Formation and fluids evaluation of heterogeneous and over pressured retrograde gas condensate low-permeability but high-reserves potential reservoirs requires use of advanced formation evaluation techniques. The Achimovskaya formation of Urengoiskoe field is one of Russia's giant low-permeability gas condensate fields. The main objectives of the pilot phase of the exploration projects in the Achimovskaya formation are reducing reservoir and fluid uncertainties, confirmation of technical and commercial feasibilities, construction of a pilot gas processing plant (UKPG), and startup of a pilot gas and condensate production. The initial formation evaluation in the first two "Achimgaz" vertical wells included an extended formation logging suite followed by formation testing and downhole PVT sampling achieved using a wireline formation tester (WFT) that had a dual packer to test a large area of open hole. Formation testing and sampling objectives were: reservoir evaluation, formation pressure profiling, direct measurements of mobility (effective permeability), downhole PVT sampling for hydrocarbons confirmation, and full laboratory studies of formation fluids before the production phase of the field. Rig time constraints were one of the main issues for the extended formation evaluation program, defining logging and testing scope.
In addition to traditional downhole sampling methods like low-shock PVT sampling (which allowed taking gas samples above the dewpoint), new approaches for logging and testing techniques were applied for the first time in Russia during this job. These were the combination of inflatable straddled dual packer wireline formation tester with nuclear magnetic resonance (NMR) logging in a single logging string and the use of two downhole pumps, also in a single string. The quality of sampling was controlled by application of downhole fluid analysis techniques. Comprehensive pre-job planning, close cooperation with drilling and operating companies' representatives, and real-time decision-making during data acquisition led to the successful completion of the operation.
This paper describes this first successful application of a dual-packer wireline formation tester to obtain PVT samples of retrograde gas in the Achimovskaya formation. As a result of the job, the presence of hydrocarbons was proved, formation pressure and mobility profiles were obtained along the tested sublayers of Achimovskaya formations, and high-quality PVT fluid samples were acquired. This provided valuable information for future testing, appraisal drilling, and field development planning. Furthermore, comparison between core permeability, formation testing fluid mobilities, and NMR permeabilities acquired in one of the exploration wells is also shown in the paper.