(756d) Effects of Reservoir Temperature and Percent Levels of Methane and Ethane on CO2/Oil MMP Values As Determined Using Vanishing Interfacial Tension/Capillary Rise

Geological CO₂ storage
combined with enhanced oil recovery (EOR) can be an economically-viable
approach to storing CO₂ while gaining the benefit of increasing oil
recoveries from a reservoir which has undergone primary (and secondary)
production.  CO₂ is widely
understood to mobilize otherwise immobilized oil via processes involving
several mechanisms including hydrocarbon solvation into the CO₂
phase, physical displacement, oil swelling, lowered oil viscosity, and the
formation of a separate "miscible" CO₂/hydrocarbon mixed
phase as the CO₂ contacts lighter hydrocarbons. This mixed phase is
characterized to form at the minimum miscibility pressure (MMP). MMP is a major
input variable for models used to optimize EOR efficiencies. Long-established methods
to determine MMP of crude oil (e.g., slim tube, rising bubble) can be costly,
slow, and subject to operational variations. In contrast, newer vanishing
interfacial tension (VIT) methods rely on a more fundamental definition of
miscibility, i.e., the conditions at which there is no interfacial tension
(IFT) between the two fluids. IFT is measured at different pressures by
observing the height of oil in a capillary tube (with accurately known internal
diameter) and the density of both fluids at each experimental pressure, a
requirement that greatly increases the cost and complexity of the
instrumentation and operation [1].

Fortunately, it is not necessary to
measure IFT at several pressures to determine when the IFT goes to zero since a
plot of the oil height in the capillary tube versus pressure will intercept the
pressure axis at the same value as the IFT data [2]. That is, both zero IFT and
zero capillary rise height occur at the same pressure. Using the simple view
cell apparatus (and a suitable pump and temperature bath) shown in Figure 1, this
approach yields linear plots, and no density measurements are needed, which
greatly reduces instrument cost and experimental complexity. With this
simplified approach, MMP values can be rapidly and cost-effectively determined allowing
the effects of multiple parameters on MMP to be investigated including (but not
limited to) the effects of various reservoir conditions such as temperature,
changing gas composition (e.g., the effect of methane in recycle CO₂),
and changes in crude oil composition on MMP. For example, methane mixed with CO₂
increases MMP, while the presence of ethane in CO₂ lowers the
MMP.  Similarly, the MMP of a crude oil
increases greatly with increasing reservoir temperature.

These tests show that dramatic oil/CO₂
interactions occur both below and above MMP, and led us to focus on such
processes by visually observing the behavior of crude oil/CO₂
mixtures at reservoir conditions as the CO₂ pressure is raised from
below to above MMP, followed by a controlled depressurization to observe the
oil behavior as CO₂ pressure is reduced (e.g., as could occur between
the injection and production wells).

High-pressure view cell videos of CO₂/oil
interactions show that achieving MMP is not a "magic" condition
required for oil mobilization, and that significant mobilization is occurring
at pressures below the crude's MMP. In addition, when the pressure is slowly
lowered to mimic the drop in pressure from an injection to production well, as
much as 1/2 of the mobilized oil is lost from the CO₂-mobilized
phase, even at pressures well above MMP.
For example, when crude from a reservoir that is currently undergoing a CO₂
flood is exposed to 5000 psi CO₂, ca. 2/3 of the hydrocarbon is
suspended in the overlying (upper) CO₂ "miscible" phase. However,
lowering the pressure to 4000 psi causes much of the mobilized oil to
precipitate into the bulk crude, even
though the pressure is still higher than the 2800 psi MMP of that oil. Methods
have also been developed to sample and analyze the hydrocarbon composition in
the upper "miscible" CO₂/oil phase, and have shown
dramatic changes both in the mass of mobilized oil, as well as the carbon
number distribution of the oil in the "miscible" phase. For example,
the mass of oil hydrocarbons from a conventional crude oil at 1500 psi (MMP is
1450 psi) was only ca. 1/2 that mobilized at the expected CO₂
injection pressure of 2300 psi, and the lower pressure showed a much higher
fraction of lower-carbon-number hydrocarbons. Similarly, when the pressure was
dropped from the 2300 psi injection pressure to just above MMP, substantial
losses of oil occurred from the "miscible" phase with the higher
molecular weight hydrocarbons showing the most deposition.

Experiments are continuing to
qualitatively and quantitatively describe CO₂/oil mobilization
processes that occur under different reservoir scenarios, including the effects
of changing gas composition on MMP, as well as video investigations on including
the CO₂ mobilization of residual oil following a water flood. The
MMP method and associated effects of changing reservoir conditions will be
presented, and videos of CO₂/oil interactions at reservoir
conditions and their interpretations will be discussed

REFERENCES

[1]        S.C.
Ayirala and D.N. Rao, J. of Canadian Petroleum Technology, 50, 2011, pp. 71-81 (SPE 99606-PA).

[2]        EERC
patent pending.

ACKNOWLEDGEMENTS

This
material is based on work supported by the U.S. Department of Energy National
Energy Technology Laboratory under Award No. DE-FC26-05NT42592.