When drilling a well and keeping the wellbore fluid gradient below that of the natural formation gradient, this is called Underbalanced Drilling (UBD) and it has its differences with traditional methods. Namely, the well can flow throughout the drilling because the bottomhole circulating pressure always remains lower than the formation pressure. As well as penetrating at a faster rate and removing the risk of lost circulation, UBD also has another benefit in that the damage of invading fluid is reduced. Even after this, drilling time gets smaller, bit life improves, and it can be easier to detect and test productive intervals. Of course, this benefit isn’t seen when the well isn’t underbalanced so this is pivotal to the whole process.

If minimizing the risk of invasion is a priority, UBD can be a valuable tool. Over time, the method has been gaining in popularity because depleting pressure levels is a common occurrence and it also reins supreme within reservoirs that are low quality or particularly complex. Today, the vast majority of UBD applications use coiled tubing systems. As a sign of its popularity, around four in every ten onshore wells used underbalanced conditions back in 2000 and this has been increasing ever since.

Drilling Fluids for Underbalanced Drilling

Within UBD operations, there are three different types of drilling fluid seen; incompressible (liquid), compressible (gaseous), and two-phase. For drilling fluid selection, this will depend on the conditions of formation fracture pressure, bottomhole flowing pressure, formation pore pressure, and borehole collapse pressure.