The toxicity of hydrogen sulphide in hydrocarbon streams is well known in the industry and considerable expense and efforts are expended annually to remove hydrogen sulphide to a safe level. In large production facilities, it is generally economical to install a regenerative system for treating sour gas streams. However, during the development stages of relatively small low-sour gas fields at remote and normally unmanned locations where regenerative systems are not practical nor economical, it is necessary to treat the sour gas production with non-regenerable scavenging processes. In the development of its low-sour Zechstein gas reserves in the Coevorden field in the North-East The Netherlands, the Nederlandse Aardolie Maatschappij (a Shell operating unit, hereafter referred to as NAM) decided to adopt continuous direct-injection of liquid scavenging agents as the lowest overall cost process having the least environmental impact and the highest energy efficiency. At the inception of the project, the operating parameters controlling the scavenging efficiency using direct injection of liquid scavengers in this system were largely unknown. Consequently, numerous field trials using different chemistries and different injection mechanics had to be carried out. In this paper we present the results of these field trials, which ultimately led to a very successful and profitable field development strategy. A variety of very challenging operational problems were encountered, and solved. Reference is made to injection nozzle blockages, fouling of glycol gas dehydration systems, severe scaling problems in production and downstream water treatment/injection facilities, inadequate hydrogen sulphide removal efficiencies and HS&E related issues. A better understanding of the fundamental relationships between operating parameters governing direct-injection processes and associated chemical development and application methods has been gained. Communication and integration of experience and knowledge between the operating unit and its chemical supplier were key success factors in this achievement, as was the endurance and continuing support of field operations staff in facilitating the resolution of difficult problems. Introduction NAM's Ten Arlo system (Fig. 1) produces gas from 32 satellite locations and four gas treatment / compression plants. The majority of the fields in the system produce sweet gas from the Limburg reservoir. However, the system's heavily compartmentalized Coevorden field also contains gas accumulations in the Zechstein reservoir with H2S concentrations up to 300 ppm(v). Developing these accumulations would require extensive modifications to existing (sweet) gas production / treatment facilities to ensure safe operations and the prevention of H2S emissions. The producible reserves were small yet economically attractive. However, due to their geographical scatter, a pipeline grid connecting these low-sour accumulations to an existing nearby plant utilizing a regenerative solvent process for gas sweetening was found to be economically unfeasible. In-field desulpherisation with, for example, a solid-bed adsorption process, was found to be technically feasible, but the required capital investment and perceived life-cycle costs could not be justified. In order to allow production of the low-sour gas without violating the Ten Arlo sales gas specification for maximum allowable H2S concentration (5 mg H2S/Nm3 or 3 ppm(v)), in-field desulpherisation utilizing a commercially attractive alternative technology was looked for.

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