Deep into the night and I have nothing really to look forward to. It is turning out to be sleepless. Boring you tube videos cannot help me and I on the other hand am supposed to study and so will jump on to show you guys this post. Something reservoir engineering, is what I thought i would keep it as. Dont mind the grammatical errors in here. This isnt exactly an ideal time when you expect attentiveness.
So, drive mechanisms are the words for the day!
And as and when the formation of a secondary gas cap comes into picture, the material balance equation starts to dawdle. This mainly happens because of the term 'm'. There isnt much the equation can do about it you see. It has been pre-decided as the gas in place to the oil in place ratio. put straight into the equation. One reason why the MBE is so simple for most volumetric reservoirs is that the term 'm', takes with itself any of the pain that might try poke its nose into your estimation. But when your reservoir starts to roll, there aint gonna be much concerning for the simplicity of calculations. There the constancy of 'm' is put to test and as the more perceptive of you might have judged by now, it sort of collapses.
So, what do we do?!
Do away with the 'm'?
Hell no.!
That'll be a nightmare! that little thing has saved us pains all through when the reservoir was static and wasn't churning out black gold. We cant just let it go cause some bloke in a shirt says it wont work !
So we resorted to what was closest to 'm' all the time. Just round the corner, we have the good old GOR (gas oil ratio for the more dumb among you) more aptly represented by 'R'. But this isn't the solution really
R only provides a means to meaning. Now, our approach has become more dynamic. Now, the formation of a secondary gas cap seems more appreciable. But we cant even move until we don't have something dynamic in hand.
So, point #1 : we need a dynamic gas oil ratio.
Hold on. The problem dosent really end there. Even after we have a dynamically adjusted GOR, we are still not sure about the now constantly and rapidly changing saturation within the reservoir. And without a proper knowledge of the saturation, even a dynamic GOR wont be of much use. All that we have at hand though is the cumulative oil and gas production.
No wait, actually, its just the cumulative oil production. Bah! save yourself the pain of a cumulative gas production, you already have that little magical kitty named the dynamic (instantaneous) GOR.
So now, just before we step into reservoir performance, we need one more small tiny weeny thing.
An equation that gives a relationship between the saturation and the cumulative oil production
and we'll be all set to go.
So, point #2 : an equation that relates the saturation to cumulative oil production.
Let's start by trying to understand the Instantaneous GOR. Clearly as the name goes, it'll be the ratio between the rate of the total gas being produced at any time (SCF/day) to the rate of total oil being produced at the same point of time (STB/day). The units arent really that big a deal, you just need to keep the consistency right. You might just use the formation factors from the lab data and write the above in terms of RB/day. Dosent really matter.
Coming down to the equation. We are well aware of the instantaneous solution GOR which we very proudly every time jot down as Rs. What will differentiate this from the instantaneous GOR though will be the free gas flow rate and the free oil flow rate once the reservoir fluid gets to the surface.
So we can have GOR = Rs + (Qg/Qo)
where GOR = 'instantaneous GOR' SCF/STB
Rs is the gas solubility or the solution gas oil ratio or whatever you like to call it.
Qg is the free gas flow rate. Mind it, thats a rate as we needed a dynamic value and so the units are SCF/day.
Qo is the oil flow rate again naturally in STB/day.
So while the units to the inst. GOR essentially remain the same as before, we need to understand the fact very carefully that what it represents has completely changed.
Now, the total gas production rate can be given by
Qg = qg/bg + Qo*Rs
pretty simple to understand i suppose.
oh and qg is the gas production rate in RB/day.
The oil production rate with hence be
Qo = qo/Bo
So, our inst. GOR or simply R = [(qg/Bg) + QoRs]/qo/Bo
or R = qgBo/QoBg + Rs
You might as well go ahead and proclaim me to be stupid enough for using the extended form of Qo= qo/Bo when i was anyways going to write them togather in the final equation. But since Mr Ahmad did give a very reasonable explanation for that.
We can now comfortably use the Darcy's Law and write
qo = 2*pi*Ko*H*deltaP/ Vo*ln(Re/Rw)
The terms here have their usual meaning except for Vo which is supposed to look like the oil viscosity. Pardon me i am too lazy to import the symbols into the post as it is already 3:10 and i am starting to have a craving for icecream and pizza.
Nevermind, using a similar equation for the gas flow rate well, we can have the more comprehensive form of the inst. GOR as
R = KgVoBo/KoVgBg + Rs
(the V again for viscosity, i skipped the complicated multiplication. its too tedious to type, and its just baby terms cancelling out each other. Something you can very well understand just by looking at it. )
but woah!
we did the stupid thing once again. For the reservoir that we are trying to get a solution to, the presence of a gas cap seriousy affects the value of H, the height term as the permeability becomes relative to the wetting phase and so there are huge changes in values with the change of the non wetting fluid viscosity.
So you can jolly well go ahead and add the term of Hg and Ho as two different terms, as if it makes the thing look any prettier.
So that ends the discussion to the solution of the first hurdle towards performance prediction of reservoirs.
I will get back to you soon enough with the second point of the equations to saturation.
PS: thats when i get it.
Till then have a nice time.!
The Sign Painter
So, drive mechanisms are the words for the day!
And as and when the formation of a secondary gas cap comes into picture, the material balance equation starts to dawdle. This mainly happens because of the term 'm'. There isnt much the equation can do about it you see. It has been pre-decided as the gas in place to the oil in place ratio. put straight into the equation. One reason why the MBE is so simple for most volumetric reservoirs is that the term 'm', takes with itself any of the pain that might try poke its nose into your estimation. But when your reservoir starts to roll, there aint gonna be much concerning for the simplicity of calculations. There the constancy of 'm' is put to test and as the more perceptive of you might have judged by now, it sort of collapses.
So, what do we do?!
Do away with the 'm'?
Hell no.!
That'll be a nightmare! that little thing has saved us pains all through when the reservoir was static and wasn't churning out black gold. We cant just let it go cause some bloke in a shirt says it wont work !
So we resorted to what was closest to 'm' all the time. Just round the corner, we have the good old GOR (gas oil ratio for the more dumb among you) more aptly represented by 'R'. But this isn't the solution really
R only provides a means to meaning. Now, our approach has become more dynamic. Now, the formation of a secondary gas cap seems more appreciable. But we cant even move until we don't have something dynamic in hand.
So, point #1 : we need a dynamic gas oil ratio.
Hold on. The problem dosent really end there. Even after we have a dynamically adjusted GOR, we are still not sure about the now constantly and rapidly changing saturation within the reservoir. And without a proper knowledge of the saturation, even a dynamic GOR wont be of much use. All that we have at hand though is the cumulative oil and gas production.
No wait, actually, its just the cumulative oil production. Bah! save yourself the pain of a cumulative gas production, you already have that little magical kitty named the dynamic (instantaneous) GOR.
So now, just before we step into reservoir performance, we need one more small tiny weeny thing.
An equation that gives a relationship between the saturation and the cumulative oil production
and we'll be all set to go.
So, point #2 : an equation that relates the saturation to cumulative oil production.
Let's start by trying to understand the Instantaneous GOR. Clearly as the name goes, it'll be the ratio between the rate of the total gas being produced at any time (SCF/day) to the rate of total oil being produced at the same point of time (STB/day). The units arent really that big a deal, you just need to keep the consistency right. You might just use the formation factors from the lab data and write the above in terms of RB/day. Dosent really matter.
Coming down to the equation. We are well aware of the instantaneous solution GOR which we very proudly every time jot down as Rs. What will differentiate this from the instantaneous GOR though will be the free gas flow rate and the free oil flow rate once the reservoir fluid gets to the surface.
So we can have GOR = Rs + (Qg/Qo)
where GOR = 'instantaneous GOR' SCF/STB
Rs is the gas solubility or the solution gas oil ratio or whatever you like to call it.
Qg is the free gas flow rate. Mind it, thats a rate as we needed a dynamic value and so the units are SCF/day.
Qo is the oil flow rate again naturally in STB/day.
So while the units to the inst. GOR essentially remain the same as before, we need to understand the fact very carefully that what it represents has completely changed.
Now, the total gas production rate can be given by
Qg = qg/bg + Qo*Rs
pretty simple to understand i suppose.
oh and qg is the gas production rate in RB/day.
The oil production rate with hence be
Qo = qo/Bo
So, our inst. GOR or simply R = [(qg/Bg) + QoRs]/qo/Bo
or R = qgBo/QoBg + Rs
You might as well go ahead and proclaim me to be stupid enough for using the extended form of Qo= qo/Bo when i was anyways going to write them togather in the final equation. But since Mr Ahmad did give a very reasonable explanation for that.
We can now comfortably use the Darcy's Law and write
qo = 2*pi*Ko*H*deltaP/ Vo*ln(Re/Rw)
The terms here have their usual meaning except for Vo which is supposed to look like the oil viscosity. Pardon me i am too lazy to import the symbols into the post as it is already 3:10 and i am starting to have a craving for icecream and pizza.
Nevermind, using a similar equation for the gas flow rate well, we can have the more comprehensive form of the inst. GOR as
R = KgVoBo/KoVgBg + Rs
(the V again for viscosity, i skipped the complicated multiplication. its too tedious to type, and its just baby terms cancelling out each other. Something you can very well understand just by looking at it. )
but woah!
we did the stupid thing once again. For the reservoir that we are trying to get a solution to, the presence of a gas cap seriousy affects the value of H, the height term as the permeability becomes relative to the wetting phase and so there are huge changes in values with the change of the non wetting fluid viscosity.
So you can jolly well go ahead and add the term of Hg and Ho as two different terms, as if it makes the thing look any prettier.
So that ends the discussion to the solution of the first hurdle towards performance prediction of reservoirs.
I will get back to you soon enough with the second point of the equations to saturation.
PS: thats when i get it.
Till then have a nice time.!
The Sign Painter
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