“Wettability of chalk and argillaceous sandstones assessed from T1/T2 ratio”
By
Konstantina Katika
Abstract:
Low field nuclear magnetic resonance (NMR) relaxation of the fluids inside the porous rock is the result of bulk and surface relaxation of the protons inside the pore fluid. Bulk relaxation is a fluid property when the solid-fluid interaction is minimized; and surface relaxation is the result of the solid-fluid interaction and is related to the mineral properties. Thus, longitudinal, T1, and transverse, T2, relaxation times should in principle be similar. However, microscopic magnetic gradients related to minerals can shorten T2 relaxation times as compared to T1 relaxation times provided the saturating fluid has high affinity to the solid. We consequently find that the T1/T2 ratio can quantify the affinity between the rock and wetting pore fluid. The affinity is a measure directly linked to wettability.
In order to investigate the T2-shortening, we performed T1-T2 maps NMR experiments on different samples of chalk, Berea sandstone, and chloritic greensand, saturated either with water, oil or oil/water at irreducible water saturation. The T1/T2 ratio obtained from T1-T2 maps reflects the T2 shortening, so we compare the T1/T2 ratio for the same type of rock, saturated with different fluids. The chalk shows high affinity for water, Berea sandstone has no clear preference for oil and water whereas chloritic greensand shows different behaviour for small and large pores as defined in the MICP throat size distribution. Small pores (fast relaxing components) have (T1/T2=2.0) when water saturated, but (T1/T2=3.8) when oil saturated indicating oil-affinity. By contrast large pores (slow relaxing components) have significant preference for water (T1/T2 = 2.2) as compared to oil (T1/T2=1.2-1.4).