Repeat Formation Tester
When formations are unconsolidated and/or mud conditions prevent running a conventional drill stem test, pressure information and formation fluid samples can be obtained by using a wireline test tool called a repeat formation tester. This tool offers the capability of taking any number of pressure-depth measurements with a single logging run. At each setting depth, a “pretest” is made in which small samples of fluid are withdrawn from the formation. The formation pressures are recorded at the surface. The “pretest” fluids are not retained. However, a larger fluid sample can be retrieved by using either or both of two larger sample chambers, 2-3/4 and 1 gallon. This capacity permits fluid samples to be obtained from two separate intervals with one logging run.
The fluid samples provide a qualitative indication of formation productivity. For example, a Yegua formation that recovered 35 cubic feet of gas and 2000 cc of mud filtrate was completed producing 1 MMCFPD with 1250 psi FTP and no water. However, because of the small volume of fluid removed from the reservoir, quantitative values for productivity are very questionable.