Method and apparatus for detection of pitting corrosion under iron sulfide deposition

1. A method for monitoring pitting corrosion of a production tubing in a hydrocarbon well under downhole conditions during a production process using a coupon segment, the method comprising the steps of:

placing the coupon segment in the hydrocarbon well at a placement depth, wherein the coupon segment is exposed to produced fluids, wherein the coupon segment comprises:

an outer layer, wherein the outer layer comprises the same metallurgy as the production tubing, wherein the outer layer comprises a known thickness,

an inner layer, and

a hollow middle layer, wherein the hollow middle layer comprises a tracer fluid, wherein the tracer fluid is operable to leak from the hollow middle layer into the produced fluids when pitting corrosion penetrates the outer layer;

operating the production process in the hydrocarbon well such that produced fluids flow through the production tubing to a surface of the hydrocarbon well, and wherein the produced fluids contact the coupon segment such that pitting corrosion occurs on the coupon segment;

obtaining a sample of the produced fluids at the surface of the hydrocarbon well;

measuring an amount of the tracer fluid in the sample; and

placing one or more additional coupon segments in the hydrocarbon well, wherein each additional coupon segment contains a different tracer fluid, wherein each coupon segment has a different thickness of the outer layer.

2. The method of claim 1 , further including:

determining a pitting corrosion rate of the coupon segment; and

estimating pitting corrosion of the production tubing based on the pitting corrosion rate of the coupon segment.

3. The method of claim 1 , wherein the step of placing the coupon segment in the hydrocarbon well comprises the steps of:

placing the coupon segment on a coupon holder; attaching the coupon holder to an anchoring segment; and

positioning the anchoring segment attached to the coupon holder in the hydrocarbon well with a placement tool.

4. The method of claim 1 , wherein the hydrocarbon well is selected from the group consisting of a sour gas well, an oil well, and a high hydrogen sulfide content well.

5. The method of claim 1 , wherein the tracer fluid is selected from the group consisting of inorganic tracer fluids, organic tracer fluids, radioactive tracer fluids, and combinations of the same.

6. The method of claim 1 , wherein the hydrocarbon well is selected from the group consisting of a sour gas well and an oil well.

7. An apparatus for monitoring pitting corrosion of a production tubing in a hydrocarbon well under downhole conditions, the apparatus comprising:

a placement tool, the placement tool configured to retrievably set a downhole pitting corrosion monitoring (DPCM) tool at a placement depth in a hydrocarbon well; and

the DPCM tool, the DPCM configured to monitor pitting corrosion, the DPCM comprising:

an anchoring segment, the anchoring segment configured to anchor a coupon holder in the hydrocarbon well;

the coupon holder physically anchored by the anchoring segment, the coupon holder configured to hold one or more coupon segments, wherein the coupon holder comprises a cylindrical housing configured to hold the one or more coupon segments such that each coupon segment is exposed to a produced fluid stream within the production tubing; and

the coupon segment anchored by the coupon holder, the coupon segment operable to corrode upon exposure to the produced fluid, wherein each coupon segment comprises:

an outer layer, the outer layer comprising a known thickness, wherein the outer layer is exposed to the produced fluid;

an inner layer; and

a hollow middle layer, wherein the hollow middle layer is defined by the annulus formed by the outer layer and the inner layer, wherein the hollow middle layer comprises a tracer fluid,

wherein each coupon segment has a different known thickness of the outer layer and wherein each coupon segment contains a different tracer fluid in the hollow middle layer.

8. The apparatus of claim 7 , wherein the placement tool is selected from the group consisting of slickline and coiled tubing.

9. The apparatus of claim 7 , wherein the anchoring segment comprises a gauge hanger.

10. The apparatus of claim 7 , wherein the means to retrievably anchor the anchoring segment is configured to expand radially outward from the anchoring segment onto the hydrocarbon well.

11. The apparatus of claim 7 , wherein the known thickness of the outer layer is between 0.5 mm and 10 mm.

12. The apparatus of claim 7 , wherein the tracer fluid is selected from the group consisting of inorganic tracer fluids, organic tracer fluids, radioactive tracer fluids, and combinations of the same.

13. The apparatus of claim 7 , wherein the hydrocarbon well is selected from the group consisting of a sour gas well and an oil well.