In high-angle wells the force of gravity pulls the drill string against the low side of the hole. This stabilizes the string and allows drill pipe to carry high axial loads without buckling. In addition to this, the small size of typical wellbores limits the deflection of buckled pipe to values that are often acceptable. These two effects make it practical to run drill pipe in compression in certain situations.
In drilling vertical wells it is common oil-industry practice to avoid loading drill pipe so that it would be unstable from a simple column-buckling point of view. Drill pipe is kept stable by making sure that the buoyed weight of the drill collars and heavy-weight pipe exceeds the weight-on-bit to be used. This practice is based on an analysis of drill-string buckling published by Lubinski in 1950.
In drilling directional wells it is common practice to use about the same bottom-hole assembly practice to use about the same bottom-hole assembly weight that would be used for a vertical well. Most operators do not add collars as hole angle increases even though they may have several thousand feet of drill pipe in compression in high-angle holes. This practice seems to work fairly well; the high practice seems to work fairly well; the high incidence of drill-string failure that would be expected if compression were really harmful does not occur. This paper is intended to justify and support this practice by showing that drill pipe can tolerate practice by showing that drill pipe can tolerate significant levels of compression in small-diameter high-angle holes because of the support provided by the low side of the hole.
The benefit of using drill pipe in compression is that the BHA weight can be kept low in high-angle drilling. This, in turn, helps reduce torque and drag, which are often operational constraints in very deep directional drilling.
This section presents two types of buckling analysis. The first analysis shows that pipe is very resistant to buckling in inclined holes. The second analysis shows that, under some conditions, pipe can buckle without harmful consequences. At pipe can buckle without harmful consequences. At the end of this section these two analyses are combined to give proposed operating guidelines for using drill pipe in compression.
Paslay and Bogy have analyzed the stability of a circular rod lying on the low side of an inclined circular hole. As is shown in the Appendix, their result may be simplified to yield the following expression for the critical compressive load:
This equation should be used to predict the onset of buckling in inclined holes. It replaces the simple Euler column-buckling condition:
The Paslay-Bogy analysis predicts a much higher buckling load than would be calculated from the Euler analysis. Consider, for example, 1000 ft of 5-in., 19.5-ppf drill pipe in an 8.5-in. hole at a 45 deg. angle. Equation (1) gives a critical load of 35,000 lb in this case while Eq.(2) gives a critical load of only 29 lb. This illustrates the profound difference between the two types of buckling analysis.
The reason that pipe in an inclined hole is so resistant to buckling is that the hole is supporting and constraining the pipe throughout its length.