Erosion resistant surface and method of making erosion resistant surfaces

An erosion resistant surface using a dense array of elastic whiskers to slow the velocity of erosive particles before impacting with the surface. A carbon nanotube forest is grown on the surface to provide the erosion resistance. In the alternative, a carbon nanotube forest is grown on a flexible substrate that is bonded to the surface.

TECHNICAL FIELD

The present invention relates to controlling erosion on apparatuses exposed to highly erosive environments. More particularly, the present invention relates to application of a dense network of elastic fibers to wellbore tools and equipment.

BACKGROUND OF THE INVENTION

Erosive wear occurs when a surface is exposed to a flow of material in a fluid. Particles within the fluid impact on the exposed surface and impart some of their kinetic energy into the exposed surface. If sufficiently high, the kinetic energy of the impacting particles creates significant tensile residual stress in the exposed surface, below the area of impact. Repeated impacts cause the accumulation of tensile stress in the bulk material that can leave the exposed surface brittle and lead to cracking, crack linkage and gross material loss.

Erosive wear is a cause for concern in applications as diverse as hydroelectric turbines, jet engine turbine blades, aircraft surfaces and wellbore drilling and stimulation environments. Each situation has its own particular challenges in mitigating erosive wear. Hydroelectric turbines are subject to high velocity flows of water mixed with various amounts of silt and sand. Jet engine turbine blades are subject to flows of superheated, high velocity gases. Aircraft surfaces must withstand high speed movement through air particulates such as rain, ice, dirt, and acidic pollution. Tools and equipment for wellbore exploration, including drilling and formation stimulation, are subject to a constant flow of mud and sand.

Typically, components that are exposed to erosive flow are subject to various hardfacing treatments to improve erosion resistance. Such treatments often include either surface preparations that harden and smooth the base material itself or bonding erosion resistant materials to the surface of the base material. Surface preparations can often make the base material more resistant to impact from particles with low kinetic energy, but these same preparations can leave the base material more brittle and thus susceptible to cracking as a result of impacts from high kinetic energy particles. Also, such surface preparations are usually applied using high-temperature processes, thus limiting their applicability only to high-temperature resistant materials such as metals and ceramics. Bonding of erosion resistant materials is typically performed using thermal spray techniques such as High Velocity Oxy-Fuel (HVOF) or Air Plasma Spray (APS). These techniques use a fuel/oxygen mixture or a DC arc to melt a metal powder and spray it onto the surface to be coated. As such, high-heat bonding techniques are amenable only for use on high-temperature resistant materials. Further, in highly erosive environments, the residual tensile stress that results from multiple impacts can accumulate at the junction of a base material and its bonded coating, leading to delamination of the coating material.

An addition issue arises when components of a device are difficult to access once put in place. For example, many devices are manufactured to be replaceable (e.g., the device may be welded, snapped or riveted together) rather than serviceable. In other instances, a device may be permanently placed in an inaccessible location, being intended to serve reliably for the lifetime of the structure (e.g., devices cemented into structure walls). In such cases, it is common to “over-design” the component such that it can reliably perform its function for the life of the device, even if the component is badly eroded. As a result, the cost of design and manufacture of such components may be significantly increased, along with their size and strength.

Erosion control is of particular concern in wellbore operations. During wellbore drilling, a drilling mud, usually consisting of significant amounts of solids such as sand, chert or other rock suspended in water, is constantly pumped into the wellbore at velocities that can exceed 50 meters per second. The drilling mud provides cooling to bottomhole assemblies, hydraulic horsepower to mud motors that rotate the drill bit, and a medium for removing the cuttings. In this environment, the mud motor rotor and stator are subject to significant erosive forces, as are the drill bit and particularly the shirttails (the exposed outer face of the roller cone-bearing journal).

Likewise, in wellbore completions, such as gravel packing or fracturing operations, a slurry of particles suspended in a liquid are pumped under high pressure into the wellbore. In gravel packing, gravel of various sizes is pumped into an angular flow diverter to pack the annulus between the wellbore and the casing with gravel, to prevent the production of formation sand. In fracturing, the slurry includes a propant, typically sand, that is pumped into the formation to stimulate low-permeability reservoirs. Here, the angular flow diverters are subject to erosive wear.

Because of the harshly erosive environment of wellbore operations, significant effort and expense is expended to mitigate erosive loss and improve wellbore tool and equipment life. Hardfacing treatments, as described above, are used extensively to protect a wide array of wellbore tools. Also, wellbore tools and equipment are often over-designed to provide adequate service life. Additional steps are often taken to treat the fluids to make them less erosive. However, all of these steps routinely prove inadequate to provide sufficient protection from erosion, and wellbore operations are often interrupted to replace broken tools that were unable to withstand the prolonged stress.

From the foregoing it will be apparent that there is a need for an improved method of providing erosion resistance to components exposed to a flow of erosive material.

DETAILED DESCRIPTION OF THE INVENTION

The description and examples are presented solely for the purpose of illustrating the preferred embodiments of the invention and should not be construed as a limitation to the scope and applicability of the invention. While the compositions of the present invention are described herein as comprising certain materials, it should be understood that the composition could optionally comprise two or more chemically different materials. In addition, the composition can also comprise some components other than the ones already cited. In the summary of the invention and this detailed description, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in this detailed description, it should be understood that any cited numerical range listed or described as being useful, suitable, or the like, should be considered to include any and every point within the range, including the end points. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, if any or all specific data points within the range, or conversely no data points within the range, are explicitly identified or referred to, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors convey possession of the entire range and all points within the range.

The disclosure shows an improved erosion resistant surface, and methods of applying the same. By applying a dense network of elastic fibers, also known as elastic whiskers, to a surface, the force of particles impacting with the surface is reduced, and so resistance to erosion is improved. A carbon nanotube forest is one example of a dense network of elastic whiskers. Tools and equipment that utilize this improved erosion resistant surface can withstand higher flow rates without experiencing increased amounts of erosion. Also, tools and equipment which are currently designed with thicker, heavier, less erosion-resistant surfaces can be redesigned to be more compact and lighter using this improved surface. Further, the low-temperature methods of surface application described herein will allow new materials to be considered for use in highly erosive environments.

INTRODUCTION

Disclosed herein are erosion resistant surfaces and methods of manufacturing them. While the disclosure is described in relation to well-drilling methods and apparatus, it must be recognized that this particular application of the disclosure is not the only possible application, and that the erosion resistant surfaces and methods described herein provide great benefit to other applications where erosion is a concern.

FIG. 1illustrates a drilling rig100with its associated mud circulation systems200and subsurface downhole assembly300. In drilling a well, drilling mud202is pumped from a surface reservoir204by the mud pump206through a hose208into a string of standpipes102into the wellbore104. At the bottom of the wellbore104the standpipes102are attached to the bottom hole assembly300that typically includes a mud motor310and a drill bit320. At the bottom hole assembly300, the drilling mud202drives the mud motor310to rotate the drill bit350. The drilling mud202then flows through and cools the drill bit350and is ejected from the drill bit350through nozzles352, to lubricate the drill bit350at the face of the formation106, and to carry the cuttings108from the formation106. The drilling mud202mixed with cuttings108flows back up the wellbore104to the surface mud system210where the cuttings108are removed from the drilling mud202. The surface mud system210typically includes a series of shale shakers, degassers, desanders and mud cleaners (all not shown) that remove the majority of the cuttings108from the drilling mud202. The clean drilling mud202is then discharged into the surface reservoir204, where it is recycled as the process begins again.

The drilling mud202can be water-based, oil-based or synthetic-based. While the surface mud system210is effective in removing most of the cuttings108from the drilling mud202, there is typically some residual amount of fine particulate matter, composed of sand, chert or other rock, that remains suspended in the drilling mud202. Therefore, no matter how clean the drilling mud202is at the beginning of drilling operations, it quickly becomes a gritty fluid that, when pumped at high velocity and high pressure, is highly erosive to the components in the mud pump206and in the bottom hole assembly300.

In particular,FIG. 2(which is presented as partial views illustrated inFIGS. 2A and 2B, respectively) shows the bottom hole assembly300that consists of a mud motor310and a drill bit350. The mud motor310is a kind of cavity pump that translates the linear flow of drilling mud202into a shaft rotation that twists the drill bit350. The mud motor310includes a power section320, inFIG. 2A, a transmission section330,FIGS. 2A and 2B, and a bearing section340,FIG. 2B. The power section320includes a rotor322with a number of vanes324spiraling along the length of the rotor322. The power section320also includes a fixed stator326with a number of lobes328. The rotor322diameter is such that the outer edges of the vanes324fit within the inner diameter of the lobes328on the stator326. In this way, the rotor322is free to rotate, but the fact that the vanes324and the lobes328remain in contact forms a seal around which the drilling mud202cannot pass.

The flow of drilling mud202under high pressure, flowing between the rotor322and the stator326, thus causes the rotor322to turn a flexible transmission shaft332in the transmission section330. The transmission section330may include a bend of from 0 to 4 degrees, to change the direction of the wellbore, as is well known in the art. The drilling mud202, after exiting the power section320, flows between the transmission shaft332and the transmission section wall334.

The bearing section340includes several thrust bearings342that permit free rotation of the transmission shaft332and bear the load of the drilling operations. The bearings are surrounded by lubricant344and enclosed by upper and lower seals346. At the transition between the transmission section330and the bearing section340, the transmission shaft332is fashioned as a hollow tube with an inner channel338. Several passages336link the annulus between the transmission shaft332and the transmission section wall334with the inner chamber338, permitting the drilling mud202to flow to the drill bit350.

The drill bit350shown is a rotary cone-type drill bit that has three wheels354attached to the shirttails356that form the outer diameter of the drill bit350. The drilling mud202flows into a chamber358with channels360to the nozzles352. The flow of drilling mud202provides cooling for the drill bit350, lubrication for the wheels354against the face of the formation106and a medium for carrying the cuttings108away from the formation.

In this context, the drilling mud202is highly erosive because it retains particles from the cuttings108that the surface mud system210was unable to remove and because it is flowing at velocities in excess of 50 meters per second. In the power section320, the contact point between the vanes324and inner surface of the stator324is particularly susceptible to erosive wear; material loss at this juncture can permit drilling mud202to flow between the rotor and the stator, resulting in less efficient operation of the mud motor310. The seals346in the bearing section340are also susceptible to erosive wear. Failure of the seals346would permit the bearings342to become exposed to the gritty drilling mud202, resulting in a seized bearing342. At the drill bit350, the seals for the wheel354bearings (both not shown) are exposed to erosive wear and are likewise susceptible to seizing. Also, the shirttails356, forming the narrowest constriction between the drill bit350and the wellbore104experience the erosive force of not just the drilling mud202, but also of the cuttings108. Erosive wear of the shirttails356can result in a broken drill bit350. All of these problems include the additional cost of pulling the entire drill string from the well to replace or repair the failing component.

Erosion Resistance

FIG. 3is a cross-section illustrating the surface of a drilling tool with improved erosion control provided by a dense network of fibers attached to the surface. In the embodiment shown, an erosion resistant surface400is made up of a dense network of fibers410attached to the surface412that requires erosion resistance.

InFIG. 3, erosive particles414are shown in various stages of impact with the surface412. The velocity of each particle is shown by an associated vector416whose direction corresponds to the direction of travel of the particle414, and whose length corresponds to the speed of the particle414. Particle414ais shown just before impact with the surface412and is traveling at a high velocity as depicted by vector416a. Particle414bis about to impact with the surface412, but is first impacting with the fibers410attached to the surface412. Because of the elastic properties of the fibers410, the fibers410deform without breaking, absorbing the kinetic energy of particle414b, and thus reducing the speed of particle414b, as depicted by vector416b. Particle414chas impacted with the surface412, but the further deformation of the fibers410has further reduced the kinetic energy and velocity of particle414c, as depicted by vector416c. Particle414dhas bounced off of the surface412and, the fibers410having imparted the stored spring energy back to the particle414d, is traveling at a high velocity away from the surface412as depicted by vector416d.

The effectiveness of the above-described erosion control mechanism may be understood by considering the factors that affect erosion rate. A general equation for the material removal rate of a brittle coating by erosive damage is given by the following equation:

Q⁢⁢Ω⁢⁢t=C⁢Mvn⁢HmKIC⁢⁢(in⁢⁢mm3⁢/hr)
Where Q is the volume of material removed per particle impact, Ω is the particle flux, t is time, M and ν are respectively the mass and velocity of the particle, H and KICare respectively the hardness and the fracture toughness of the surface, and C is a geometrical scaling factor. The velocity exponent, n, is typically 2.4 to 3.2, and the hardness exponent, m, is typically −0.5 to 0.1. Because the material removal rate varies with particle velocity to an exponent of 2.4 to 3.2, even small reductions in the velocity of the particle414before impacting with the surface412will lead to significantly reduced erosion rates. For example, assume the erosion rate is proportional to the particle's414speed to an exponent of 3.0, and the network of fibers are capable of reducing the speed of the particle414from 10 meters per second to 7.5 meters per second before the particle414impacts with the surface412. Before impact, the particle414would cause erosion proportional to 1000 meters3/second3(1000=103). With the reduced velocity, the particle414will cause erosion proportional to 422 meters3/second3(422=7.53). Therefore, in this example, the effect of the erosion resistant surface400is to reduce the erosion rate experienced by the surface412by 58%. Thus, because of the exponential relationship between speed and erosive force, even modest reductions in speed can result in a significantly lower erosion rate. With all impacting particles considered cumulatively, the reduced speed at impact produces dramatic improvements in erosion resistance.

If the erosive particles414exist in a flowing fluid (not shown), then, in addition to the elastic properties of the fibers410, the surface412is further protected by the effect of the impacting particle414extruding the fluid from between the fibers410. Here, the fluid is free to flow between the fibers410. In this case, the fluid between the fibers410has a higher viscosity than the fluid outside of the fibers410, because of the restriction to free flow created by the network of fibers410. Therefore, when a particle414impacts the fibers410, the particle414not only deflects the fibers410, but also displaces the higher viscosity fluid from between the fibers410. This added resistance improves the ability of the erosion resistant surface400to reduce the speed of incoming particles414, and provides further improvement in erosion resistance.

Carbon Nanotubes

In order to confer sufficient erosion resistance, the network of fibers410should be densely packed and strongly bonded to the surface412. In addition the fibers410should have a high elastic modulus and be hard enough to resist the cutting action of the faceted edge of typically erosive particles414, such as sand. In one embodiment of the present invention, the network of fibers410consists of a forest of carbon nanotubes, as shown inFIG. 4. Carbon nanotubes are different allotropes of carbon that consist of either a single one-atom thick sheet of graphite rolled into a seamless cylinder (single-walled nanotubes [SWNTs]) or multiple sheets of graphite rolled into a tube (multi-walled nanotubes [MWNTs]).

Several methods are available for synthesis of multi-walled nanotubes that make these very attractive for surface treatments. One such method is Chemical Vapor Deposition (CVD), where the surface of the article to be treated is seeded with catalyst particles, and is exposed at high temperature to a carbon-containing gas such as acetylene or ethylene, thus growing the multi-walled nanotubes on the catalyst particles. The diameter, surface density and structure of the multi-walled nanotubes is related to the size and surface density of the catalyst particles. Catalysts commonly include nickel, cobalt or iron. Carbon nanotube forests can be grown with lengths in excess of 2.5 millimeter, and with distances between nanotube of between 0.2 micron (0.2×10−6meter) to 2 microns (2.0×10−6meter) or more. Other methods of forming multi-walled nanotubes are available; including arc-discharge or laser ablation, and other methods may be developed in the future. Therefore, the method of carbon nanotube synthesis used, and the length and surface density of the nanotubes are not intended to form a limitation on the scope of the present invention.

In another embodiment, shown inFIG. 5, a flexible carbon nanotube forest500is attached to the surface requiring erosion resistance520through an appropriate bonding material522. In that embodiment, the flexible carbon nanotube forest500is fashioned by synthesizing multi-walled nanotubes510on a flexible substrate512. The bonding material522is chosen to best fasten the flexible substrate512to the surface520, such as an epoxy, and may be used in combination with other processing steps to pre-condition either the flexible substrate512or the surface520to better adhere to the bonding material522.

Example

Assume that an incoming erosive particle buckles a carbon nanotube in a perfectly elastic manner. The buckling stress for a cylinder F, is:

f=π2⁢EI4⁢L2
where E is the elastic modulus of the carbon nanotube, L is the length and I is the moment of inertia. A carbon nanotube is a hollow cylinder so I is given as:

I=π⁡(r24-r14)4
where r1is the inner radius and r2is the outer radius. Assume that the force prior to buckling is negligible and that the post-buckling force is constant. If a carbon nanotube is deflected to half its height by an impacting particle, then the energy absorbed by the nanotube, W, will be:

Assume that the individual carbon nanotubes within the forest are arranged in a square matrix on the substrate and separated by a distance of 2r2. Further, assume that a cubic sand particle, with length d on each edge, and density ρ, is traveling with velocity ν, and impacts normal to the surface. Then the number of carbon nanotubes that will absorb the energy of impact, N, is given as:

N=d216⁢r22
Therefore, the total energy absorbed by the collision, WT, will be:

WT=π3⁢Ed2⁡(r24-r14)512⁢Lr22
The kinetic energy of the particle, J, will be:

J=ρ⁢⁢d3⁢v22
Therefore, the maximum velocity of the particle that can be completely stopped, νmax, is:

For a typical carbon nanotube forest, the length of the nanotubes, L, is about 1 μ-meter (=1×10−6meter), the outer diameter, r2, is 100 n-meter (=100×10−9meter), the inner diameter r1, is 50 n-meter (=50×10−9meter) and the elastic modulus, E, is 1 T-Pascal (=1×1012Pascal). Further, assume a cubic sand particle of length, d, on each edge of 1 millimeter, and a density, ρ, of 3 grams per cubic centimeter. From the calculations above, the maximum particle velocity, νmax, that can be completely cushioned is ˜20 meters per second. Or, viewed another way, if the surface is exposed to a maximum particle velocity of 50 meters per second, but the erosion resistant surface of the present invention can reduce the particle velocity to 30 meters per second, then the erosive wear, being a function of velocity to the third power (as discussed above), is reduced by 78%.

The improvements in erosion resistance achieved using the herein-described technologies permit new and various approaches to the design of components that must withstand highly erosive environments. For example, applying a carbon nanotube forest to existing designs enables those designs to withstand higher flow rates without sacrificing service life. Furthermore, devices employing carbon nanotube forests as described herein may be designed with less concern for erosive wear, thus allowing for lighter and smaller design. Also, particularly when using the flexible carbon nanotube forest500, erosion resistance formerly available only to high-temperature materials is now readily applicable to a wide range of low-temperature materials, because many bonding processes, and in particular, epoxy processes, are low temperature processes.

The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. In particular, every range of values (of the form, “from about A to about B,” or, equivalently, “from approximately A to B,” or, equivalently, “from approximately A-B”) disclosed herein is to be understood as referring to the power set (the set of all subsets) of the respective range of values. Accordingly, the protection sought herein is as set forth in the claims below.