Summary The formation-rate-analysis (FRASM) technique is introduced. The technique is based on the calculated formation rate by correcting the piston rate with fluid compressibility. A geometric factor is used to account for irregular flow geometry caused by probe drawdown. The technique focuses on the flow from formation, is applicable to both drawdown and buildup data simultaneously, does not require long buildup periods, and can be implemented with a multilinear regression, from which near-wellbore permeability, p * and formation fluid compressibility are readily determined. The field data applications indicate that FRA is much less amenable to data quality because it utilizes the entire data set. Introduction A wireline formation test (WFT) is initiated when a probe from the tool is set against the formation. A measured volume of fluid is then withdrawn from the formation through the probe. The test continues with a buildup period until pressure in the tool reaches formation pressure. WFTs provide formation fluid samples and produce high-precision vertical pressure profiles, which, in turn, can be used to identify formation fluid types and locate fluid contacts. Wireline formation testing is much faster compared with the regular pressure transient testing. Total drawdown time for a formation test is just a few seconds and buildup times vary from less than a second (for permeability of hundreds of millidarcy) to half a minute (for permeability of less than 0.1 md), depending on system volume, drawdown rate, and formation permeability. Because WFT tested volume can be small (a few cubic centimeters), the details of reservoir heterogeneity on a fine scale are given with better spatial resolution than is possible with conventional pressure transient tests. Furthermore, WFTs may be preferable to laboratory core permeability measurements since WFTs are conducted at in-situ reservoir stress and temperature. Various conventional analysis techniques are used in the industry. Spherical-flow analysis utilizes early-time buildup data and usually gives permeability that is within an order of magnitude of the true permeability. For p* determination, cylindrical-flow analysis is preferred because it focuses on late-time buildup data. However, both the cylindrical- and spherical-flow analyses have their drawbacks. Early-time data in spherical-flow analysis results in erroneous p* estimation. Late-time data are obtained after long testing times, especially in low-permeability formations; however, long testing periods are not desirable because of potential tool "sticking" problems. Even after extended testing times, the cylindrical-flow period may not occur or may not be detectable on WFTs. When it does occur, permeability estimates derived from the cylindrical-flow period may be incorrect and their validity is difficult to judge. New concepts and analysis techniques, combined with 3-D numerical studies, have recently been reported in the literature.1–7 Three-dimensional numerical simulation studies1–6 have contributed to the diagnosis of WFT-related problems and the improved analysis of WFT data. The experimental studies7 showed that the geometric factor concept is valid for unsteady state probe pressure tests. This study presents the FRA technique8 that can be applied to the entire WFT where a plot for both drawdown and buildup periods renders straight lines with identical slopes. Numerical simulation studies were used to generate data to test both the conventional and the FRA techniques. The numerical simulation data are ideally suited for such studies because the correct answer is known (e.g., the input data). The new technique and the conventional analysis techniques are also applied to the field data and the results are compared. We first review the theory of conventional analysis techniques, then present the FRA technique for combined drawdown and buildup data. A discussion of the numerical results and the field data applications are followed by the conclusions. Analysis Techniques It has been industry practice to use three conventional techniques, i.e., pseudo-steady-state drawdown (PSSDD), spherical and cylindrical-flow analyses, to calculate permeability and p* Conventional Techniques Pseudo-Steady-State Drawdown (PSSDD). When drawdown data are analyzed, it is assumed that late in the drawdown period the pressure drop stabilizes and the system approaches to a pseudo-steady state when the formation flow rate is equal to the drawdown rate. PSSDD permeability is calculated from Darcy's equation with the stabilized (maximum) pressure drop and the flowrate resulting from the piston withdrawal:9–11 $$k {d}=1754.5\left({q\mu \over r {i}\Delta p {{\rm max}}}\right),\eqno ({\rm 1})$$where kd=PSSDD permeability, md. The other parameters are given in Nomenclature.

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

Wireline Formation Testers (WFT) can provide valuable, cost-effective information on undisturbed reservoir pressure (P*), vertical pressure gradients, in addition to formation fluid samples, and an estimate of near-wellbore permeability. Various analysis techniques have been borrowed from the well testing studies and adapted to analyze WFT-measured drawdown and buildup data. Spherical-flow analysis utilizes early-time data and usually gives a reliable estimate of permeability. For P* determination, cylindrical-flow analysis is preferred because it focuses on late-time buildup data. However, the cylindrical-flow analysis has its drawbacks. Late-time data crucial for cylindrical-flow analysis, especially in low-permeability formations, but long testing periods are not desirable because of potential tool "sticking" problems. Even on long tests, the cylindrical-flow period may not occur or may not be detectable on WFTs. When it does occur, permeability estimates derived from the cylindrical-flow period may be incorrect and their validity is difficult to judge. We introduce a new analysis technique that simplifies the interpretation of WFT pressure-transient data. The theory is based on the geometric factor concept, which is equally valid for drawdown, stabilized flowrate, and buildup data. The technique is less sensitive to data quality than other methods, does not require long buildup periods for low-permeability formation testing, and can be implemented with a simple graph of pressure vs. formation flowrate, from which both near-wellbore permeability and P* are readily determined. Three numerical simulation cases are set to test the theories of spherical, cylindrical, stabilized drawdown and the newly introduced pressure vs. flowrate analysis techniques. Occurrence of these flow regimes are tested by comparing their predicted slopes with the slopes of simulation run data. Conventional and new analysis techniques are applied to two field-measured data sets and the results are compared. Background A wireline formation test is initiated when a probe from the tool is set against the formation. A measured volume of fluid is then withdrawn from the formation through the probe. The test continues with a buildup until the pressure stabilizes. Pressure in the tool is continuously monitored throughout the test. WFTs provide formation fluid samples and produce high-precision pressure and permeability profiles and vertical pressure gradients, which, in turn, can be used to locate fluid contacts. Wireline formation testing is much faster compared with the regular pressure transient testing. Total drawdown time for a formation test is just a few seconds and buildup times vary from less than a second (for permeability of hundreds of mD) to half a minute (for permeability of less than 0.1 mD), depending on the ratio between drawdown volume and formation permeability. Wireline formation testing can also generate detailed profiles of formation heterogeneity (degree of damage to the formation, existence of thin shale laminations, and natural or induced fractures). Because the investigated volume can be small (a few cm3), the details of reservoir heterogeneity on a fine scale are given with better spatial resolution than is possible with conventional pressure transient tests. Furthermore, WFTs may be preferable to laboratory core permeability measurements because WFTs are conducted at reservoir in-situ conditions and provide formation fluid samples. P. 343

Wireline Formation Test (WFT) can provide a valuable, cost effective source of near wellbore formation damage data. Interpretation of WFT data can be improved with numerical simulation and laboratory experimental data. A 3-D numerical simulator for WFT was developed and a comprehensive study of problems associated with the wireline test was made. The simulator results are interpreted with the concept of geometric factor. Case studies include sensitivity to formation damage, internal probe diameter, fluid compressibility and viscosity, formation permeability, and drawdown tank volume. The radial grid system used in finite element method is particularly amenable to handling practical problems with real WFT flow geometries and configurations. The simulated test pressures match well with actual field data. In addition to the numerical simulations, an experimental setup which models measurements on a downhole damaged formation was built and attached to a laboratory probe permeameter equipment. This experiment enabled us determine dependence of the measured permeability of the damaged formations on the severity and the depth of damage. The results of the experiments are also in good agreement with the 3-D numerical simulator results.

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