Some Technical and Economic Aspects of Stabilizer Placement
Authors: Walker Bruce H. 
Published: December 2007

Here is a new procedure for arriving at a bottom-hole assembly that, with optimum bit weight, will control hole deviation and reduce the severity of dog-legs. An assembly selected on the basis of this theoretical analysis can improve drilling performance and reduce drilling costs.

Introduction

The pros and cons of different bottom-hole assemblies have been debated for more than 20 years. Theoretical and experimental studies have suggested a diverse set of techniques to improve drilling performance and reduce drilling cost. Recently, a performance and reduce drilling cost. Recently, a comprehensive economic justification for using sufficient stabilization with diamond bit drilling has been completed. Several of the parameters used to define sufficient stabilization can be combined by a new theoretical analysis of down-hole activity. An examination of the trends leading towards this theory, a description of the theory, and a consideration of the theory's applications can yield the next building block in the development of improved bottom-hole assemblies. In the early history of rotary drilling, a bottom-hole assembly often was made up simply of plain drill collars. The rig was located over the target area, and by varying bit weight and by redrilling, the path of the borehole was controlled until total depth was reached. In the early 1900's, the industry was just beginning to understand and correlate the variables affecting hole deviation. Occasional excessive deviation caused the industry to feel that hole deviation should be limited to a range of 3 degrees to 5 degrees. This limit demanded using very low bit weights, often resulting in very low penetration rates. Two methods of increasing penetration rate by improved understanding of down-hole penetration rate by improved understanding of down-hole activity evolved from numerous papers and speeches of the 1950's and early 60's.

Dog-Legs vs Gradual Hole Deviation

The first method evolved from the contractual maximum allowable hole angle of 3 degrees to 5 degrees, which was criticized severely in more than 25 papers published before 1962. It was reasoned that the hole published before 1962. It was reasoned that the hole angle itself was not nearly so important as the rate of change of hole angle. A rapid change in hole angle or direction may, for example, cause key-seats, failures in drillpipe or collar connections, or excessive wear on the drillstring. A gradual change in hole deviation has little influence on drilling, completion, or production. It was reasoned, therefore, that a rapid change in deviation or direction (rather than hole deviation of itself) was usually the basis of hole problems. Theories on the measurement of the over-all change in hole deviation and direction, on dog-leg severity, and on the limits of these dog-legs were available by 1962. These limits are indicated in API's "Recommended Practice for Drill Stem Design and Operating Limits." As a result of the published arguments and the developments in well-surveying practices, it is now generally considered acceptable practices, it is now generally considered acceptable to allow holes to deviate gradually to obtain higher penetration rates. If the formation characteristics are penetration rates. If the formation characteristics are known, this can be accomplished by locating the rig to allow for the expected wellbore drift. Alternatively, the size of the planned target area may be increased. Modern drilling practices are directed toward limiting dog-leg severity while using high bit weights, even at the expense of gradual hole deviation.

JPT

P. 663