Discussion about oil migration to gas cap

[DAH7542]

Hey fellows, I would like to hear opinions about the physical reasons that make oil migration into the gas cap a problem... I've checked couple of books that say something like "oil migrating to gas cap is consequently lost".... but why?, please refer some books of articles to read..... thanks...

[vinomarky]

I wouldn't necessarily say ALL oil that migrates to the gas cap is lost, but generally a large quantity is.... A primary gas cap (ie not formed through dropping saturated oil pressure down) generally is pretty dry.... It may have some residual water in it, but no oil. When you allow oil to migrate into it you will as a MINIMUM loose a quantity relating to the residual oil saturation of that rock ie you may flow 10mm bbls of oil into the gas cap volume, but (for a 20% Sorw) will only ever hope to get out at maximum 80% of that volume... in effect you are saturating additional gross rock volume with residual oil volume which means unavoidable losses

Add into the that the significant differences in mobility between gas and oil when you try to flow it out means that of that of the 80% you can theoretically get back, only a small portion will actually come back

[DAH7542]

Thanks vinomarky, nice explanation.....

[ELgharib]

Vinomarky, what could be the reason of having oil migrating into the gas cap?

[vinomarky]

Per pm - If you have an oilfield with water drive and gas cap, and through gas coning (or gas production if you only had a thin oil rim and this was not the primary target) you produce a significant volume of gas, then the water support can push the oil column up

Please either pm me, or post a question on the forum - but not both in future. Generally I prefer the forum, as everyone then benefits (hence why I repeat my answer here)

[vinomarky]

And to your followup pm question

Thanks, and it is clear. My question is this now : Can you have water saturates the primary gas cap?

I am not 100% clear on what you are asking.... if you are asking if water can move up into the gas cap volume as the pressure depletes, then the answer is absolutely. If you are asking can you have residual (connate) water saturation in the gas cap, then the answer is also absolutely - water/gas capillary pressure works exactly the same way as water-oil capillary pressure, albeit with different IFT and contact angles along with far greater density differences, so you still expect to see some sort of transition zone through to Swir - whether or not this transition zone is relevant compared to your reservoir size and other uncertainties depends upon many things including permeability & relief.

[ELgharib]

Dear Vinomarky,

Normally I should have created a new thread...But will explain it here.
Do you have any material to analyse Pressure BU in gas lift well. I have observed strange behaviour as the well is affected by Annulus pressure and seem to have created 2 auccessive Wellbore storage.

[vinomarky]

ELgharib - Interesting problem

Short answer unfortunately is no, I don'y have any material (or indeed experience) analyzing PBU tests in gas lift wells. I could well believe that you'd see two wellbore storage trends, masking your formation response quite badly. You may have to do something like running in a memory gauge hanging below a blanking plug to set in the x-nipple at the bottom of your tubing via slickline (hopefully below your last GL valve). Otherwise..... hmmm... you may be better off trying to model it with a RTA (rate transient analysis) package such as Fekete FAST RTA, which takes your flowing rates and estimates BHP's from fluid properties and lift curves to figure out formation parameters... at least then the wellbore storage should not be changing so radically.

Good luck

*Update: Have done some investigation, and the more appropriate tool apparently is FAST VirtuWell - you can get a demo download from

[link Point to another website Only the registered members can access]
I have not used this software though.....