Question

how can closed Porosity be measured?answer in one ☝page​

Answer 1

Answer:

Porosity measurement is difficult when the fluid is gas. The neutron porosity is calibrated with water-filled pores. Gas molecules are less dense than oil or water. Thus, the neutron tool will measure a low porosity when gas filled. On the other hand, the density porosity will read high. In the nearby water or oil leg, they should read essentially the same. Consequently, one way of distinguishing between oil and gas is this “crossover” of the two traces in the case of gas.

All of the four measurements described previously are plotted side by side in a vertical mode, with formation depth on the vertical axis. This plot is known as a log. Fig. 7.1 is a schematic example without some features found in actual logs. Qualitative interpretation is done first, using the gamma to identify the likely reservoir rock. In Fig. 7.1, this is the left trace in green. At approximately 4185 feet, the gamma drops to the level associated with sandstone. The deep reading resistivity shows as high from approximately 4185 to approximately 4195 feet. This indicates some sort of hydrocarbon. Shifting to the third trace, neutron porosity is shown in blue and density porosity in red. The traces cross over in the portion shown by the orange shading. This is a region with gas, and the relatively high resistivity supports this finding. At approximately 4191 feet, the porosity traces come together and the resistivity remains high. Thus, this is an oil zone. At approximately 4195 feet, the resistivity drops but the gamma stays low. Thus, this is sandstone with water in it. The porosity traces come together in the clean sandstone after approximately 4191 feet as they should