Estimation of rock properties by NMR relaxation methods

Two often used permeability models, which are based on logarithmic mean of relaxation time distribution and irreducible water, were examined. The model based on irreducible water was found to be more suitable than the model based on mean value of relaxation time distribution when oil is present. The NMR response of North Burbank with partial saturation of air and water is different from those of other sandstones. The increase in the amplitude of the microporosity part of the relaxation time distribution after desaturation was observed for North Burbank. The clay lining a pore is diffusionally coupled with the large pores when 100% water saturated. After desaturation with air, the water in the microchannel is isolated and relaxes like water in an isolated micropore. It is generally believed that when the rock is water-wet, there is tendency for water to occupy the small pores and contact the majority of the rock surface. Water is typically relaxed by contact with grain surface, but oil at the center of the pore has no access to these surfaces and therefore can only relax by bulk processes. According to this study, $T\sb1$ distributions under partial saturation with brine/Soltrol followed the above behavior. However, for $T\sb2$ distributions, we found the Soltrol peaks for chlorite-coated North Burbank and highly shaly sandstones were broadened and shortened to shorter relaxation times due to diffusion and internal gradients effects. The diffusion effect can be supported by the same observation in 100% S$\rm\sb{w}$ condition. This suggests even if oil is prevented from contacting the grain surface, oil will not necessarily relax as a bulk oil in water-wet system