CMG PVT table

[RavenKrofft]

Hello!

I'm new to this forum and I've been reading interesting posts for over two hours now

I happen to be fairly new to reservoir simulation (CMG specifically) and I have ran into a problem while defining the PVT properties of a regular black oil fluid sample. Typical curves will have Rs be constant above the bubble point, and Bo decrease above the bubble point.

CMG thinks there is an error every time I try to keep Rs constant and/or have Bo decrease above bubble point. While I believe the problem has to do with oil compressibility, I am lost as to how to 'convert' my PVT table into the CMG 'language'.

I hope this makes sense. If anybody (surely most people here) has any idea of the actual cause of this problem, and how to overcome it, I'd really appreciate it if it could be explained to me.

Thanks.

[samadali20]

Hi RavenKrofft,

As you said,that the issue you are facing is probably with oil compressibility,It would be better,If you can keep a check on the oil compressibility,as to whether it yields a negative value during simulation?

You can calculate the compressibility of saturated oil by this calculation.

Co = [(-1/Bo)*(DelBo/DelP] + [(1/5.615)*(Bg/Bo)*(delRs/DelP)]

where P is the pressure,Bo is FVF,Bg is Gas FVF,Rs is GOR solution


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I wanted to know what does the error message says when you enter the Rs and Bo tables against pressure??

There could be possible solutions for your problem:

1. If you want a constant Rs(that is Dead Oil),then you do not need to define a Gas phase in the Simulation,as that would be inactive.For CMG(IMEX),you then need to use the *MODEL*OILWATER option.

2. If the error comes out to be negative compressibility,which actually means that the gas redissolves in oil,then you need to use some keyword that sets the change of increase in Rs to a value(u can put it to be zero,for No resolutio).Unfortunately,I do not know the option to do this in IMEX,but in ECLIPSE people use the DRSDT keyword.

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I hope this helps .But I am still confused about your problem,It would be better ,If you could explain the issues and specify the error .

Regards

Samad

[RavenKrofft]

Hello Samad,

Thanks for your reply! I will try to explain my problem in detail now.

I have oil PVT properties that are similar to any typical black oil values. They look like the attached images (Rs and Bo)

[link Point to another website Only the registered members can access]
As you can see, they are just your typical Reservoir Engineering I curves. The problem is that when I put these values into CMG, CMG gives me the following error:

"Values of Rs in PVT Table (PVT Region 1) should be monotonically increasing.
Values of Bo in PVT Table (PVT Region 1) should be increasing."

This is the first time I have been told that Rs and Bo should be 'increasing' after bubble point pressure! In other words, CMG expects PVT tables to look linear all the way through. But as you can see in the attached diagrams, they are not and should not be increasing after bubble point occurs.

I have been reading the CMG manual, and they mention the same thing that you said about compressibility. But, they don't mention why or how this affects simulation.

I have changed my PVT tables to look like straight lines before and after bubble point, and I can run the simulation with no problems. But I just don't understand how it is possible that Rs and Bo keep on increasing after the bubble point pressure.

Thanks for your time!

[samadali20]

Hi RavenKrofft,

Thanks for your explanation.

I believe,the issue is with the manner you are inputting your Rs and Bo values.And Obviously,you are right ,that the Rs values should be constant above the Bubble point pressure(Its as correct,as the Sun rises in the east...)

----------------------------------------------------------------------------------------------


Now,say for exmaple we have a data table,that we want to enter in a simulator:

==============TABLE-1================================================= ====================

RS(GOR) Pressure(B.p) FVF(Bo) Viscosity(Mu)
0 14.7 1.0725 0.909
0.1291 500 1.1616 0.3362
0.2636 1000 1.238 0.2914
0.5569 2000 1.3966 0.2217
0.9212 3000 1.5875 0.17

The Simulator actually takes the values of Rs,Bo and Viscosity at one single bubble point pressure(as in the table,above),and also requires values of Bo and Viscosity at that Rs and pressures above bubble point(undersaturated oil).

For Example,The simulator will ask for minimum one set of values above Bubble point pressure for the constant Rs,that is consider that Rs is 0.1291 in the second row at abubble point pressure of 500,then the simulator aska a value of Bo and viscosity at pressures above 500 for the same Rs(0.1291)---------------and that would make your table look a bit like this:

==============TABLE-2================================================= ====================

RS(GOR) Pressure(B.p) FVF(Bo) Viscosity(Mu)
0 14.7 1.0725 0.909
----------------------------------------------------------------
0.1291 500 1.1616 0.3362
0.1291 1000 1.1517 0.3601 ''''''Undesaturated Values for Rs = 0.1291'''''''''''''
0.1291 2000 1.135 0.4063
0.1291 3000 1.1215 0.4507
----------------------------------------------------------------
0.2636 1000 1.238 0.2914
0.5569 2000 1.3966 0.2217
0.9212 3000 1.5875 0.17


And,You have to do this for all Rs values defined with a Bubble Point pressure.After you have done that,your table would look something like this:

==============TABLE-3================================================= ====================

RS(GOR) Pressure(B.p) FVF(Bo) Viscosity(Mu)
----------------------------------------------------------------
0 14.7 1.0725 0.909
0 500 1.07 0.9324 ''''''Undesaturated Values for Rs = 0.0'''''''''''''
0 1000 1.0676 0.9559
0 2000 1.0633 1.0005
0 3000 1.0594 1.0426
0 4000 1.0559 1.0822
---------------------------------------------------------------
0.1291 500 1.1616 0.3362
0.1291 1000 1.1517 0.3601 ''''''Undesaturated Values for Rs = 0.1291'''''''''''''
0.1291 2000 1.135 0.4063
0.1291 3000 1.1215 0.4507
0.1291 4000 1.1101 0.4933
----------------------------------------------------------------
0.2636 1000 1.238 0.2914
0.2636 2000 1.2159 0.334 ''''''Undesaturated Values for Rs = 0.2636''''''''''''''
0.2636 3000 1.1984 0.375
0.2636 4000 1.184 0.4147
----------------------------------------------------------------
0.5569 2000 1.3966 0.2217
0.5569 3000 1.3674 0.255 ''''''Undesaturated Values for Rs = 0.5569'''''''''''''
0.5569 4000 1.3444 0.2875
-----------------------------------------------------------------
0.9212 3000 1.5875 0.17
0.9212 4000 1.5496 0.1952 ''''''Undesaturated Values for Rs = 0.9212'''''''''''''

================================================== ==============================

Now, If you see in Table-1,the values of Bo are increasing down the table which is actually what is required,and also the values of Rs are monotonically increasing.

The way data is entered in the simulator,which is actually Table-3,accounts for the variation of Bo and Viscosity for the undersaturated oil for a constant Rs. I have attcahed a plot of the data in Tabl1-1 and Table-3 for your asistance,Please have a look at it.I hope that eases your peoblem a bit.TAble-3 is the manner in which you enter the values in ECLIPSE simulator

[link Point to another website Only the registered members can access]

================================================== =============================

Now,as you are using CMG(IMEX),the way you can enter your PVT data:

Suppose,you have the same data as Table-1 and Table-3,then in IMEX,you have to use,the following Table-4:

------------------------------------------TABLE-4-------------------------------------------------
*PVT*EG
** RS(GOR) Pressure(B.p) FVF(Bo) GEF(Eg) Viscosity(Mu)
0 14.7 1.0725 1.000 0.909
0.1291 500 1.1616 5.000 0.3362 """"" GEF(Eg) is the Gas Expansion factor,which is Reciprocal of Bg""
0.2636 1000 1.238 50.000 0.2914
0.5569 2000 1.3966 100.000 0.2217
0.9212 3000 1.5875 200.000 0.17
-------------------------------------------------------------------------------------------

After you have entered this table,then you need to define your values of Bo for underasaturated oil(that is above the bubble point,In IMEX you can do that by using the keyword *BOT

the Table will look like this then:

====================TABLE-5========================
-----------------------------------------------------
*BOT
**p Bo
14.7 1.0725
500 1.07
1000 1.0676
2000 1.0633
3000 1.0594
4000 1.0559
--------------------
*BOT
**p Bo
500 1.1616
1000 1.1517
2000 1.135
3000 1.1215
4000 1.1101
--------------------
*BOT
**p Bo
1000 1.238
2000 1.2159
3000 1.1984
4000 1.184
-------------------
*BOT
**p Bo
2000 1.3966
3000 1.3674
4000 1.3444
-------------------
*BOT
**p Bo
3000 1.5875
4000 1.5496
------------------------------------------------------------------------------
================================================== ====

Now for Viscosity ,above the bubble point pressure you have to use the Keyword *VOT,and in the same way as the used for Bo,


and now you will have three tables,Table-4,table-5 and then table for viscosity that you need to input in the IMEX(CMG) simulator,to get you going........


I hope this helps,

If it doesn't,please put a reply,I will try to explain again.

Thanks for sharing an easy (yet really really difficult to sort out) problem.

Regards
Samad


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[Fernanda Reis]

Hello,

I´ve just started to learn about IMEX and I had the same question.
But I don´t know that much about reservoirs and my english skills aren´t very good as well.
If possible can you explain a little bit more, something like "for dummies" please.

Thank you very much!

[DAH7542]

This is my read of this situation: there is not any problem with IMEX or any other simulator when they warn you for "monotonically increasing values of Rs and Bo" even above your expected or measured bubble point pressure (Pb). It's completely right that for pressure above Pb, the dissolved gas ratio (Rs) and Bo are constant and decreasing respectively. However, as in ECLIPSE, IMEX input formats for PVT tables use the so-called "variable bubble point pressure option". This format requires that the saturated and undersaturated oil PVT data to be input separately with both tables covering a wide range of PVT data for different (i.e. variable) bubble point pressures to ensure that the fluid properties, and the physics within the reservoir, are properly represented when, for example, re-solubilization (by re-pressurization) effects occur after the reservoir pressure have dropped below Pb, or any other phenomenon when the bubble point pressure varies. I encourage you to read the theory on this "variable bubble point pressure option" in the Eclipse Technical Manual, and then read the IMEX user manual to fully understand how the PVT data must be input for saturated and undersaturated PVT data.

[fered]

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