Mechanical behavior of drillstrings in curved boreholes with local geometrical imperfections

Abstract

The problem of elastic bending of drillstrings in deep curved boreholes with geometrically imperfect axial lines is stated based on the theory of curved flexible bars. The dependence of the force resisting the motion of the drillstring on the amplitude, spacing, and location of geometric imperfections is analyzed numerically. The numerical results are discussed Introduction. Three well drilling methods are most widely used in developing offshore oil and gas fields. One of the methods weakly depends on the seabed depth over the producing field and the offshore distance. The method employs special ocean vessels and was used to drill a 4000 m well through hard rock under the ocean bottom (3050 m) in the Gulf of Mexico [12]. Using fixed offshore platforms for well drilling at relatively shallow depths is another method. It is less labor-intensive and more popular. It is obvious that on-land drilling is the simplest method. The last two methods drill curved boreholes that reach the field at some distance from the surface location. There are extended-reach wells with a horizontal departure of more than 12,000 m, and even 15,000 m was planned [6].It is significant that curved extended-reach wells make it possible not only to target remote underground oil and gas reservoirs, but also to considerably increase the well productivity and oil/gas recovery factor. For example, the maximum recovery factor is 37% for vertical wells and 55% for curved wells. However, the directional well drilling technologies are accident-prone, and since the fraction of trouble-free wells drilled all over the world is only 67% [11], the implementation of deep-well drilling technologies necessitates theoretical simulation of the mechanical phenomena accompanying drilling operations to prevent accidents.The majority of easily accessible fields of oil, gas, and gas condensate in Ukraine are at the final stage of development. At the same time, prolific offshore hydrocarbon fields were located at depths of 7 km and more in the Black and Azov seas. Since directional deep-well drilling technologies are of relevance for the energy sector of Ukraine, it may be concluded that the mathematical simulation of the mechanical behavior of drillstrings for deep wells of various configurations is of theoretical and practical importance.The bending stability of vertical deep-well drillstrings was studied in [8, 10]; their bending and torsional vibrations were analyzed in [7]; the mechanical behavior of drillstrings in curved boreholes was studied in [5,6,[11][12][13][14] by modeling them by perfectly flexible cables and in [9] by formulating direct and inverse problems of bending of a curved elastic rod. Relevant reviews can be found in [2,3].Here we set out to determine the forces resisting the motion of a drillstring in a curved borehole with local geometrical imperfections of its axial line. We will analyze cases where these imperfections considerably reduce the mobility of the drillstring (DS) and cause its sticking (freezing).1. Elastic Bending of a Drillstri...