Shear modulus of heavy oils, rheometer measurements: confinement effect and amplitude dependence

Abstract

The success of seismic and other acoustic monitoring techniques is based on the ability of geophysicists to accurately model acoustic waves propagation. True "fluids" are not capable of supporting shear stress; however, heavy oils are viscoelastic and allow the transmission of shear waves. The shear modulus of heavy oils is highly dependent on frequency which adds a major difficulty since the data we study are obtained at different frequencies. Measuring elastic properties at high frequencies has been done for many years and it is much simpler than low frequencies measurements; specifically in bulk heavy oils. The equipment deforms the sample at frequencies from 3 to 3000 Hz and it works well for solid-like samples but cannot be used for liquid-like samples. An alternative method to measure the shear modulus of heavy oils at low frequencies is the rheometer which measures the shear modulus at frequencies from 0.01 to 100 Hz with strains amplitudes in the order of 10-4, two or three orders of magnitude larger than tension/compression. Linear viscoelasticity theory indicates that measurements done in the linear viscoelastic regime should be consistent between techniques as elastic properties are independent of amplitude. My research is focused on understanding rheometer measurements, and validating them against the tension/compression technique. However, in this work I identified two aspects of the rheometer that make their measurements not consistent with tension/compression results. The first is the presence of the two solid-liquid interfaces provided by the parallel plates; and the second is the increased strains amplitudes in the rheometer. The solid-liquid interfaces cause reorientation of the surface active components of the heavy oils, increasing the shear modulus near the interface in a scale that can be measured by the rheometer. This was evidenced by an increased of the shear modulus when reducing the gap between the rheometer plates. On the other hand, the increased amplitudes on the rheometer cause decrease in the shear modulus with respect to what is measured in seismic techniques. Heavy oils are aggregates that at rest or under low shear, behave as a solid due to weak bonds between the aggregate particles. As strain amplitude is increased the weaker bonds are broken and the heavy oil behaves liquid-like. This was seen by the increased values measured by the tension/compression results at low amplitudes in comparison to the much lower values measured by the rheometer at higher amplitudes. This indicates that two linear viscoelastic regimes can be present in the heavy oil at different amplitudes. The main two implications of this work are the fact that under confinement heavy oils properties change and the shear modulus and viscosity increases affecting not only the measurements but also the flow in porous media where heavy oils are confined between grains. On the other hand, the use of the rheometer for seismic or acoustic applications is limited as the higher strain of the rheometer can measure values that are lower than those measures with seismic techniques.