T2-store-t2 Relaxation Exchange Nmr to Charcterise Effect of Asphaltenes on Wettabilty Dynamics in Siliceous Systems

The asphaltene fraction of crude oils is one of the main factors defining wettability conditions and ultimate oil recovery. At reservoir scale asphaltenes may cause reservoir compartmentalization and at pore scale govern wettability phenomena. To reproduce reservoir conditions, aging in oil is a common step in laboratory core analysis. Oils relevant to the origin of the plugs are the apparent choice for aging, while for outcrop rocks various hydrophobic chemicals and anti-wetting agents or arbitrary oils are often used. We evaluate alteration properties of synthetic oils represented by various proportions of commercially available bitumen, aromatics and alkane for studies requiring wettability alteration. Low-field NMR relaxation measurements have been applied in the past to characterise the wettability of rocks by introducing an NMR wettability index. However, the latter requires multiple reference measurements at end-point saturation states. NMR correlation techniques have a higher prediction capacity, e.g. T2-store-T2 (REXSY) experiment is naturally sensitive to spatial variation of physical properties by detecting diffusion exchange between different environments. It has been applied to study the connectivity of the pore space in aqueous systems such as gypsum, cement pastes, soils, etc. We applied REXSY to study effect of asphaltenes deposition on wettability of siliceous systems. The change of wettability over aging time in different synthetic oils was tracked using T2 relaxation measurements, providing estimates of aging dynamics useful in designing wettability-related experiments. Quantitative information about fraction of altered surface area and deposition pattern was inferred from combination of T2 experimental and numerically simulated responses and from T2-store-T2 experiments. Results show that the wettability alteration process is strongly sensitive to both chemical composition of synthetic oils and asphaltenes origin (light or heavy oil). It can be performed in controlled manner to set variety of heterogeneous wetting conditions. Elements of resulting deposition pattern and wetting state of the core were identified using low-field NMR relaxation and relaxation exchange techniques.