Modeling/Forecasting Reservoir Souring in a Field Rajasthan, India with an Extremely Low Indigenous Volatile Fatty Acid VFA Concentration

E. Burger, P. Venkat, Saumya H Mittal
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Abstract

The Rajasthan Field has been undergoing waterflood with produced water reinjection (PWRI) using makeup water with a moderate sulfate (≈500 mg/L) and negligible organic content since 2010. Initial analyses of the formation water indicated that the volatile fatty acid (VFA) content was quite low (≈ 20 mg/L), suggesting a priori that the levels of H2S biogeneration and production would not be problematic. However, after less than four years the H2S production rate from the field was over 1000 kg/day and the H2S concentration in the composite separator gas was about 200 ppmv. Consequently, studies were carried out using the H2S forecasting model previously discussed in four SPE papers to determine the cause for the high level of souring and to estimate future levels and trends of H2S production in the field. The mechanistic reservoir souring model considers H2S biogeneration due to water-soluble VFAs and/or primarily oil-soluble organics such as BTEX components, the effects of H2S-siderite geochemical reactions within the reservoir to scavenge H2S, flow of H2S (and other components) through the reservoir to the surface, and partitioning of H2S into the oil, water and gas phases within the reservoir and in the surface separators. Also included in the Rajasthan model were the use of power water to lift the well production since it affects partitioning at the surface; and, the effect of chemical H2S scavengers added in selected well flowlines to maintain H2S partial pressures at safe levels. The model determined that the observed H2S production was not possible even with complete consumption of the indigenous VFAs by sulfate-reducing bacteria and that only with the majority of their organic nutrients being provided by the BTEX-type components were the historical H2S production levels able to be matched. The model results have indicated that H2S production rates have already peaked in the field, primarily due to the reduction in makeup water which provides most of the sulfate being injected into the reservoir. Sulfate is the limiting microbial reactant since the oil-soluble organic supply is essentially infinite. This study has shown even in non-seawater waterfloods and with minimal organic acids in the formation water that reservoir souring can occur, resulting in the need to handle significant levels of H2S on the surface. The significance of oil-soluble organics as a potential SRB nutrient must be considered when planning a waterflood if sulfate is injected.
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在印度拉贾斯坦邦一个具有极低本地挥发性脂肪酸VFA浓度的油田建立油藏酸化模型/预测
自2010年以来,Rajasthan油田一直在进行采出水回注(PWRI)注水,使用含有中等硫酸盐(≈500 mg/L)和可忽略有机含量的补充水。对地层水的初步分析表明,挥发性脂肪酸(VFA)含量相当低(≈20 mg/L),这表明H2S的生物生成和生产水平不会有问题。然而,在不到4年的时间里,该油田的H2S产量超过了1000 kg/天,复合分离器气体中的H2S浓度约为200 ppmv。因此,利用之前在SPE的四篇论文中讨论过的H2S预测模型进行了研究,以确定高酸化程度的原因,并估计该油田H2S产量的未来水平和趋势。储层酸化机理模型考虑了由于水溶性VFAs和/或主要是油溶性有机物(如BTEX组分)、储层内H2S-siderite地球化学反应清除H2S的影响、H2S(和其他组分)通过储层流向地表、H2S在储层内和表面分离器中的油、水和气相分配。拉贾斯坦邦模型还包括使用动力水来提高油井产量,因为它影响了地表的分配;在选定的管线中加入化学H2S清除剂,将H2S分压维持在安全水平。该模型确定,即使硫酸盐还原菌完全消耗了本地VFAs,也不可能产生观察到的H2S产量,只有当它们的大部分有机养分由btex型组分提供时,H2S产量才能与历史水平相匹配。模型结果表明,油田的H2S产量已经达到峰值,这主要是由于补充水的减少,而补充水提供了注入储层的大部分硫酸盐。硫酸盐是限制微生物反应物,因为油溶性有机供应基本上是无限的。该研究表明,即使在非海水水驱和地层水中有机酸含量极低的情况下,储层也可能发生酸化,因此需要处理表面大量的H2S。如果注入硫酸盐,在规划注水时必须考虑油溶性有机物作为潜在SRB营养物的重要性。
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