Improved Performance with New Casing while Drilling Bit Design Elements - Development and Case Studies

Abstract

Abstract Casing while Drilling (CwD) is an effective means of mitigating hole stability and lost circulation problems in many parts of the world. In recent years this technology has become a standard part of a drilling engineer's toolkit resulting in a reduction of flat time and more cost effective means of drilling surface and intermediate hole sections. The popularity of Casing while Drilling stems from the ongoing success in the Asia Pacific region, where softer rock strength and high rig rates make it a cost effective technology. In many parts of the world, operators are faced with significant drilling hazards in areas with harder or more abrasive formations. However, unlike conventional drilling, the current selection of CwD bits has been limited for these types of applications. The design of the casing bit must improve drilling performance and effectively enhance the economics, while still retaining the ability to be drilled out without damaging the polycrystalline diamond compact (PDC) cutting structure of the drill-out bit. There are a number of Casing while Drilling bits on the market with various types of cutting structures. Some are readily drillable, utilizing soft materials but with limited longevity. Others are designed with robust cutting structures, delivering enhanced performance in hard formations, but may require additional drill-out, milling, and clean-out trips. Finding a balance between durability and ease of drill-out is an ongoing challenge. This paper will introduce new CwD bit design elements, development testing, operational efficiencies, and several case histories, and will also highlight a number of key successes - particularly the optimized drilling performance and ease of drill-out without the need for a specialized drill-out bit. Introduction Well construction costs have been escalating significantly over the past decade especially in the offshore segment where rig costs have more than tripled (Figure 1), spurring both operators and drilling contractors to search for methods of reducing drilling flat time. The century old Casing while Drilling (CwD) technology hence received renewed interest and has been further improved and deployed in many geological areas since the late 1990s. There are two mainstream CwD technologies, namely the retrievable and non-retrievable systems, sometimes referred to as the latch system and cement-in-place system, respectively (Fisher et al, 2004). As the name implies the latch system involves latching the bottom-hole assembly (BHA) into the first joint of casing. Drilling torque is transmitted to the bit either from surface via the casing string, or generated by a downhole motor as a component of the BHA, enabling the bit to turn independently of the casing. Upon reaching total depth (TD), the latched BHA (including the bit) is pulled out of the hole prior to cementing.