Determination of water saturation (Sw) from conventional resistivity and porosity logs has proved difficult in several gas fields within Bolivia. To address this challenge, a method was developed to determine Sw profiles based on pseudo-capillary pressure curves (Pc) derived from transversal relaxation times (T2 distribution) measured by nuclear magnetic resonance (NMR) logging tools.
Some of the Bolivian sand reservoirs, producing gas and condensate with a high gas/oil ratio, are characterized by a lack of resistivity contrast above and below the oil-water contact. This problem is attributed to complex mineralogy, including thin shale laminations within sand bodies of variable petrophysical quality, and mostly to very fresh formation waters that average a salinity of 5000 ppm of total dissolved solids. Also, the lack of lateral extension of each reservoir makes cross-well correlation very difficult.
The core hypothesis of this original method is to assume that the relation between capillary pressure and pore throat sizes is similar to that between T2 values and pore-size distribution. A consistent scaling factor is used to derive a pseudo-capillary pressure from an NMR T2 distribution. After the pseudo-pressure is calibrated with capillary pressure measurements from laboratory-derived core data, it is combined with density (difference between the produced hydrocarbon and the formation water) and free-water level (FWL) information to compute Sw along the wellbore trajectory.
Even so, in general terms, resistivity-based models are still the most consistent and documented foundation in order to compute water saturation; Examples demonstrate the success of this new standalone method of uncovering unforeseen hydrocarbon in place and obtaining accurate Sw as an alternative to the standard, but dubious or inadequate, resistivity method in Bolivia where cretaceous and carboniferous rocks with intergranular porosity are found within several sedimentary basins.