A Case Study of High H2S Aquifer Production Testing : Planning and Execution

Abstract

This paper shares a case study of production testing of a large Middle East dolomitic aquifer containing high concentration of H2S. The aim of this paper is to discuss on the HSE aspects primarily on wellsite and operation safety by sharing the issues, challenges and mitigations at various stages of planning and execution. As part of field X development strategy, water flooding is required to maximize recovery. However, due to water scarcity Operators had to look into ground aquifers to support water injection leading to dolomitic reservoir being identified as the best source. The initial planning was based at 30 ppm H2S limit centered on samples obtained from nearby field located 400 km from field X. Based on actual bottomhole samples from field X, it was found that the reservoir contained higher H2S concentration which requires the operator to re-strategize the production testing operation within a limited time frame to cover safety and security aspects. The operation involved multidisciplinary collaboration encompassing DST design, process safety and security. As the well is located in a densely populated area, a first of its kind H2S dispersion study in aqueous solution was introduced. The DST operation was successfully conducted with no major HSE and security incident recorded. The well was tested up to a maximum rate of 20 kbl/d and through the introduction of multiple injection point, the concentration H2S was successfully reduced to 2 ppm. The H2S dispersion study in aqueous phase played a pivotal role, assisting in risk identification and quantification. Through the study, many critical HSE aspects were addressed and resolved, expediting the planning stage and improved the whole system. The modelling was based on worst case scenario where all 18,000 ppm of H2S will disperse to air. On the other hand, for normal operation, some portion of H2S is expected to dissolve in water during discharge. It is also noteworthy to mention that understanding the limitation of the model is equally important to assist in planning and executing the operation effectively.