Reservoir Pressure Calculation

[ahmedj9579]

Good morning Pet people

i have one question:
in order to estimate reservoir pressure from ESP downhole gauge sensor @ certain depth or from static fluid level, we need specific gravity "SG" of oil, water and gas.
this SG was usually calculated from static gradient survey. But every time we get a different value of this SG from static gradient to an other or comparing to PVT sampling.
After many years producing and in order to get the most appropriate reservoir pressure, should we use value from PVT or from last calculated value from static gradient survey?
please i need your help!
regards

[ahmedj9579]

pleas i need your help!
thanks

[kochichiro]

To properly measure static pressure gradient, you need to follow specific order.

Fist of all, before measuring static pressure gradient your well should be in shut-in at least for one or two days.

During measurements you need to record pressure at well head with same recorder that used for pressure recording in wellbore.

Final point of pressure measurement should lie at the maximum possible depth that can be reached in your well. Having some pressure steps with equally spaced depth intervals would be nice.

After that, averaging pressure gradient you can calculate average specific gravity of wellbore fluid.

You need to measure static pressure gradient each time you want to calculate bottom-hole pressure, because it is dynamic parameter which depends on wellbore water content, amount of free gas in wellbore, etc. You can easily perform it at the end of build-up during well test.

Unfortunately, PVT experiment is idealized representation of reservoir and wellbore processes. Therefore, static pressure measurements should be preferred over PVT data during bottom-hole pressure calculation.

[kochichiro]

To properly measure static pressure gradient, you need to follow specific order.

Fist of all, before measuring static pressure gradient your well should be in shut-in at least for one or two days.

During measurements you need to record pressure at well head with same recorder that used for pressure recording in wellbore.

Final point of pressure measurement should lie at the maximum possible depth that can be reached in your well. Having some pressure steps with equally spaced depth intervals would be nice.

After that, averaging pressure gradient you can calculate average specific gravity of wellbore fluid.

You need to measure static pressure gradient each time you want to calculate bottom-hole pressure, because it is dynamic parameter which depends on wellbore water content, amount of free gas in wellbore, etc. You can easily perform it at the end of build-up during well test.

Unfortunately, PVT experiment is idealized representation of reservoir and wellbore processes. Therefore, static pressure measurements should be preferred over PVT data during bottom-hole pressure calculation.

[ahmedj9579]

thank you for these information.
if i understand you well, you mean we need to rely on specific gravity extarcted from static pression gradient survey else from PVT data in order to calculate bottom hole pressure? right?

[kochichiro]

Yes, you understand me right.

[ahmedj9579]

thank you very much.
but SG calculated from pressure survey are every time changing???? is there any explanation for this?
regards

[kochichiro]

It is changing during production, i.e. from month to month or even from year to year? Or it is changing even during well test?

When it is changing during production it is normal process because all reservoir parameters are dynamic ones. During prolonged production from reservoir more water coming from aquifer, or more gas come out of solution, so the wellbore fluid gravity is changing from year to year or even from month to month.

When it is changing during well test it showing that you have multiphase flow in wellbore. On each flow sequence with different rate you would get different specific gravity, because of relative phase friction and phase redistribution. During build-up you would get static phase redistribution, i.e. lighter fluid (associated gas) at low depth, oil at medium depth and water at higher depth. That is why you should measure static gradient at the end of build-up and pressure recorder should be lowered at maximum possible depth to cover all density changes in wellbore (from lighter fluid through medium to heaviest one).

[ahmedj9579]

yes, it is changing during production. when we did a static gradient survey, each time i find different value.
so i think it is better to use the last calculated value of SG else than from PVT???

[kochichiro]

Yes, as I said before you should use SG value from pressure survey.

[Petrorafo]

Well, as a contribution I'd like to post this video where we can see the importance of an Downhole Shut-in tool in the measure of reservoir pressure