Relaxation and relaxation exchange NMR to characterise asphaltene adsorption and wettability dynamics in siliceous systems

Abstract

The fraction of asphaltenes in crude oils is among the major concerns in upstream and downstream petroleum engineering. At reservoir scale asphaltenes may cause compartmentalisation and at pore scale they may create barriers to flow, change wettability conditions and relative permeability; and as a result affect ultimate oil recovery. Rock core ageing in oil is a common step in laboratory core analysis. Where possible, crude oils relevant to the origin of cores are used, while the use of arbitrary oils, various hydrophobic chemicals and anti-wetting agents is not uncommon. Published ageing time and temperature vary broadly. This fact motivates us to evaluate the applicability of synthetic oils for studies requiring wettability alteration. We systematically study the precipitation kinetics of the heavy-end oil fraction and wettability alteration properties of mixtures comprised by various proportions of commercially available bitumen, aromatics and alkane components.Low-field NMR relaxation measurements have been applied to characterise wettability of rocks by introducing an NMR wettability index. The latter requires multiple reference measurements at end-point saturation states similar to a standard Amott-Harvey workflow. Furthermore, petrophysical interpretation of T2 relaxation data is prone to be affected by diffusional coupling effects. NMR correlation techniques have a higher prediction capacity, e.g. the T2-store-T2 (REXSY) experiment is naturally sensitive to the spatial variation of physical properties by detecting diffusion exchange between different relaxation environments. We applied a combination of relaxation and relaxation exchange techniques to study the effect of asphaltene deposition on pore-space morphology and wettability for two siliceous systems with different surface topology. The change of wettability over ageing time in different synthetic oils was tracked using T2 relaxation measurements, providing estimates of ageing dynamics useful in designing wettability-related experiments.Results show that the kinetics of asphaltene deposition and wettability alteration processes are strongly dependent on chemical composition of synthetic oils, asphaltene origin (light oil or bitumen) and solid phase morphology. Elements of the resulting deposition pattern and wetting state of the cores were inferred using the proposed approach, utilising low-field NMR T2 relaxation and T2-store-T2 relaxation exchange experiments combined with numerical simulation of relaxation responses. The knowledge of deposition pattern and dynamics obtained mainly by the mean of combination of NMR relaxation techniques contributes to the improved design of core wettability alteration steps and potentially to enhanced petrophysical application of low-field NMR technology.