Drilling horizontal wells are always in the development phase and people know the geological area pretty well. Additionally, they can accurately determine reservoir pressure of the target sand for the horizontal candidate. Hence, drilling engineers can plan the well with less chance of being underbalance condition. However, there are several scenarios where the well control occurs in horizontal wells. We will need to understand what circumstances can create well control situations in the known well bore pressure like the horizontal wells.
Swabbed Kick
The swab in the horizontal wells is similar to swabbing in normal well. Swabbing effect can occur when the pipe is pulled off bottom for making up connection or when tripping out of hole. High rate of swab entering into the well can happen if swabbing occurs when tripping out of the hole. The length of horizontal section will be exposed to the differential swab pressure. On the other hand, the swab will be small when the pipe is pulled off bottom for making connection. In this case, you may see several small gas behind the bit which will show on the surface later.
Figure 1 – Swabbed Kick
You can swab the kick in the horizontal section while moving the pipe but the well may not flow because the vertical height of mud column does not change. It means that you still have the same hydrostatic pressure. Additionally, gas in the horizontal section will migrate up to high side of the wellbore only. The well will start flow when the gas is moved into deviated and/or vertical section.
Secondary Kick
This is when you have induce the second kick into the wellbore because of poor bottom hole pressure control resulting in hydrostatic pressure in the wellbore less than formation pressure. This problem can be mitigated by ensuring that the correct procedures are performed.
Figure 2 – Secondary Kick
Penetrate into New Formation
The well control in the horizontal section can be occurred when the well is drilled into new virgin reservoir which the reservoir pressure is unknown. This is very important that the horizontal well path will not cross the fault into virgin reservoir. For this case, you will see the response like the vertical or deviated wells. The positive well control indicators (well flow with pump off, increase in flow, pit gain) can be observed for this kind of kick. Once the well is shut in, you will see shut in casing pressure and drill pipe pressure. It is not difficult to detect this kick when compared to swabbed kick.
Another issue if you take this kick is underground cross flow from the virgin formation (higher pressure) to the lower pressure sand. If the cross flow happens, you will casing pressure raising up to a certain level and maintain. It indicates that kick is pushed into the lower pressure barrier zone. As you can see, taking a high pressure kick and flowing into the low pressure zone can create a lot of confusion and the normal well control procedure may not applicable. What’s more, in order to stop cross flow, barite plug, LCM or heavy pill cannot be utilized effectively in the horizontal zone. Therefore, it is very important to design the well which will not intersect the new reservoir with higher pressure than the current reservoir.
Figure 3 – Penetrate into New Formation
Barite Sag in Horizontal Section
Barite sag is a situation when barite falls down to the low side and the lighter fluid moves up due to the differences in density. This becomes an issue especially in highly deviated or horizontal wells because when the lighter fluid is circulated into vertical section, it reduces the hydrostatic pressure. It might be lower enough to create an underbalance condition in which wellbore fluid will be able to move into the well. The longer static period, more chance to have the barite sag issue. In order to minimize this issue, driller’s method is recommended because the first circulation can start right away. If the wait and weight is used, the sag issue can be worse because the wait and weight takes longer time to prepare drilling fluid before circulation gets started.
Reference books: 
Well Control Books