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Reservoir Conditions and Stock Tank Conditions in Oil and Gas Exploration

## Reservoir Conditions and Hydrocarbon Phase Behavior

Introducing reservoir conditions and stock tank conditions Thus far we have considered the domain of temperature T and pressure p that defines the phase envelope for a given hydrocarbon mixture. There are however other sets of T and p values that are relevant to consider relative to the phase envelope. These are respectively the reservoir conditions (i.e. the conditions Tres and pres in the geological reservoir in which the oil and/or gas is found) and the stock tank conditions (Tstock and pstock i.e. the ambient conditions at the surface at which oil is stored). Where these reservoir or stock tank conditions are located relative to the phase envelope affects the phase in which the hydrocarbons are found respectively in the reservoir or stock tank, and this depends on the hydrocarbon mixture. Another way of saying this is that the type of hydrocarbon mixture affects the location of the phase

envelope relative to the reservoir and/or stock tank conditions. A heavy oil mixture (mostly long carbon chains, usually with a small proportion of shorter carbon chains mixed in) typically has a large critical temperature Tc (and consequently also a large cricondenbar temperature Tbar and a large cricondentherm temperature Ttherm). Almost certainly then Tres ? Ttherm. Heavy oils are not very volatile meaning that the bubble point pressure pbub (which is a function of temperature) when evaluated at Tres, almost certainly is less than the reservoir pressure pres. However the reservoir pressure pres (despite exceeding pbub) could still be less than the cricondenbar pressure pbar (since bubble point pressure pbub at reservoir conditions is necessarily less
than pbar).

1. Sketch a phase envelope (based on the ones already sketched in previous tutorials) and show, in the case of a heavy oil, where the reservoir conditions are located relative to the phase envelope.
i. In what phase is the hydrocarbon mixture within the reserv oir?
Stock tank conditions are typically at a temperature Tstock lower than the reservoir temperature Tres, and at a pressure pstock that is much less than pres. As a result, pstock tends to be less than pbub evaluated at Tstock.

2. On your phase envelope sketched above, indicate where the stock tank conditions are relative to the phase envelope in the case of heavy oil.
i. What physical process must occur as the hydrocarbon mixtu re is brought from reservoir condi?tions to stock tank conditions?
ii. What does this then imply for the volume of liquid oil collected in a stock tank, compared to the volume of liquid oil present originally in the reservoir?

## Stock Tank Conditions and Hydrocarbon Volume

Now consider the case of a light oil. This has shorter hydrocarbon chains than heavy oil. Lighter oils tend to be more volatile than heavy oils: one consequence of this is that their critical temperature Tc tends to be smaller. In this case, even though reservoir temerature Tres does tend to be smaller than Tc (and likewise smaller than cricondenbar temperature Tbar and cricondentherm temperature Ttherm), typically Tres might not be much smaller than Tbar, just slightly smaller.

Also because the light oil tends to be quite volatile, the pressure in the reservoir pres will only be very slightly above the bubble point pbub evaluated at temperature Tres. However since pbub at reservoir conditions will necessarily be less than criconcendar pressure pbar, this means pres will also likely be less than pbar.
iii. Sketch a phase envelope for light oil, and show where the reservoir conditions are with respect to the phase envelope.
Stock tank conditions tend (as mentioned earlier) to have temperature Tstock lower than Tres and pressure pstock much lower than pres, remembering also that for light oil pres itself is only very slightly above pbub.

4. Again on your sketched phase envelope indicate where stock tank conditions lie for light oil.
i. Again what physical process must occur as the (light oil) h ydrocarbon mixture is brought from reservoir conditions to stock tank conditions?
ii. What does this then imply for the volume of liquid oil collected in a stock tank, compared to the volume of liquid oil present originally in the reservoir?

So far we have considered only oil reservoirs, respectively for heavy and light oil.
Gas reservoirs are another matter. These have very short chain hydrocarbons. Compared to the case of oil reservoirs, critical temperature Tc tends to be very small in gas reservoirs. Reservoir tem?perature Tres therefore tends to be larger than Tc and likewise larger than cricondenbar temperature Tbar and cricondenbar Ttherm. However reservoir pressure pres would often still be less than critical pressure pc.

There are two types of gas reservoir: dry gas and wet gas. In the case of dry gas, reservoir temperature Tres tends to be far above cricondentherm temperature Ttherm. This means that stock tank temperature Tstock (which is always less than Tres) can still be greater than Ttherm. Meanwhile stock tank pressure pstock is less than reservoir pressure.
5. Sketch a phase envelope for a dry gas, and indicate on the s ketch where reservoir conditions and stock tank conditions are relative to the phase envelope for a dry gas.