EVALUATING TIGHT GAS RESERVOIRS WITH NMR - THE PERCEPTION, THE REALITY AND HOW TO MAKE IT WORK

摘要

Nuclear magnetic resonance (NMR) well logging of tight gas formations poses unique challenges with regards to data acquisition and interpretation. Low formation porosity combined with the low hydrogen index of gas results in signals near the detection limit of NMR logging tools. In addition, the high diffusion coefficient of gas results in rapid signal decay. Accurate interpretation of NMR logs and subsequent derivation of formation properties, such as total porosity, fluid type, gas & water saturation, and permeability trends, relies on assumptions made about the NMR properties of bulk fluids and their respective interactions with the surrounding rock matrix. Based on measurements of resistivity indices as well as interfacial tension in gas-bearing reservoirs, it is widely assumed that gas is typically a non-wetting phase in the pore system. Accordingly, in the interpretation of NMR log data, the NMR relaxation time of formation gas is often assumed to be that of the bulk gas phase. Our measurements have shown that the relaxation time of gas in the pore space can be significantly shorter than expected. The implication of this behavior reaches beyond the simple understanding of NMR logs in tight gas sands. It may result in an incorrect interpretation of fluid and gas phases present in the zone of investigation, as well as an erroneous calculation of total porosity, and irreducible fluid saturations. At the same time, our experiments have indicated that information on wettability as derived from conventional core analysis may not adequately represent the more complex transfer of magnetic energy and entropy that occurs across the interfaces of grain surface, bound water layer, and bulk gas phase. Consequently, conventionally- and NMR-derived wettabilities might not directly correlate for all rockfluid systems. This paper summarizes initial results of an integrated study of laboratory NMR measurements, core analysis, and log interpretation. NMR measurements of methane-bearing sandstone and tight rock samples were conducted at reservoir pressure to study the nature of this behavior. NMR logs were run in the Pinedale Anticline gas field in Wyoming. Low frequency, low gradient NMR logging tools were used to differentiate the gas signal from the non-movable fluid region based on relaxation time. Incorporating the very short relaxation time of the formation gas and the core-calibrated values of the bound/movable cutoff significantly improved the log interpretation.