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Abstract

During the last two decades, several simple models have been developed to predict the onset of reservoir souring in seawater injected reservoirs. The key mechanism to generate hydrogen sulfide is a biological reaction between sulfate in the injection water and volatile fatty acids in the formation water in the presence of sulfate reducing bacteria (SRB). The produced hydrogen sulfide interacts with rock and partitions between oil and water phases. Comparison of field data with reservoir souring model predictions often shows inconsistent results. We present the development of a comprehensive reservoir souring model in a chemical flooding simulator that accounts for these mechanisms.  

Being able to estimate the likelihood and timing of the onset of H₂S production would permit more realistic assessments of project economics. We incorporated mechanisms of generation and transportation of H₂S in porous media to develop a reservoir souring simulator to predict the onset of souring in oil reservoirs.

We implemented a general souring model in a 3D finite difference compositional non-isothermal reservoir simulator. The results indicate that depending on the type of SRB, the temperature propagation in the formation determines the onset of biological reactions and consequently, the generation of different hydrogen sulfide concentrations. The lag in the temperature front with respect to the injection front can cause a delay in the observed souring if the temperature is not favorable for SRB activation.

A predictive model would enable operators to make better decisions for remedial actions to either prevent souring or to mitigate its impact. The developed finite difference simulator is 3D and accurately accounts for variation and impact of in-situ concentrations, temperature, pressure, and reservoir heterogeneity on H₂S transport and production.

Introduction

Reservoir souring is the process of the production of hydrogen sulfide in a seawater injected reservoir. Using the knowledge of the mechanisms of generation and transportation of hydrogen sulfide in the reservoir, several reservoir souring models have been developed (Ligthelm et al., 1991; Sunde et al., 1993; Eden et al., 1993). The degree of exactness and reliability of these models depend on their capabilities to mimic the essential parameters which determine the generation and transportation of the hydrogen sulfide in the porous media.

Figure 1 shows the process of reservoir souring. While injecting cold sea water which contains sulfate, nitrate, phosphate, and SRB into the hot formation, which provides organic acids, in the presence of SRB, sulfate reacts with organic acids to produce hydrogen sulfide. The produced hydrogen sulfide interacts with rock surfaces and partitions between oil and water phases (Ligthelm et al., 1991). The expected concentrations and temperature profiles are shown in Figure 2. The temperature distribution ranges from sea water (T_w) to the reservoir (T_res) temperatures. The activities of SRB, which are responsible for souring, depend on the temperature distribution and available nutrients (Herbert et al., 1985). At low temperatures, mesophiles, and at high temperatures thermophiles or hyperthermophiles, are activated and the biological reaction between sulfate and organic acids will initiate. Table 1 shows the range of activation of the discussed SRB.