We observe two different events happening in a Matagorda Island area. Faults may act as a seal and enhance the shale sealing. Changes in pore pressure calculated from sonic/resistivity logs are steep, “transitional” zone is about a 100–150′ thick and almost invisible at a whole well scale. Because of such sharp changes these zones are hard to recognize on MWD, however typically one casing point required to separate it from upper normally pressured section.
Another option — fault causes partial damage, possibly through micro-fractures in shales. Shale sealing properties are decreased pore pressure in reservoir sands is lower, that in neighboring blocks. Sealing zone possess gradual changes in pore pressure with lower values that expected. Such zones are up to 2000′ thick and could be observed on MWD as well as on conventional log diagrams. Several casing points required in order to drill through such zone.
By incorporating this information into the geological model one can predict whether the fault will act as a pressure-formation liquids “semi-conductor” or as a good seal. Faults that damage the seal above overpressured zone might result in unexpected shallow overpressure and/or shallow gas. Such faults should be determined, mapped and avoid when possible from drilling.