T_1/T_2 NMR Surface Relaxation Ratio for Hydrocarbons and Brines in Contact with Mature Organic-Shale Reservoir Rocks

摘要

NMR measurements have been made under ambient conditions on seven finely ground mature organic samples. Each sample was subdivided into three identical subsamples. One subsample was saturated with brine, one with dead light-crude oil, and one with dodecane. T_1 and T_2 measurements were made on each subsample. The T_2 measurements were repeated with several echo spacings to allow diffusion relaxation to be removed from the relaxation. NMR measurements were also made on the bulk fluids. After removing bulk and diffusion relaxation it was found that the average value of the T_1/T_2 ratio for brine was 2.21, for crude 4.10, and for dodecane 3.93. This difference between the brine and hydrocarbon ratio observed in these experiments suggests that analysis of the T_1/T_2 ratio as a function of relaxation time may provide a method to separate fast relaxing brine and fast relaxing hydrocarbon signals in NMR measurements of mature organic-shale reservoirs. Careful analysis of the brine ratios shows that the ratios measured on the ground up samples should be similar to ratios that would be measured on whole fresh-state plugs and in reservoir logs.