer chain scission, surface abrasion resistance to drilling fluid solids and rotor metal finish and fluid swelling resistance. Each of these properties can provide synergies that allow for far greater downhole performance and longevity when compared to prior designs in the industry.
Although the concepts disclosed herein have been described in connection with the preferred form of practicing them and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made thereto. Accordingly, it is not intended that the scope of these concepts in any way be limited by the above description.
| Number | Date | Country | |
|---|---|---|---|
| 61941269 | Feb 2014 | US |
