Integration of an optical FBG sensor into a nuclear magnetic resonance core flooding system

Abstract

A new platen was designed and developed for a nuclear magnetic resonance (NMR) core flooding cell for the integratation of fibre Bragg grating (FBG) strain sensors. The platen is made from carbon fibre material with a special channel to insert the FBG inside the cell, and it is compatible with an NMR overburden system (i.e. no metal, and also resistant to pressure, temperature and reactive chemicals) to allow, for example, advanced chemical experiments. This development has allowed us to acquire both fluid dynamics behaviour, from NMR, and deformation/elastic properties, from FBG sensors, during NMR core flooding experiments for advanced hydro-mechanical-chemical coupling characterisation. Generally, pore fluid redistribution occurs while changing the surrounding stress and/or temperature conditions of the tested material, and will generate elastic and geomechanical responses. The impact of rock–fluid interactions during NMR core flooding has not been evaluated until now, mainly due to technical limitations. Fortunately, the FBG sensing technique is a localised/discreet micro-strain gauge (8 mm long and 125 micro thick), and it is applicable to the monitoring of pseudo-tomography during core flooding experiments. We recently managed to attach eight FBG sensors on a carbonate rock plug to monitor strain alteration during brine imbibition under 800 psi confining pressure. The results validate the ability of the FBG sensors to track mechanical strength alterations due to pore pressure changes and also to detect waterfront velocity during injection. The integration of FBG sensors into the NMR core flooding system has added a new capability to the NMR overburden system, enabling it to simultaneously monitor material deformation with FBG sensors during pressure/temperature alterations as a first basic application.