Field observations of reservoir souring development and implications for the Extended Growth Zone (EGZ) souring model

Abstract

Microbial generation of H2S in-situ due to injection of seawater, so-called reservoir souring, can induce severe negative impact in terms of health, safety, maintenance and economics in mature oil fields. Historical H2S data have been collected for decades at the Norwegian Continental Shelf fields. The Gullfaks field in the North Sea has been negatively impacted by severe H2S production, whereas another comparable North Sea field, called Field B, only has minor H2S challenges. The difference in cumulative H2S development in Gullfaks wells are described as Type I, II and III by an empirical classification. Type I wells show rapid increase in the H2S concentration produced (<1 year or <1 mill. Sm3produced water), whereas Type III show a significant delay and slowly rising H2S production (>10years or >10 mill. Sm3produced water). Type II can be considered as a mixture of Type I and III. To understand the difference between these types, a microbial reservoir souring model called the Extended Growth Zone (EGZ) model has previously been introduced. Differences in H2S development described by the three different well types are supported by the consequences of the EGZ model for both fields on field, platform/installation and well level. The results support that the overall drainage water injection strategy implemented is the single-most important cause governing the development of produced H2S over the lifespan of a field: •High H2S production is expected when cold seawater is injected directly into oil bearing zones (Type I behavior). •Relatively lower H2S production is expected when cold seawater is injected in the water zone below oil bearing zones (Type III behavior). The EGZ model describes the development of reservoir souring based on two main hypotheses (1) Degradation of crude oil as carbon source (2) The importance of temperature on the rate of H2S generation as summarized: •The expected H2S development according to the EGZ model agrees with observed data from Gullfaks both on platform level (GFA>GFC>GFB) and for individual wells. •The observed slow development of H2S for Field B observed for platform and well bore level is explained by the presence of Type III wells only. •Type III wells in Gullfaks and Field B behave similarly within the uncertainty range for H2S production.