Effects of hydrostatic pressure on strain measurement with distributed optical fiber sensing system

Abstract

This paper presents effects of hydrostatic pressure on strain measurements by using distributed optical fiber sensing system. Laboratory experiments on strains of metallic plates and a cylindrical sample of Berea sandstone were performed under hydrostatic pressure by using a system that measures strains by the frequency shifts of Brillouin and Rayleigh scattering in an optical fiber, together with conventional strain gauges. The measurements revealed that the radial strain of the optical fiber due to the hydrostatic pressure considerably affect the frequency shifts as well as the strain along the fiber line. The effects of two strains on the Brillouin or Rayleigh frequency shifts were clearly confirmed and provided important insights when converting both frequency shits into the fiber strains along the linear and radial directions. We have established the method to determine the Brillouin or Rayleigh frequency shift-strain coefficients and our experimental technique successfully applied to strain measurement sandstone under static pressure and to monitor deformation of a high permeable target layer during injection of fluids. These results strongly suggest the usefulness of distributed strain measurement with an optical fiber sensing system.