[rockdoc123]

I'm no expert on the question. But isn't it a bit easier to say than to do? The US went through various EOR techniques after what was considered a peak in production rate in 1970. But until recently (40 years after this "peak"), the production rate was decreasing, even with the more recent discoveries and exploitation of offshore production.

Apples and oranges. Recovery factors in the conventional reservoirs were already much higher (averaging primary recoveries around 35%). The tendency in those reservoirs is to drill enough wells to get them to a plateau rate (usually governed by amount of pipeline capacity available, reservoir considerations etc) and gradually let them decline over time. Water or natural gas injection might be considered during the plateau period if there is too much pressure decline. It isn't until sometime after the wells have already declined considerably that full on water injection or EOR would normally be considered. Hence the fields will never reattain their peaks, they do increase in production for sometime but not to the peak rate. In the shales it is a bit of a different story because the recovery is so small the issue is not as much the problem of residual oil saturation do to wettability changes or adverse mobility it is mainly due to the fact the fractures which are created can only access a certain amount of the formation. Imagine a swimming pool size of shale that has high porosities which allows for lots of storage of oil that has been generated in-situ but with nano-darcies of permeability (that's 10*9 smaller than most conventional reservoirs). You hammer on the rock with a big sledge and create a big fracture across the width of the pool with a bunch of subsidiary fractures that fan out for a metre or so around the large fracture. What that does is it allows all of the oil which has direct access to the big fracture (via smaller fractures or microcracks) to be extracted. Yet the vast majority of that swimming pool of rock is untouched. So in the case of shales or marls the issue is that there needs to be more of the reservoir intersected where permeability can be increased. The technology is different and the economics are different.

[Plantagenet]

Imagine a swimming pool size of shale that has high porosities which allows for lots of storage of oil that has been generated in-situ but with nano-darcies of permeability (that's 10*9 smaller than most conventional reservoirs). You hammer on the rock with a big sledge and create a big fracture across the width of the pool with a bunch of subsidiary fractures that fan out for a metre or so around the large fracture. What that does is it allows all of the oil which has direct access to the big fracture (via smaller fractures or microcracks) to be extracted. Yet the vast majority of that swimming pool of rock is untouched. So in the case of shales or marls the issue is that there needs to be more of the reservoir intersected where permeability can be increased. The technology is different and the economics are different.

Yup.

One way they are doing this is by stacking wells, so multiple wells are drilled one right on top of the next to create a dense array of fractures.

But because the "sweet spots" get drilled out first, as time goes on the remaining shale/marl contains less oil and is generally less productive, creating an inexorable problem for TOS oilcos. This makes it harder and harder to increase or even maintain oil production over time at TOS plays.

You can see this working out in the Bakken where production reached a peak in 2015, and has declined significantly since then.

Cheers!

[rockdoc123]

But because the "sweet spots" get drilled out first, as time goes on the remaining shale/marl contains less oil and is generally less productive, creating an inexorable problem for TOS oilcos. This makes it harder and harder to increase or even maintain oil production over time at TOS plays.

the sweet spots are not about storage capacity (i.e. level of porosity and hence oil in place) what they are about is a couple of things, presence or absence of natural fractures (including grain boundary dislocations), amount of clay content and ratio of silica to carbonates both of which determines whether the shale behaves with plastic or brittle response when fracked. There is still a large amount of oil sitting in the areas outside of the sweet spots. They just do not respond as well under current fracking technology and hence need a bit higher commodity price to access them.
That will change with time. Just as the sweet spots weren't economic back in 2003 before all of the initiatives regarding fracking, supply chain management and operations were applied.

[Plantagenet]

the sweet spots are not about storage capacity (i.e. level of porosity and hence oil in place) what they are about is a couple of things, presence or absence of natural fractures (including grain boundary dislocations), amount of clay content and ratio of silica to carbonates both of which determines whether the shale behaves with plastic or brittle response when fracked. There is still a large amount of oil sitting in the areas outside of the sweet spots. They just do not respond as well under current fracking technology and hence need a bit higher commodity price to access them.
That will change with time. Just as the sweet spots weren't economic back in 2003 before all of the initiatives regarding fracking, supply chain management and operations were applied.

Yup. No doubt with higher oil prices they'll go after just about anything that has oil in it.

Hope you had a great Christmas holiday rockdoc.

Cheers!