Characterization of viscoelastic properties of an epoxy composite and simulation of time dependent deformation of an annulus seal under load.

Abstract: A new sealing stand-alone tool, i.e. CannSeal to produce a barrier packer or zonal isolation in oil/gas wells has been developed by a Norwegian company (AGR CannSeal AS). The tool allows injection of a fluid into the annulus between the production casing and the surroundings in an oil/gas well. After injection the fluid will solidify and form an annulus solid plug which will prevent/reduce axial flow in the annulus volume. During operation and testing of well integrity the plug may be subjected to high pressure gradients, e.g. 250 bar. AGR CannSeal has developed epoxy formulations for different applications for use with the CannSeal tool. After curing the epoxy will form a solid, or more specifically a cross-linked polymer. Since the epoxy plug may be exposed to large differential pressure for long times it is of great interest to study the viscoelastic properties and estimate the severity of the time dependent deformation (creep) by FEA simulations, considering the load scenarios expected in an oil or gas well. An annulus shaped epoxy plug intended for use as a barrier seal at 70°C was manufactured by injection of an epoxy formulation at 70°C and 150 bar hydrostatic pressure into the annulus of two steel tubings (5 1/2" and 9 5/8") on top of a defect packer. Curing was interrupted after 3 weeks well beyond the Gel point. Test specimens were prepared and subjected for post-curing at 120°C for 2 days after which extensive material testing at 70°C was performed, i.e. tensile testing, relaxation testing, creep testing and compression testing. The degree of curing (conversion) was estimated to be 67 % and the glass transition to about 80°C by means of DMA, DSC and HFC. Different material models were evaluated of which the Generalized Maxwell model, which assumes linear viscoelastic properties, was found to adequately describe all experimental results. The model parameters were determined by means of the method of Non-linear least squares. FEA simulations were made using the model selected and the parameters determined. Two load cases, one simulating the most sever predicted load scenario of an epoxy plug and the second simulating loading under ideal circumstances were evaluated. The simulations predicted that the most severe load case of the epoxy plug would have a safety factor of approximately 1.8 - 2,3 before reaching the estimated fracture strain, after one year of exposure to a differential pressure of 250 bar.