A laboratory program designed to determine the effects of tubular expansion and method of tubular expansion upon the mechanical properties and pipe performance of selected grades of carbon steels and martensitic stainless steels has been conducted.
Well completion design that incorporates downhole tubular expansion, a relatively new technology, requires a complete understanding of the mechanical properties and performance of the expanded tubular. The dissemination of this knowledge is required for the informed use of this technology, which can permit access to hydrocarbons that cannot be reached by conventional drilling and completion schemes.
Tubular expansion is a cold working process and alters the mechanical properties of the OCTG. Alloy system, expansion method, pipe loading conditions during expansion, and post expansion thermal treatments and its kinetics were studied. This laboratory study demonstrated that each shows pronounced effects on the final mechanical properties of the steel (i.e., its stress-strain behavior, yield, ductility, fracture toughness, and defect tolerance), residual stress in the OCTG, resistance to sulfide stress cracking, and performance of the expanded OCTG (e.g., collapse pressure).
The results of this study can be used to plan a program of materials qualification of a steel as an expandable to assure that the pipe properties and performance are fit-for-service downhole after expansion. Further, the results of this study can be used to infer the mechanical properties and performance of expanded tubular products for downhole use, e.g., for collapse.
Downhole tubular expansion is a relatively new, but growing, method of well construction that is a means to decrease the costs of drilling and completion as well as enabling the safe exploration of deeper hydrocarbon reserves. Over 325,000 feet (99,100 m) of solid pipe have been expanded downhole since the first installation on November 25, 1999. The first commercial installation was the in situ expansion of 1,000 feet (305 m) of 7-5/8 inch OD (194 mm), electric resistance seam welded, "80" grade ("550 MPa") carbon steel pipe in a well in the Gulf of Mexico.
Tubular expansion produces extensive plastic deformation (? 10 to 25%, a function of the application) in the pipe; therefore, a complete study of the effects of downhole exposure must be included for an understanding of the performance of expanded tubulars in the wellbore. Exposure to oil and gas wellbore temperatures affects the mechanical properties of carbon and low alloy steels. Cold work has been shown to adversely affect the mechanical properties and resistance to sulfide stress cracking (SSC) of these steels[2-9]; strain aging has been shown to exacerbate these effects[3,6].
In addition to the introduction of significant plastic deformation, tubular expansion increases the diameter to wall thickness ratio (D/t) of the pipe, hence its collapse pressure decreases.