Reservoir Souring Management in a Mature Waterflooded Offshore Field Malaysia: Case Study of Field-D
Authors: Ismail W. , Mithani A. , Shah T. Tengku Zainal Alam , Mawardi M. , Lajawai M. T. , Tahir M. M. , Bak A. A. , Koyyalamudi L. , Runham G. , Nor H. M. 
Published: October 2023

Field-D is a mature offshore asset in Malaysia producing since 1991. Following the startup of production operations at Platform-X4 in 1994, waterflooding was introduced to the field in 1996. Initial fluid sampling and analysis during field development indicated no H2S presence (sweet reservoir). However, in recent years 2014 onwards excursion of H2S was observed at elevated levels up to 1000ppm at well producers and processing facilities on topside. This resulted in closure of high H2S content wells with an impact of oil production ca. 800bpd. The facilities were re-classified as sour service in the year 2020. This paper will highlight the challenges in managing reservoir souring in a mature waterflooded field. A workflow was developed to understand the reservoir souring phenomenon and its impacts.

The workflow consists of 8 stages starting with data management, followed by situational assessment, souring modelling, H2S forecast, mitigation evaluation, treatment selection, execution, or implementation and finally performance monitoring. The elements within each stage will be further explained in this paper based on the experience and lessons learnt from Field-D operation which is currently at the stage of mitigation option execution and performance monitoring. The reservoir souring mitigation solutions i.e. H2S scavenger injection at topside facilities and nitrate injection to reservoir have been deployed at this stage. Hence this paper will also touch based on selection and deployment process of the H2S scavenger and nitrate injection strategy. Apart from that, the paper will highlight the role of different disciplines involvement in reservoir souring studies and management that is deemed crucial in ensuring successful approach holistically.

Finally, following the workflow, a set of conditions were identified at which biogenic reservoir souring will occur in a waterflooded field related to reservoir pressure, water chemistry, operating temperature, natural scavenging elements, presence of SRB and deoxygenation process. Improvement in operations philosophy is required to ensure data availability and on time analysis or assessment to ensure safety and integrity. The increase in H2S concentration is expected if the field is producing on waterflood, and the increase can be immediate based on observation. Hence, the implementation of mitigation options is crucial to be deployed soonest to gradually manage the increasing H2S concentration. By adopting this method, Field-D has managed to open 3 out of 5 high H2S wells by 2023 to uplift ca. 500bopd.

Based on the case study of Field-D, other mature fields will be able to adopt the same workflow in identifying souring and managing it operationally. The lessons learnt from Field-D can be utilized as a case study to green fields in ensuring that reservoir souring in waterflooded fields will not come as a surprise and early intervention can be performed.