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Produced water injection wells are used to re-inject produced water as a byproduct from gas production back into a subsurface reservoir formation. Originally, both Arun and South Lhok Sukon (SLS) Fields [See Figures 1 and 2] had Produced Water Treatment Plant (PWTP) to treat produced water prior to discharging to the environment. The typical PWTP consists of tanks for sedimentation and storage, pits, chemical injection, Air Flotation Unit (AFU) to recover condensate and Biological Oxidation Pond (BOP) to reduce oil in water content to meet regulatory requirements. The PWTPs at Arun and SLS were converted to water injection facilities to resolve reliability issues at Arun and new ammonia discharge limits at SLS. In both cases, conversion to produced water injection was more economic than refurbishment or upgrade of the existing PWTPs. Another advantage of produced water injection well implementation is zero hydrocarbon discharge to the environment. An important factor in the decision to convert the PWTPs to injection was that both Arun and SLS Fields had available wells that could be easily utilized for water injection. At Arun, this was achieved by converting unutilized gas injection wells. At SLS, a shut-in production well was utilized. The converted wells were selected based on their subsurface mechanical integrity, reservoir characteristics and surface locations. In addition, they had to comply with government regulation (PERMEN LH No. 13/ 2007). Permission from Ministry of Environment (MoE) was mandatory prior to implementing the water injection with concurrence from DITJEN MIGAS. The produced water injection system at Arun was commissioned in 2005 while the produced water injection system at SLS was commissioned in November 2008. Besides compliance with new regulations, implementation of water injection has also simplified operations surveillance and maintenance activities, which in the end has reduced operating expenses. This paper discusses the technical details of the modifications that were made including well selections to meet permit requirements. There is no new technology involved in the facilities modification.