Sampling for MEOR: Comparison of surface and subsurface sampling and its impact on field applications

Abstract

The increasing demand for oil and also increasing numbers of mature oil reservoirs have attracted attention to enhanced oil recovery (EOR) in which microbial enhanced oil recovery (MEOR) is one of the commercial options. In order to develop a MEOR strategy for a given field it is extremely important to evaluate the microbial community present, as this will be the basis for a customized nutrient mixture. The objective of this study was to assess how sampling strategies (surface vs. subsurface) can accurately reflect the microbial community and microbial activity in a reservoir. In addition to fluid samples the analysis of sand deposits, also collected from the well bottom, was included to get insight into presence and identity of attached microbes. Fluid samples were collected at the wellhead (surface) and in-situ at a depth of approximately 780m tvdss (true vertical depth subsea) under a reservoir pressure of 31 bars (subsurface). The pressure was released either quickly (within minutes) or slowly (stepwise, within 11 days) to assess the potential activity loss of microorganisms sensitive towards rapid pressure changes. Quantitative PCR (polymerase chain reaction) for determining cell numbers showed higher amounts of microbial cells in the subsurface sample compared to the surface water. The composition of the microbial community was only slightly more diverse in the subsurface sample than in the surface sample. Sand material, however, showed the most diverse community. Biogenic CO2 formation rates were determined under atmospheric and high pressure conditions with highest rates, i.e. highest microbial activity, in the wellhead fluid and the solid sand sample. In conclusion no substantial differences were observed between fast and slowly depressurized subsurface fluids and also between surface and subsurface fluid samples in terms of microbiological activity and diversity. Therefore, using wellhead fluids for microbiological analysis and MEOR development seems to be a valid option although it has always to be carefully considered not to miss out on information of solid samples regarding various aspects such as community and biofilm formation.