PVT and Flow course - Gas or Oil Reservoir?

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Whitson Academy is probably one of the most useful YouTube channels for petroleum engineers.

The 2016 PVT and Flow Course is a great series of lectures. And here I'll summarize the main ideas from the videos.

Gas or Oil Reservoir? Part 1

Sometimes it is really hard to tell if it is a gas or an oil reservoir.

What's in common between a Gas and an Oil reservoir?

• mobile hydrocarbon
• pores in a rock
• connate water

What determines a gas or oil reservoir?

• Fluid composition:
  • type of the component (methane, ethane, propane, etc., + CO₂, H₂0, H₂S, N₂, etc)
  • amount of the component
• Reservoir pressure
• Reservoir temerature

With the fluid composition we need to know:

• Saturation pressure at reservoir temperature
• Type of Saturation pressure (bubbple point or dew point)

It is an oil reservoir if the reservoir pressure is above (or equal to) the bubble point.
It is an gas reservoir if the reservoir pressure is above (or equal to) the dew point or reservoir temperature is above (or equal to) cricondentherm.
It is both oil and gas reservoir if the reservoir pressure is below the bubble point/dew point.

Gas or Oil Reservoir? Part 2

One more thing that determines the hydrocarbon state in the reservoir

The state of the reservoir system may be:

• Equilibrium - the state of balance between gravity and all other forces.
• Non-equilibrium - the state where gravity has not yet reached equilibrium.

Gravitational segregation of individual components leads to a variation of the composition with depth (simply put, methane is lighter than benzene, so the lighter components will tend to segregate upward due to gravitational effects, and the heavier components will tend to gravitate downward because they're heavier). If the system reaches some kind of an equilibrium where gravity and chemical forces are all in equilibrium then you can get the variation with depth of composition that actually puts gas on the top and oil in the bottom.

If the reservoir system hasn't come to equilibrium, it might turn into a gas reservoir or into an oil reservoir. But today the reservoir is in a state of non equilibrium and the state of non equilibrium says that today there's gas and oil today even though all the laws of physics would say that it's going to be only an oil or all near gas. It takes geologic time to bring the system to a true equilibrium.

And there are examples around the world of their either being gas or oil because of the non-equilibrium, where if you left it alone forever it would turn into just a gas or just an oil you also have the opposite case where you have only what looks like a gas but if you wait long enough it might split into a gas and an oil.

Why Reservoir Gas vs. Reservoir Oil might be important for field development

Viscosity

• Gas viscosity ~ 0.01 - 0.1 cP
• Oil viscosity ~ 0.1 - 10⁶ cP

The higher the viscosity, the higher the pressure drop, that's both in the reservoir and the pipe. The higher the pressure drop the more cost in general, the more expensive it is to develop a reservoir.
The major cost items related to viscosity is the number of wells. How many wells you need to drain the reservoir? If you've got 1 centipoise, you might need fifty wells if you got 0.02 centipoise, it's a gas, you might need five wells to empty the reservoir in 25 years. The diameter of the pipes also depends on the viscosity.

Density

• Gas density ~ 1 - 800 kg/m³ at standard conditions.
• Oil density ~ 300 - 1000 kg/m³ at standard conditions.

The higher the density in general the more expensive to develop the field. You have to get the stuff from the bottom of the well to the top of the well and the more dense it is, more likely that you're going to have artificial lift to help get it out of the reservoir to the surface.

Both viscosity and density can have a big role in how you develop the field.

Gas expansion vs. Oil shrinkage

When it goes from the reservoir to the surface, reservoir oil will always shrink. You start with one barrel of reservoir oil and what you get as the sellable oil will be less than that one barrel. Typically it'll be anywhere from five percent less to a factor of three less. Gas, on the other hand, expands.

• Oil shrinkage ~ 0.95 - 0.3
• Gas expansion ~ 50 - 500

Condensation

The reservoir gas comes to the surface, it goes to a lower pressure or lower temperature and you get some liquid out. And those molecules sold as a liquid at the surface actually have more value than if they're sold just as part of the gas.

Property question

Sometimes what you call the reservoir a gas or an oil has a big consequence on who owns what (for example agreements in the Middle East). And this has a big effect on value to the state versus to the the companies.

Gas or Oil Reservoir? Part 3

The reservoir composition is a very important to how the reservoir is developed, how many wells are needed, whether you're going to do gas injection, etc. A lot of things are affected by the so called PVT behavior, phase behavior of the of the reservoir fluids both when they're flowing in the reservoir as they flow into the production tubing to the surface and certainly when they're being processed at the surface and they're creating sellable products. And everywhere in the system the local composition is important to determine how much of each phase (gas or oil), density of each phase and viscosity of each phase. The density and viscosity are always needed for engineering calculations of transport in the Rock and in the pipe. Viscosity and density are the sacred properties to do really any kind of engineering calculation. They're dependent on the composition of the oil and gas, how much gas and how much oil you have.

Hydrocarbon components are described by the number of carbons: C1, C2, C3, C4, C5, C6. Then they stop separating them into carbon numbers and they just say: this is what's left, the heavy stuff. It depends a little bit lab to lab and also historically, so before 1980 lab reports only gave C7+, they didn't separate C8. And then in the 80s Norway was one of the leading reasons why this went up to C10+, maybe C12+. And now pretty much you get C36+ as a standard the heaviest component in the analysis.
C6, C7, C8, C9, etc - these are all really mixtures of several hydrocarbons. And there are some other mixtures:

• waxes - typically this the alkane compounds (C20+) that precipitate out of the oil because of lower temperature. It's not going to precipitate in the reservoir but it'll precipitate in the production tubing maybe, certainly in the process facilities where the temperatures reduce. Wax can precipitate from both oils and gases.
• asphaltenes are often huge mixture of really large compounds that can have molecular weights that are greater than a thousand. Asphaltenes will precipitate from oils as the pressure approaches the bubble point and below. Gas lift can cause asphaltenes precipitating. Mainly asphaltenes precipitation a function of the pressure approaching the bubble point and basically the composition of the oil changing because of this adding the gas to some other composition.

Symbols used for compositions

Molar compositions:

• z_i - number moles of the component i over, the total moles in the entire mixture
• y_i - number of moles i in the gas phase over the total moles in the gas phase
• x_i - number of moles i in the oil phase over the total moles in the oil phase

Mass composition:

• w_i - the mass of component i in the total mixture over the total mass

Molecular weight:

• Ḿ - average molecular weight of the mixture
• M_i - molecular weight of the i component

zi  = (Ḿ/Mi) * wi 

wi =  zi  * (Mi/Ḿ)

Molecular weight (M_i) of hydrocarbons are known up to about C5. Molecular weights of C6 + has an uncertainty.

Gas or Oil? Rules of Thumb

General statement: If the reservoir temperature is above the critical temperature, you have a gas reservoir, otherwise it is oil. The lower the content of C1 ("solution gas"), the further the reservoir temperature is from the critical temperature (critical temperature is too high).

Reservoir characteristics (@ TR, PR)	Oil reservoir	Gas Reservoir
Viscosity	≥ 0.3 cP	≤ 0.05 cP
Density	≥ 400 kg/m3	≤ 250 kg/m3
Saturation pressure type	Bubble point @ TR	Dew point @ TR or no Dew point

Surface (Processed) Characteristics	Oil reservoir	Gas Reservoir
GOR = 1/OGR	≤2500 scf/STB	≥3000 scf/STB
OGR = 1/GOR	≥400 STB/MMscf	≤350 STB/MMscf
API gravity	≤45	>50

See also

Curtis Whitson Petroleum Engineering Videos

Other lectures from the PVT and Flow course

• Blog:PVT and Flow course - Gas or Oil Reservoir?
• Blog:PVT and Flow course - Single Component Vapor Pressure.
• Blog:PVT and Flow course - Two-Component Phase Behavior
• Blog:PVT and Flow course - Multi-Component Phase Diagrams
• Blog:PVT and Flow course - K Values
• Blog:PVT and Flow course - Flash Calculations
• Blog:PVT and Flow_course - Surface Separation Processing
• Blog:PVT and Flow course - Sampling
• Blog:PVT and Flow course - PVT Lab Tests
• Blog:PVT and Flow course - OBM Decontamination
• Blog:PVT and Flow course - Lab PVT Tests CCE
• Blog:PVT and Flow course - LAB PVT Tests Multistage SEP
• Blog:PVT and Flow course - Lab PVT Tests DLE
• Blog:PVT and Flow course - Lab PVT Tests CVD

Class notes developed during lectures are available as PDF files, named with the format yyyymmdd.pdf located on: http://www.ipt.ntnu.no/~curtis/courses/PVT-Flow/2016-TPG4145/ClassNotes/

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