Underbalanced coiled tubing drilling (UBCTD) is one of the keys to unlock the true potential of the tight and abrasive sandstone.

The sandstone formation is mainly characterized by hard and abrasive sandstones interbedded with shale and siltstone. Typically, the sandstone formations are encountered at very deep depths. They are the most difficult sandstone formations to drill, with rock unconfined compressive strength (UCS) of up to 35,000 psi and internal friction angles ranging from 25 to 60 degrees. These conditions make it very difficult to drill and reach optimal lateral lengths. Additionally, it is very difficult to perform an openhole sidetrack (OHST) in such hard and abrasive formations, compared to more porous carbonates.

This paper will cover some of the key steps in optimizing underbalanced coiled tubing drilling in hard sandstones from different aspects such as drilling challenging profiles at high build rates, optimizing the rate of penetration (ROP), and also increasing formation contact by successfully performing OHST to deliver multiple laterals. The paper covers the following topics:

• Optimizations in well design for lowest tortuosity and best wellbore access
• ROP optimization and reducing well delivery time
• Optimization of openhole sidetracks procedures