Sour Gas Dehydration in Mobile Bay

Abstract

Abstract An evaluation of Mobil's different methods of sour gas development utilized offshore and processes selected are presented with the maintenance history. The conclusions are based on field data showing past performance with a correlation made for future development and the selection criteria for dehydration Introduction Mobil's offshore operation in Alabama is divided into two main fields: The Lower Mobile Bay-Mary Ann Field and the Mobile OCS Block 823 Field. The Mary Ann Field was the first Alabama offshore production and started in April of 1988. Located in state waters, the field spans the entrance and shipping channels 01Mobile Bay. The original field consisted of four producing wells, three production platforms and one auxiliary platform which served as a pipeline gathering facility. The Mobile 823 Field is located southwest of Mobile Bay and south of Dauphin Island. This was the first production from OCS waters offshore Alabama and occurred during September 1991. These fields have been designed, constructed, and produced independent of each other due to the gascompositions, rates, and timing. The average hydrogen sulfide content of the Mary Ann field is approximately 5 mol% while that of the Mobile 823 field is considerably less at approximately 150 ppm. Bottom hole pressures and temperatures of the wells differ with the higher values associated with the Mobile 823 field. Different methods for development have been chosen based on philosophy, operational requirements, and information which were available at that time. Now with the information and knowledge gained, all future production must go through a rigorous evaluation for the most cost effective method to produce and which system to incorporate that production. Design Philosophy The design philosophy for the Mary Ann field was based on the technologies available for that time period. The final design incorporated the cooling, separation and dehydration of the sow gas offshore to allow for the use of carbon steel materials for the surface and pipeline components. A corrosion inhibition system was also incorporated in the design to protect the carbon steel equipment upstream of the dehydration system and also to solubilize any element sulphur deposition which might occur on the platform. Both glycol and desiccants were evaluated for sour gas dehydration with silica gel being chosen as the final design philosophy. The silica gel was chosen over other desiccants due to lower regeneration temperatures, less coking, and little or no carbonyl sulphide(COS) formation. The carbonyl sulphide formation had little to no affect for offshore, however this would have increased the sweetening unit load, the sulfur recovery unit, and the tail gas treater at the onshore plant. Some disadvantages noted of silica gel were the need for a regeneration media and the mechanical switching valves required for the cyclic service. Each of these added to the complexity of the desiccant operation and maintenance requirements.