An Integrated Approach to Explain Anomalous Gas Production from an Oil Bearing Reservoir

We discuss anomalous gas production data observed downdip of oil production in a thick oil reservoir. At the onset of production, the wells under consideration exhibited a sharp rise of the GOR with large disparity between estimated gas in solution and actual gas production. This was followed by a declining GOR period. After declining, the GOR rose and then fell off again in one of the wells. Such a behavior leaves little doubt that free gas exists. The reservoir pressure is approx. 20 bars above the bubble point, so these observations contrast with the general assumption that gas below the gas/oil contact must be in solution in the oil. The fact that free gas comes in peaks suggests the presence of large bubbles suspended in the oil at variable distances from wells. In order to understand how long a gas bubble can live in undersaturated oil and the conditions under which it can be mobilized, we analyze the system in terms of gravity-capillary-viscous force equilibria and investigate the kinetics of gas diffusion into the oil. We show that bubbles that are sufficiently large in width (hundreds of meters) and sufficiently small in thickness (a few meters) can move horizontally but not vertically (meaning that they remain suspended in the oil and are producible) and take hundreds of thousands of years to dissolve into the oil. The presence of these bubbles is compatible with the available geochemical data, which provide evidences that the source rock of gas is more mature than that of oil and probably still in the generation and expulsion phase. This unusual non-equilibrium situation was reproduced by means of 3D simulation approach, to confirm the conceptual model adopted.