Constraining the full stress tensor from observations of drilling-induced tensile fractures and leak-off tests: Application to borehole stability and sand production on the Norwegian margin

Abstract

Presently, conventional geomechanical stress measuring instruments can only provide measurements at a single point. A fiber-optic (FO) based stress-sensor has the potential to make quasi-distributed stress measurements along its length. This paper reports on the development of a practical distributed fiber-optic (FO) stress sensor for geologic material. Initially, the sensitivity of various stress-sensitive FO cables was determined by subjecting them to direct strain in the laboratory. Also, the response of the fibers grouted in a number of different materials was evaluated. From these initial laboratory tests, it was clear that the range of light-loss sensitivities offered by the various cables would make it possible to custom design a stress sensor for a wide range of geologic materials, grouts, and conditions. Next, a full-scale prototype fiber-optic stress sensor was constructed, installed, and tested in a “coalcrete” test block. This prototype installation in the man-made coalcrete allowed researchers to simulate an underground installation under laboratory-controlled media and stress conditions. With this prototype, the location and magnitude of two artificially generated stress concentrations were successfully determined using an optical time domain reflectometer. Finally, the initial design of the prototype stress-sensor was modified using experience gained during the coalcrete tests, and a second generation FO stress-sensor was installed and tested in an underground coal pillar. The outcome of this research was the demonstrated capabilities of distributed fiber-optic stress sensing in geologic material.