Application Of Repeat Formation Tester Pressure Measurements In The Middle East
Authors: Pelissier-Combescure J. , Pollock D. , Wittmann M. 
Published: December 2007

The Repeat Formation Tester which can make any number of accurate pressure tests in one run in open hole, has applications in the determination of reservoir pressure, fluid density, fluid contacts, differential depletion, reservoir inter-communication and effective permeabilities of invaded and uninvaded zones. The effect of filtrate invasion called "supercharging" may sometimes affect the pressure measurements.

In the paper these applications and the phenomenon of supercharging are discussed and field examples illustrating them are presented.

Introduction

The Repeat Formation Tester can take an unlimited number of formation pressure measurements and up to two fluid samples in a single trip in open hole. To achieve a high degree of accuracy, the RFT* tool makes use of advanced electronic circuitry and digital pressure readout. With special techniques and calibration against a dead-weight tester, an absolute accuracy of up to 13 psi for a 10,000 psi gauge is achievable.

This capability of fast, accurate, point-by-point pressure measurement has opened up a new series of pressure measurement has opened up a new series of reservoir analysis methods. The technique of point-by-point evaluation of reservoir pressure can be point-by-point evaluation of reservoir pressure can be used to determine pressure profiles, fluid density, fluid contacts, differential depletion and reservoir intercommunication. Analysis of the drawdown and build-up curves can provide measurement of effective permeabilities in the invaded zone and in the uninvaded zone. permeabilities in the invaded zone and in the uninvaded zone. This technique can be adversely affected if, at the time of testing, the formation is still overpressured or supercharged by the invading mud filtrate.

For sampling purposes, the ability to evaluate the formation permeability before committing a sample chamber allows the selection of the most suitable sampling level and increases the chance of formation fluid recovery.

TOOL CHARACTERISTICS

The Repeat Formation Tester uses an electrically driven hydraulic pump to provide pressure to operate its various systems. This hydraulic circuit is controlled from surface to provide repeated set-retract capability. Two sample chambers are available in the tool to take samples from two different zones, or to take a segregated sample from one zone. A strain gauge pressure transducer is located in the flowline to monitor pressure continuously during test. Two pretest chambers of 10 cc each are automatically opened after the tool is set to withdraw 20 cc of fluid at two different rates. (Fig. 1) The pretest samples are not saved. Pressure drawdowns during this period and the following build-up curve are recorded at the surface and provide data for formation pressure and permeability calculations. pressure and permeability calculations. The RFT tool overcomes several drawbacks of other wireline testing systems. When the tool is set, a packer moves out on one side, and back up pistons packer moves out on one side, and back up pistons move out on the opposite side. Thus the body of the tool is held away from the borehole wall to reduce the chances of differential sticking. A filter in the flowline probe prevents sand entry into the tool and a piston cleans the filter when the tool is retracted. piston cleans the filter when the tool is retracted. Thus flowline plugging is substantially reduced. Because the RFT tool does not depend on mud pressure for its operation it can be set even at very pressure for its operation it can be set even at very shallow depths and still have enough setting force to provide a seal at the packer. provide a seal at the packer. Fig. 2 shows a typical pretest pressure record. The pressure initially is that of the mud column. As the pressure initially is that of the mud column. As the tool is set, the packer compresses the mud cake causing a pressure rise.