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Abstract Most drilling problems have been accepted as an inherent part of the drilling process. As a result, there has been a great amount of effort put forth to find solutions for said undesirable events after they occur. Much of the industry's current drilling activity is happening in challenging, problematic areas such as depleted sands, HPHT wells, shale gas, deep water, etc. Preventing drilling problems is essential to the economics of such projects. Casing Drilling has been successfully practiced for more than a decade, and has proven to be a unique approach in reducing drilling problems and associated NPT. Some of the most notorious drilling problems are stuck pipe, well control events, lost circulation, wellbore instability, and drilling induced formation damage. Since there is no tripping of the drill string during Casing Drilling, this helps with many of the above mentioned problems. Most importantly, this helps with well control issues. Also, the small annulus brings about a high annular velocity that eliminates a lot of cutting transport issues, especially at critical angles. Other benefits of Casing Drilling relate to casing contact with the wellbore wall and the well-known, "Plastering Effect". The large diameter of the casing ensures that the casing will contact the wellbore wall smoothly and smear the cuttings, filter cake, and the added LCM into the pore spaces. This creates a very high quality, thin, almost impermeable mud cake that isolates the formation. The plastered filter cake prevents stuck pipe and lost circulation, improves wellbore stability, and reduces the formation damage caused by filtrate invasion. Casing Drilling has proven to be a reliable option to eliminate drilling problems. It saves time, cost, and the frustration of unplanned events. It also significantly increases drilling performance and keeps rigs drilling. Introduction Casing Drilling is a process in which a well is drilled and cased simultaneously. The original purpose of developing Casing Drilling technology was to eliminate Non Productive Time (NPT) associated with tripping and running casing. During early implementation of the technology, other benefits were seen while drilling with large diameter casing as less drilling problems were observed even though it was originally thought that drilling with the large pipe would instigate more problems. Years of drilling and exploiting petroleum reservoirs has left the drilling industry with a much more complex environment. Current drilling applications are frequently located in troublesome zones, depleted reservoirs, and wells with severe wellbore instability. Casing Drilling has been used in numerous difficult wells and to drill through troublesome well sections that would not have been possible with conventional drilling techniques. The big question is; what happens when we drill with casing instead of conventional drill pipe. In other words, how can we explain the Casing Drilling benefits in regards to wellbore stability? In this study, the authors try to answer this question in two sections. The first section relates the benefits of Casing Drilling methodology and inherent differences with conventional drilling. Plastering Effect is introduced and analyzed as a dominant contributor to the unexplained advantages of Casing Drilling. |
