Integrated study of reservoir souring in chalk reservoirs

Flooding oil reservoirs with water alters the environment of the reservoir in many ways, such as lowering the temperature and disturbing the ionic composition of the formation water. Such alterations may create a favourable environment for microorganisms to grow inside the reservoir and cause microbial reservoir souring, i.e., the reduction of sulfate to hydrogen sulfide by Sulfate Reducing Bacteria (SRB). Microbial sulfate reduction has been demonstrated in the laboratory, and various microbial strains with sulfate reduction function have been identified. Batch and small-scale core or sand-pack flooding experiments have also been simulated using different bio-reactive models. On the other hand, only a few large-scale simulations have been done, usually using simplified reservoir models. Moreover, a proper tunned model against historical production data requires the consideration of various operational complexities, such as the interference of gas lift and the conditions of sampling. This project is an attempt to develop a modelling approach and a simulation toolbox considering real-world complexities of reservoir souring and perform a case study on a specific field in the Danish North Sea.

Chapter 2 investigates the effect of souring intensity and mitigation on the severity of barite scale formation inside the reservoir. The results show that the effect of barite scale formation on reservoir souring is small (a maximum of 4 percent reduction in the total generated hydrogen sulfide) whereas reservoir souring and nitrate treatment significantly influence barite formation. Depending on the case, souring can cause a complete removal of barite scale around the production well (positive effect) or a maximum of 8 times increase in barite scale amount around the production well (negative effect). Chapter 3 investigates nitrate treatment in a sector scale hydrocarbon reservoir in the Danish North Sea through a non-isothermal multi-phase multi-component bio-chemical model and field observations. The common expectation is that the higher the concentration of nitrate or nitrite in the injection seawater the less hydrogen sulfide production. However, the results show that slowing down the microorganisms (or mitigation through nitrate treatment) may cause higher hydrogen sulfide production from production wells. That is because it is possible that insufficient amounts of nitrate cause the generation zone to move toward the production wells. Chapter 4 investigates the potential benefits of removing sulfate from the injection seawater after around 20 years of untreated seawater injection in a sector of an oil field in the Danish North Sea. The results show that, desulfation results in a significant decrease in the amount of co-produced water for a given oil production rate. Moreover, desulfation is considerably more effective than nitrate treatment in mitigating microbial reservoir souring. Furthermore, the possibility of scale formation is considerably decreased due to desulfation, which further encourages produced water re-injection. Chapter 5 investigates real field data of several wells and presents a modified reservoir souring model coupled with a full-field reservoir model for a case in the Danish North Sea. Under various sets of assumptions, the model is matched against the history of the production data. The results suggest the presence of multiple SRB strains with different optimum growth temperatures in different parts of the reservoir. Moreover, different sectors of the same field show different souring behaviours and macro-scale growth rates, which is attributed to different elements that affect flow patterns, such as the presence of Darcy-scale heterogeneity and fractures. Chapter 6 concerns the effect of several factors, including the incomplete mixing and the inability of coarse grid models in accurately capturing species and heat transport in the medium, on large-scale simulations of bio-reactive transport. This chapter demonstrates why macro-scale constitutive relations are required for capturing how the local processes in the presence of heterogeneity manifest themselves at the macro scale. Moreover, a mathematical approach that can calculate an upscaled (effective) growth rate for microorganisms in homogeneous cases is developed.

The high-level look of this study at microbial sulfate reduction through field-scale modelling and considering the real-world complexities enhances our understanding of the activities of microorganisms in the reservoirs underground and points to the most important parameters affecting microbial reservoir souring and mitigation.