GEOG 4891 Lecture Notes - Lecture 8: Resistivity Logging, Polyethylene Terephthalate, Overburden
Document Summary
In drilling especially deep wells, it is vital to predict formation pressures before drilling, and during drilling this prediction proceeds as well. Formation pressure gradient defines mud weight against blow out. Fracture pressure gradient defines the need for protective casing. All methods predicting formation pressures are based on compaction (primarily in shales). If not, then most probably formation is over pressured. Seismic provides sound velocity in formations (so do sonic logs): denser the formation, higher the sound velocity, prediction of the top level of abnormally high pressured formation by seismic data can be erroneous as a few hundred feet. Seismic data is the only data to predict formation pressures for wildcat drilling. In abnormal pressure formations, resistivity log tends to decrease ohm-meter (shale resistivity) while conductivity log tends to increase meter-mho (shale conductivity) due to salinity of formation waters, and induction log tends to decrease giving less formation resistivity factor (f).
