Management of Reservoir Souring While Water Flooding of Major Reservoirs in North Kuwait: Threat, Tracking & Tackling

Abstract

Abstract Reservoir souring is the in-situ generation of H2S in the reservoir itself due to waterflood operations using seawater. Some degree of reservoir souring is expected, as seawater is being injected into traditionally sweet reservoir like Mauddud. Thus, a close monitoring was required to track reservoir souring on continuous basis. One of Mauddud wells showed up H2S as first indication of reservoir souring, which was an alarm bell for production facilities designed for only sweet crude. Surface/ Bottomhole fluid samples were required to confirm whether it is localized or reservoir-wide. In order to track reservoir souring and monitor on continuous basis, a process for tracking of reservoir souring periodically was developed and initiated for the first time in 2006. Subsequently, this has been made part & parcel of fluid study requirements each year. Water flooded reservoirs in North Kuwait has been monitored and tracked in regular basis to know the souring level in the reservoir. In light of long term HSE & facility compatibility threat, a special focus was provided by initiating a task force in 2006 to track and timely tackle the issue and create a standard work flow process. All tools available in the industry for sampling, chemical dosage, H2S Measurement methods & prevention aspects with the best industry practices were comprehensively encapsulated. As of 2014, the average H2S level in Mauddud has been managed to be stabilized in an average of (10-20 PPM in gas phase) by effective containment & prevention via the adoption of the work flow process developed. In addition, improved procedure for souring sampling management was implemented for the first time in North Kuwait to capture the bottom hole samples during static as well as dynamic periods of water flooding. Expected H2S levels being very low (10-100 ppm), risk of losing and not capturing these concentrations using conventional samplers/ bottles due to absorption/ reaction with the metallurgy of the samplers was felt, thus posing a challenge for obtaining a representative bottomhole samples for the analysis of H2S. A comprehensive sampling program was made, using the non-reactive internally coated samplers for capturing bottom hole samples; performing the onsite analysis, followed by immediate shipment to the local fluid analysis laboratory and conduct all necessary analysis with expert supervision & care. As per the recommendations based on the sample analysis, water injection wells were treated for the first time in KOC with a biocide to inhibit the growth of SRB and reduce the microbial induced corrosion inside the wellbore. The combination of laboratory tests, various chemical doses and operational practices were defined and integrated with the injection water management teams. The alerts on H2S levels due to reservoir souring has been deemed as one of the best practices while progressing ahead with water flooding giant reservoirs in North Kuwait with sea water, till alternative injection fluids such as Effluent water from GCs or formation water from aquifers or options such as Nitrate treatments are adopted. Value was added to North Kuwait Water flood management by timely knowing the onset of reservoir souring through a comprehensive work flow process built and implemented to contain the threat with periodic tracking/ evaluation and accordingly tackling with tangible actions.