NMR T1-T2 Response of Moveable and Non-Moveable Fluids in Conventional and Unconventional Rocks.

摘要

Recent publications have shown that liquid rich shales can also contain a significant proportion of high viscosity hydrocarbons that are not producible in tight rocks such as shales [1],[2]. NMR theory of fluid relaxation suggests that the ratio of T1/T2 can be used to differentiate low viscosity fluids(moveable)and highly viscous fluids. In the present study we have investigated the NMR response of moveable and non-moveable fluids in conventional and unconventional reservoir rocks using T1-T2 correlation maps obtained with a TE = 100μsec. Berea sandstone,Lyons sandstone,and Bossier tight sands were saturated at 3000 psi with 2.5% KCl brine. The corresponding T1-T2 maps show that T1/T2= 1.5. In two Wolfcamp shale samples from an oil producing well,we observed for the”native”state a NMR signal with a T1/T2 ratio of 1.3. An increase in amplitude of the”native”state NMR signal was observed after the shale samples imbibed 2.5% KCl brine for 24h. However,the imbibition of dodecane for 24h by the shale samples gave rise to a NMR signal with a T1/T2 ratio of 3 for one shale sample and 2.5 for the other sample. These experiments show that moveable brine in rocks has a T1/T2 ratio close to 1.5 while the T1/T2 ratio for moveable oil is close to 3. The study of non-moveable hydrocarbons was conducted on solid bitumen and wax. In their bulk solid state,solid bitumen and wax have T1/T2 ratios of 20 and 400,respectively. However,these ratios decreased and approached a value of 2 to 3 after the hydrocarbons were liquefied. Liquid wax was also introduced in a Berea sandstone sample,and allowed to solidify within the rock. The NMR T1-T2 map of Berea saturated with solid wax show a T1/T2 ratio of 400. These results imply that non-moveable hydrocarbons are not affected by surface relaxivity and have T1-T2 ratios higher than moveable hydrocarbons.