Reservoir Implications of a Spill-Fill Sequence of Reservoir Charge Coupled with Viscosity and Asphaltene Gradients from a Combination of Water Washing and Biodegradation

Abstract

Reservoirs with multiple processes that impact the quality and distribution of crude oils can be complex. Here, a series of seven reservoirs in the North Sea contain a viscosity profile that is strongly affected by a spill-fill sequence of reservoir charging, biodegradation, water washing, and variation in thermal maturity. Mapping the viscosity gradient and its origins is critical to understand how each factor contributes to the complexity of these reservoirs. Additionally, evaluation of reservoir connectivity is key. Combined studies of downhole fluid analysis (DFA) and high-resolution compositional analysis by comprehensive two-dimensional gas chromatography (GC×GC) can be used to unravel the contributions of various effects and provide an improved understanding of the reservoir. The seven reservoirs examined are consistent with a simple description of the multiple processes that contribute to the viscosity gradient and connectivity analysis. The maturity of charge in this spill-fill sequence has continued to increase with time; consequently, crude oil in deepest reservoir is most mature and the shallowest, the least mature. The oil spills from the oil-water contact (in these injectite reservoirs); consequently, the deepest reservoir is the least biodegraded and the shallowest reservoir the most biodegraded. Observations here are consistent with both biodegradation and water washing being dominated by in-reservoir confinement of the oil as opposed to during migration. The deepest reservoir exhibits mild biodegradation and mild water washing, while the nearby shallowest reservoir exhibits severe biodegradation and severe water washing. Finally, in this spill-fill process, the replacement of new oil for old, especially in the deepest reservoirs, indicates excellent reservoir connectivity.

Reservoir Fluid Geodynamics: The Chemistry and Physics of Oilfield Reservoir Fluids after Trap Filling

“…Thus, shallower reservoirs fill with more biodegraded oils, as validated in Figure . Specifically, all seven reservoirs are consistent with the spill-fill model, with biodegradation occurring increasingly in shallower reservoirs …”

Overview of Asphaltene Nanostructures and Thermodynamic Applications

“…RFG has proven to be of great value to address all manners of reservoir complexities in ∼50 oilfield RFG studies to date. Key issues that have been addressed include reservoir connectivity , and its inverse compartmentalization, , baffling, , viscous oil and tar mat formation, , upstructure mobile tar, aquifer support, asphaltene onset pressure, , wax precipitation, solution gas gradients, timing of charge, fault block migration, , biogenic gas charge into oil, and spill–fill trap filling, , along with traditional geochemical concerns of biodegradation, ,, gas washing, and water washing. ,, These wide-ranging concerns treated in various RFG oilfield studies establish that the development and application of asphaltene thermodynamics extends far beyond issues that are directly impacted by asphaltenes, such as tar mat formation. Instead, RFG exploits thermodynamics, which itself is unlimited in its applications. ,,,− …”

Biodegradation of the aromatic fraction from petroleum diesel fuel by Oerskovia sp. followed by comprehensive GC×GC-TOF MS

“…The most noticeable decrease is found in the area up to a 40 min retention time, where the lower molecular weight compounds are found. This was expected, since these compounds are more volatile and more susceptible to microbial degradation [33,34]. Many studies have shown that microbial consortia are more efficient in biodegradation than single cultures, especially in the case of complex pollutant mixtures [10,35,36].…”

“…To assess the degradation of different classes of aromatic compounds, selective ion chromatograms were created using ±0.1 of the theoretical mass. Figure S2-15) was dependent on the degree of alkylation, as the highly alkylated compounds were not efficiently degraded [34]. [34,37,38].…”

“…Figure S2-15) was dependent on the degree of alkylation, as the highly alkylated compounds were not efficiently degraded [34]. [34,37,38]. The higher molecular weight PAHs can be degraded after the lower weight ones, which are the preferred substrates, are depleted or they can be co-metabolized with other substrates [39,40].…”