Biodegradation and water washing in a spill-fill sequence of oilfields

Abstract

A recent paper delineated the variations in crude oil in five oilfields (seven reservoirs); large differences in biodegradation and water washing were reported in accord with expectations of the spill-fill sequence of trap filling [1]. Here, the deepest oilfield, Catcher, with the best quality crude oil is examined in detail. Among different oilfields, the extent of water washing is tightly coupled to the extent of (ongoing) biodegradation. Moreover, in-reservoir gradients of biodegradation and water washing are also highly correlated supporting the previous finding that, for cases of ongoing biodegradation, water washing can be assisted by biodegradation. Further support for this process is obtained with a detailed analysis of alkylbenzenes and alkylnaphthalenes that show elimination in accordance with water solubility, yet scale with the extent of biodegradation. Ongoing biodegradation accelerates water washing; once water-soluble components enter the aquifer, they are consumed by microbes maintaining a flux of these components into the aquifer and eliminating the corresponding need for (slow) diffusion of these components away from the oil-water contact. In addition, the oil gradient in the Catcher oilfield is affected by the change in biodegradation of the crude oil spilling into Catcher from a deeper, subsiding reservoir causing a large gradient at the top of the oil column. Factors that determine oil type in charge are discussed. This paper extends the range of measurement of water washing in these reservoirs to a factor 10,000 in water solubility. All data herein are consistent with biodegradation and biodegradation-assisted water washing occurring in reservoir, not in migration.