Selection of Materials for Sour Service in Petroleum Production

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area,these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Summary. This paper provides a technical review of the materials selectionprocess for sour service. It highlights H2S corrosion processes that occur onhigh-strength steels and corrosion-resistant alloys. In addition, procedures toperform materials evaluation and selection to minimize the deleterious effectsof H2S are discussed.

Introduction

As shallow reserves are depleted, the worldwide search for new sources ofhydrocarbons is turning to deeper reservoirs. Drilling and producing thesereservoirs requires the use of new materials not previously used. In manysituations, drilling procedures and completion schemes are now being dictatedby materials technology.

Deeper drilling means that higher pressures will be encountered and that thereservoir fluids will be at a higher temperature than normal. The likelihood ofencountering acid gases (CO2 and H2S) as a reservoir component is alsoincreased. Formations containing 25% H2S + CO2 at 200 deg. C [392 deg. F] witha bottomhole pressure of more than 140 MPa [20,000 psi] are not uncommon.

The mechanical requirements for materials used for production equipmentincrease substantially with well depth. Tensile loads for tubulars are greaterbecause of the greater hangoff loads, and hoop stresses increase because ofpressure. Elevated temperatures have a pronounced detrimental influence onmechanical properties.

The tradeoff in economics is often between higher-strength(higher-price-per-pound) materials and thicker-walled equipment (more pounds).In most cases, higher-strength materials would be favored economically were itnot for the greater susceptibility of high-strength materials to environmentaldegradation in reservoir fluids. This paper discusses (1) the environmentalprocesses associated with H S that affect the performance of high-strengthmaterials, (2) the alloys that are used (or are being considered) to overcomethe adverse effects of corrosive environments, and (3) procedures to selectmaterials for sour service. The more difficult frontier areas where wells aredeep and hot and contain acid gas have been emphasized.

Historical Background

Since the early 1950's, the petroleum industry has been developing sour-gasfields. Initially, much of this activity was centered in western Canada. Asfield operations began, it was quickly realized that there was a major problemwith brittle fracturing, later called sulfide stress cracking (SSC), ofhigh-strength (high-hardness) steels when exposed to sour productionenvironments. This problem was particularly associated with the failure ofdownhole production tubulars. However. other consequences of exposure to sourenvironments were quickly recognized.

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