Evaluation of Seal Materials for High-Temperature H2S Service
Authors: Ray Thomas W. , Ivey Carl E. 
Published: March 2010

The drilling of deeper wells in the exploration for oil and gas has imposed new demands on seal materials used in the completion of these wells. Materials used in the past for the completion of shallow sweet wells well not perform satisfactorily in the deep sour hot environment. These materials undergo serious chemical and physical degration under the conditions imposed by these wells.

Seal materials have been tested in the laboratory at various temperatures, from 200 to 450 °F, in 35% H °S, 15% CO2 50% CH4 and pressures from 1,000 to 15,000 psi. The materials have been used in actual field conditions with temperatures up to 425 °F and an 8,000-10,000psi differential across the seals.

This paper describes the laboratory evaluation of these materials and the field tests used to develop reliable seals for the harsh environment.

Introduction

The first well in the Thomasville field, near Piney Woods, Mississippi, was completed with a conventional packer having a sliding-seal assembly set deep in the well. After the production test, it was found that the elastomers in the packer, tubing hanger and valve stem packing had failed. Extensive research revealed that no currently available elastomer could withstand the environment of hot, high-pressure, sour gas for any appreciable period of time. Consequently, well configurations were altered to aboid the use of elastomer seals in the well(1). Research and development of polymeric seal materials at Otis Engineering has resulted in a significant development for oil and gas completions. Laboratory tests were designed to simulate as near as possible the condition encountered in a downhole situation. Sour gas mixtures were specified to contain 35%, H2S (hydrogen sulphide), 15% CO2, (carbon dioxide) and 50% CH4, (methane). Sour crude was obtained and kept under pressure for use in elevated~temperature evaluations. The existing materials were pushed as far as possible in temperature (450 °F) and pressure (15,000 psi) to determine if any were suitable for the severe conditions. The commonly used oil seals were found to undergo serious physical and chemical deterioration.

The specific objectives of the test program were:to evaluate various elastomers and thermoplastic materials to determine which were the most resistant to hot, sour gas and fluid conditions;to determine if a combination of materials would perform better than anyone material alone; andto field test qualified materials under the most severe conditions available.

Laboratory Tests

The environment used in the test program consisted of a gas mixture containing 35% hydrogen sulphide, 15% carbon dioxide and 50% methane. The sour crude contained approximately the same composition as the gas, As the test fixture was charged with gas, a liquid phase of fresh water and diesel oil was also introduced, because it was assumed that these are produced in the formation. The lest chamber (Fig. 1) is made of C-75 and Inconel material for H2S testings. This fixture is heated by an oil bath with careful control of the desired temperature.

The major portion of the testing was conducted in a static condition.