Formation Evaluation Using Wireline Formation Tester Pressure Data
Authors: Smolen J.J. , Litsey L.R. 
Published: December 2007

This paper describes the Repeat Formation Tester, a tool that can make on one open-hole trip an unlimited number of pressure determinations. Down-hole pressure data from the tool are used to monitor and enhance the effectiveness of a waterflood in Rangeley Field, CO. Data from this tool also are used in a technique to evaluate permeability; results in U.S. Gulf Coast wells are compared with those from sidewall cores.

Introduction
One key to meeting our future energy requirements is more efficient production of new and remaining reserves. To this end, information is needed on conditions down hole, including accurate down-hole formation pressures. The Schlumberger Repeat Formation Tester (RFT) is an open-hole wireline device capable of providing such pressure data with minimal demands for drilling-rig time. pressure data with minimal demands for drilling-rig time.The RFT may be set any number of times during a single logging run. At each setting depth, a "pretest" is made in which small samples of fluid are withdrawn from the formation. During this pretest, the fluid pressure in the formation adjacent to the wellbore is monitored until equilibrium formation pressure is reached. These RFT pressure data are recorded at the surface on both analog pressure data are recorded at the surface on both analog and high-resolution digital scales.
The pretest fluid samples are not saved. However, after the pretests in a zone of interest, another larger fluid sample can be taken optionally and retained, with the possibility of retrieving two such fluid samples per trip in possibility of retrieving two such fluid samples per trip in the hole. In this paper, however, interest is directed to the large number of pressure measurements that can be made by setting the tool and going through the pretest cycle at successively different levels.
Recent experience of Chevron U.S.A. Inc., in the Rangely Field of Colorado is described to demonstrate the quality of the pressure measurements and the reliability of tool operation. Chevron applies the pressure information to the planning and monitoring of pressure information to the planning and monitoring of a secondary-recovery waterflood project. Pressure data, in conjunction with other data available during the drilling of infill wells, were used to predict which flooded zones would produce with a high water cut. By eliminating these zones from production and by injecting into essentially unflooded zones, the effectiveness of the flood could be enhanced. The pressure measurements have been used with open-hole and mud-log data to predict the expected water cut. Significant pressure predict the expected water cut. Significant pressure overbalance suppresses hydrocarbon shows on the mud log. Pressure underbalance exaggerates hydrocarbon shows. Both lead to erroneous water-cut predictions. Knowledge of the pressures makes it possible to allow for such errors.
Pressure profiles through the Weber sandstone reservoir were determined in a number of wells in the Rangely Field. Reservoir pressures were found to vary greatly and to be distributed erratically both vertically and horizontally. This is attributed to the field's long history of production and water injection and to the fact that many of the permeable zones are discontinuous. Plotting these pressure permeable zones are discontinuous. Plotting these pressure data on contour maps delineates areas requiring increased flooding to maintain the effectiveness of the waterflood program. program.During the drawdown phase of the pretest, when fluid is being extracted from the formation, the pressure behavior is indicative of the minimum local permeability at that depth. A simple technique is described for computation of permeability from the pressure data, based on a steady-state spherical flow model.
JPT
P. 25