By Irina Slav
Oil well shut-ins are the new black. Everyone, especially in the U.S. shale patch, seems to be shutting in wells in response to what is shaping up to be the Great Glut of 2020. Now, many are asking how all these wells will be restarted once prices improve.
The answer? Nobody knows.
Shutting in oil wells is markedly different from flipping a switch. It is a job that has to be done with extreme care based on the characteristics of the formation into which the well is drilled, its rate of production, and the specificities of the oil that flows from it.
But even with careful planning, there is a risk of permanent damage if the well remains shut-in for more than a couple of weeks.
Here are some of the problems shutting-in could cause in oil wells.
There is a kind of routine well shut-in that aims to increase the output of oil and/or gas by letting the pressure in the rock that contains the hydrocarbons build up. These shut-ins never last months, however. They are a short affair, and they are only suitable for wells drilled in rocks that have the “proper” pressure–that is, pressure that has the potential to build up.
This is not the case with low-pressure wells. In low-pressure wells, experts from the Journal of Petroleum Technology write, a shut-in could negatively affect the permeability of the oil-bearing rock.
What is permeability? The tiny little pores in the rock where the oil and gas hide. High reservoir pressure pushes them out relatively easily. Low reservoir pressure keeps them in. The problem is solvable with chemicals, the experts note, but let’s not forget that chemicals cost money, which would increase the cost of reviving the shut-in wells.
Crossflow refers to the flow of oil and gas from high-pressure areas in the rock formation to low-pressure areas. (Yes, rock formations are not as homogenous as one might hope) On the one hand, this is a problem because the recovery of oil and gas from a low-pressure area in the formation is more difficult. On the other, crossflow is problematic because it results in the undesirable mixing of different kinds of hydrocarbons from different areas in the formation. Separating the different hydrocarbons after the fact may not be possible.
The undulating wellbore
Undulations of the wellbore are common in horizontal wells. One of the great advantages of horizontal wells is the increased contact between the well and the reservoir. The purpose of wellbore undulations is to further increase the contact between the well and the reservoir and improve output. However, things get tricky with shut-ins.
Water buildup in the well is one problem that experts warn could render the well unusable eventually. If enough water collects in the well, pumping it all out to get back to the oil may become uneconomical, the JPT authors note. Other experts note that water is not the only substance that would build up in a horizontal well if it is shut-in for a lengthy period. Heavier hydrocarbon fractions such as paraffin and tar could also accumulate in the wellbore and affect the future performance of the well negatively.
Not all wells are created equal
Undulations are not the only problem for shale well shut-ins. The bigger problem is that shale wells shut-ins are terra incognita for the industry, while in conventional oil drilling, there is a lot of information on how to properly shut in wells.
Because of the specific nature of shale wells, there is a host of problems that could be caused by a lengthy shut-in, one expert in that particular field told the JPT, from problems with the on-ground equipment to unexpected chemical reactions in the reservoir rock. In any case, water will build up as tends to happen with all shut-in wells.
Picking the best well to shut in seems to be a very tricky business, and even if you are careful with the picking, you could still get a nasty surprise when you try to resume pumping from that well. And this is where the most unpleasant difference between conventional and shale oil wells seems to lie: conventional wells don’t necessarily need to be shut in.
As the chief of Russia’s Zarubezhneft recently told local media, with a conventional well, all you need is to adjust the flow rate by 2-3 percent lower, and you could get a total output reduction of 200,000 to 300,000 bpd. This does not seem to be an option in the shale patch where all reports are either about well shut-ins or the suspension of new well drilling and fracking.
The United States is on track to reduce its crude oil production by 1.7 million bpd. Most of this would come from well shut-ins in the shale patch. Because of the lack of evidence about the most likely effects of these shut-ins on production, the next few months will be interesting. Perhaps there won’t be much damage and producers would be quick to ramp up once prices improve. Or maybe many of those wells will need to be plugged and abandoned, and new wells would need to be drilled.
Uncertainty still reigns.
By Irina Slav for Oilprice.com