Optimization of Underbalanced Coiled Tubing Drilling in Hard Sandstones

Abstract

Scope & Objective 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. Methods Procedures & Process 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: Results, Observations, Conclusions The combination of technology and process has so far resulted in successful delivery of many sandstone wells with multiple laterals drilled on most of the wells, totalling more than 35000 ft in this hard and abrasive formation. The first-time success rate of openhole sidetracks was 96%. The average ROP and bit run lengths have also improved consistently. Novel/Additive Information It is expected that the recent improvements and consistency in delivering the sandstone UBCTD wells, will enable engineers to expand the horizon of UBCTD to several other fields, and tap the true potential of sandstone formations.