ABSTRACT

Nuclear Magnetic Resonance (NMR) is now a firmly established formation evaluation service available in logging while drilling (LWD) mode. Key NMR products including lithology-independent porosity, fluids, and producibility answers are broadly accepted in the industry. Existing commercial services focus either on T2or T1acquisition modes, each of which has its own advantages and disadvantages. This paper presents initial results of a new generation LWD NMR service for slim holes which offers the benefits of both T2and T1measurements.

The new slim hole NMR technology introduced in this paper has been designed with service efficiency as a primary objective, along with hardware reliability, data quality, and answer products objectives. Using novel acquisition electronics and an optimized magnet configuration, the sensor is highly tolerant to variations in borehole salinity and operates with a short echo spacing. The new tool delivers simultaneous T1and T2distributions while drilling, along with all derived NMR answer products in real time while the BHA is either rotating or sliding. The simultaneous T1-T2capability allows good definition of both slow and fast-relaxing components, enabling more accurate differentiation of different fluid environments. Because of the high quality raw data and an efficient T1-T2acquisition scheme, NMR answers are delivered at high resolution even at moderate drilling speeds, with formation features in the order of 1 ft thick being accurately identified. A common processing workflow ensures that all real-time answers are equivalent to post-acquisition recorded mode products.

The potential effects of lateral motion, a concern for all LWD NMR tools, are largely mitigated in the new slim hole tool through optimization of the magnet and antenna and by adopting a short echo spacing for data acquisition. The standard acquisition of T1data also helps to minimize the risk of motion effects compared with T2-only measurements. In addition, a workflow has been developed which integrates NMR physics with drilling dynamics so that BHA configurations may be optimized to minimize motional modes at the NMR sensor, as part of the overall job planning exercise.

Results are presented from two field tests with prototype tools. The first was in a vertical hole at a test facility. Multiple runs were acquired while drilling both with and without a motor and in washdown mode. The second field test was performed in an operator well on the North Slope, Alaska. Here, the new NMR tool was run in a horizontal 6.75in hole drilled through a sand-shale formation with the primary objective of identifying permeable pay zones in real time as targets for subsequent wireline fluid sampling. Additionally, this data set was used to understand the complex fluid distributions and properties within an isolated fault block.

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