Abstract
The phase behavior in liquids rich shale reservoirs continues to be an area of investigation in the industry. In conventional reservoirs with permeabilities on the order of millidarcies, production data transition to bubble point occurs differently than what is observed in liquids rich unconventionals. Published findings over the past seven years cover a comprehensive view on expected gas to oil ratio (GOR) trends. With large developments in unconventionals, GOR trends are also used as a qualitative indicator to determine depletion, and there are industry efforts to develop predictive models as well.
In this review, the conventional system used to compare GOR trends with liquids rich shale reservoirs is a vertical well, black-oil, solution gas drive reservoir. The initial production will start at the solution GOR, and as the average reservoir pressure declines to less than the bubble point, then a gas saturation begins to form. GOR dips due to critical gas saturation, and once critical gas saturation is surpassed, free gas flows from the reservoir to the surface. Assuming an unfractured well, GOR is driven by the average reservoir pressure (Jones 2017). The liquids rich shale reservoir is assumed to be a black-oil reservoir, and the GOR is driven by flowing bottom hole pressure. In multifractured horizontal wells (MFHW) with matrix permeabilities on the order of microdarcies to nanodarcies, GOR’s will go through several stages:
Early GOR is constant at initial solution GOR, while flowing bottom hole pressure is above bubble point.
GOR rises when flowing bottom hole pressure goes below bubble point
Transient GOR plateaus, characteristic of linear flow
This review concludes that:
From observations and simulation results, bubble point suppression is observed in liquids-rich shale reservoirs. Bubble point suppression is caused by capillary pressure effects and mid-confined nanopores. However, acceleration to deplete the reservoir below bubble point is also observed through aggressive drawdown strategies in other liquids rich unconventional plays.
Factors that affect GOR trends: Flowing bottom hole pressure (drawdown strategies), PVT property suppression, pressure-dependent permeability and changes of critical gas saturation, fracture geometry (planar versus network), pore-size distributions, and gas-oil relative permeability curves.
Despite differences in GOR trends between unconventional and conventional reservoirs, the industry will continue to use modified hyperbolic, as this still matches the empirical data well.
This review will also cover PVT considerations when sampling versus observing GOR trends and reservoir simulation efforts to date to best characterize GOR. Additionally, modeling techniques to have varied pore size distribution, capillary pressure, and defined matrix and enhanced permeability zones provide a comprehensive context on GOR trends in liquids rich shale reservoirs—there is continued work, but some physical effects are explained based on matches obtained from these findings. With abnormal trends of GOR’s, drainage area and pore sizes ought to be taken into consideration. More drainage means that nanopores from the shale matrix will deplete, increasing GOR’s.
