Assume that we drill a vertical well, but when we get a survey of borehole trajectory we find that there is some deviated angle and azimuth. Some people say that, because of the weight of an anticline core, the but does as a plummet and get a deviation to crest of anticline. But I want to know the effects of stresses and rock strength independently from bed layering. Is it true that a well makes a deviation to low stress concentration?
Hi,
As you mentioned, a well tends to be oriented towards minimal stress. But usually due to stress balance, the impact of structural parameters are much more in deviation process.
Alireza, the fundamentals say that the rock surrounding a wellbore is subject to three main stresses and if these exceed the strength of the rock, the rock will fail (Zoback 2008). This implies that wherever the failure occurs around the wellbore then a deformed zone is created and might develop to become a "breakout". Please note that rock strength is dependent on both chemical(Fluid-rock interaction) and mechanical(Mud weight) effects acting while drilling and therefore they need to be considered. I hope it helps.
I suspect that the issue is related to oil and gas wells. I agree with Juan. Please note that basic mechanics suggest that the well's column will deviate towards the area of least resistance. That is either towards failed rock material as Mohsen suggests, or away from non uniform high lateral stress vector (non uniform confinement). In situ stresses are redistributed radially, tangentially (hoop) and along the well's lining and any lateral stress differential will have immediate effect. I image that stress analysis in oil & gas applications uses total stress (full pore pressure) conditions, which is more difficult to predict. If the well's lining is strong enough, then high lateral stress (and hence pressure) differential will force the well column to deflect in the opposite direction, depending on lining's material stiffness. Since direct in situ stress measurements are very difficult to obtain and indirect tools are used to design, I suggest that the solution lies with the real time monitoring of the well's directionality, especially if data from other wells in the same area exist. But this is already outside my speciality. Good luck.
Since, you have mentioned Vertical well, what is the maximum deviation you have observed in your well.? As Juan and Georgios mentioned the well will follow the least resistance. As far as the drilling parameters are in control, deviation would be minimum. High WOB will KOP of the well unintentionally towards the fractures, high angle bed interface, which are not necessarily follow regional stress pattern. Local analysis must be carried out to understand the cause and how to control it.
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