Corrosion Testing of Highly Alloyed Materials For Deep, Sour Gas Well Environments
Authors: Watkins M. , Greer J.B. 
Published: December 2007

Completing sour gas wells with materials that possess innate corrosion resistance may be more practical than to attempt chemical corrosion inhibition of steel alloys. A program of screening tests determined that alloy systems containing large amounts of nickel, cobalt, chromium, and molybdenum have exceptional corrosion resistance for this environment.

Introduction
A family of highly alloyed, super austenitic stainless tubular goods, virtually inert to corrosion and having yield strengths in the range of 150,000 to 250,000 psi, may soon appear in the oil patch. This new development was brought about by (1) the need to safely produce gas from deep (20,000 ft), high-pressure (20,000 produce gas from deep (20,000 ft), high-pressure (20,000 psi), high-temperature (400 to 500 deg. F) reservoirs psi), high-temperature (400 to 500 deg. F) reservoirs containing high percentages of H2S and CO2 along with salt water, and (2) the inability to effectively inhibit conventional steel tubing from corrosion at these conditions.
The cost of a 20,000-ft string of 3 1/2-in. inert metal tubing likely will be on the order of $3 million. Because of its ultrahigh strength and concurrent thin wall design, however, a doubling of production rate per well is possible when compared with steel tubing in an identical possible when compared with steel tubing in an identical casing program (Fig. 1). This, along with freedom from the need to inject large volumes of corrosion inhibitor and freedom from workovers, makes the use of such tubular goods economically attractive.
A second possible use of ultrahigh-strength tubular goods would be for small-diameter workover strings where a high-tensile overpull capacity and thin wall design are desirable.
This paper summarizes the results of a comprehensive testing program designed to determine if alloy systems exist that could be used in a deep, sour gas well without protection from a corrosion inhibitor. Those alloys found to be inert to all the tests are candidates for manufacture as tubular goods. A family of ultrahigh-strength, super austenitic stainless alloys composed principally of nickel (and/or cobalt), chromium, and principally of nickel (and/or cobalt), chromium, and molybdenum successfully passed all laboratory tests.

Experimental Program

Alloys Tested
The nominal chemical compositions of the alloys selected for testing are given in Table 1, and their mechanical properties are given in Table 2.

Test Program
The selection of alloys that are inert to the deep, sour gas well environment requires a screening program for the literally thousands of corrosion-resistant metals and alloys currently available. Although there are considerable technical data on the corrosion performance of many of these alloys, most have very limited applicability to the deep, sour gas well. For example, there are scarcely any data available on environments containing chloride and sulfide ions at temperatures above 250 deg. F, especially for high-strength materials.
There are two general types of materials problems anticipated in the deep, sour gas well environment - corrosion and embrittlement. There is weight-loss corrosion caused by CO2, H2S, and brine, and also pitting and crevice corrosion. The second problem, embrittlement, comprises both sulfide stress cracking and chloride stress cracking. Materials for the deep, sour gas well must be resistant to both corrosion and embrittlement.
JPT
P. 698