Impact of Formation Heterogeneity on Efficiency of Hole Enlargement While Drilling (HEWD) Operations

ABSTRACT: The search for new oil and gas reserves has taken Operators to ultra-deepwater regions with water depths exceeding 3000m. This requires deeper wells and complex lithology, which necessitates multiple casing sizes and subsequently, hole enlargement operations. Hole enlargement poses challenges to the drilling operation such as low rates of penetration, vibration resulting in damage to the drilling assembly, and wellbore stability. The durability, stability, and drilling efficiency of the drillstring, drill bit and underreamer blocks are essential to delivering a project on time and on budget. Using a standard drilling assembly commonly used in the region as a base, an advanced 4D (3D + time) finite element transient drillstring dynamics modeling package was used to improve the cutting structures, and drilling dynamics, between different drilling assembly configurations. A detailed post-drill analysis is presented for the stability, reliability/robustness, and measurement quality with respect to the different formations encountered. This work resulted in the implementation of an improved, fully digital bit design and drillstring workflow that is formation-specific, integrating calibrated 4D dynamic simulation model, service provider and oil company experience to reduce risks when enlarging challenging post-salt formations in Brazil. 1. INTRODUCTION Drilling can be described as the process of making hole in the ground by means of a surface drive, downhole drive, or a combination of both. Drilling is essential to extracting and unlocking the potential of underground resources such as Oil & Gas. However, Oil & Gas resources become scarcer, they become less accessible, resulting in more complicated drilling (Ultra-deepwater, hard formations, high pressure, high temperature, extended reach wells etc.). The complicated nature of these wells results in more casing strings and the need to enlarge the hole while drilling. Hole enlargement while drilling (HEWD) is the process of using a reamer above the bit to enlarge the pilot hole drilled by the bit. HEWD is primarily used to enlarge the hole below a casing shoe to a size equal to or larger than the previous casing OD or to drill a pilot hole with a small diameter required for logging while drilling. This has pushed operators and service providers to improve the speed and length of which a bottom hole assembly (BHA) drills, also known as drilling efficiency.