2010
WEAR RESISTANT COATING
FOR DOWNHOLE TUBULARS AND TOOLS
Joe L. Scott WearSox, LP 1626 S. Cherry Street Tomball, Texas 77375 Abbe Doering Consulting Engineer Kingwood, TX Grant Folkmann Devasco International, Inc.1626 S. Cherry St. Tomball, Texas 77375
ABSTRACT
The oil and gas industry has long been in need of a wear mitigation solution for the tube body of drill pipe that did not manifest the metallurgical problems of welding. Thermal spray
provides an alternative application method to welding, but is typically limited to very thin
deposits (1mm or less) and is most often associated with corrosion resistance rather than wear resistance.
A new coating has been developed that is wear resistant, robust and is capable of being applied in very thick deposits, from 0.010 inches up to 3.0 inches (.25 – 76 mm).
This material has been applied to the body of drill pipe without any deleterious effect to the pipe, and the coating is being used to create centralizers and stabilizers for casing drilling and casing running, as well as other selected areas on down-hole tools and devices.
This paper examines: the coating/deposit properties; application of the coating onto a substrate; and certain industrial exploitations of the coating.
Keywords: Wear, wear mitigation, coating, thermal spray
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INTRODUCTION
There have been attempts to address the challenges of wear mitigation and friction reduction in drill string and other mechanical endeavors by application of hardbanding to the mechanical components. Current coatings and application methods have both economic and engineering limitations. Application via traditional welding, such as used for the hardbanding of tool joints, is not acceptable due to potential fatigue failure from both metallurgical notches and
dimensional changes from the heat of the process. The coating and thermal spray method of application described herein can be relied on to overcome many of the limitations presented by currently available alloys and the traditional arc welding process.
MATERIAL PROPERTIES
Prior to deposition the material is in the form of metal-cored wires supplied on spools. The deposited alloy is a complex iron based carbide/boride system with typical composition and bulk properties shown below (Table 1). The microstructure of the material was observed using scanning electron microscopy (SEM) (Figure 1).
APPLICATION
The coating is applied via a Twin Wire Arc Spray (TWAS) thermal spray process - Twin Wire. The TWAS process employs two wires fed simultaneously through a gun, where one wire has a positive direct-current (DC) electrical potential and the other a negative potential. The wires are angled such a manner that they touch, creating an electrical arc, which melts the ends of the wires. Compressed air applied through the gun atomizes the molten metal and propels it onto a substrate. Prior to deposition it is vital that the substrate be cleaned and grit blasted to a rough, angular anchor pattern of a minimum of 5 mils.
The molten metal droplets collide with the surface of the substrate and conform to the anchor pattern, freezing instantaneously. The material is applied in consecutive passes and deposits approximately 0.006 inches of material with each pass. This process is shown in below (Figure 2).
The application temperature typically does not exceed 250F (121C), which is low enough to ensure that metallurgical changes to the base material will not occur. The material has been applied successfully to a variety of base materials including aluminum drill pipe. Moreover, the temperature of the base material can be kept even lower with in situ cooling further expanding the substrate selection that can be serviced with the coating.
Figure 2. Photograph of four guns applying coating to 5-7/8 inches (149.2mm) drill pipe.
LABORATORY AND FIELD TESTING
In order to assess the suitability of the coating for use in drilling and completed wells a number of laboratory and field tests have been completed. These tests and their results are
summarized below. Fatigue tests
Fatigue tests have been performed using the resonant bending test. In this test a joint of pipe was coated in the center and strain gauges were applied (Figures 3,4). The pipe was
precessed to simulate drilling through the build and horizontal sections of an extended reach well. The coating was undamaged, through four million cycles at bend stresses up to 25 degrees of bending per 100 feet of pipe length.
Figure 3. Resonant bending test - strain gage placement
Shock testing
Shock testing was performed using a new modification of the ASTM Drop Weight Test (ASTM E 208)1. This modified Drop Weight Test is used to characterize materials, procedural
variables and to qualify operators. Acceptance criteria are based on the drop-height required to initiate a crack in the coating or to spall the coating. Typically, the coating does not
experience cracking or spalling until the drop-height exceeds 18 inches. In this test, a short section of pipe was sprayed to the required thickness, and placed in a special fixture. A 100-pound weight was dropped from successively greater heights until cracking occurred. Wear
Abrasive wear was assessed using the ASTM G-65 Dry Sand Abrasion Test, Pin-on-Disc and the DEA 42 Open Hole/Casing Wear Test 2,3,4. Results are summarized below (Table 2).
Table 2. Wear and friction data
Metallurgical changes to the base material
Metallurgical changes to the base material were evaluated by micro-hardness scans of the bond area, optical metallography and scanning electron microscopy. Even on the aluminum base metal, no changes were detected.
CORROSION
Several corrosion tests have been performed in order to better define the behavior of the coating in corrosive environments.
Autoclave stress corrosion cracking (SCC) test
An autoclave SCC test (NACE TM0177-96 Method “B” (2005 laboratory testing of metals for resistance to sulfide stress cracking and stress corrosion cracking in H2S environments) as modified by ASTM G 39 – 1999 reapproved 2005 – Standard practice for preparation and use of bent-beam stress-corrosion test specimens) was conducted in order to determine the susceptibility of the coating to corrosion. The test was conducted at 80% stress level at 1000 psi (50 psi H2S, 400 psi CO2, balance N2) for a period of 7 days. SEM analysis was conducted
in order to determine whether cracks developed during testing. No cracking was observed (Figures 5,6).
Figure 5. Optical image of thermal spray coating after autoclave SCC test
Figure 6. SEM image of cross-section of coating and base material after Autoclave SCC Test
– no cracking was observed
Salt spray (fog) test
A salt spray corrosion test (ASTM B117 – 2009 – Standard practice for operating salt spray (fog) apparatus) was conducted in order to determine the materials’ susceptibility to salt water corrosion. The specimen was tested for a total of 312 hours. SEM analysis was conducted and no evidence of corrosion was observed (Figures 7,8).
Figure 7. Optical image of thermal spray coating after 312 hour salt spray (fog) test
Figure 8. SEM image of cross-section of coating and base material – no evidence of
corrosion observed
APPLICATIONS
Drill pipe applications
Drill pipe comes in two fundamental lengths – 30 feet and 45 feet. When the longer 45 foot pipe is used in extended reach wells, the pipe’s tube body will contact the sides of the hole or casing thereby inducing wear. Wear can also occur when the pipe is run in high compression in either straight or deviated hole paths. When the pipe is worn beyond usable thickness it is scrapped.
To mitigate such wear and the corresponding cost of having to scrap expended pipe, the thermal spray coating is typically applied as a series of bands in the center or nodal contact points of the pipe (Figure 9).
Figure 9. Drill Pipe as Applied and after drilling 12,000 feet
Application to casing
The coating is unique in that it can be used to create stabilizers and centralizers on the casing tube itself (Figures 10 and 11). The ribs or blades can be built up to any required height and length with curvatures ranging from 0-360 degrees. Additionally, they can be placed at the optimum locations on the pipe for maximum stabilization during casing drilling or casing/liner running.
The centralizers have very low impedance to cement flow, but can also be applied to create flow disturbances. For example, on the pipe in Figure 11, the coating has been applied in alternative right-hand-left hand spiraled blades to promote turbulent flow.
Use of this coating can eliminate the need for expensive threaded integral blade stabilizers (IBS), which add cost, make-up time on the rig and a potential failure point in the string. The coating is also being used to add stabilizers directly on the shank of certain bits,
eliminating the need for integral blade stabilizers. Drilling reports indicate that this method of near bit stabilization allows for much straighter hole paths without drift.
Figure 10. Straight blade centralizers on 2-3/8 inch casing for a side track through a
narrow window
Figure 11. Heavy stabilizers for casing drilling in Wyoming, S. Texas
Application to down-hole tools
The coating is being used successfully to create wear pads on tools used for measurement while drilling (MWDs) and other tools as a cost effective wear mitigation solution (Figure 12).
The low thermal signature of the application process can negate the need to remove internal sensors, conductors, etc.
Figure 12. MWD non-magnetic tool prior to application to wear surfaces
Application to bow spring centralizers
When the coating is used to create bow spring stabilizers, it is applied to the inside of the end rings as stops (Figure 13). This allows the centralizer to be “pulled” rather than pushed, which is the case with stop rings beyond the ends of the centralizers. API and full destruction tests indicate that this anchoring method will withstand casing and liner running through very narrow well heads, etc.
Figure 13. Application of internal anchors on bowspring centralizers
CONCLUSIONS
The coating has proven to be wear resistant and robust in laboratory and field use. It is
advantageous for both wear mitigation and creating in-situ centralizers and stabilizers for many applications, by lowering net cost and/or allowing flexibility in drilling and completion design strategies.
ACKNOWLEDGEMENTS
John Gammage
Rick Watts, ConocoPhillips Phil Kirk, AccuTest Labs
Robert Zand, Corrmet Engineering Services Jack Smith, Stress Engineering Services, Inc. Salah E. Mahmoud, MTL Engineering, Inc. Bairu Liu, MTL Engineering, Inc.
REFERENCES
1. ASTM Standard E208 – 06: Standard Test Method for Conducting Drop-Weight Test to Determine Nil-Ductility Transition Temperature of Ferritic Steels.
2. ASTM Standard G65 – 04: Measuring Abrasion Using the Dry sand/rubber wheel apparatus.
3. ASTM Standard G99 – 05: Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus.
4. “Improved Casing and Riser Wear Technology,” Maurer Engineering, Report DEA-42, Houston, TX, 1997
