i have a serious issue regarding the development of a very tight sand stone reservoir - relatively deep +/- 3700 m on shore. solution gas drive with almost no aquifer support. reservoir so far, developed via drilling 3 vertical oil producers and 3 horizontal producers. the main issue, we noticed sever decline in the reservoir pressure and lack of sustainable oil production as wells start producing nicely for about 6 month then ceased flow even with gas lift. latest core data from one of the new wells showed that very high contrast in permeability. as only one plug on the top of the reservoir showed very high permeability and other plugs showed very very low permeability. very light oil we have and production wise 3 vertical wells almost produced similar to the other 3 horizontal wells.
the challenge we have now is how to produce the oil from this reservoir and how to maintain the production. are there any technology can be used, we have tried hydraulic fracture and only one well out of 6 was successful and good results been shown.
i am hoping some one can help on this
let us have a brain storming together
many thanks in advance
Greetings Mohamed. Have you considered that your fluid might have originally been a gas in the reservoir and that the oil is a condensate? Is it possible that your reservoir pressure has dropped enough to allow the gas to condense into liquid in the reservoir rather than at the surface? I know of such cases where very-light oil wells become low-rate gas wells when the liquid condenses because there is not enough permeability to allow liquid to move. I think the only solution is to inject gas to raise the reservoir pressure and maintain it above the condensation point. If you have confidence in your estimates of hydrocarbon in place and the recovery factor based on current production is ridiculously low, this might be the case.
Thank you Keith Winfree for your great discussion and ideas. However, considering the reservoir is originally gas already questionable. as per our understanding, he source rock is above as we do not have gas cap in top of the reservoir. reservoir thickness as i mentioned around 30 m and only high permeable in the top 2 meters and in top of that directly hot shale. we thought the charge came from top to bottom. let me know your thoughts please.
best regards
Mohamed
Was the reservoir already treated with hydraulic fracture stimulation? This is important to know.
Without knowing more, it sounds like the high K layer is very thin (with very limited storage volume) and sandwiched by the very tight rock. The rock matrix surrounding the high K streak apparently has very low permeability, thus cannot feed the thin high K streak at commercial rates. Once the pressure in the thin conduit drops below bubble point, the high K streak gasses out quickly and Kro essentially goes to 0.
It would be critical to understand the oil saturation in the surrounding very tight rock. If there is low very total water saturation (bound+free) and the reservoir is thick enough, it might require massive hydraulic fracture stimulation in multiple stages along the horizontal. Without knowing more however, it is impossible to know for sure whether this might work.
Trying to list and even experiment solutions without first analyzing root cause(s) upfront bears a high failure probability. The first thing you should do is analyze whether there is a problem or not and if there is a problem whether it derives from near or far. Your description points first and foremost to pressure tests and pressure transient analysis. If skin is high, some solutions might help (e.g. fracturing [that you apparently tried], acidizing, ....), others if skin is evolving (e.g., scale removal), and others solutions (e.g. injection) in other configurations. You do not give enough information to point materially to what is amiss (if anything, pressure decline can be expected in general) and what solution to consider.
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