A Basin Modeling Approach to the Prediction of Oil Biodegradation

Tømmerås, Are, Hermann M. Weiss, SINTEF Petroleum Research, Trondheim, Norway

A modelling tool for predicting risk of encountering biodegraded oil in undrilled prospects has been designed and built into the basin modelling simulator SEMI. The simu­lator accounts for mixing of biodegraded and fresh oils during migration and filling of reser­voirs and predicts the physical properties of the trapped biodegraded oil.

The biodegradation module assumes that biodegradation in the reservoir occurs within a relatively thin zone above the oil/water contact, as suggested by observations of compo­sitional gradients in biodegrading oilfields. The field-wide biodegradation rate is therefore influenced by the ratio between the oil/water contact area and the oil volume.

Oil columns can be treated as compositionally uniform (i.e. assume instant homogeni­sation within the reservoir) or vertical compositional variations can be modelled using a dif-fusion-based approach.

The simulator allows for definition of biodegradation processes at component (i.e. com­pound or compound group) level as sequential and/or simultaneous reactions. It can inter­act with a water-flow model and thus simulate the supply and consumption of oxidants and/or nutrients dissolved in the aquifer by the biodegradation process.

The rates at which the degradation processes are modelled to occur are controlled by a temperature-dependent microbial activity function and a maximum rate for each process. The rates are further limited by the amounts of degradable fluid components in the biodegra­dation zone. It is also possible to model pasteurisation of traps, i.e. to stop all biodegrada­tion processes once a reservoir has reached a specific threshold temperature.