Encountering gas pay at a given depth frequently raises the question:
"Could there be an oil leg down-dip from this gas?"
If a gas is found to be entirely bacterial in origin, then the likelihood of a down-dip oil leg is small. However, if a gas is thermogenic in origin, or is of mixed thermogenic/biogenic origin, it may or may not have an associated oil leg.
One clue that there may be a down-dip oil leg is provided by the composition and quantity of condensate in a gas. Specifically, if a gas has a down-dip oil leg, then the gas must be saturated with components of that oil (i.e., the gas must be at dew point) at the reservoir temperature and pressure. However, finding that a gas is at dew point says nothing about the size of the underlying oil leg, and, in fact, the oil leg may be vanishingly small. Nevertheless, if a gas is found to have a thermogenic component, then determination of whether or not a gas is at dew point is an excellent first step in assessing the likelihood of a down-dip oil leg.
Calculation of Dew Point
The quantity of condensate that a gas can hold in the gas phase (at a given temperature and pressure) depends on both the gas composition and the condensate composition. The drier the gas (i.e., the higher the methane content, and the lower the ethane, propane and butane content) the less condensate the gas can hold at a given temperature and pressure. Similarly, the waxier the condensate (i.e., the more high-molecular-weight paraffins it contains), the lower the solubility of that condensate in a gas of a given composition. Therefore, when one is determining whether or not a gas is at dew point, what is really being determined is: "how much condensate of this particular composition can a gas of this particular composition hold in the gas phase at this particular reservoir temperature and pressure." If the calculated value is 20 bbls per MMcf, and the actual condensate yield of the gas is 17-23 bbls per MMcf, then the gas is probably at dew point. An alternative way of presenting the same information is to say "the particular gas/condensate mixture found in this zone at this temperature would be at dew point at what reservoir pressure?" The calculated pressure can then be compared with the actual reservoir pressure to assess whether or not the gas is at dew point.
If the pressure drawdown (i.e. the difference between the reservoir pressure and the pressure in the well bore) is large during production of a gas, then condensate may drop out of a gas down hole, causing the condensate yield of the produced gas (measured at the test separator) to appear lower than it actually is. Therefore, a gas may appear not to be at dew point simply because it lost some of its condensate down hole. As a result, the magnitude of pressure drawdown during the gas production should be recorded.
In summary, to calculate whether a gas is at dew point, the following input data are needed:
- Reservoir temperature
- Reservoir pressure
- Observed condensate yield of the discovered gas
- A compositional analysis of the gas
- A compositional analysis of the condensate
- Pressure drawdown during production of the gas
These data are put into an equation-of-state model to determine if the gas is at dew point.
The value of this approach becomes especially apparent when you are dealing with a dry gas that contains a waxy condensate. In such a situation, the gas may be at dew point when it contains only 5 bbls condensate per MMcf of gas. Without the dew point calculation, such a low condensate yield might lead an explorationist to assume erroneously that there could not be a down-dip oil leg, when actually there could be underlying oil or underlying retrograde condensate.
Appropriate gas sample collection methods depend on for which gas species the samples are being collected. Protocols for collection of samples for hydrocarbon gas analysis differ from the protocols for the collection of samples for helium analysis, which differ from the protocol for collection of samples for H2S analysis. Visit our Geochemical Sampling Procedures page for an overview of sampling procedures.
In addition to the techniques described above, other complementary approaches can also be applied to further assess the likelihood of a given gas having an associated oil leg.
For more information on the techniques described here, or to discuss a specific project, e-mail us at firstname.lastname@example.org, or
call us at U.S. (214) 584-9169.