Summary Wireline formation testers (WFTs) collect fluid samples for pressure-volume-temperature analysis through a probe set against the borehole wall. Filtrate contamination is reduced prior to sampling by either pumping the mixture of filtrate and reservoir fluid from the formation to the borehole or flowing the mixture into one or more WFT chambers. The cleanup is monitored at the surface. The time to reach the level of acceptable contamination (LAC) depends on the depth of invasion, pumpout rate, and various fluid and rock properties. Generalized guidelines predict time to first oil based on simple volumetrics but do not predict the rate of cleanup. Excessive cleanup time increases costs and the risk of differential sticking of the tool/cable. In some cases it may not be practical to attempt the operation as the LAC may take too long to achieve. A numerical simulator was used to investigate the characteristics of the contamination level versus time curve and to define the variables governing cleanup. The model was validated using data from five wells from two fields with differing rock and fluid properties. One hundred and fifty simulation runs were made with different invasion depths, flow rates, and rock and fluid properties. An equation was developed for field use that estimates filtrate contamination fw as a function of cleanout time t. An alternative approach for WFT sampling is also suggested, using not one but two probes. With this method, both cleanup time and the final level of filtrate contamination can be substantially reduced. Introduction Formation fluid is drawn through the probe and pumped into the wellbore for periods ranging from a few minutes to several hours or more. In this way, formations that were previously too contaminated with mud filtrate to yield useful samples can now be sampled by pumping out fluid until the level of contamination has dropped to an acceptable level. This can provide samples for pressure-volume-temperature analysis from a number of zones before the well is completed. Drill stem or production tests whose main objective was to collect formation fluid samples may no longer be required as a result. The contamination level is monitored continuously until it has reached the level of acceptable contamination (LAC), at which time a set of samples can be taken. The results of a study of near wellbore fluid flow during wireline formation tester (WFT) cleanup prior to sampling are presented. The objective was to predict cleanup time. In this way, it becomes possible to ascertain whether an operation is feasible, in terms of the time to reach LAC, the likely time/cost of the operation, and the consequential associated risks of stuck tools. The proportion of filtrate fw in the formation fluid pumped can be approximated by a function of the following form:1 f w = 1 − • [ 1 − ( t 0 / t ) n ] , where t0 and n are functions of radius of invasion, flow rate, and rock and fluid properties. Geometry of Flow The invaded zone can be visualized as a cylinder, with a hollow center representing the wellbore, and its outer wall representing the end of the invaded zone. Zero fluid movement through the mudcake is assumed. As filtrate is removed from around the probe, it is replaced by fluid in an elliptical flow regime, the geometry of which is dependent upon rock permeability anisotropy. The outer wall of the cylinder, which is the filtrate/oil interface, is also drawn in by the shrinking filtrate body toward the probe leading to a filtrate saturation distribution around the wellbore similar to the shape of an hourglass. The simulator results confirm this. Fig. 1 shows the radial section adjacent to the probe and Fig. 2 shows the section opposite the probe. Water filtrate (cooler colors) can be seen feeding in from above and below, while oil forms a cone directed toward the probe. Both figures show the development of the hourglass filtrate saturation distribution centered upon the probe. Assumptions This study is limited to sampling in vertical wells set in horizontal beds. The rock is homogeneous, and anisotropic. The mud is water based, and the filtrate is brine. The filtrate and the oil are immiscible, and the degree of contamination during cleanup is represented by the watercut. The rock is water wet. Corey relative permeability curves were used in the simulation runs used to test the effect of various parameters on cleanup time. The appropriate field relative permeability data were used when validating the model against the modular formation dynamics tester (MDT)*2 data sets. Appendix A describes the model setup. A Representative Invaded Zone The invaded zone was simulated by injecting water for a duration and rate that gave a radius of invasion obtained from resistivity logs. Pseudoised relative permeability curves were used to create a shock front. Watercut Prediction Five sets of MDT field data were used to validate the model. The optical fluid analyzer (OFA)*2 monitors the relative proportions of oil and water in the flowline with an accuracy of 5%. The observed water fraction during cleanout was compared to that predicted from the simulator. The watercut vs. time can be approximated to a particular function of time. Two points on the watercut curve have been selected as reference points: t0 the time when first oil starts to flow into the probe, and t1 the time when the watercut drops to 10%. Figs. 3 through 8 compare the observed water cut from the OFA data to the simulator results. Matching the Model with Field Data Wells X1, X3, and Y1 are from a field characterized by heavy viscous oil and high permeability rock. The field relative permeability data and fractional flow curves were used to generate the pseudoized relative permeability curves input to the simulations.

Stratified or layered carbonate reservoirs are common in the UAE and throughout the Middle East. For field development and reservoir management it is important to describe the layering in terms of the vertical and horizontal layer permeabilities and the vertical communication between layers. Vertical communication between layers affects the overall development plan, the economics and management of enhanced recovery projects. The number and placement of both injector and producer wells will require an accurate description of the layering.In this paper, we discuss the use Interval Pressure Transient Testing (IPTT), which is also commonly called Vertical Interference Testing, and Vertical Pressure Profiling in a major oil field in UAE. The objective was to characterize the vertical and horizontal layer permeabilities and vertical communication through the main producing reservoir section.In a newly drilled and uncompleted well, eight IPTT tests were conducted with a Formation Tester Dual Straddle Packer and two Probe Modules. The vertical pressure profile along the wellbore was also obtained for information about the geology and flow units as well as depletion.The interval pressure test at each location was interpreted using an appropriate geological model with the packer and probe pressure and flow rate measurements. The interpretation procedure consisted of identification of the different flow regimes followed by history (type curve) matching for the estimation of vertical and horizontal permeabilities for each layer. The interpretation of two of the interval pressure transient tests is presented.

This paper describes early UKCS experience of a new production logging sensor, the Digital Entry and Fluid Imager, or DEFT * tool.Deviated and horizontal wells can cause complex flow regimes to arise that render interpretation of conventional production logging sensors difficult or sometimes impossible.The DEFT tool is a new type of production logging tool which facilitates the understanding of the nature of well bore flow and improves confidence in interpretation. The tool makes four independant measurements of the phase holdup, distributed in differring quadrants of the pipe cross section, Results to date show that the DEFT tool gives a direct count of the dispersed phase bubbles. Each probe can give an indication of the flow type, the absolute velocity of the dispersed phase, and the bubble flow direction. As the four measurements are distributed at different locations within the pipe cross section, an image of pipe flow may be interpreted, Early results also show how the DEFT tool can be used to define the location of oil entries and to pin-point circulatory flow.The paper briefly describes the tool principles, Interpretation is illustrated with example logs made during its field introduction.

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Wireline formation testers (WFT) collect fluid samples for PVT analysis through a probe set against the borehole wall. Filtrate contamination is reduced prior to sampling by either pumping the mixture of filtrate and reservoir fluid from the formation to the borehole or flowing the mixture into one or more WFT chambers. The cleanup is monitored at the surface. The time to reach the level of acceptable contamination (LAC) depends on the depth of invasion, pumpout rate and various fluid and rock properties. Generalized guidelines predict time to first oil based on simple volumetrics but do not predict the rate of cleanup. Excessive cleanup time increases costs and the risk of differential sticking of the tool/cable. In some cases it may not be practical to attempt the operation as the LAC may take too long to achieve. A numerical simulator was used to investigate the characteristics of the contamination level versus time curve and to define the variables governing cleanup. The model was validated using data from five wells from two fields with differing rock and fluid properties. One hundred and fifty simulation runs were made with different invasion depths, flow rates, and rock and fluid properties. An equation was developed for field use, that estimates filtrate contamination fw as a function of cleanout time t. An alternative approach for WFT sampling is also suggested, using not one but two probes. With this method, both cleanup time and the final level of filtrate contamination can be substantially reduced. P. 27