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Abstract

Wireline formation testing (WLFT) is a downhole testing technique used to determine the formation pressure, the formation pressure gradient, and the formation permeability and to collect formation fluids. Current WLFT tool are designed for high to medium permeability reservoirs. In this study, the results from WLFT runs in eight wells drilled in cooperation with the Gas Research Institute are presented. The tests were conducted by three major service companies. The merits of the WLFT tools. the information provided, and the possible misinterpretations of provided, and the possible misinterpretations of the data are reported. A brief description of each formation tested is made in each well: rock type, shaliness, porosity, permeability, saturation, and format ion porosity, permeability, saturation, and format ion pressure. The test results are characterized by pressure. The test results are characterized by the flowrate, the chamber size, the duration of drawdown, and the duration of the pressure build-up. Typical curves are presented. A statistical study of the successful tests is attempted. The results obtained with a slow flow wireline tester (SFWT) are also presented. Based on the analysis of the SFWT' runs and using a new modeling technique, a new interpretation method is proposed. The results indicate that the current proposed. The results indicate that the current WLFT tools will have to be modified to allow a better analysis of the drawdown period. This study shows that meaningful data can be obtained with the WLFT tool in many instances, especially when a slow flow rate is used. The drawdown portion as well as the build-up portion of the pretest can be interpreted, and the drawdown data can confirm or, to some extent, replace the build-up data. The evaluation of low permeability gas formations will be greatly enhanced by using the new slow flow testers and the new interpretation technique.

Wireline Formation Testers

The wireline or sidewall formation tester was invented around 1950. The purpose of the tool was to isolate a small portion of the borehole wall from the mud and extract a fluid sample. The first commercial tool, designed and built by one of the authors, was equipped with a pressure gauge on the flowline. Thus, a flowing pressure curve during drawdown could be recorded as well as a build-up curve when the sampling chamber was full. The formation pressure could be read directly at the end of the pressure build-up in high permeability zones and extrapolated with the Horner plot technique in low permeability intervals. The sampling chamber size was one gallon (3.785 liters) or 2.75 gallons (10.4 liters). A water cushion flowing through a small choke kept a constant flowing pressure. This tool worked well in medium permeability formations (10 md to 1 d). In highly unconsolidated formations, however. the formation would flow in the tool, and the seal would be lost. In low permeability formations, the flowrate was too low permeability formations, the flowrate was too low to fill up the sampling chamber in a reasonable time and no formation pressure could be recorded. In 1962, a "snorkel," i. e., a tube which extends from the pad face when the formation forms a crater, and a screen to prevent the sand flow were added. Much better results were obtained in soft formations. and the seal pad size could be reduced to the size of a doughnut. Circa 1974 a major service company designed and marketed a tool capable of repeating several pressure tests during the same run as well as pressure tests during the same run as well as collecting several fluid samples. During the first 20 years of wireline formation testing, it was recognized that the pressure measuring capability was more important than the fluid sampling capability.

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