Vertically distributed point pressure measurements from a Repeat Formation Tester (RFT™) are used to understand the fluid dynamics anddistribution in the drainage volume surrounding a wellbore. The RFT tool sets apacker at the chosen depth thereby isolating the tool hydraulically from themud column. The RFT probe withdraws fluid from the formation into a fixedvolume resulting in a pressure drawdown. The ensuing build-up is analyzed usingspecific solutions to the diffusivity equation. This test by test analysis, which can also be done at the well site, uses solutions to the diffusivityequation in spherical and cylindrical co-ordinate systems. An extrapolatedpressure for that depth and an estimate of spherical mobility is obtained. Ifthe effect of supercharging (a localized zone of over-pressure caused by thefiltration process) is significant, the extrapolated pressures for that to becorrected to obtain the true pressure at that depth. These extrapolated andcorrected point pressure measurements, when plotted versus true vertical depth.give invaluable information in both virgin and developed reservoirs.
This paper will describe the test by test interpretation and discuss modelsavailable to estimate excess pressures due to supercharging. It will alsoillustrate, using examples where possible, the estimation of fluid gradients, de-lineation of transition zones and insight into rock wettability by carryingout a Free Water Level Analysis on RFT pressures from virgin reservoirs. Fordeveloped reservoirs examples of differential depletion and non-uniformwater-flood conformance will be presented.
The predecessor of RFT was the wireline formation tester commonly known as(FIT™). To analyse the pressure-time behaviour during the pressurebuild-up which followed the drawdown of fluid into the FIT, spherical flowsolutions tothe diffusivity equation were required. Moran and Finklea(1) solvedthe diffusivity equation for spherical flow geometry for both constant How rateand constant pressure at the inlet to the tool. The equation to analyse thebuild-up following the constant flow rate period was also presented. Stewartand Wittman[2] used the constant flow rate spherical geometry Solution[1] andformulated the build-up equation which can analyse the pressure-time historyafter two constant flow rate periods. These two constant Bow rate periods arecharacteristic. of RFT and are also known as the two pre-tests.
The RFT tool was presented in the petroleum engineering literature by Schultzet. al.[4]. They gave field examples of pressure measurements and sampling.Emphasis in their paper was the multiple setting capability of the tool forpressure measurements. Reservoir Engineering applications of verticallydistributed pressure measurements were presented by Smolen &: Lirsey[5].Based on RFT pressure profiles they were able to monitor a waterflood better.Applications for the Middle East and Canada were presented[6][7] in 1979.
This paper in addition to reviewing the interpretation theory for an individualtest will also present some recent field example from Canada, These examplesare from exploration and development wells. Vertical pressure profiling andestimate of effective permeabilities are the basic products from RFTmeasurements. This paper will propose new applications which using RFTmeasurements, may give insight into the wettability characteristic of therock.
