Determining the in situ effective mobility and the effective permeabilty of a formation

A method for determining the average in situ permeability of a formation layer traversed by a cased borehole, the steps comprising:

• a) creating a plurality of perforation sets in cased borehole, to place the cased borehole in fluid communication with the formation layer;
• b) selecting a set of locations in the formation layer;
• c) selecting from the set a first location;
• d) lowering a formation test tool in the borehole to the location, the tool comprising a central conduit having an inlet and being provided with a pressure sensor, a fluid receptacle in fluid communication with the central conduit, a optical fluid analyzer, a fluid sample chamber in fluid communication with the central conduit, and means for discharging fluid, the tool further having an upper and a lower inflatable packer, disposed above and below the fluid inlet, the upper and lower packers, when inflated, isolating the selected set of perforations from the remaining perforation sets;
• e) inflating the upper and lower packers;
• f) establishing an exclusive fluid communication between the formation layer and the inlet of the central conduit;
• g) inducing formation fluid to flow from the formation layer through the central conduit, entering the fluid receptacle, and measuring formation fluid pressure build-up;
• h) determining formation permeability and formation fluid mobility from the pressure build-up;
• i) deflating the upper and lower packers and positioning the tool near a next perforation set and repeating steps e) through h), determining the formation fluid mobility for the perforation sets;
• j) determining the perforation set having the highest mobility, the effective mobility, calculating the permeability for this perforation set based on viscosity of uncontaminated formation fluid, and determining viscosity for contaminated formation fluid using the permeability and the effective mobility for that perforation set;
• and
• k) recalculating the permeabilities for the other perforation sets using the viscosity of contaminated formation fluid and the mobility measured in step h), for that location and calculating the average of the permeabilities for all perforation sets.