“On any given day, when you look at it over the life of the well,” said AAPG member Steve Jones, “Gas/oil ratio behavior is different in unconventional reservoirs than in conventional reservoirs.”
To fully understand, then, the meaning of GOR, which is the ratio of the volume of gas that comes out of solution to the volume of oil at standard conditions, especially as it relates to unconventional wells, will require a new understanding.
Jones, a reservoir engineering adviser for Cimarex Energy Co., in Tulsa, spoke recently on the topic at this year’s AAPG Global Super Basins 2019: The Permian.
The past is not necessarily prologue.
In a program entitled, “Making Sense of Producing Gas-Oil Ratio in Unconventional Oil Reservoirs,” he discussed the unique performance characteristics in unconventional reservoirs, and how GOR models, which are predictive in conventional wells, may or may not be accurate in deterring the success or failure of the unconventional.
One thing is certain: relying on GOR models from conventional wells is not, according to Jones, the way to go about it.
“People have struggled to understand how and why GOR behaves the way it does,” he said, and “if you don’t understand the difference, you can get wrong ideas about whether your oil is ‘good or bad.’”
An example: in some unconventional reservoirs, the GOR will quickly go up as probable pressure is decreased, leading to a false negative, if you will, which might mistakenly be read as a small reservoir. The problem is that in a conventional reservoir, the GOR is a pretty good predictor of such outcomes.
“But not in an unconventional reservoir where we’ve drilled a horizontal well,” said Jones.
In fact, he said, it’s normal in an unconventional well for the GOR to jump up shortly after production is started. In and of itself, though, such a reading is not indicative of the future performance of the reservoir. Making matters even more complicated, while unconventional wells behave differently from convention wells, especially as it relates to GOR readings, unconventional wells don’t all act the same.
Jones sees a clear reason for the disparity between conventional and unconventional.
“The reason I say that is because the conventional reservoirs are usually developed with vertical wells and the frac’s are not as big as they are in horizontal wells, so the flow characteristics are different,” he said.
When you drill a vertical well, the oil is going to flow radially – it makes a circle around the well – and that causes the GOR to mainly depend on average reservoir pressure. In tighter reservoirs, those wells drilled horizontally, it may take months or years for the pressure wave to get out even 100 feet.
Jones mentioned that in some unconventional wells, like the Permian formations, the GOR may not go up for a year – and that’s true in part of the Bakken, as well.
Predicting Oil Recovery
Jones, a registered professional engineer, said the concern, obviously, regardless of the type of well drilled, is how much of the oil will be recovered and to what extent the GOR can predict that.
In conventional wells, it’s not such a mystery.
The GOR in unconventionals does not act like a crystal ball in terms of future output, owning to its unique overall behavior
Determining what that paradigm is presents a challenge.
Further, when multiple horizontal wells are drilled close to each other during development, the long-term GOR behavior can be different from the original parent well that was drilled to hold the lease. This difference in long-term GOR behavior means the child wells have been drilled close enough to affect each other, which helps determine what the optimal well spacing should be.
“This is due to the parent well being ‘unbounded’ (i.e. its drainage area is not limited by nearby wells) and the child wells being ‘bounded’ by the other wells close to it,” said Jones, adding, “It’s not that you won’t see the effect on oil rates and declines, but understanding this GOR behavior will give you another confirmation.”
Conclusions from GOR Readings
Jones has studied the issue over the years to determine what controls these behaviors and what conclusions can be drawn from the GOR readings at any given unconventional site. He is convinced the rise in the GOR should not be taken as proof that the reservoir in question is of limited reservoir or that oil production is going to crater.
“That’s not the way it works,” he said.
Ironically, Jones said those who have only worked on unconventionals may have less of a learning curve because, without the preconceived notions of the relationship between GOR and reservoir production in conventional wells, these new scientists will not fixate on GOR ratios because they’ve seen its limited use in predicting future output.
This business with GOR and the unconventional wells, Jones has said in the past, “is nothing they taught us in school.”
So what is the new paradigm for the GOR in unconventional wells?
There are analogs one can look at, but Jones reiterated that GOR behavior has a lot to do with flow and fluid storage in rock properties.
“And we won’t know about these,” he said, “until you drill the well and produce it for several months.”