Patent Publication Number: US-2022235647-A1

Title: Downhole coupon holder

Description:
BACKGROUND 
     Coupon tests are used in the oilfield to assess the nature and degree of corrosion, erosion, and scale damage of materials intended for use in making a wellbore tubular or other metallic component to be deployed into a wellbore. Coupons are specimens of the materials of interest. In tests, coupons are characterized when pristine and after a predetermined deployment period in a wellbore environment. 
     Coupons are supported in coupon holders for deployment into the wellbore. Traditionally, a coupon holder that can accommodate multiple coupons includes multiple perforated tubes that are capped and connected end to end, each perforated tube typically containing one coupon. One of the challenges with the traditional coupon holder is that each addition of a coupon demands lengthening the coupon holder. A traditional coupon holder could be three to five feet long to hold three to five coupons. In some cases, there may be a limitation on the length of the coupon holder, which will limit the number of coupons that can be deployed into the well at any given time and the number of tests. A single deployment period could be three to six months. As such, limiting the number of coupons that could be deployed to a small number may result in significant delays in testing, validating, and certifying materials. Another challenge when multiple coupons are carried by the traditional coupon holder is that the coupons are arranged end to end along an axial axis of the holder, which means that the coupons will be at different positions along the axial axis of the holder. In this case, the coupons may experience different flow regimes or conditions since the coupons will be at different depths within the wellbore. In some cases, the perforated tubes alter the true nature of the hydrodynamic flow profile, especially at the edges of the perforations and open channels within the tube, so that the coupons do not experience the actual dynamic flow in the wellbore. 
     SUMMARY 
     In a first summary example, a coupon holder includes an elongated body having opposed first and second ends and one or more coupon holder unit disposed between the opposed first and second ends. Each coupon holder unit includes a coupon holder body, which has an elongated core and an external surface circumscribing the elongated core. The elongated core is axially aligned with a coupon holder axis. The external surface defines a plurality of coupon mounting surfaces with different radial orientations relative to the coupon holder axis. The plurality of coupon mounting surfaces are circumferentially aligned around the elongated core and have geometric centers with the same axial position along the coupon holder axis. In certain cases, a plurality of slots may be formed in the external surface. The base walls of the plurality of slots may define the plurality of coupon mounting surfaces. Alternatively, a plurality of facets may be formed on the external surface. The plurality of facets may define the plurality of coupon mounting surfaces. In certain cases, the plurality of mounting surfaces may be selected from planar surfaces and curved surfaces. In certain cases, the plurality of mounting surfaces may have a rectangular shape. In certain cases, a first end connection may be coupled to the first end of the elongated body, and a second end connection may be coupled to the second end of the elongated body. Each of the first end connection and the second end connection may include a threaded surface for forming a threaded connection. Alternatively, an end connection may be coupled to the first end of the elongated body, and an end fitting may be coupled to the second end of the elongated body. The end connection may include a threaded surface for forming a threaded connection, and the end fitting may have a bull nose shape. In certain cases, the elongated core may be a hollow core. Alternatively, the elongated core may be a solid core. In certain cases, the elongated body may include a plurality of the coupon holder units coupled in series between the first and second ends of the elongated body. In certain cases, a partition wall may be disposed between each adjacent pair of coupon holder units. In certain cases, the coupon holder body of each coupon holder unit may be made of a metallic material. 
     In a second summary example, a coupon assembly includes the coupon holder as described in the first summary example and a plurality of coupons having back surfaces in mating contact with the plurality of coupon mounting surfaces of at least one of the coupon holder units of the coupon holder. The coupons are fastened to the coupon body of the respective coupon holder unit. In certain cases, the back surfaces of the coupons in mating contact with the coupon mating surfaces may be coated with a corrosion-resistant material. In certain cases, the coupons may be selected from planar coupons, plano-curved coupons, and curved coupons. In certain cases, each of the coupons may be selected from a corrosion coupon, an erosion coupon, and a scale coupon. 
     In a third summary example, a coupon holder includes an elongated support structure that is axially aligned with a coupon holder axis and a coupon holder body that is positioned to have an end face in a plane that is transverse to the coupon holder axis. The coupon holder body has a plurality of slots disposed in a circular arrangement around the coupon holder axis. The plurality of slots have open ends on the end face. The plurality of slots receive end portions of a plurality of coupons through the open ends and position the plurality of coupons in parallel to the coupon holder axis and around the elongated support structure. In certain cases, the plurality of slots may include cylindrical slots to receive end portions of cylindrical rod coupons. Alternatively, the plurality of slots may include flat-sided slots to receive end portions of flat rod coupons. Alternatively, the plurality of slots may include curved slots to receive end portions of curved coupons. In certain cases, the plurality of slots may have shapes to receive end portions of at least two different types of coupon shapes selected from flat rod coupon shape, cylindrical rod coupon shape, and curved coupon shape. In certain cases, the coupon holder body may be a ring body having a central opening, and the plurality of slots may be disposed in a circular arrangement around the central opening. In certain cases, the coupon holder may include a coupon holder housing having a bore. The coupon holder body may be disposed within an annular cavity formed within the bore. An end portion of the elongated support structure may extend through the central opening in the coupon holder body into the bore. In certain cases, the coupon holder housing may include a threaded surface for forming a threaded connection. In certain cases, the elongated support structure may be a cylindrical tube. Alternatively, the elongated support structure may be a cylindrical rod. In certain cases, the coupon holder may include an end connection coupled to one end of the coupon holder body and an end fitting coupled to one end of the elongated support structure. The end connection may include a threaded surface for forming a threaded connection, and the end fitting may have a bull nose shape. In certain cases, the coupon holder body and the elongated support structure may be made of a metallic material. 
     In a fourth summary example, a coupon assembly includes a coupon holder as described in the third summary example and a plurality of coupons having end portions mounted in the plurality of slots of the coupon holder body of the coupon holder. The plurality of coupons are positioned in parallel to the coupon holder axis and around the elongated support structure by the slots. In certain cases, each of the plurality of coupons may be selected from cylindrical rod coupons, flat rod coupons, and curved coupons. In certain cases, each of the coupons may be selected from a corrosion coupon, an erosion coupon, and a scale coupon. 
     In a fifth summary example, a coupon assembly includes a first coupon holder body that is axially aligned with a coupon holder axis. The first coupon holder body has a first end face in a plane transverse to the coupon holder axis. A plurality of first slots are formed in the first coupon holder body. The plurality of first slots are disposed in a circular arrangement around the coupon holder axis and have first open ends on the first end face. The coupon assembly includes a second coupon holder body having a second end face in a plane transverse to the coupon holder axis. The second end face of the second coupon holder body is positioned in opposing relation to the first end face of the first coupon holder body. A plurality of second slots are formed in the second coupon holder body. The plurality of second slots are disposed in a circular arrangement around the coupon holder axis and have second open ends on the second end face. The coupon assembly includes a plurality of coupons with first end portions mounted in the plurality of first slots and second end portions mounted in the plurality of second slots. The plurality of coupons are positioned in parallel to the coupon holder axis and around the elongated support structure by the first and second slots. In certain cases, the plurality of coupons may be selected from cylindrical rod coupons, flat rod coupons, and curved coupons. In certain cases, each of the coupons may be selected from a corrosion coupon, an erosion coupon, and a scale coupon. 
     In a sixth summary example, a downhole tool includes a coupon assembly as described in the second summary example or the fourth summary example or the fifth summary example and a downhole corrosion measurement tool coupled to the coupon assembly. 
     The foregoing general description and the following detailed description are exemplary of the invention and are intended to provide an overview or framework for understanding the nature of the invention as it is claimed. The accompanying drawings are included to provide further understanding of the invention and are incorporated in and constitute a part of the specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The following is a description of the figures in the accompanying drawings. In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not necessarily drawn to scale, and some of these elements may be arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn are not necessarily intended to convey any information regarding the actual shape of the particular elements and have been solely selected for ease of recognition in the drawing. 
         FIG. 1A  is a perspective view of a planar coupon in a rectangular shape. 
         FIG. 1B  is a perspective view of a planar-curved coupon. 
         FIG. 1C  is a perspective view of a planar coupon in a disk shape. 
         FIG. 1D  is a perspective view of a curved coupon. 
         FIG. 1E  is a perspective view of a cylindrical rod coupon. 
         FIG. 1F  is a perspective view of another cylindrical rod coupon. 
         FIG. 1G  is a perspective view of a flat rod coupon. 
         FIG. 2A  is a perspective of a downhole coupon holder configured to hold coupons with flat back surfaces. 
         FIG. 2B  is a cross-sectional view of one of the coupon holder units (CHUs) in the downhole coupon holder of  FIG. 2A  on a plane transverse to a coupon holder axis. 
         FIG. 2C  is a perspective view of a coupon assembly formed with the downhole coupon holder of  FIG. 2A . 
         FIG. 3A  is a perspective view of a downhole coupon holder with an axial bore. 
         FIG. 3B  is a cross-sectional view of one of the CHUs in the downhole coupon holder of  FIG. 3B  on a plane transverse to a coupon holder axis. 
         FIG. 4A  is a perspective view of a partially assembled coupon assembly formed with a downhole coupon holder configured to hold coupons with flat back surfaces and coupons with curved back surface. 
         FIG. 4B  is a perspective view of a fully assembled coupon assembly formed with a downhole coupon holder of  FIG. 4A . 
         FIG. 4C  is a cross-sectional view of a CHU in the downhole coupon holder of  FIGS. 4A and 4B  on a plane transverse to a coupon holder axis. 
         FIG. 4D  is a cross-sectional view of the coupon assembly of  FIG. 4B  on a plane passing through a CHU configured to hold coupons with curved back surfaces. 
         FIG. 5A  is an elevated view of a downhole coupon holder configured to hold coupons with flat back surfaces in a compact body. 
         FIG. 5B  is a cross-sectional view of  FIG. 5A  along line  5 B- 5 B. 
         FIG. 5C  is an elevated view of a coupon assembly formed with the downhole coupon holder of  FIGS. 5A and 5B . 
         FIG. 5D  is an elevated view of another coupon assembly formed with the downhole coupon holder of  FIGS. 5A and 5B . 
         FIG. 6A  is an elevated view of a downhole coupon holder configured to hold coupons with curved back surfaces in a compact body. 
         FIG. 6B  is a cross-sectional view of  FIG. 6A  along line  6 B- 6 B. 
         FIG. 6C  shows curved coupons mounted on the curved facets of the downhole coupon holder of  FIGS. 6A and 6B . 
         FIG. 6D  is a cross-sectional view corresponding to  FIG. 6B  and shows slots with curved base walls to hold coupons with curved back surfaces. 
         FIG. 6E  shows curved coupons mounted in the slots shown in  FIG. 6D . 
         FIG. 7A  is a perspective view of a downhole coupon holder configured to hold cylindrical rod coupons. 
         FIG. 7B  is a cross-sectional view of the downhole coupon holder of  FIG. 7A . 
         FIG. 7C  is an end view of the downhole coupon holder shown in  FIGS. 7A and 7B . 
         FIG. 8A  is a perspective view of a coupon assembly formed with the downhole coupon holder of  FIGS. 7A-7C . 
         FIG. 8B  is a cross-sectional view of the coupon assembly of  FIG. 8A . 
         FIG. 9A  is a perspective of a coupon assembly including coupons mounted between two coupon holder bodies of a downhole coupon holder. 
         FIG. 9B  is a cross-sectional view of the coupon assembly of  FIG. 9A . 
         FIG. 10A  is a cross-sectional view of a coupon assembly including coupons mounted between two coupon holder bodies of a downhole coupon holder having an adapter to extend the holding capacity of the holder. 
         FIG. 10B  is a cross-sectional view of a coupon assembly including two sets of coupons mounted in a downhole coupon holder having double coupon holding capacity. 
         FIG. 11A  is a cross-sectional view of a coupon assembly with cylindrical rod coupons. 
         FIG. 11B  is a cross-sectional view of a coupon assembly with flat rod coupons. 
         FIG. 11C  is a cross-sectional view of a coupon assembly with curved coupons. 
         FIG. 11D  is a cross-sectional view of a coupon assembly with curved coupons and cylindrical rod coupons. 
         FIG. 12A  is an elevated view of a coupon assembly formed with a downhole coupon holder that supports coupons around a central rod. 
         FIG. 12B  is an exploded view of the coupon assembly shown in  FIG. 12A . 
         FIG. 12C  is a cross-sectional view of  FIG. 12A  along line  12 C- 12 C. 
         FIG. 12D  is a cross-sectional view of  FIG. 12A  along line  12 D- 12 D. 
         FIG. 12E  is a cross-sectional view of  FIG. 12A  along line  12 E- 12 E. 
         FIG. 13  is a schematic diagram showing a coupon assembly attached to a downhole tool. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, certain specific details are set forth in order to provide a thorough understanding of various disclosed implementations and embodiments. However, one skilled in the relevant art will recognize that implementations and embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, and so forth. In other instances, related well known features or processes have not been shown or described in detail to avoid unnecessarily obscuring the implementations and embodiments. For the sake of continuity, and in the interest of conciseness, same or similar reference characters may be used for same or similar objects in multiple figures. 
     Coupons of interest herein may be corrosion coupons, erosion coupons, and scale coupons (or deposition coupons). Corrosion coupons provide a qualitative measure of relative corrosion rates of a material in the presence of wellbore fluids, such as hydrocarbons and other substances that could damage a metallic component. Erosion coupons provide a qualitative measure of relative erosion rates of a material in the presence of wellbore fluids. Scale coupons provide a qualitative measure of relative deposition rates of substances on a surface in the presence of wellbore fluids. For coupon testing in a wellbore, the coupons are prepared or treated with the materials of interest. Typically, the coupons are made of metallic materials. Treatment or coatings, such as corrosion inhibitors and performance coatings, applied to the coupons may be metallic or non-metallic. The coupons are pre-weighed and then deployed into the wellbore environment for a predetermined period. After the coupons are removed from the wellbore, they are weighed again to determine material loss or gain and may be subjected to other types of assessments. 
     Various downhole coupon holders capable of holding multiple coupons, e.g., any combination of corrosion coupons, erosion coupons, and scale coupons, at a given fixed length are described herein. The downhole coupon holders support coupons radially from a coupon holder axis. Several coupons can have the same axial position along the coupon holder axis, allowing the coupons to experience the same dynamic flow when the coupon assembly is deployed in a wellbore. Each downhole coupon holder has the capability to hold a significant number of coupons in a compact size. The downhole coupon holders described herein may hold coupons with the same shape or coupons with mixed shapes. Coupon shapes may be generally classified as planar, curved, or rod. Examples of coupon shapes are illustrated in  FIGS. 1A-1F . However, these examples do not constitute an exhaustive list of coupon shapes that could be supported by the downhole coupon holders described herein. 
       FIG. 1A  shows a planar coupon  100  with a front surface  102 , a back surface  104 , which is in opposing relation to front surface  102 , and an edge surface  106  extending between front surface  102  and back surface  104  and around a perimeter of the coupon. Front and back surfaces  102 ,  104  are planar and parallel to each other. Edge surface  106  may be straight as shown or may be beveled or chamfered in other examples. Front surface  102  is the side of coupon  100  that would face outwards when coupon  100  is mounted on a planar surface of a coupon holder, and back surface  104  is the side of coupon  100  that would mate with the planar surface of the coupon holder. Coupon  100  has fastener holes  108  extending through a thickness of the coupon. Fastener holes  108  have apertures at the front and back surfaces  102 ,  104  to allow insertion of fasteners, such as bolts or screws, through the coupon. Typically, the number of fastener holes  108  in coupon  100  will match the number of fastener holes in the respective coupon mounting surface of the coupon holder. In the illustrated example of  FIG. 1A , front surface  102  and back surface  104  have rectangular shapes. In another example, front surface  102  and back surface  104  may have square shapes. The term “rectangular coupon” may describe both a planar coupon with a rectangular shape and a planar coupon with a square shape since a square is a special case of a rectangle.  FIG. 1B  shows a variation  100 ′ of the planar coupon including a front surface  102 ′ that is curved and a back surface  104 ′ that is planar, forming a plano-curved coupon. The size of planar coupon  100  or plano-curved coupon  100 ′ will vary with the intended application. One example size is a length (L) in a range from 2.0 to 5.0 inches, a width (W) in a range from 0.5 to 2.0 inches, and a thickness in a range from 0.1 inches to 0.5 inches. 
       FIG. 1C  shows a planar coupon  110  having a front surface  112 , a back surface  114 , which is in opposing relation to front surface  112 , and an edge surface  116  extending between front surface  112  to back surface  114  and around a perimeter of the coupon. Front and back surfaces  112 ,  114  are planar and parallel to each other. Edge surface  116  may be straight as shown or may be beveled or chamfered in other examples. Front surface  112  is the side of coupon  110  that would face outwards when coupon  110  is mounted on a planar surface of a coupon holder, and back surface  114  is the side of coupon  110  that would mate with the planar surface of the coupon holder. Coupon  110  has a fastener hole  118  extending through a thickness of the coupon. Fastener hole  118  has apertures at front and back surfaces  112 ,  114  to allow insertion of a fastener, such as a bolt or screw, through the coupon. Coupon  110  may have more than one fastener hole in some cases. In the illustrated example of  FIG. 1C , front and back surfaces  112 ,  114  have circular shapes. In another example, front and back surfaces  112 ,  114  may have oval shapes. The term “disk coupon” may describe both a planar coupon with a circular shape and a planar coupon with an oval shape. The size of disk coupon  110  will vary with the intended application. One example size is a diameter or width in a range from 0.5 to 2.0 inches and a thickness in a range from 0.1 to 0.5 inches. 
       FIG. 1D  shows a curved coupon  120  having a front surface  122 , a back surface  124 , which is in opposing relation to the front surface, and edge surfaces  126   a ,  126   c ,  126   c  extending between front surface  122  and back surface  124  and along edges of the coupon. Edge surfaces  126   a ,  126   b  are in opposing relation. Front and back surfaces  122 ,  124  are curved and may have the same radius of curvature. Edge surfaces  116   a ,  116   b ,  116   c  may be straight as shown or may be beveled or chamfered in other examples. Curved coupon  120  may have fastener holes  128  extending through a thickness of the coupon. Fastener holes  128  have apertures at front and back surfaces  122 ,  124  to allow insertion of fasteners, such as screws or bolts, through the coupon. The number of fastener holes  128  may be dictated by the number of fastener holes in the respective coupon mounting surface or the number of fasteners to ensure a secure connection of the coupon to the coupon mounting surface. The size of the curved coupon will vary with the intended application. One example size is a length of 2 to 6 inches, a width of 0.5 to 2.0 inches, a height of 0.5 to 1.5 inches, and a thickness of 0.1 to 0.5 inches. 
     It can be difficult to source coupons of desired metallurgy. Even when the metallurgy is available, it is typically in the form of a pipe. Manufacturing coupons with flat surfaces from pipes can be challenging. In this regard, curved coupons may have an advantage over planar coupons, or coupons with a flat back surface, in that curved coupons can be prepared directly from a pipe with the desired metallurgy. For example, a single cut in a plane parallel to the axis of the pipe will create two curved coupons that are ready to use. Another cut, parallel to the pipe axis but 90 degrees offset from the previous cut, would create four coupons that are ready to use. On the other hand, planar coupons, or coupons with a flat back surface, will require that curved sections formed by cutting a pipe are further machined to form flat surface(s). 
       FIG. 1E  shows a cylindrical rod coupon  130  having an end portion  133  with a pin  135  that may be inserted in a cylindrical slot of a coupon holder. The other end portion  137  may also include a pin  139  for insertion in a cylindrical slot in a coupon holder. In some cases, pins  135 ,  139  may be omitted, and end portions  133 ,  137  may be inserted directly in cylindrical slots in a coupon holder.  FIG. 1F  shows an alternative cylindrical rod coupon  130 ′ having end portions  133 ′ with threads  135 ′ that may engage threads inside a cylindrical slot of a coupon holder. The other end portion  137 ′ may also have threads  139 ′. In another example, a cylindrical rod coupon may have threads at one end and a pin at the other end. In yet another example, end portions of a cylindrical rod coupon may have holes to receive fasteners, e.g., in order to enable locking of the cylindrical rod to a coupon holder with fasteners. The size of the cylindrical rod coupon will vary with the intended application. One example is a diameter in a range from 0.1 to 0.5 inches and a length in a range from 2 to 6 inches. 
       FIG. 1G  shows a flat rod coupon  140 , i.e., where both sides  142 ,  144  of the rod are flat. Flat rod coupon  140  has end portions  143 ,  147  that may be inserted in flat-sided slots of a coupon holder. In some cases, end portions  143 ,  147  may have holes  145 ,  149  to receive fasteners that allow the end portions to be locked to the coupon holder. The size of the flat rod coupon will vary with the intended application. One example is a width in a range from 0.1 to 0.5 inches, a thickness in a range from 0.1 to 0.5 inches, and a length in a range from 2 to 6 inches. 
       FIG. 2A  shows a downhole coupon holder  200  according to one illustrative implementation. Downhole coupon holder  200  has a coupon holder axis  201 . Coupon holder axis  201  will be aligned with a wellbore axis when downhole coupon holder  200  is deployed in a wellbore. Downhole coupon holder  200  includes one or more coupon holder units (CHUs)  204  arranged in series along coupon holder axis  201 . Partition walls  205  may be disposed between adjacent CHUs  204 . CHUs  204  and any intervening partition walls  205  are connected together to form an elongated body  202  that is axially aligned with coupon holder axis  201 . In some cases, CHUs  204  and any intervening partition walls  205  may be integrally formed into a single piece. In the illustrated example, downhole coupon holder  200  includes four CHUs  204 . In general, downhole coupon holder  200  may include one or more CHUs  204 , e.g., one to six CHUs  204 . The length of elongated body  202  may be a sum of the lengths of CHUs  204  measured along coupon holder axis  201  and the thicknesses of any partition walls  205  measured along coupon holder axis  201 . In one example, the length of elongated body  202  may be anywhere from six inches to forty-eight inches. The diameter of elongated body  202  may be dictated by the largest diameter of CHUs  204 , which in turn depends on the number and sizes of coupons to be supported by the CHU. In one example, the diameter of elongated body  202  may be in a range from 0.75 inches to 2.5 inches. 
     Downhole coupon holder  200  includes end connections  206 ,  208  that are disposed at opposite ends of elongated body  202 . End connections  206 ,  208  may be integrally formed with, or otherwise attached to, CHUs  204  at opposite ends of elongated body  202 . In one example, end connections  206 ,  208  include threaded surfaces  206   a ,  208   a  for forming threaded connections with other components. For example, a hanger may be coupled to downhole coupon holder  200  by threaded surface  206   a  to enable deployment of downhole coupon holder  200  into a wellbore on the end of the hanger. Similarly, a measurement tool, such as an electrical resistivity tool, may be coupled to downhole coupon holder  200  by threaded surface  208   a  to allow deployment of the measurement tool with the downhole coupon holder  200  into the wellbore. One example of forming downhole coupon holder  200  may include machining a rod made of a suitable material to form CHUs  204  and any intervening partition walls  205 . End portions of the rod may be machined to form end connections  206 ,  208 . 
     Referring to  FIGS. 2A and 2B , each CHU  204  includes a coupon holder body  207  having an elongated core  211  (in  FIG. 2B ) and an external surface  213   a  circumscribing core  211 . Elongated core  211  is axially aligned with coupon holder axis  201 . External surface  213   a  has n sides, where the number n is greater than 1. In one example, n may range from two to eight, preferably from three to eight. The n sides have different radial orientations relative to coupon holder axis  201 . Each of the n sides contains a slot  210 . In the example shown in  FIGS. 2A and 2B , each CHU  204  includes three (n=3) slots  210  corresponding to a three-sided CHU (the three sides are visible in  FIG. 2B ). However, as previously noted, n is not limited to three, and CHU  204  may have more than three slots or less than three slots. Assuming that each slot  210  carries at least one coupon, then the number of slots  210  in each CHU  204  will define the holding capacity of the CHU. In a downhole coupon holder with multiple CHUs  204 , such as illustrated in  FIG. 2A , CHUs  204  may have the same or different coupon holding capacities. 
     Each slot  210  has a base wall providing a coupon mounting surface  212 . In the example of  FIG. 2A , each CHU  204  includes three slots  210  and three corresponding coupon mounting surfaces  212  (the three slots are visible in  FIG. 2B ). In each CHU  204 , coupon mounting surfaces  212  formed in adjacent slots  210  are angularly offset from each other. In other words, coupon mounting surfaces  212  formed in adjacent slots  210  of a CHU are not on the same plane. In one example, coupon mounting surfaces  212  formed in adjacent slots  210  of a CHU may be angularly offset from each other by 360/n degrees, where n is the number of coupon mating surfaces in the CHU, which may mean that the coupon mounting surfaces are evenly distributed about a circumference of coupon holder body  207 . In some cases, there may be an alternating arrangement of coupon mounting surfaces on an external surface of a CHU, such as a wide coupon mounting surface followed by a narrow coupon mounting surface. In this case, even distribution about a circumference of the coupon holder body may be local to each size represented in the coupon mounting surfaces, e.g., the wide coupon mounting surfaces are evenly distributed about the coupon holder body, and the narrow coupon mounting surfaces are evenly distributed about the coupon holder body.  FIG. 2A  shows an example of three coupon mounting surfaces  212  formed in external surface  213  of coupon holder body  207  and angularly offset from each other by 120 degrees. The angular offsets are indicated by the circular arrows in  FIG. 2B . However, this example is not intended to be limiting since there could be more or less than three coupon mounting surfaces per CHU. 
     Returning to  FIG. 2A , coupon mounting surfaces  212  are shown as planar surfaces and have rectangular shapes. Coupon mounting surfaces  212  with these features can mate with coupons having a flat back surface, such as the coupons shown in  FIGS. 1A to 1C . Fastener holes  214  are formed in coupon mounting surfaces  212 . These holes extend to some depth within elongated core  211  (in  FIG. 2B ). These holes help register coupons on coupon mounting surfaces  212  as well as receive fasteners to secure the coupons to coupon holder body  207 . The size of each slot  210  will be dictated by the size of the coupon to be arranged in the slot, or vice versa. In one example, the sizes of slots  210  in any one CHU  204  are the same such that coupons having the same size can be supported in the CHU. In addition, all the coupon mounting surfaces  212  in any one CHU  204  are aligned circumferentially about elongated core  211  (in  FIG. 2B ) such that they have the same axial position along coupon holder axis  201 . For the purpose of determining whether coupon mating surfaces  212  have the same axial position on coupon holder axis  201 , the geometric centers of the coupon mounting surfaces  212  may be used as a reference. When the geometric centers of coupon mating surfaces  212  in a CHU have the same axial position on coupon holder axis  201 , the coupon mating surfaces may be considered to have the same axial position along coupon holder axis  201 . Another way of expressing this is that a polygon having vertices at geometric centers of the coupon mounting surfaces  212  will lie in a plane that is orthogonal to coupon holder axis  201 . One of the benefit of this arrangement is that when the downhole coupon holder is disposed in a wellbore, all the coupon mounting surfaces  212  in any one CHU  204  will be at the same depth within the wellbore and will experience the same flow conditions within the wellbore. 
     Any of the coupon shapes with flat back surfaces may be mounted in the slots of CHUs  204  to form a coupon assembly. For illustrative purposes,  FIG. 2C  shows a coupon assembly including rectangular coupons  100  (previously shown in  FIG. 1A ) mounted in the slots ( 210  in  FIG. 2A ) of CHUs  204  in downhole coupon holder  200 . The back surfaces ( 104  in  FIG. 1A ) of coupons  100  are in mating contact with the coupon mating surfaces ( 212  in  FIG. 2A ) of CHUs  204 . Front surfaces  102  of coupons  100  are exposed for testing. Coupons  100  may be secured to the coupon support bodies ( 207  in  FIG. 2A ) by fasteners  216  inserted through aligned fastener holes in the coupons and coupon mating surfaces. Fasteners with flatter heads than shown for fasteners  216  in  FIG. 2C , e.g., flat head screws, may be used to minimize hydrodynamic flow disturbance around the coupons. 
     Coupons  100  and/or the slots ( 210  in  FIG. 2A ) of CHUs  204  may be sized such that there are no substantial air gaps between the edge surfaces ( 106  in  FIG. 1A ) of coupons  100  and the side walls of the CHU slots ( 210  in  FIG. 2A ). In addition, the edge surfaces of coupons  100  may be coated with a non-metallic material, such as a fluoropolymer, e.g., Xylan fluoropolymer, to close any air gaps between the edges surfaces of the coupons and the side walls of the CHU slots due to manufacturing tolerances and to prevent galling between metal parts. The material of coupons  100  will depend on the desired coupon test. In some cases, it may be useful to have each CHU  204  contain the same type of coupons. In the example of  FIG. 2C , each CHU  204  holds three coupons, and there are four CHUs for a total of twelve coupons. If each CHU has a length of about six inches, this would be twelve coupons in an elongated body with a length of about two feet. 
     Downhole coupon holder  200  may be modified to include a central passage for fluid flow. As illustrated in  FIGS. 3A and 3B , downhole coupon holder  200 ″ may include an axial bore  215  extending along coupon holder axis  201  and through each of elongated cores  211 ″ of coupon holder bodies  207 ″, partition walls  205 ″, and end connections  206 ″,  208 ″. Comparing  FIG. 3B  to  FIG. 2B , core  211  in  FIG. 2B  is a solid core, whereas core  211 ″ in  FIG. 3B  with a portion of bore  215  is a hollow core. Other than the axial bore  215  in downhole coupon holder  200 ″, downhole coupon holder  200 ″ is identical to the downhole coupon holder described with reference to  FIGS. 2A and 2B . 
     Downhole coupon holder  200  may be modified to hold curved coupons and mixed types of coupons. As illustrated in  FIG. 4A , downhole coupon holder  200 ′ includes one or more CHUs  204  and one or more CHUs  204 ′ arranged in series and aligned axially to form an elongated body  202 ′. Partition walls  205  may be disposed between adjacent CHUs  204 ,  204 ′. CHUs  204 ,  204 ′ and any intervening partition walls  205  may be integrally formed. End connections  206 ,  208  are disposed at opposite ends of elongated body  202 ′. CHUs  204 , partition walls  205 , and end connections  206 ,  208  are as described for the downhole coupon holder in  FIG. 2A . Each CHU  204 ′ includes a coupon holder body  207 ′. Coupon holder body  207 ′ has an elongated core  211 ′ and an external surface  213   a ′ circumscribing core  211 ′. Elongated core  211 ′ is axially aligned with coupon holder axis  201  and has n sides, where n is greater than 1. The n sides have different radial orientations relative to coupon holder axis  201 . As shown more clearly in  FIG. 4C , each of the n sides contains a slot  210 ′. Thus, CHU  204 ′ is structurally similar to CHU  204 , with the exception that the base wall of slot  210 ′ provides a curved coupon mounting surface  212 ′. For illustrative purposes, rectangular coupons  100  are arranged in CHUs  204  as previously described with reference to  FIG. 2C . In an alternative example, plano-curved coupons  100 ′ in  FIG. 1B  may be arranged in CHUs  204 . Curved coupons  120  are arranged in CHUs  204 ′. In  FIG. 4A , some slots  210 ′ in CHUs  204 ′ are deliberately left without curved coupons  120  to allow the coupon mounting surfaces  212 ′ of the CHUs to be visible.  FIG. 4B  shows the full coupon assembly with all the slots in CHUs  204 ′ occupied by curved coupons  120 . 
     In  FIGS. 4A and 4B , CHUs  204 ,  204 ′ hold their respective coupons  100 ,  120  such that the front surfaces  102 ,  122  of the coupons are exposed for testing. The back surfaces of the coupons mate with the coupon mounting surfaces. Coupons  100 ,  120  are secured to the coupon holder bodies of the respective CHUs  204 ,  204 ′ by fasteners  216 ,  216 ′. In the illustrated example, each of CHUs  204 ,  204 ′ is capable of holding three coupons (see the three coupon mounting surfaces  212  for CHU  204  in  FIG. 2B  and the three coupon mounting surfaces  212 ′ for CHU  204 ′ in  FIGS. 4C and 4D ). However, each of CHUs  204 ,  204 ′ could have more than three coupon mounting surfaces/slots to hold more than three coupons. Moreover, the number of coupon mounting surfaces/slots in CHU  204  could be different from the number of coupon mounting surfaces/slots in CHU  204 ′, i.e., CHU  204  and CHU  204 ′ could have different coupon holding capacities. As in previous examples, the coupons and the slots that hold the coupons may be sized such that there are substantially no gaps between the edge surfaces of the coupons and the sidewalls of the slots that hold the coupons. If needed, gaps may be filled with a non-metallic material, as previously described. 
       FIG. 5A  shows a downhole coupon holder  300  according to another illustrative implementation. Downhole coupon holder  300  has a coupon holder axis  301 . Coupon holder axis  301  will be aligned with a wellbore axis when downhole coupon holder  300  is deployed in a wellbore. Downhole coupon holder  300  includes a CHU  304 , an end connection  306  attached to one end of CHU  304 , and a bull nose fitting  308  attached to the other end of CHU  304 . Bull nose fitting  308  is an end fitting with a bull nose shape. End connection  306  has a threaded surface  306   a  for making up a threaded connection with other components. For example, a hanger may be coupled to downhole coupon holder  300  by threaded surface  306   a  to enable deployment of downhole coupon holder  300  into a wellbore on the end of the hanger. Bull nose fitting  308  may serve to protect CHU  304  from direct impact with a surface, e.g., when deploying downhole coupon holder  300  into the wellbore. In some cases, bull nose fitting  308  may serve as an interface for connection to other components, such as a downhole tool. End connection  306  and bull nose fitting  308  may be integrally formed with, or otherwise attached to, opposite ends of CHU  304 . One example of forming downhole coupon holder  300  may include machining a rod made of a suitable material to form CHU  304 . End portions of the rod may be machined to form end connection  306  and bull nose fitting  308 . 
     CHU  304  includes a coupon holder body  307  having an elongated core  311  and an external surface  313  circumscribing elongated core  311 . Elongated core  311  is axially aligned with coupon holder axis  301 . External surface  313  has n facets  310 , where the number n is greater than 1. In one example, n may range from two to eight. The n facets have different radial orientations relative to coupon holder axis  301 . In the example shown in  FIG. 5A , external surface  313  includes six facets  310  corresponding to a six-sided CHU (the six facets are visible in  FIG. 5B ). Each facet  310  provides a coupon mounting surfaces  312 . Adjacent coupon mounting surfaces  312  are angularly offset from each other since facets  310  are on different planes. In one example, adjacent coupon mounting surfaces  312  may be angularly offset from each other by an angle 360/n, where n is the number of facets. The sizes of coupon mounting surfaces  312  may be the same or different. In one implementation, coupon mounting surfaces  312  are aligned circumferentially about elongated core  311  such that they have the same axial position along coupon holder axis  301 . For the purpose of determining whether coupon mounting surfaces  312  have the same axial position on coupon holder axis  201 , the geometric centers of the coupon mounting surfaces  312  may be used as a reference. When the geometric centers of coupon mounting surfaces  312  have the same axial position on coupon holder axis  301 , coupon mounting surfaces  312  may be considered to have the same axial position along coupon holder axis  301 . Fastener holes  314  are formed in coupon mounting surfaces  312 . Fastener holes  314  are useful for securing coupons to coupon mounting surfaces  312  and may aid in mounting coupons such that they are circumferentially aligned about elongated core  311 . 
     Any of the coupon shapes with flat back surfaces may be mounted on coupon mounting surfaces  312  to form a coupon assembly. For illustrative purposes,  FIG. 5C  shows rectangular coupons  100  (previously shown in  FIG. 1A ) mounted on coupon mounting surfaces  312 , with the back surfaces of the coupons mate with coupon mounting surfaces  312 . Front surfaces  102  of the coupons are exposed for testing. Coupons  100  may be secured in place by fasteners  316 . Coupons  100  have the same axial position along coupon holder axis  301  when attached correctly to coupon holder body  307 . Downhole coupon holder  300  provides a compact form that can be six inches or less in length while carrying a significant number of coupons, e.g., up to eight coupons. 
     For illustrative purposes,  FIG. 5D  shows disk coupons  110  (previously shown in  FIG. 1C ) mounted on coupon mounting surfaces  312 , with the back surfaces of the coupons in mating contact with coupon mounting surfaces  312  and the front surfaces  112  of the coupons exposed for testing.  FIG. 5D  illustrates the possibility of having several sets of circumferentially aligned coupons within a CHU. For example, coupons  110  in this illustration are staggered to provide several sets of coupons, each set having the same axial position along coupon holder axis  301 . For example, four axial positions  315   a ,  315   b ,  315   c ,  315   d  are indicated in  FIG. 5D . There will be a plurality of coupons at each axial position. For example, if there are six coupon mounting surfaces  312  positioned about the core of coupon holder body  307 , and each adjacent pair of coupon mounting surfaces  312  holds coupons as shown in  FIG. 5D , then there will be three coupons at each of the axial positions  315   a ,  315   b ,  315   c ,  315   d.    
     Downhole coupon holder  300  may be modified to hold curved coupons. As illustrated in  FIGS. 6A and 6B , downhole coupon holder  300 ′ includes CHU  304 ′, end connection  306 , and bull nose fitting  308 . End connection  306  and bull nose fitting  308  are as described for the downhole coupon holder in  FIGS. 5A-5D . CHU  304 ′ includes a coupon holder body  307 ′ having an elongated core  311 ′ and an external surface  313 ′ circumscribing core  311 ′. External surface  313 ′ has n curved facets  310 ′, where the number n is greater than 1. In one example, n may range from two to eight. In the example shown in  FIGS. 6A and 6B , CHU  304 ′ has six curved facets  310 ′ (seen more clearly in  FIG. 6B ), each of which provides a coupon mounting surface  312 ′. Coupon mounting surfaces  312 ′ are aligned circumferentially about core  311 ′ such that they have the same axial position along coupon holder axis  301 . The main difference between CHU  304 ′ and previously described CHU  304  (in  FIGS. 5A-5D ) is that the curved facets  310 ′ of CHU  304 ′ provide curved coupon mounting surfaces  312 ′. In comparison, the facets  310  (in  FIGS. 5A-5D ) of CHU  304  provide planar coupon mounting surfaces  312  (in  FIGS. 5A-5D ). As shown in  FIG. 6C , curved coupons  120  can be mounted on curved coupon mounting surfaces  312 ′.  FIG. 6D  shows an alternative coupon holder body  307 ″ where slots  310 ″ with curved base walls are formed instead of curved facets. The curved base walls provide curved coupon mounting surfaces  312 ″ in the same manner as curved facets  310 ′.  FIG. 6E  shows curved coupons  120  mounted in slots  310 ″ and in mating contact with curved coupon mounting surfaces  312 ″. Coupon holder body  307 ″ shown in  FIGS. 6D and 6E  may be used instead of coupon holder body  307 ′ shown in  FIGS. 6B and 6C  in the downhole coupon holder of  FIG. 6A . 
     In one example, the various components of the downhole coupon holders ( 200 ,  200 ′,  300 ,  300 ′) illustrated in  FIGS. 2A to 6E , e.g., the coupon holder bodies ( 207 ,  307 ), the partition walls ( 205 ), the end connections ( 206 ,  208 ,  306 ), and the bull nose fittings ( 308 ), may be made of a corrosion-resistant metal or alloy, such as Grade 17-4 Stainless Steel (UNS S17400) or other metallic material that will not corrode when disposed in a wellbore that is undergoing production or treatment. To use the downhole coupon holders in a wellbore environment, the threaded surfaces ( 206   a ,  208   a ,  306   a ) of the end connections may be coated with an anti-galling agent, such as high purity Goop, and/or high temperature silicone grease that will not be destroyed by the application of torque and that is resistant to corrosion in the wellbore environment. 
     The downhole coupon holders illustrated in  FIGS. 2A to 6E  are of the type where the coupon mounting surfaces are intended to be in mating contact with the back surfaces of coupons. In these holders, the coupon mounting surfaces are shaped to match the back surfaces of the target coupons. When coupon assemblies built with the downhole coupon holders of  FIGS. 2A to 6E  are deployed into a wellbore, the back surfaces of the coupons will not be exposed directly to fluids in the wellbore environment.  FIGS. 7A through 12C  relate to downhole coupon holders that allow all sides of the coupon to be fully exposed to fluids in a wellbore environment. This is achieved by end-mounting of the coupons around a common core and providing spaces around the coupons for movement of fluid. 
       FIGS. 7A and 7B  show a downhole coupon holder  400  according to another illustrative implementation. Downhole coupon holder  400  has a coupon holder axis  401 . Downhole coupon holder  400  includes an elongated support structure  406  that is axially aligned with coupon holder axis  401 , which means that the axial axis of elongated support structure  406  coincides with coupon holder axis  401 . In one example, elongated support structure  406  is a cylindrical tube. Downhole coupon holder  400  includes a coupon holder housing  403 , which may be axially aligned with coupon holder axis  401 . Coupon holder housing  403  is mounted on an end portion  406   a  of elongated support structure  406 , or end portion  406   a  is inserted into a bore  405  of coupon holder housing  403 . In an enlarged portion of bore  405 , an annular cavity  409  is formed. Annular cavity  409  is situated between an inner surface of coupon holder housing  403  and an outer surface of end portion  406   a  of elongated support structure  406 . A coupon holder body  407  is disposed in annular cavity  409 . Coupon holder housing  403  may have a threaded surface  404  for making a threaded connection with other components. 
     Coupon holder body  407  may be axially aligned with coupon holder axis  401 . Annular cavity  409  is disposed adjacent to an end  403   b  of coupon holder housing  403 , allowing coupon holder body  407  to be accessible from end  403   b . Coupon holder body  407  is in the form of a ring having a central opening  410 . End portion  406   a  of elongated support structure  406  extends through central opening  410  into bore  405  of coupon holder housing  403 . Coupon holder body  407  includes slots  412  to receive end portions of coupons. In one example, slots  412  are cylindrical slots to receive end portions of cylindrical rod coupons. The open ends of slots  412  through which end portions of coupons are received are located at an end face  413  of coupon holder body  407 . End face  413  is in a plane transverse to coupon holder axis  401 . The open ends of slots  412  at end face  413  are arranged in a circular pattern around central opening  410  and around coupon holder axis  401  (shown more clearly in  FIG. 7C ). Slots  412  extend in the same direction as coupon holder axis  401  such that coupons having end portions inserted into slots  412  extend parallel to elongated support structure  406 . 
     Coupons may be mounted in slots  412  to form a coupon assembly. As shown in  FIGS. 8A and 8B , when end portions of cylindrical rod coupons  130  (previously shown in  FIG. 1E ) are disposed in slots  412 , cylindrical rod coupons  130  will extend in the same direction as elongated support structure  406  and will form a pattern circumscribing portion  406   b  of support structure  406  that extends outside of coupon holder housing  403 . Elongated support structure  406  provides structural stability to coupons  130 . When support structure  406  is a tube having bore  415 , support structure  406  also serves as a fluid conduit. As shown more clearly in  FIG. 8B , the positioning of slots  412  relative to elongated support structure  406  is such that there are spaces  416   a  between the outer surface of elongated support structure  406  and coupons  130 . Also, the spacing between slots  412  along the circular pattern results in spaces (indicated at  416   b  in  FIG. 8A ) between adjacent rod coupons  130  along the length of elongated support structure  406 . Spaces  416   a ,  416   b  are connected and form flow paths around rod coupons  130 , allowing rod coupons  130  to be fully exposed to wellbore fluids when deployed in a wellbore environment with downhole coupon holder  400 . 
       FIGS. 9A and 9B  show that downhole coupon holder  400  may include an additional coupon holder housing  403   a  and coupon holder body  407   a . Coupon holder housing  403   a  and coupon holder body  407   a  may have the same structures described for coupon holder housing  403  and coupon holder body  407 , respectively. Coupon holder housing  403   a  may include a threaded surface  404   a  for making a threaded connection with other components. In this implementation, one end portion of elongated support structure  406  is inserted into coupon holder housing  406 , and the other end portion of elongated support structure  406  is inserted into coupon holder housing  406   a . Coupon holder body  407  is disposed in an annular cavity inside coupon holder housing  406  as previously described. Coupon holder body  407   a  is similarly disposed in an annular cavity inside coupon holder housing  403   a . Each of coupon holder bodies  407 ,  407   a  have slots  412 ,  412   a  to receive end portions of coupons. This arrangement secures coupons  130  between coupon holder bodies  407 ,  407   a.    
       FIG. 10A  shows that downhole coupon holder  400  may include an adapter  440  to enable extension of the holding capacity of the coupon holder. Adapter  440  may include a sleeve  443  and a coupon holder body  447  mounted inside sleeve  443 . Coupon holder body  447  has slots  452   a ,  452   b  on opposite sides to receive end portions of coupons. In  FIG. 10A , elongated support structure  406  has one end inserted through coupon holder body  407  into the bore of coupon holder housing  403 , as previously described. The other end of support structure  406  is received inside coupon holder body  447  of adapter  440 . End portions of coupons  130  are mounted in slots  412  of coupon holder body  407  and slots  452   a  of adapter  440 . Coupons  130  extend parallel to elongated support structure  406 . The pattern of slots  452   a  in coupon holder body  447  matches the pattern of slots  412  in coupon holder body  407  to enable coupons to be arranged between coupon holder bodies  407 ,  447  as shown. 
       FIG. 10B  shows another elongated support structure  446  with one end received in coupon holder body  447  of adapter  440 . The other end of support structure  446  is inserted through an opening at the center of coupon holder body  407   a  into the bore of coupon holder housing  403   a . Elongated support structure  446  may be, for example, a cylindrical tube. End portions of a second set of coupons  130  are mounted in slots  452   b  of coupon holder body  447  and slots  412   a  of coupon holder body  407   a . The second set of coupons  130  extends parallel to support structure  446 . As in the previous example, the pattern of slots  452   b  matches the pattern of slots  412   a  to enable the second set of coupons to be mounted between coupon holder bodies  447 ,  407   a  as shown. In the example shown in  FIG. 10B , adapter  440  allows doubling of the capacity of the downhole coupon holder. As many adapters as needed may be arranged in series to extend the holding capacity of the downhole coupon holder. 
     Coupon holder bodies  407 ,  407   a ,  447  may be configured to support various arrangements of coupons. Examples of configuration of coupon holder  407  is shown in  FIGS. 11A to 111D . These example configurations can be applied to any of the other coupon holder bodies described in  FIGS. 9B, 10A, and 10B . For illustrative purposes,  FIG. 11A  shows coupon holder body  407  with cylindrical slots to receive end portions of cylindrical rod coupons  130 .  FIG. 11B  shows coupon holder body  407  with flat-sided slots to receive end portions of flat rod coupons  140 .  FIG. 11C  shows coupon holder body  407  with curved slots to receive end portions of curved coupons  120 .  FIG. 11D  shows coupon holder body  407  with cylindrical slots to receive end portions of cylindrical rod coupons  130  and curved slots to receive curved coupons  120 . 
       FIGS. 12A and 12B  show a downhole coupon holder  500  according to another illustrative implementation. Downhole coupon holder  500  has a coupon holder axis  501 . Downhole coupon holder  500  includes a coupon holder body  507 , an end connection  504  attached to one end  507   a  of coupon holder body  507 , and an elongated support structure  506  attached to another end  507   b  of coupon holder body  507 . In one example, elongated support structure  506  is a cylindrical rod. End connection  504 , coupon holder body  507 , and elongated support structure  506  are axially aligned with coupon holder axis  501 . End connection  504  includes a threaded surface  504   a  for making a threaded connection. The length of support structure  506  projecting from end  507   b  of coupon holder body  507  may be selected to be greater than the length of rod coupons  130  to be supported by the holder. 
     Coupon holder body  507  includes a central opening  505  (in  FIG. 12C ) to receive an end portion of elongated support structure  506 . The end portion of support structure  506  may be retained in central opening  505  using any suitable method. Coupon holder body  507  includes slots  512  (in  FIG. 12C ) to receive end portions of rod coupons  130 . As shown in  FIG. 12C , slots  512  have open ends at an end face  513  of coupon holder body  507 . End face  513  is on a plane transverse to coupon holder axis  501 . In the illustrated example, slots  512  have a circular cross-section to receive end portions of cylindrical rod coupons. In alternative implementations where flat rod coupons are to be supported, slots  512  may have a rectangular cross-section. Similarly, if curved coupons are to be supported, slots  512  may have curved cross-sections. The open ends of slots  512  at end face  513  are arranged in a circular pattern around central opening  505  and around coupon holder axis  501 . When end portions of rod coupons  130  are inserted into slots  512 , rod coupons  130  will extend in the same direction as support structure  506  and will form a circular pattern circumscribing support structure  506 . 
     To lock rod coupons  130  in place, spacer tubes  508  may be mounted on pins ( 139  in  FIG. 1E ) at the ends of rod coupons  130 . Spacer tubes  508  are inserted into slots in a spacer connector  518 . The slots in spacer connector  518 , shown as  519  in  FIG. 12D , can be similar in arrangement pattern to slots  512  in coupon holder body  507 . Spacer connector  518  includes a threaded surface  518   a  for forming a threaded connection with a bull nose fitting  520 . Spacer connector  518  also includes a bore for passage of a tip of support structure  506 . The tip of support structure  506  includes a hole  521  (in  FIG. 12B ) to receive a fastener. Similar holes are in spacer connector  518  and bull nose fitting  520 . These holes align when all the parts are assembled to receive a fastener. V-grooves  522 ,  524 ,  526  may be formed in outer surfaces of coupon holder body  507 , spacer connector  518 , and bull nose fitting  520  for passage of fluid around rod coupons  130 .  FIG. 12E  shows that coupons  130  as assembled into coupon holder  500  are spaced from each other and from the support structure to allow fluid to move freely around the coupons. 
     In one example, the various components of the downhole coupon holders ( 400 ,  500 ) illustrated in  FIGS. 7A to 12E  that will be exposed to wellbore fluids, e.g., the coupon holder housings ( 403 ,  403   a ), the coupon holder bodies ( 407 ,  407   a ,  447 ,  507 ), the elongated support structures ( 406 ,  506 ), the end connection ( 504 ), the spacer tubes ( 508 ), the spacer connector ( 518 ), and the bull nose fitting ( 520 ), may be made of a corrosion-resistant metal or alloy, such as Grade 17-4 Stainless Steel (UNS S17400) or other metallic material that will not corrode when disposed in a wellbore that is undergoing production or treatment. To use the downhole coupon holders in a wellbore environment, the threaded surfaces ( 404 ,  404   a ,  504   a ) for making threaded connections may be coated with an anti-galling agent, such as high purity Goop, and/or high temperature silicone grease that will not be destroyed by the application of torque and that is resistant to corrosion in the wellbore environment. 
     The coupons mounted in the downhole coupon holders ( 200 ,  200 ′,  300 ,  300 ′,  400 ,  500 ) of  FIGS. 2A to 12E  can be any combination of corrosion coupons, erosion coupons, and scale coupons as previously described. For corrosion study with the downhole coupon holders ( 200 ,  200 ′,  300 ,  300 ′) of  FIGS. 2A to 6E , the back surfaces of the coupons that will come into mating contact with the coupon mounting surfaces of the downhole coupon holders may be coated with a corrosion-resistant material such that only the front surfaces of the coupons will be allowed to corrode for accuracy of corrosion calculation. The corrosion-resistant material used on the surfaces of the coupon may be selected from polymeric, resin, epoxy, organic, powder, metallic, and nano-composite coatings. 
     Various coupon assemblies may be configured using the downhole coupon holders ( 200 ,  200 ′,  300 ,  300 ′,  400 ,  500 ) described in  FIGS. 2A to 12E . These coupon assemblies may be deployed into a wellbore alone for coupon testing or attached to a downhole tool and then deployed into the wellbore with the downhole tool. For illustration purposes,  FIG. 13  shows a coupon assembly  600  including coupons  100  fastened to downhole coupon holder  300 . Coupon assembly  600  is fitted to an end of a downhole tool  604 . A gauge hanger (not shown) may be attached to end connection  306  of downhole coupon holder  300  and deployed into a wellbore using a slickline. Once the gauge hanger is anchored in the wellbore, the slickline may be retrieved. Coupon assembly  600  and downhole tool  604  will remain in the wellbore, suspended at the end of the gauge hanger. Production can be carried out in the wellbore while coupon assembly  600  and downhole tool  604  are suspended in the wellbore. In one example, downhole tool  604  may be a downhole corrosion measurement tool including an electrical resistance probe  608  to measure metal loss due to corrosion. Downhole tool  604  may include an instrumentation module  612  to record the data from the probe. In the example downhole coupon holder  300 , the front surfaces of coupons  100  will be exposed to wellbore fluids while the assembly is in the wellbore. Damage to the coupons can be accessed when the coupon assembly is retrieved to the surface. 
     The detailed description along with the summary and abstract are not intended to be exhaustive or to limit the embodiments to the precise forms described. Although specific embodiments, implementations, and examples are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art.