Learnings from Reservoir Souring Assessment on an Offshore Malaysian Field

Abstract

MY01 is an offshore field located in Malaysia. Although souring mitigation by nitrate injection was applied from the start of the seawater flood, hydrogen sulphide (H2S) has been detected in several producers after injected seawater breakthrough. The objectives of this work are to understand the causes of reservoir souring in MY01 and to provide key considerations for improving souring prediction in low temperature reservoirs, as generalised from the learnings from this field. Reservoir souring potential was assessed using a Joint Industrial Project (JIP) developed program (SourSimRL). SourSimRL is a reservoir model post-processor which simulates the microbial generation, scavenging, adsorption and transport processes of H2S in fields subjected to waterflood, based on fluid dynamics, reservoir conditions and water chemistries as dictated by the reservoir model. The MY01 reservoir model was split into two parts: (i) a history matched model that allows calibration on reservoir souring simulator input parameters; and (ii) a forecast model on which souring development is predicted. Extensive sensitivity studies were conducted to define key factors promoting or inhibiting H2S production to match the actual H2S levels seen in the producers. Furthermore, the application of microbiological analysis to understand the reservoir souring behaviour, including screening of bacteria present using DNA-based techniques is also discussed. The simulations demonstrated that carbon is the souring limiting factor in MY01. To match the H2S field data, other metabolisable carbon sources should be available in addition to the volatile fatty acids (VFA's) in the formation water. The souring development in such field could be driven by the mechanism of the carbon supply. Therefore, it is critical to identify the type and quantify the level of dissolved organic carbon, including oil-derived BTEX or microbial degradation products. Since MY01 reservoir conditions are favourable for souring activities, microbial development is likely to take place both in the biofilm that forms at the injector face as well as at flood fronts deeper into the reservoir.