Patent ID: 7857049

Claim:
A method for operating a guarded sampling probe in a wellbore to sample formation fluids from a formation surrounding the wellbore, comprising: lowering a wellbore tool coupled with the guarded sampling probe into the wellbore, wherein the guarded sampling probe comprises a sampling probe and a guard probe, and wherein the sampling probe is coupled with a sampling flowline and a sampling pump and the guard probe is coupled with a guard flowline and a guard pump; urging both the guard probe and the sampling probe into contact with a sidewall of the wellbore; operating the guarded sampling probe in a first mode, which comprises controlling the sampling pump and the guard pump so that the sampling flowline and the guard flowline have equivalent characteristics and operate as essentially a single flowline, such that pressures in the guard probe and the sampling probe are equal; using the sampling probe and the guard probe operating as essentially a single flowline to remove fluid from the formation, wherein the fluid comprises at least one fluid selected from the group consisting of the formation fluid, drilling fluid and a filter cake; using a sensor to generate signals in response to variations in a concentration of contamination present in at least one of the fluid in the sampling flowline and the fluid in the guard flowline, wherein the contamination comprises substances other than the formation fluid; using a processor to process a contamination concentration from the signals; and operating the guarded sampling probe in a second mode when the processed contamination concentration in at least one of the fluid in the sampling flowline or the fluid in the guard flowline falls below a threshold value, wherein, in said second mode, fluid is withdrawn through the sampling probe and sampling flowline without coming into contact with fluid withdrawn through the guard probe, and collecting a sample of the formation fluid from the sampling flowline.