Fiber-optic monitoring of evaporation-induced axial strain of sandstone under ambient laboratory conditions

This paper presents the fiber-optic monitoring model and scheme for utilizing a fiber Bragg grating (FBG) sensor array to measure relative humidity. The effect of evaporation on reservoir rock involves various physical and chemical reactions under ambient laboratory conditions, which will result in a significant influence on the mechanical properties, especially for the surface of rock blocks in the fields. In this experiment, it was of great importance to better understand the effects induced by deionized water (DIW) evaporation occurring on the surface of a DIW-saturated sandstone core in terms of the induced small strains. The primary objective of this paper is to identify the applicability and feasibility of using the fiber Bragg grating sensors to monitor small strains induced by natural evaporation. This work also brings novel insights into the capability of high-precision measurements to predict the deformation mechanism of outcropped rocks, which often are subjected to natural evaporation. In this work, two arrays of fiber Bragg grating sensors were employed in the real-time monitoring of strain changes along the axial surface of a cylindrical sandstone core and to record the potential implications of the wavelength shifts induced by the evaporation factor. The experimental results could provide novel insights into the influences of natural evaporation on a rock monitored by FBG sensing technology under ambient laboratory conditions.