Formation tester with position monitoring of drawdown piston to maintain constant drawdown rate or drawdown pressure

摘要

A method of testing a downhole formation uses a formation tester (10, fig 2) on a drill string. The tester is disposed downhole on a drill string and a formation test is performed by forming a seal between a formation probe assembly 50 and the formation 9. A drawdown piston 170 then creates a volume within a chamber (172, fig 3A) to draw formation fluid into the volume through the probe assembly. The position of the piston is monitored during the procedure. The operation of the piston is controlled in order to maintain a constant drawdown rate or steady drawdown pressure. Controller 190 uses the positional information to calculate the drawdown rate of the piston 170. Based on the calculations, the controller determines if adjustments need to be made in the hydraulic system 200 during the drawdown of the piston 170 to maintain a constant drawdown rate. The controller similarly adjusts the drawdown rate or volume to maintain a certain pressure.