Wireline Formation Test (WFT) tools are used to measure formation pressures at discrete depths and to obtain fluid samples. From these pressure tests, fluid density and fluid contacts can be derived. The transient pressure response created by a WFT pressure test has typically been analyzed by methods that originated from well test analysis. In 1996 a new method based on the formation flow rate was developed that simultaneously considers both drawdown and buildup portions of the data. Because the new method is based on material balance from the beginning of the test, it accounts for the storage effect in the tool. By a simple, graphical technique or multi-regression analysis, the method determines fluid compressibility, mobility, and undisturbed formation pressure (P*). Pressure tests in low-permeability formations have been hampered by problems, such as excessively long buildup time, supercharging, and loss of packer seals. In addition, pressures with insufficient drawdown volume are confused with tight formation tests. These problems result in abandonment of tests or incorrect formation pressures that eventually give false pressure gradients, and waste rig time. We have analyzed numerous field and test-well jobs and have found that good pressure tests in a low-permeability formation can be achieved in less time by implementing quality control procedures through the new method. The ability to repeat a pressure test provides a good basis for applying quality control measures while logging. When the new method indicates a tight formation for the initial test, the drawdown rate for a subsequent test is controlled to a very low rate (less than 0.5 cc/s) and a real time cross plot of formation rate versus pressure is monitored to ensure the quality of the data. The new method can also detect tool plugging and the presence of pressure supercharging. These abilities can assess the validity of a test during testing and thereby provide a way to assure that a test is successful. Introduction Wireline formation testers (WFT) are used to measure formation pressure at discrete depths along a wellbore. From these data information on formation fluid type, fluid contact depth, and formation connectivity to another wells is eventually obtained. In probe-type wireline formation tests, a tube or conduit is extended to the formation and a small volume of fluid, typically 10 cc or less, is withdrawn from the formation. An initial-test starts from the hydrostatic pressure and repeat-tests thereafter start from the prior buildup pressure (presumably the formation pressure). The pressure change during the drawdown and the buildup period are recorded and then analyzed for the formation pressure, mobility, and the fluid compressibility. Various conventional analysis techniques, drawdown, spherical-flow, cylindrical-flow analyses, have been used in the industry.1,2,3 Analysis methods with storage and skin based on a spherical model was developed and supported by numerical simulation studies.4,5 These techniques are designed to analyze tests after the job is complete, or just to estimate the pressure and mobility of the formation during the job, but they do not provide any information on whether the test itself is valid or not. WFT pressure tests can be hampered by various reasons, including insufficient drawdown volume, tool or formation plugging during a test, imperfect sealing, or pressure supercharging, that result in false pressure information. Pressure tests with a drawdown rate that is too fast or that provide insufficient drawdown volume should be avoided. Too fast of a drawdown rate can cause excessive pressure drop and long buildup time. If the formation cannot provide enough fluids, the pressure response will be dominated by the compressibility of fluid in the tool. The pressure can go below the bubble point pressure and gas will evolve from the solution occupying the flowline of the tester.

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Accurate PVT data are crucial to well completion and production, formation evaluation and reservoir characterization. This is especially true for initial reservoir characterization where the PVT sample needs to be obtained prior to production. It is essential that the fluid sample be recovered as closely as possible to in-situ conditions whether by drill stem or wireline formation tester. The need to remove drilling mud filtrate prior to collecting a sample has been widely recognized. Wireline testers which can pump fluid from a formation until filtrate is reduced to a minimum overcome this problem. While reducing sample contamination has been addressed, little emphasis has been placed on the need to control inlet pressure during filtrate removal or during sampling. Reducing contamination is important; however, there is equal need to determine the critical sampling pressure. The purpose is to prevent phase separation in the formation by regulating the sampling process based on this information and thereby obtain a more representative reservoir fluid sample. A recently introduced wireline instrument provides the capability of measuring the critical pressure prior to sampling, of controlling the sample pressure and of increasing the pressure in the sample container to compensate for temperature decline during delivery of that sample to a testing laboratory. Example of pressure tests while pumping and during pressure buildup are presented along with indicated sample properties. Introduction Wireline Formation Testers (WFT) provide an cost effective means to determine pressure as a function of depth and to recover samples of fluid from formations at selected depths. No other method can provide this type of information. Pressure data are used to estimate mobility, fluid contact and fluid density. Samples are used to verify fluid type, measure fluid properties, and to develop the phase and precipitation behavior. The importance of obtaining samples which are truly representative of the formation has been emphasized in developing the next generation of WFT. Background on Wireline Fluid Sampling This method of testing was originally developed to recover a fluid sample. Over the following three decades several improvements were made. Means were added for pressure measurement, multiple pressure tests on one run, to reduce loss of packer seats during sampling, and a pressure control system to regulate pressure during sampling. In the same time period, resolution and accuracy of pressure transducers were improved two orders of magnitude and successful sample recovery improved from one out of three to nine out of ten. Although these improvements are significant, sample quality has improved only marginally. While filtrate and drilling mud problems have been reduced, concern is now being expressed for maintaining fluid composition prior to analysis. The use of samples has changed from demonstrating that hydrocarbons could be recovered to predicting phase behavior and conditions under which waxes and/or asphaltines precipitate. These uses require that contaminants be eliminated or at least significantly reduced. It also emphasizes the need to recover the sample without causing changes in composition. Mud filtrate invades the formation as a result of the drilling process. Preventing this fluid from being in a sample is difficult at best and can be near impossible when the filtrate is miscible with the formation fluid. Previous WFT were limited to removing fixed volumes of filtrate because one of the two sample tanks had to be used. P. 871

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