Iron sulphide scale can be deposited in production tubing, topsides facilities due to reservoir souring associated with water injection. One of the main mechanisms of reservoir souring is believed to be the reduction of sulphate in the injected water by sulphate reducing bacteria (SRB). To be active in the reservoir, the SRB need to be able to utilize fatty acids, which are present in the formation water, often in the cooled region of the reservoir in the vicinity of the water injection well. Consequently the location and the degree of mixing of the injected water with the in-situ brines in the oil zone and the water zone is a significant factor in the potential for reservoir souring and subsequent iron sulphide scale generation.
Brine mixing occurs to very different extents, depending on the reservoir geometry and on the presence or absence of an aquifer (water zone). When water injection is into the oil zone, banking of the connate water occurs ahead of the injected water, resulting in limited mixing of the two waters in the reservoir. When water injection is into the water zone (aquifer), the degree of water mixing is much greater, increasing the potential for biogenic reservir souring.
Based on the above discussion, water injection into the aquifer should result in a greater degree of reservoir souring than injection into the oil zone. For water injection into the oil zone water mixing in the reservoir is restricted and biogenic sulphate reduction may be delayed until the waters mix in the production facilities.
The investigation of waterflood induced reservoir souring and subsequent iron sulphide scale generation is an important part of the integrated water management of an oilfield development (Fig. 1).
The risks of reservoir souring and the associated problems need to be addressed in the early phases of field appraisal and development.
The risks of reservoir souring need to be incorporated into the decision analysis for the asset development as a whole.
The biogenic generation of H2S can have a significant impact on the asset Net Present Value (NPV).