PROBLEMS WITH PERCEPTION
In many cases, horizontal wells can overcome low productivity from vertical wells. An extended reach well increases flow capacity by penetrating a longer interval of reservoir rock, provided of course that you stay inside the reservoir. If natural fractures are present, aiming perpendicular to them will often improve flow capacity, but not if the fractures also penetrate water below the oil or gas above it. It's a 3-dimensional problem and it needs to be understood before the well is planned in detail. You ignore structure, stratigraphy, and fluid contacts  at your peril.

Petrophysical analysis can reduce the risks of horizontal wells dramatically. Unfortunately, many such analyses come too late and are merely post-mortem autopsies. I call these projects forensic log analysis. a real whodunit detective story usually unfolds.

In the three examples that follow, the horizontal wells were planned because the vertical wells were not producing as well as their neighbours. In all three cases, the reservoir was thinner and lower structurally than those high production neighbours. In all three cases, the horizontals were economic failures and one was a complete write-off from the beginning.

These projects date back about 30 years so the people and companies involved  are long gone, so I doubt anyone reading my commentary will be too embarrassed. The wells shown are 0.5 to 1.0 mile apart.


The operator of the well second from the right compared his well performance with his luckier neighbours and came up short. Stratigraphic correlation looks pretty good but his structural position is terrible, due to differential salt solution during and after deposition. That coal bed was horizontal once. The common oil water contact in the channel sand confirms the current structural levels. He decided to drill the horizontal well anyway and penetrated only the upper poor quality sand and the coal, never touching the better sand below
the coal. This reduced the risk of penetrating water but the result was dry hole. This cross section was made after the fact, and the operator appeared completely surprised by the structural situation.


The complexity of carbonate lithology coupled with structural concerns makes horizontal well planning a bit difficult. Same operator, same problem as the previous example - his well at the right is producing less than his neighbours, so let's do a horizontal. It produced at a rate about the same as the original vertical well and never made a positive cash flow. The zone is thinner, lower porosity, and closer to the water, so it was a risk at best, but fortunately not a total disaster. Note that the well at far left is in a completely separate oil pool as the water contact is quite different.


Yup. Same client, same problem, another post-mortem. His well is on the right. You can write the prognosis yourself. The zone is thin and low porosity, sitting on higher porosity water. What did you expect to happen? Again, the well on the left is in a separate oil pool.
 

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