Apparatus for anchoring downhole corrosion coupons

The present invention relates to a method and apparatus for anchoring corrosion coupons downhole in the production tubing of a producing oil and/or gas well. More particularly, the present invention is directed to a method and apparatus which makes it possible to closely duplicate the actual conditions experienced by the production tubing to enable a more accurate evaluation of corrosion rate and, hence, operative life of the tubing, to be made.

Monitoring the corrosive effects of well fluids using corrosion coupons allows 1) a determination of the corrosion problem downhole to be made in the actual environ­ment (pressure, temperature, etc.) to which the production tubing is exposed, 2) an evaluation of the effectiveness of the corrosion mitigation program to be conducted, and 3) an assessment of the life of the production tubing to be made.

Failure to monitor corrosion rates and to take appropriate measure to slow those rates can reduce the operative life of the well. Accordingly, monitoring the corrosive effects of the corrosives entrained in the well fluids (such as CO₂, H₂S and O₂) is becoming increasingly important. A number of coupon holders for positioning corrosion coupons downhole are currently available on the market today. The prior art systems of which applicant is aware suffer from one or more of the following defects: 1) the coupon holder can only be used where the well is capable of accomodating specially configured tubing (i.e., a side-pocket mandrel); 2) the conditions (pressure, flow rate, etc.) experienced by the coupon are not representative of those experienced by the tubing - thus, the corrosion rate may also differ; 3) the anchoring technique within the tubing may be unreliable and/or potentially harmful to the interior surface of the tubing; 4) the manner of securing the coupon in the holder is such that it effectively alters the corrosion rate impairing the accuracy of the monitoring system; and 5) the holder may unduly restrict the flow of fluids therethrough, which both reduces the rate of recovery of fluids and alters the flow and corrosion conditions experienced by the coupon (i.e., gives a faulty indication of corrosion rate).

Viewed from one aspect the invention provides apparatus for anchoring at least one corrosion coupon at a preselected location downhole in a tubing string of a producing well, said apparatus comprising:

• a) a locking mandrel for positively locating the or each corrosion coupon adjacent said preselected location by interlocking within a portion of said tubing string, said locking mandrel having a predetermined minimum inner diameter;
• b) a cylindrical housing surrounding said corrosion coupon(s), the minimum dimension defined by the region of said housing which is occupied by said coupon(s) being not less than said predetermined minimum inner diameter of said locking mandrel;
• c) means for connecting said cylindrical housing to said locking mandrel; and
• d) means for securing said corrosion coupon(s) in said cylindrical housing.

In a preferred embodiment, a preferably cylindrical coupon is anchored in one or more predetermined locations within the production tubing using state-­of-the-art nipples and locking mandrel. The holder which houses the coupon is threadingly attached to an adaptor which, in turn, is threaded onto the end of the locking mandrel. The coupon is maintained in position within the cylindrical housing by end retainers which telescopically receive the ends of the coupon and which are compressively engaged by the end of the housing and the end of the adaptor. This compressive force is sufficient to firmly engage the coupon but is not excessive to the point of deforming the coupon or the end retainers. The amount of compressive force exerted is limited by metal to metal engagement between the adapter and the coupon holder. Further, the metallic coupons are maintained in positions out of contact with any other metallic surface or object.

Preferably, two or more coupons are used. In this manner, it is possible to cross-check the corrosion experienced by each. The two coupons can be made of the same material for purposes of cross-checking or they may be made of different materials in order to determine how different alloys are affected by the same environment. With two coupons, a center retainer is telescoped over the adjoining ends of the coupons to maintain proper alignment. The minimum dimension inside the housing occupied by the coupon is not less than the minimum internal dimension of the interlocking mandrel so as to minimize the impediment to flow caused by the holder.

Accordingly, at least in a preferred embodiment, the present invention may provide a corrosion monitoring system that 1) is firmly anchored in position, 2) does not damage the internal surface of the casing, 3) does not require specially configured tubing but, rather, utilizes a conventional nipple that is ordinarily positioned within the tubing string to permit other types of periodic downhole operations, 4) minimizes the obstruction fluid flow, 5) substantially duplicates the flow conditions experienced by the actual tubing so as to provide an accurate reading of corrosion rate and, 6) is secured by means not requiring a penetrating metallic fastener or contact by any other metallic object which can unduly alter corrosion due to cathodic reaction or the resulting internal strain within the coupon and, 7) has a securing means that permits back-flow around the coupon to increase the surface area exposed to the corrosive fluids.

Viewed from a second aspect the invention provides a method of monitoring the downhole corrosive effects of well fluids on the production tubing of a producing oil or gas well, comprising positioning a cylindrical coupon within a coupon holder, securing said coupon in said holder utilizing non-penetrating fastening means, anchoring said holder within said production tubing at a predetermined location for a predetermined period of time without significantly impeding fluid flow, retrieving said coupon holder from said anchored position, and determining the amount and nature of corrosion that has occured in said predetermined period of time.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:

• Figure 1 is a cross-sectional side view with portions broken away, of a coupon holder in accordance with the present invention suspended in a nipple by a locking mandrel;
• Figure 2 is an enlarged sectional side view depicting the end and center retainers of the coupon holder in greater detail;
• Figure 3 is a lateral cross-section as seen along line 3-3 of Figure 2; and
• Figure 4 is a lateral cross-section as seen along line 4-4 of Figure 2.

A corrosion coupon anchoring system is depicted in Figure 1 generally at 10. While this corrosion coupon anchor system cannot be used with rod pumped wells, it could be utilized in virtually every other type of producing oil or gas well. Cylindrical housing 12 threadingly engages adaptor 14 which is, in turn, threaded on to the end of a locking mandrel 11. The use of the adaptor 14 enables the corrosion coupon holder of the illustrated embodiment to be utilized with any commercially available locking mandrel without the need to modify the tool. One such mandrel, by way of example and not limitation, is identified as type 'X' lock and is sold by Otis Engineering Corporation. This locking mandrel 11 is lowered into a tubing string by a wireline and is set into a type 'X' nipple 13 which has been inserted in the tubing string at a predetermined location to permit various downhole operations such as this to be subsequently performed, as necessary. Mandrel 11 has a minimum internal dimension D₁. Resilient packing 17 forms a fluidic seal between mandrel 11 and nipple 13 preventing fluid flow outside of mandrel 11.

A groove 16 interrupts the thread on adaptor 14. Groove 16 receives an O-ring 18 which exerts pressure against the internal surface of cylindrical housing 12 discouraging thread disengagement. These are preferably at least two coupons 20 and 22 secured within the housing 12 (shown here in their preferred cylindrical configuration). While these coupons could be manufactured of different materials to determine which is more corrosion resistant in a particular environment, it is preferred that the two coupons both be made of the same material and that that material be the same material as the tubing. Examples of suitable materials are 1020 and 4140 alloy steels. The use of two coupons of the same material enables a cross check to be made on the nature and rate of corrosion.

The distal ends 24 and 26 of coupons 20 and 22 are each engaged by identical end retainers 28 and 30. Each end retainer comprises a cylindrical sleeve 32 which slides over the end 24 or 26 of coupon 20 or 22. Shoulder 34 limits the distance which sleeve 32 can slide over its respective coupon end. Lateral slots 36 and 38 provide flow paths for the corrosive liquids on the exterior of the coupon. This back flow has the benefits of a) increasing the surface area of the coupons exposed to the fluids and b) will provide a means to distinguish between the combined corrosion/erosion effects inside the coupon and the almost purely erosive effects outside the coupon. The opposite end of end retainers 28 and 30 are beveled as at 40 for engagement by correspondingly beveled surfaces 42 and 44 provided on the end of adaptor 14 and cylindrical housing 12, respectively.

When two coupons are utilized, a center retainer 50 telescopically receives the proximate ends 25 and 27 of coupons 20 and 22. Center rib 52 fits between ends 25 and 27 to keep the coupons separated. Retainers 28, 30 and 50 are preferably made of a plastic material such as Ryton (a registered trademark of Phillips Petroleum Co.) polymer. Non-metallic fasteners are used in order to avoid cathodic reaction which may occur as a result of two dissimilar metals coming in contact. In fact, end retainers 28 and 30 and center retainer 50 securely suspend coupons 20 and 22 out of contact with even the cylindrical housing 12 which may be made of, by way of example, Inconel 318 (a registered trademark of International Nickel Company) alloy steel. The minimum internal dimension D₂ of the portion of housing 12 that contains the coupons 20 and 22 (usually inner diameter of the coupons themselves) is not less than D₁.

To utilize the coupon holder of the illustrated embodi­ment, a properly sized locking mandrel 11, adaptor 14 and cylindrical housing 12 are selected for the type and size of nipple 13 and tubing 15. The corresponding sizes of coupons 20,22 (preferably cylindrical) and end and center retainers 28,30 and 50 are thereby determined. The corrosion coupon anchoring system 10 is configured by assembling end retainers 28 and 30, center retainer 50 and corrosion coupons 20 and 22 within cylindrical housing 12. Adaptor 14 is then threaded into housing 12. When the threads are fully seated (i.e., when the end of adaptor 14 dogs down against the internal shoulder in holder 10), the beveled surface 42 on adaptor 14 in conjunction with beveled surface 44 on housing 12, maintain a firm compressive force on the coupon assembly but do not cause deformation or signi­ficant internal strain in any component. The coupon anchoring system 10 of the illustrated embodiment forms no greater restriction of flow than the locking mandrel 11 since its minimum internal dimension D₂ is not less than the minimum internal dimension D₁ of said mandrel 11.

Adaptor 14 is then threaded onto the appropriate locking mandrel and the entire system 10 is lowered downhole using a wireline apparatus. When the corrosion coupon anchoring system is in the vicinity of the nipple, locking fingers 46 are sprung into engagement position as shown in Fig. 1, by using a conventional jarring technique to shear a retainer pin (not shown). The anchoring system is, then, left downhole during normal production operations for a period of several months. System 10 can then be fished out of the wellbore using conventional wireline techniques. The coupons are then weighed individually (after cleaning) and their net weights compared to their original gross weights at the time of insertion into the well. The coupons can also be subdivided and microscopically examined to determine the type and source of corrosion, as well as the rate.

A prototype coupon holder of the illustrated embodiment was tested in a gas well that is approximately 150 miles off the Louisiana coast. Two coupons, both of 1020 alloy steel and indentified as C-1018-1 and C-1018-2, were inserted at the 6142 foot mark of a well that is in excess of 9000 feet deep using an Otis 'X' locking mandrel in a type 'X' nipple. These coupons were downhole for a period of 67 days, 21 days of which the well was producing (the well was periodically shut in for the remainder of the time for rate control). The well, which was flowing at a rate of 11 million cubic feet of gas and 260 barrels of condensate per day, experienced no appreciable flow reduction due to the insertion of the coupon holder.

The coupons were removed from the holder and after a thorough cleansing, each coupon was weighed to determine the amount of weight lost due to corrosion/erosion effects. The weight loss was plugged into the following formula to determine corrosion rate: where

c is the corrosion rate in mils per year (mpy),

k is a unit adjusting constant (3.45 x 10⁶ for mpy),

Δm is the mass loss in grams,

A is the surface area exposed in square centimeters

t is the time of exposure in hours, and

ρ is the density in grams per cubic centimeter

The corrosion rates for the two coupons calculated out to be:

No pitting was observed in either coupon. Since the only difference in the calculation of corrosion rate between the 21 day and 67 day exposures is the length of time, the 67 day data naturally shows a slower rate. The differences between the corrosion rate of coupon C-1018-2 and C-1018-1 might have resulted from erosional effects on C-1018-2, the coupon nearer the leading edge of the coupon holder. In any event, the difference is well within experimental error. As a means of comparison, a rod-type coupon inserted into a highly turbulent zone near the surface, experienced corrosion rates of 35.09 mpy and 11.0 mpy during the same 21 and 67 day periods. Such high rates (which are obviously predominantly erosion, rather than corrosion), if believed, would mislead the Corrosion Engineer regarding the effectiveness of his/her corrosion mitigation program and suggest a higher (more expensive) application rate.

By accurately duplicating flow conditions within the tubing string while minimizing the restriction to flow, the corrosion coupon anchoring system of the present invention enables a more accurate assessment of actual corrosion rate to be made. Accordingly, more accurate assessments of 1) the nature of the downhole corrosion problem, 2) the effectiveness of the corrosion mitigation program and 3) the life of the tubing, can be made.

Various changes, alternatives and modifications will be apparent following a reading of the foregoing application. For example, although the coupon holder discussed above is depicted as housing two coupons, the length of the holder could be increased to accomodate anywhere from 3 to 6 coupons of various materials by simply adding additional center retainers. This would enable a variety of materials to be used in virtually identical environments to determine which material performed best in these downhole conditions for application to design considerations in associated wells. Accordingly, it is intended that all such changes, alternatives and modifications which may be apparent to those of relevant skill be considered part of the present invention.