How come ?!!

[jikzo]

A well was perforated in 2 reservoirs .. The log shows that the first reservoir is watered out ..
However, SPT and PLT show that 90% of oil production is coming of this reservoir (first one ) ??
although, this reservoir has the worst characteristics compared to the second one.


so, how come ?
what possible explanations ??

PS: The well passes through a fault .. it maybe explained that the fault is acting as a conduit .. but what if it's a sealing ?

[ELgharib]

How is your completion? Open hole or cased hole? Selectivity? Can you isolate one reservoir and measure the flowrate?

[barryn56]

If by "watered out" your open hole logs are showing high water saturation, that doesn't mean you can't flow clean oil - is this reservoir produced elsewhere? The rock may be fine grained and have a high residual water saturation, but no permeability to water. Or you may have a completion issue - need a lot more information to come with a unique answer, but it should be possible. barryn56@yahoo.com

[jikzo]

How is your completion? Open hole or cased hole? Selectivity? Can you isolate one reservoir and measure the flowrate?

This well was side tracked from the original well .. but then it was abandoned because they couldn't reenter it ..
so now am talking about the side tracked well ..it's cased in reservoir 2 .. but open hole in reservoir 1 with the highest production, ( the original well was completed in the same way as well )
yes they isolated one of the reservoir so they can know the split of the oil production .. and this was the result ..
the reservoir with the worst characteristics and watered out.. is the one with highest production

[jikzo]

If by "watered out" your open hole logs are showing high water saturation, that doesn't mean you can't flow clean oil - is this reservoir produced elsewhere? The rock may be fine grained and have a high residual water saturation, but no permeability to water. Or you may have a completion issue - need a lot more information to come with a unique answer, but it should be possible. barryn56@yahoo.com

actually .. when i read the well report .. its written exactly like this " This log shows that reservoir 2 is mainly flooded except the upper part of it " ..
and when i asked my mentor about the meaning of "flooded" he said it means "watered out" ! which means it was producing mainly water !

as i said this well was side tracked from an original well .. but then the original well was abandoned .. so the sidetrack is only producing now ..
its only producing from these 2 reservoirs.

what more information you need ?

[barryn56]

OK, as the result is from a log (I presume some form of PNC or resistivity) then flooded means it has the lower part with high water saturation - the upper part of Reservoir 2 still has oil in it, and based on the logging has good water drive (pressure support). If it is not completed across the watered out section, then you could expect very good oil production, until the water reaches the perforations. From your description, reservoir 2 is above reservoir 1. It would be useful to have a description of the well completion depths (casing shoe and where perforated and where log shows oil water contact). If the whole of reservoir 2 is perofrated, then the cnditions under which the log was run is important (well flowing/shut in, etc.) and what type of log was it.

[jikzo]

OK, as the result is from a log (I presume some form of PNC or resistivity) then flooded means it has the lower part with high water saturation - the upper part of Reservoir 2 still has oil in it, and based on the logging has good water drive (pressure support). If it is not completed across the watered out section, then you could expect very good oil production, until the water reaches the perforations. From your description, reservoir 2 is above reservoir 1. It would be useful to have a description of the well completion depths (casing shoe and where perforated and where log shows oil water contact). If the whole of reservoir 2 is perofrated, then the cnditions under which the log was run is important (well flowing/shut in, etc.) and what type of log was it.

The log that was performed is TDT.
and yes it makes sense now coz all the upper part was perforated with a very small part of the lower section ..
but how can the high saturation act as a water drive ? we're not sure if it's active or no ! how can i prove it ?

reservoir 2 is producing more water that reservoir 1 as well .. this is due to the perforation as i said earlier ..

and i can say that usually the cased hole has a better flow than the open hole ? so reservoir 2 has more production !

but now help me to prove the water drive thing !


thank you very much i really appreciate it !

[barryn56]

Hi, the TDT log is normally run as a time lapse, but if this is the first run, you can compare the current saturation with the original open hole logs to determine how saturation has changed with production, and thus show this is a water encroachment/drive. You don't mention under what well conditions the TDT was run (flowing or shut in?). There is a possibility that the water is coning, if flowrates are high. The lower reservoir may be damaged, so you could try perforating. You say the water saturation is low in the open hole reservoir - is this result coming from the TDT? You need to make much higher wellbore corrections for open hole compared to cased hole for TDT, so be careful with the analysis.

[jikzo]

Hi, the TDT log is normally run as a time lapse, but if this is the first run, you can compare the current saturation with the original open hole logs to determine how saturation has changed with production, and thus show this is a water encroachment/drive. You don't mention under what well conditions the TDT was run (flowing or shut in?). There is a possibility that the water is coning, if flowrates are high. The lower reservoir may be damaged, so you could try perforating. You say the water saturation is low in the open hole reservoir - is this result coming from the TDT? You need to make much higher wellbore corrections for open hole compared to cased hole for TDT, so be careful with the analysis.

Qw is almost 5000 stb/day , and Qo is almost 700 stb/day
so water flowrates are high !
but how can water conning affect the production ??

[ELgharib]

Please, if you trust your Logs, check the skin damage around the wellbore. It can explain this discrepancy.

[jikzo]

Please, if you trust your Logs, check the skin damage around the wellbore. It can explain this discrepancy.

How can i check the skin damage from the logs ??
and how can this be useful ?

am sorry am asking alot of questions .. but am still a student .. and am here to learn .. so execuse me

[barryn56]

Hi,
the comment about damage from logs comes from an estimate of invasion profile from mud during drilling - this assumes you have contracts in resistivity between the formation and drilling fluids. It really applies to new wells - this sounds like a well that has bene on production for some time, but if not, and there is resistivity contrast, then you shoul dbe able to determine the invasion depth. The premise is that the mud invasion blocks the formation permeability, while perforating might reach beyond the damaged zone (nothing stopping you perforating the open hole, by the way).

Water coning occurs when the drawdown at the well exceeds the pressure difference required between the oil and water density and the vertical permeability pressure drop due to flow at the wellbore (assuming flow of water is not due to communication through poor cement!). So instead of keeping the water in the formation to provide reservoir drive, you are producing it, which costs money and reduces the oil production. You can check this by producing the well at different rates and run TDT logs - if you see the water contact receed, you can identify coning, and you can check the oil and water productivity to see if it changes. If you have a cement channel, then you will still produce the same water productivity (rate over drawdown). Ideally, with active water drive, you use a horizontal well in the upper part of the reservoir, or other methods to minimize the drawdown at the wellbore.