Abstract
The Nuclear Magnetic Resonance (NMR) logging is one of the most efficient technologies in reservoir evaluation and characterization due to its wide range of applications. The efficiency of this technology comes from the mineralogical independent acquisition that is only measuring and reflecting the formation fluids but not the rock matrix. This advantage differentiates NMR measurements from conventional wireline electric logging where their measurements, which are affected by the rock matrix and fluid-filled pores and are more sensitive to the rock matrix than to the pore fluids.
Such NMR applications are porosity, permeability, saturations, fluids identification as well as pore size distributions reflecting the corresponding grain size of the rock. The NMR T2 bins distribution reflect the pore sizes distribution of the rock in the presence of only a single fluid in the pores i.e. 100% water saturated rock of water wet nature. In a short inter echo spacing experiment is used, the surface relaxation becomes dominant and then the NMR T2 decay becomes directly proportional to pore size. By assuming the similarity of shape geometries, the largest pores should have the highest T2 values and lowest S/V ratio, and as the pores become smaller, the T2 values become lower and the S/V ratio increases. Therefore, the average pore diameter can be predicted from the measured NMR T2 values, S/N ratio and the rock relaxivity.
The paper explores an efficient and accurate method for determining the average pore size of rock using the NMR data of both the Halliburton-MRIL and Schlumberger-CMR tools calibrated to core analysis. The method is based on identifying the pore shape using the thin sections core chips and using the shape geometry in mathematical approach for determining the average pore diameter based on the reservoir relaxivity and the NMR decay time logarithmic mean. This paper will also detail the application of this method in three different reservoirs, where two are gas-bearing, located in the deep water of the Mediterranean Sea and the third light oil-bearing reservoir located in the Nile Delta depositions.
