Summary
In compositional simulation of gas-injection processes, it is often observed
that gridblock oil saturations decrease far beyond the user-defined residual
oil saturation, even under immiscible conditions. This numerical phenomenon
occurs because oil components are allowed to vaporize into the gas phase as
much as the phase equilibrium obtained with an equation of state (EOS) permits.
Especially in the vicinity of gas injectors, an oil saturation of zero is
sometimes predicted. On the other hand, such significant low oil saturation is
rarely seen in laboratory data such as coreflood experiments and slimtube
tests.
The reason for the discrepancy between the simulation results and the
laboratory results described above is that bypassed oil located in dead-end
pores or caused by subgrid-scale heterogeneities is not considered in the
current compositional-simulation practice. To overcome this, we developed an
innovative method of incorporating laboratory-based residual oil saturations.
The proposed method can restrict the excessive vaporization and maintain the
prescribed residual oil by accommodating a novel application of the transport
coefficient (Barker and Fayers 1994).
Introduction
Incorporation of the “true” residual oil saturation into the gasflood
compositional simulation has been a critical problem in the industry. On the
other hand, most compositional simulators allow oil saturations to be as low as
an EOS predicts, but no rigorous method has been proposed to honor laboratory
observations usually indicating nonzero residual oil saturations. This is why
immiscible gas injection is predicted to achieve a good recovery factor despite
the fact that even miscible coreflood experiment results rarely show 100%
recovery.
Not only laboratory experiments but also field observations indicate that
bypassed oil occurs even after the miscible injectant passes through above the
minimum miscibility pressure (MMP), as stated by Stalkup (1983) and McGuire et
al. (1995). Dead-end pore volume and precipitation can cause such bypassed oil
under gas injection with no prior waterflood history. Although the mass
transfer in microscopic scale like molecular diffusion can partially recover
such bypassed oil, as described by Burger et al. (1996), there still remains
the oil behind the miscible front.
For this reason, even under the miscible-flooding condition, the residual
oil saturation will not reach 0% in the real heterogeneous reservoir. This
residual oil is referred to as miscible flood residual oil saturation (Spence
and Watkins 1980). In the conventional compositional simulation, there is no
facility to actively define “true” residual oil (nonvaporizing oil) and, hence,
the excessive vaporization of oil components into the gas phase is predicted.
Consequently, the miscible flood residual oil saturation could not be
represented in the simulation model.This frequently has caused optimistic
results in which all the oil in the gridblock can be stripped by the injected
gas.
The first attempt to restrict the excessive vaporization described above was
made by Fayers et al. (1992), who proposed the concept of dual-zone mixing. In
this concept, the phase equilibrium is established between the hydrocarbon
trapped in dead-end space (uncontacted oil) and that in the permeable portion
of a simulation block. Analogous to a dual-porosity model, uncontacted oil is
allowed to disperse only when it is vaporized into the contacted oil or gas.
The concept of the transport coefficient (Barker and Fayers 1994) was adopted
to incorporate the component dependence in mass transfer. However, the work of
Fayers et al. (1992) has not been used widely, primarily because of the
complexity of the theory and, hence, special coding was required to accommodate
the dual-zone mixing.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
7 December 2004
- Meeting paper published:
10 October 2004
- Revised manuscript received:
30 May 2006
- Manuscript approved:
10 October 2006
- Version of record:
20 February 2007
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