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NOTICE CONCERNING COPYRIGHT RESTRICTIONS
This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material.
The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material.
Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement.
This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law.
Hot and highly corrosive environments encountered in geo-thermal-well construction are just some of the complications operators are facing regularly. The selection of super-duplex, stainless-steel casing for these demanding environments pro-vides exceptional strength and corrosion resistance. This alloy has excellent resistance to chloride stress corrosion cracking, high thermal conductivity, and a low coefficient of thermal expansion. In addition, unusual slimhole-drilling applications and the need for versatile and robust casing attachments are essential to help ensure a successful primary cementing opera-tion. The limited clearance in slimhole applications could cause problems while running casing, especially in rough boreholes. In these scenarios, proper standoff is required to achieve op-timum cement distribution as well as to allow the option of circulation when running in the hole, and for proper hole clea-nout. This paper presents a case history of the application of an innovative, composite-ceramic-centralizer technology that is mechanically formed and chemically bonded directly to the casing surface. This technology provides a reliable centraliza-tion option for close-tolerance wellbores, uninterrupted flow during circulation, extreme abrasion and impact resistance, and CO2 and H2S resistance. Enabling a homogeneous cement-slurry distribution that prevents the casing from expanding or buckling when heated and helping prevent corrosion is critical for the life of a geothermal well.
Introduction
Geothermal-well construction in hot, high-salinity brines containing CO2 and H2S gas creates one of the most corrosive drilling environments. The severe downhole corrosion caused by these brines has been a major impediment to the development
of geothermal wells. A test program, which involved exposure of alloy tubulars to the downhole environment, identified a stainless-steel alloy that is resistant to the corrosive brines, and can be a cost-effective alternative to titanium and the frequent replacement of carbon-steel strings. In geothermal wells, all strings are cemented to the surface, which prevents buckling caused by thermal expansion while protecting the pipe from external corrosion. A challenge created by the stainless-steel metallurgy of the tubular was to help ensure there was no damage to the integrity of the super-duplex coating through the effects of galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are connected electrically and are in contact with an electrolyte solution. One of the two metals is corroded preferentially; this metal is the anode and the unattacked metal is the cathode in the galvanic couple. Because brine is an effective electrolyte solution, any casing attachments with dissimilar met-allurgy present the risk of galvanic corrosion, thereby threatening the integrity of the well plan and the well life. This presents a specific challenge with respect to cementing the casing string because a degree of standoff is critical to mud displacement and proper placement of cement.
A solution to centralization without adding any risk of galvanic corrosion was to use composite centralizers that were molded directly onto the tubular without any form of attachment.
composite ceramic centralizers
The centralizers are made from a composite of ceramics and carbon fiber in a novolac resin matrix. The material is entirely nonmetallic and effectively inert. The carbon fiber is designed to improve structural support in terms of tensile, shear, and tough-ness, while the spherical ceramics provide abrasion resistance and impact resistance. The combination of novalac resins and two-stage-hardener systems provide chemical resistance and high temperature resistance, both of which were important to the project. The catalyzed composite material has been tested in temperatures in excess of 450°F, without effects on its mechanical characteristics. Additionally, the material has also been tested as nonreactive to alkali and acids.
composite ceramic centralizers — An Innovative solution for Geothermal Well construction
in Highly corrosive Environments: case History
rafael Hernández, Lyndon chandarjit, and Iain Levie
Halliburton
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Hernández, et al.
The centralizer blades are bonded directly to the pipe through a process that includes proper preparation of the casing. The process includes sand blasting the pipe, mold installation, and injection of the composite material.
computer Modeling
Before the application of the centralizers, the well was modeled and a placement analysis was conducted showing the standoff at the centralizers and at the mid-span of the pipe, allowing engineering and design of the cement slurry and proper placement of the cement (Figure 1).
The analysis showed that placing two centralizers every three joints of 40-ft casing provided a standoff of approximately 80% at the mid-span and in excess of 90% at the centralizer.
Because the blades of the centralizer were molded directly onto the tubular, they were not placed at the same location on the pipe but were staggered down the pipe and offset from each other, allowing a greater area available for fluid bypass; their spiral design and placement acts to encourage turbulence, further assisting with mud displace-ment and cement placement. The composite material also has a coefficient of friction 40% less than that of steel, which coupled with the geometric design of the blades, acted to assist the liner to targeted depth (Figure 2).
Advantages of composite ceramic centralizers
The design versatility and mechanical properties of the carbon-fiber ceramic material allow this technology to provide a number of substantial differences from using conventional centralization methods. Some of those benefits are listed below.
Customized for specific jobs; it is molded onto cus-• tomer’s casing.Designed to fit specific well applications.• Can be installed on any grade pipe, including CRA • alloys.Provides smooth, uninterrupted flow during circula-• tion because of the absence of any banded product placed around the casing.Ideal for deviated sections of borehole because mate-• rial provides a low coefficient of friction.Reduces centralizer stiffness.• Can be used in conventional or slimhole applica-• tions.Provides resistance to extreme abrasion and impact • resistance at -25°F.Composite material is easily milled.• Field installation is al• most always achievable.
Installation
The blade-installation procedure is outlined in the fol-lowing basic steps. The material is molded directly onto the tubular
using an injection-molding system. Figures 3 through 10 describe the application process.
Figure 2. Customized, molded centralizers on casing.
Figure 1. Centralization program. Analysis of results.
Figure 3. Mold geometry is designed.
Figure 4. Master mold is made and plastic molds are stamped out from the master molds.
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Mobile application units allowed the centralizers to be applied to the casing at the pipe-manufacturer’s facility in Germany, minimizing transportation of the pipe and the risk of damage. The sensitivity of the high-chromium content of the pipe was such that only rubberized forks were used to lift and move the pipe.
case History
Before the application, adhesion of the composite material to the Super Duplex 2507 pipe, it was tested using a pull-off adhesion test in accordance to ISO 4624. Once the application was completed, the casing joints were packed in baskets and transported to the rig site. The pipe was run-in successfully to depth without any reported difficulties, and the cement was placed after mud displacement. Operators have encountered excessive running forces when using welded, hinged centralizers. In some cases, the casing had to be pulled from the hole and several of the centralizers had to be removed before casing could be run to its landing depth.
Wellbore Details. The following parameters ap-plied to this well.
Carbon-fiber ceramic-composite technology is becoming a solution for traditionally encountered problems in the wellbore-drilling industry. It allows an enhanced cementing solution that can be engineered to almost any wellbore configuration. Problems, such as differential sticking, high torque, lean profile, extend-ed-reach drilling, and horizontal and high-pressure/high-temperature applications, can now be battled with this new approach. Carbon-fiber ceramic-composite technology centralizers do not provide an electrically conductive path between the molded blades and the casing, reducing the potential of galvanic corrosion as-sociated with dissimilar metals that come into contact.
Adequate pipe centralization is an important factor in obtaining high displacement efficiency. In sections
Figure 5. The pipe is blasted to a minimum of 80 microns. The pipe OD is treated and tested to the desired profile.
Figure 6. Plastic molds are attached to pipe.
Figure 7. Injection-molding machine is used to inject composite material into mold. Injecting resin into place.
Figure 8. After material has catalyzed, plastic molds are re-moved, leaving blade now integral to pipe.
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where there is poor pipe standoff, the cement displays a strong tendency to bypass drilling fluids. Centralizers improve pipe standoff, thereby equalizing the distribu-tion of forces exerted by the cement slurry as it flows up the annulus. Otherwise, cement tends to follow the path of least resistance. Voids in the annular space are potentially disastrous because of the expansions of fluid behind pipe and the pipe itself when exposed to highly elevated temperatures. Proper centralization is critical for good zonal isolation. If proper isolation is not achieved, pipe movement and casing collapse can occur when the well is put in service.
Acknowledgements
Halliburton is the licensed distributor for the com-posite centralizer technology developed and patented by Eni.
referencesAcosta F., E. Webb, and F. Zausa, 2009. “Carbon-Fiber
Ceramic Products—An Innovative Solution that allows Centralization-Design Versatility.” Paper presented at the Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, 25–27 March.
Figure 9. Hardness is tested using Shore D hardness test. OD gauging completed after molding.
