Page 180 - Hydrocarbon

Reservoir Description 167

Permeable intervals can be identified from a number of logging tool
measurements, the most basic of which is the caliper tool. The caliper tool is
used to measure the borehole diameter which, in a gauge hole, is a function of the
bit size and the mudcake thickness. Mudcake will only build-up across permeable
sections of the borehole where mud filtrate has invaded the formation and mud
solids (which are too big to enter the formation pore system) plate out on the
borehole wall. Therefore, the presence of mudcake implies permeability.
Mud filtrate invasion is normally restricted to within a few inches into the
formation, after which the build up of mudcake prevents further filtrate loss. If
resistivity tools with different depths of investigation (in the invaded and non-
invaded zones) are used to measure formation resistivity over the same vertical
interval, then separation of the log curves can indicate invasion and hence
permeability (Figures 6.55 and 6.56).

Figure 6.55 Measurement of mudcake.

Figure 6.56 Permeability indications from resistivity logs.