 In 1956, Schlumberger acquired Johnston Testers and continues to perform drill stem tests and wireline formation tests in both open and cased holes.Drill stem testing is an oil and gas exploration procedure to isolate, stimulate and flow a downhole formation to determine the fluids present and the rate at which they can be produced.The main objective of a DST is to evaluate the commercial viability of a zone's economic potential by identifying productive capacity, pressure, permeability or extent of an oil or gas reservoir.