Rob Badry Calgary, Alberta, Canada Derrel Fincher Sugar Land, Texas Oliver Mullins Bob Schroeder Ridgefield, Connecticut, USA Tony Smits Fuchinobe, Japan
(FT) Formation (RFT) Repeat Tester Formation Tester 1955 1975
(MDT) Modular Formation Dynamics Tester 1992
In the past, wireline formation samplers have not been able to see the fluid theywere sampling. Downhole optical analysis of fluid before sampling removes the blindfold to reveal oil, water or gas. The sample chamber needs to be opened only when the desired fluid is present. Bringing formation fluid samples to the surface for examination was a novel wireline advance when it was introduced in the early 1950s (right ). Run in open hole or cased hole, the Formation Tester (FT)took a sample of formation fluid where analysis of earlier runs of resistivity and porosity logs showed promising zones. The FT consisted of a sealing packer and probe system that could be set against the formation. Once this was set and opened, formation fluid drained into a sample chamber. The entire sampling operation, from set to retract, was monitored using a pressure gauge. The sample chamberwas closed only when pressure stopped increasing—implying the chamber was full and at formation pressure.1
For help in preparation of this article, thanks to Hifzi Ardic, Schlumberger Wireline & Testing, Montrouge, France and Robert Gabb, Schlumberger Wireline & Testing, Livingston, Scotland. MDT (Modular Formation Dynamics Tester), OFA (Optical Fluid Analyzer) and RFT (Repeat Formation Tester)are marks of Schlumberger. 1. Finklea EE: “Use of Pressure Buildup Curves from the Formation Tester for the Evaluation of Permeability and Reservoir Pressure,” The Technical Review 9, no. 3 (August 1958): 30-35.
Electric power module
Hydraulic power module
Dual-probe module Flow control module
OFA Optical Fluid Analyzer module
Multisample modulenEvolution of wireline formation testers. The Formation Tester (FT) had a pressure gauge to monitor sampling into a single chamber. Pretest chambers introduced with the RFT Repeat Formation Tester tool allowed a check on seal integrity and gave an indication of permeability before sampling into one of two chambers. The introduction of the dualpacker module, with the MDT Modular FormationDynamics Tester tool, allows sampling when seal failure might be a problem. The pumpout module is used along with the OFA Optical Fluid Analyzer module to confirm the presence of desired formation fluid before one of several sample chambers is opened.
The FT’s probe and packer could be set only once per trip in the hole. Thiscreated a couple of problems. If the formation has low permeability, the sample chamber could take hours to fill, delaying rig operations and increasing the risk of the tool becoming stuck. Sampling in low-permeability formations was therefore often aborted. But sampling also had to be aborted if the seal between packer and borehole wall failed, indicated by a sudden increase in sampling pressureto hydrostatic. The only remedy was to pull out of the hole, redress the tool and try again. The next generation of testers addressed these difficulties. The RFT Repeat Formation Tester tool, introduced in the second half of the 1970s, allowed an unlimited number of settings or pretests before sampling was attempted. Pretest chambers were used to indicate the permeability and to check for sealfailures. During a pretest two small volume chambers opened producing pressure drawdowns. Knowing the amount of drawdown for each chamber gave two estimates of permeability. Once the pretest chambers were filled, formation permeability could also be calculated from the subsequent buildup to formation pressure. 2 Sudden increase to hydrostatic pressure during a pretest showed seal failure. Testing...