Nuclear Magnetic Resonance Log for Unconvetional Reservoirs
Well logging, a technique to evaluate drilled formations has progressively improved its usefulness to estimate formation porosity, fluid saturation and permeability. However, with deeper formations & complex lithologies, conventional logging like neutron-density, gamma ray et cetera have their limitations.
Formations like shales which are unconventional in nature due to their tightness and very low permeability cause complications for well logs to determine their petrophysical properties like fluid saturation, wettability and liquid filled porosity. Wettability and fluid saturation affects hydrocarbon recovery as wettability affects relative permeability, capillary pressure and ultimate recovery. Traditional methods of determining wettability such as documented by Amott and the U.S. Bureau of Mines (USBM) are difficult to apply on shales due to their very low permeability, usually in nano-darcies. In all this mayhem, Nuclear Magnetic Resonance (NMR) has proven to be a good alternative for measuring reservoir engineering properties of these unconventional hydrocarbon pools.
Unlike conventional petrophysical calculations, laboratory NMR measurements on core samples are limited by the availability of well-preserved samples and by the ability to restore fluid content into non-preserved samples, which is often a tedious task. Following is a sample NMR log taken from Crain’s Petrophysical Handbook.
Under the hood, NMR measures the effect of oscillating radio-frequency, working in a large induced static magnetic field, observing the effect it has on the spin of hydrogen nuclei. The measurement produces a pulse-echo relaxation spectrum T2, which is influenced only by the hydrogen nuclei in free water (or hydrocarbons) in the formation therefore hydrogen nuclei combined in the matrix, shale or in bound water, are not detected. The tool measurements do not produce a directly interpretable log, but the pulse-echo spectrum of relaxation times can be successfully modelled and analyzed to give a number of key petrophysical parameters. These parameters include the total liquid filled porosity (from the magnitude of NMR response), the pore size distribution (from the relaxation spectrum shape), the movable fluids or FFI (free fluid index) (from cut-offs applied to the relaxation spectrum) and permeability (from FFI and pore distribution data).
Although an expensive log for the current oil price scenario, this log nevertheless provides invaluable insight of petrophysical properties of tight reservoirs where all other tools fail to provide required information.
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