ALBERT

Beschreibung: Microbial enhanced oil recovery (MEOR) is an economically attractive tertiary recovery technique and fermentative bacteria are frequently suggested for MEOR, partly because microbially produced organic acids have the potential for rock matrix dissolution. In this study, the metabolic activity and the community shift of a diverse microbiome of a high-salinity oilfield upon supplying MEOR nutrients was investigated in dynamic sandpacks set-up with and without crude oil using pure quartz sand and two types of reservoir rock. Geochemical characterization of the porous media included specific surface area (SSA), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). During the experiments, substrate and metabolites, incremental oil and differential pressure were monitored and the microbial community shift was investigated via Illumina sequencing. Fermentative Halanaerobiales outcompeted other microbes and led to an incremental oil recovery of 24.5 ± 9.6 %OOIP in reservoir rock. Microbial-induced dissolution of surface minerals was indicated by a decrease in SSA and surface-bound ferrous iron and concluded to be an important MEOR mechanism. Fermentation of sucrose was primarily limited by an insufficient acid neutralization capacity (ANC), but a carbonate content of 12% sustainably buffered the pH in a favorable growth range. Even minor amounts of other non-inert minerals (1% pyrite and calcite) facilitated microbial growth significantly, resulting in six-fold higher acetate production rates compared to quartz sand. Besides emphasizing the relevance of accessory minerals in MEOR, our results imply that the ANC could serve as potential screening parameter for predicting the performance of fermentation - based MEOR in the field.

Beschreibung: Souring induced by sulfate reducing microorganisms (SRM) represents a severe problem in the petroleum industry. In addition to conventional biocides and nitrate, alternative SRM inhibitors such as molybdate have been proposed recently for controlling microbial souring. We used oilfield-derived microbial consortia, rock and fluids to test molybdate as a specific SRM inhibitor for a microbial enhanced oil recovery (MEOR) application where souring might occur as a side effect. SRM cells were quantified and dissolved molybdate, sulfate and gaseous hydrogen sulfide were measured under different dynamic conditions in sandpacks with and without residual oil. In batch experiments, 0.5 mM molybdate inhibited SRM growth whereas hydrogen sulfide and mineral precipitations were observed in bottles amended with 100 mM nitrate. However, significant molybdate adsorption onto reservoir rock occurred and maximum Langmuir saturation was estimated to be ≥ 34 μmol g−1. Residual oil allowed a further propagation of molybdate into sandpacks, but a pH 〈 6 and sulfide concentrations 〉11 μMH2S aq limited the efficiency of molybdate due to rapid adsorption. Under favorable souring conditions, we also observed the localized formation of macroscopic iron sulfide precipitations. These resulted in a four-fold permeability decrease after the injection of SRM substrates for 40 days and a calculated mean sulfate reduction rate of 52 μM h−1. However, delayed sulfate reduction in molybdate-preflushed sandpacks suggests an inhibitory effect even if molybdate is partially adsorbed. Sulfate reduction was not detected when molybdate was continuously injected with MEOR nutrients into sandpacks demonstrating its inhibitory efficiency in case it is applied in early phases of field operations with a potential risk of souring.

Beschreibung: In the course of the submission, revision and publication of the research paper, author affiliations changed to some extent. The authors regret that these affiliation changes are not reflected in the final publication and would like to apologise for any inconvenience and confusion caused thereby. The corrected affected affiliations are as follows: Felix Kögler, RWS IP Services, Chalfont St. Peter, UK; formerly Wintershall, Ante Borovina, OMV AG, Austria; formerly Wintershall, Foppe Visser, retired; formerly Wintershall

Beschreibung: As a dynamic screening tool for a high-salinity oilfield (186 g/L), anaerobic sandpacks were established to simulate Microbial Enhanced Oil Recovery (MEOR) under defined laboratory conditions. Glass beads, quartz sand or crushed reservoir rock were used to produce porous media which varied in permeability, wettability, homogeneity and geochemistry. In total, 14 sandpacks were flooded with oil and inoculated with indigenous fermentative bacteria of the order Halanaerobiales. After waterflooding, these were treated with nutrients in different injection scenarios during which incremental oil recovery, permeability, microbial activity and produced metabolites were measured. Our results indicate that the efficiency of MEOR is dependent on the type of porous medium used: Both glass beads and outcrop quartz sand were found to be no suitable analogue to reservoir material because not all potential MEOR effects were accounted for. MEOR was least efficient in quartz sandpacks with a recovery factor of 7.0 ± 1.7% with respect to the original oil in place (IRFOOIP), attributed mainly to fluid-fluid interactions. In sandpacks with reservoir rock, wettability alteration, matrix dissolution and bioplugging were additional MEOR mechanisms and resulted in an incremental recovery which was almost three-fold higher compared to pure quartz sandpacks (IRFOOIP = 23.2 ± 6.4%). Bioplugging was not detected in sandpacks with a permeability of 8–10 D, although cell retention was observed. Mean pore sizes of these sandpacks were calculated to be in the range of 100 μm, thus considered to be too large to allow for significant plugging. Our findings support the use of MEOR as potential tertiary recovery method but also emphasize the importance of carefully designing laboratory experiments. We argue that porous medium properties such as permeability, pore size, wettability and mineralogy play a crucial role during dynamic MEOR feasibility studies, because they directly influence incremental recovery.