Assessing Production Potential Of Heavy Oil Reservoirs From The Orinoco Belt With Nmr Logs

Although conventional logs easily identify oil in the Orinoco reservoirs, they provide little information on producibility. Production tests show a relationship between API gravity and production and confirm the existence of a few reservoirs with near-immovable hydrocarbons. These reservoirs will produce large amounts of water and little oil. Optimization of the field development strategy in a cold production framework therefore requires not only determination of oil in place, but also characterization of oil quality and mobility. Resistivity logs do not always provide this information. New nuclear magnetic resonance (NMR) tools respond to oil viscosity and can distinguish between moderately heavy, heavy and extra-heavy oils in the Orinoco belt. These tools are able to acquire very fast, low noise echoes and reach this measurement resolution through a combination of vertical averaging and repeated short-echo-spacing pulse sequences. This acquisition method yields a stable T2 oil peak that is correlated to in-situ viscosity. NMR tools quantify the invading mud filtrate. With a shallower depth of investigation, NMR tools can identify displaceable hydrocarbons where microresistivity tools do not. The location of the filtrate peak changes with sand quality, which is inferred from electrical images. This ability to identify sand quality suggests that the T2 distribution still indicates pore size, which is used to zone reservoirs into homogeneous flow units. NMR logs are combined with conventional logs to provide porosity and oil, filtrate and water volumes and are used to assess whether a sand is at irreducible saturation. After subtraction of the oil volume, the conventional NMR bound-fluid volume measurement still provides an acceptable estimate of irreducible water. Several sands show free water at the base, a source for the observed water production. By providing information that is not available from conventional logs, NMR logs help answer fundamental producibility questions in the Orinoco belt: How viscous is the oil? Can it be displaced? Which are the better sands? Is water production likely? This information is fundamental for reservoir mapping and cost-effective field development.