Stress Reorientation in Unconventional Plays

This is the first in a series of articles I am going to write about fracturing in unconventional wells.

The first large-scale shale play is acknowledged as the Barnett Shale in N. Texas. It was originally developed as a vertical play before horizontal wells were drilled. Vertical wells have the benefit of being relatively inexpensive to drill compared to horizontals, but the surface area of the wells is miniscule compared to what a horizontal well can give you. 

In the beginning, the vertical wells were drilled and fraced , and normal bi-wing fracs were developed and confirmed through microseismic. As all shale wells do, the falloff rates were very high, and the wells quickly paid out. Their cash flow past the first year was fairly low. Mitigation efforts included acidizing and refracs. These refracs yielded new reserves. To determine what was causing this, microseismic was run on the wells, and it was found that the new fracs were taking off in a different direction to the original fracs near wellbore. The fracs would then reorient themselves and parallel the original fracs. It was found that the wells could be refraced up to 3 times which each showing the same thing. This was also the case in the core of the DJ Niobrara many years later. These were called tri-fracs.

What was causing this? Fracs propagate perpendicular to the regional stress field and yet the near wellbore fracs were taking off in a different direction. I postulate that is in the depletion near wellbore that caused differential compaction in the shale thus altering the stress field very near wellbore. I call this stress reorientation. It is strictly a localized phenomenon.

The same thing happens near faults in the earth. The regional stress field goes crazy right around an active fault due to tectonic stress. The larger the fault the farther out this happens. Notice in horizontal wells that fracs placed in the vicinity of faults generally yield very little production if anything. This is because the fracs go crazy due to the stress field.

You can argue that the regional stress field dominates, and this is true. However, the local stress field is what predominates in the near wellbore region.

Anyone else experience this?