ANOMALOUS NMR RESPONSES IN HIGHLY PERMEABLE SANDSTONE RESERVOIRS: A CASE STUDY

摘要

Two field examples are presented of NMR logs in sandstone reservoirs with high porosity and permeability. The NMR logs show a total of 160 meters of anomalous porosity and T2 distribution response, when compared to conventional logs and formation test results. The NMR porosity is approximately 12 p.u. lower than the density porosity using a 2.65 g/cc grain density. The T2 log mean distribution falls almost entirely below the 33ms cutoff limit. Consequently, these NMR responses would normally indicate the predominance of capillary bound water porosity with little free fluid porosity, suggesting poor reservoir quality. This is in contrast to high resistivities and low shaliness of the sandstones in the study area, which normally suggest good permeability. Indeed, on a drill stem test (DST) the well flowed at the rate of 3,000 bbl/day of oil with a measured permeability of 6 Darcies.rnIn the two examples, the theoretical resistivity, obtained from the relationship between the free fluid porosity and the total NMR porosity, agrees very well with the Rxo measurement, but conversely is 20 times lower than Rt. The NMR measurement may be related to some kind of effect within the flushed zone. Furthermore, the DST interpretation indicates the formation is highly damaged, with the damage ratio varying from 17 to 6. Consequently, mud solids within the reservoir pore space of the flushed zone causes the reduced NMR porosity and the T2 distribution response.rnIn conclusion, it is inadvisable to use the NMR as a replacement for the density and neutron logs in reservoirs where mud solid invasion may occur. This near wellbore effect is almost entirely within the depth of investigation of the NMR tool. Evidence will also be presented that in reservoirs where mud solids invade the formation, the density and neutron logs can be affected, although to a lesser extent. The shallower the depth of investigation of a logging tool the higher the influence of the damage zone. As a result, mud solid invasion results in an underestimation of the volume of oil in place.