Tight Gas and Shaley Sand Examples

This is a classic case of low resistivity pay revealed by NMR. This well, drilled in the Gulf of Mexico, missed the target sand and encountered only the shaley sand shown below which has resistivity ranging from 0.4 to 0.9 ohm-m.Water saturation ranges from about 50% at the top to 80% at the bottom. It appears to be a transition zone with water production likely even at the top. Not a likely candidate for completion. The NMR log shows very high bound water. This means that although the sand has a high water content, none will be moveable. The interval was completed at 2000 bopd with no water. The NMR device acquired data in the "total porosity" mode, gathering the early T2 data characteristic of clay bound water. The volume of clay bound water is shown as the green shading.

nmr petrophysics gas and shale example

Tight Gas Sand Case Study

This is a well in the Cretaceous J Sand (tight gas) in the Rockies. The recognized zones were the sand lobes below 7570 which feature high resistivity and some neutron-density crossover. The NMR log reveals and additional sand at 7550 to 7570.  Note that this additional sand has similar resistivity to the wet sand at 7900 to 7910. The client included this zone in the completion and doubled production. A well in the area was accidentally completed in the additional sand lobe and produced similar volumes compared to adjacent wells completed in the traditional target sand. The off-depth perforations were later discovered. Standard log evaluation based on SP and Gamma Ray response does not indicate the presence of any sand, although resistivity and neutron-density response do show some subtle clues. The high (erroneous) porosity readings above 7535 are due to borehole washout.
nmr tight gas sand example